Assessment of the Ergonomic Quality of Hand-Held Tools and Computer Input Devices [1 ed.] 9781607502777, 9781586037888

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Copyright © 2007. IOS Press, Incorporated. All rights reserved.

ASSESSMENT OF THE ERGONOMIC QUALITY OF HAND-HELD TOOLS AND COMPUTER INPUT DEVICES

Assessment of the Ergonomic Quality of Hand-Held Tools and Computer Input Devices, IOS Press, Incorporated, 2007. ProQuest

Ergonomics, Human Factors and Safety

Copyright © 2007. IOS Press, Incorporated. All rights reserved.

Volume 1

ISSN 1874-8694

Assessment of the Ergonomic Quality of Hand-Held Tools and Computer Input Devices, IOS Press, Incorporated, 2007. ProQuest

Assessment of the Ergonomic Quality of Hand-Held Tools and Computer Input Devices

Edited by

Helmut Strasser

Copyright © 2007. IOS Press, Incorporated. All rights reserved.

Ergonomics Division, University of Siegen, Germany

Amsterdam • Berlin • Oxford • Tokyo • Washington, DC

Assessment of the Ergonomic Quality of Hand-Held Tools and Computer Input Devices, IOS Press, Incorporated, 2007. ProQuest

© 2007 The authors and IOS Press All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without prior written permission from the publisher. ISBN 978-1-58603-788-8 Library of Congress Control Number: 2007934606 Publisher IOS Press Nieuwe Hemweg 6B 1013 BG Amsterdam Netherlands fax: +31 20 687 0019 e-mail: [email protected]

Copyright © 2007. IOS Press, Incorporated. All rights reserved.

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LEGAL NOTICE The publisher is not responsible for the use which might be made of the following information. PRINTED IN THE NETHERLANDS

Assessment of the Ergonomic Quality of Hand-Held Tools and Computer Input Devices, IOS Press, Incorporated, 2007. ProQuest

Assessment of the Ergonomic Quality of Hand-Held Tools and Computer Input Devices H. Strasser (Ed.) IOS Press, 2007 © 2007 The authors and IOS Press. All rights reserved.

v

Copyright © 2007. IOS Press, Incorporated. All rights reserved.

Preface The quality of products and the management processes employed for their production, represents an important criterion that can set a company apart from competitors. This among other things is one reason the evaluation and certification of products, companies, and people have become quite popular nowadays. Also, the International Ergonomics Association (IEA) is currently developing standards for Ergonomic Quality in Design (EQUID) which primarily intend to promote ergonomics principles and the adaption of a process approach for the development of products, work systems and services. Whereas certification criteria are being defined which require the comprehensive and systematic application of human factors considerations throughout the product development cycle, hitherto, no specified generally applicable criteria in EQUID exist which address specifically the ability of products to meet user needs and their compatibility with user limitations and capabilities. Probably, in the future this will be difficult to achieve. Furthermore, certification bodies all too often use only formal and clearly understandable criteria because “real” quality is difficult to quantify. Since the term ergonomics – which suggests quality – is sometimes used arbitrarily in a trend which may be called “Ergomania,” skepticism about so-called ergonomic products is appropriate. People often think that, if they understand, for instance, the size of a person’s hand for a hand-held product, then they can call it an ergonomic product. Yet, beneath the surface of a pretty design, the quality of a product which claims to be ergonomic is often questionable. Ergonomics is more than just anthropometric considerations, as many engineers and designers often think. According to the definition of the IEA, it is a scientific discipline concerned with the understanding of interactions among humans and other elements of a system and a profession that applies theoretical principles, data, and methods to design in order to optimize human well-being and overall system performance. Thus, it is important to assess the ergonomic quality of products, hand-held tools and computer input devices via interaction through working processes that represent reality. Well-designed working tools can be expected to reduce or eliminate fatigue, discomfort, accidents and health problems and can lead to improvements in productivity and quality. Furthermore, absenteeism, job turnover, and training costs can positively be influenced by the working tools and the environment. Not all these short-term and long-term issues of working tools can be quantified in pragmatically oriented ergonomic research approaches. But multi-channel electromyography, which enables the measurement of the physiological costs of the muscles involved in handling tools during standardized working tests, and subjective assessments of experienced subjects enable a reliable insight in the essential ergonomic criteria of working tools and products. In this respect it is advantageous to provide a test procedure, in which working tests can be carried out alternatingly both with test objects and reference models. The introductory Chapter 1 describes a systematic approach for the analysis and ergonomic design of hand-held tools and controls. Striving for holistic rather than sectoral goals and considering interdependencies between the various design criteria, a systematic ergonomic layout of the hand side of tools with respect to shape, dimensions, materials, and surface must always be preceded by a thorough analysis that examines, for example, what needs to be performed with the tool, under what conditions, and where and which type of grip and coupling will be required. In addition to this European approach, Chapter 2 presents an approach to ergonomics evaluation, design and testing of hand tools from an American point of view. As visualized already by some reallife examples in Chapter 1, for the purpose of this chapter, ergonomics evaluation of a hand tool deals with an existing (non-ergonomically designed) tool of a particular kind to identify the shortcomings or deficiencies for designing or redesigning an ergonomically sound hand tool, in terms of selected criteria. Such criteria are identified as mechanical output of the tool, and impact of the tool on the operator in terms of working posture, fatigue, type of grip used, local hand pressure and injury risk. For a true ergonomic design of hand tools, in both the introductory chapters special emphasis is given to

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Preface

the physiology and anthropometric characteristics of the hand and hand grip. Compatibility between human factors, i.e., the dimensions of the hand, fingers and finger phalanges as well as type and movement range of the joints in the hand-arm-shoulder system, and technical system elements is used as guiding principle. Chapter 3 describes a knowledge-based system for utilizing electromyography as a powerful objective method for the evaluation of the ergonomic quality of hand-held tools. Utilizing multichannel recording devices, comprehensive physiological responses of muscles involved in manual work can be quantified in figures and numbers, whereby more or less ergonomically designed tools lead to different physiological costs in terms of muscle strain associated with work. Since repetitive manual movements have to be carried out both during operating hand-held tools and during materials handling (in assembly lines or in supermarket checkouts), from an ergonomics point of view it is important to reduce strain of the operator by avoiding unfavorable and providing favorable movement directions of the hand-arm system. For this reason, Chapter 4.1 contains fundamental information on electromyographically determined physiological costs associated with translatory movements of the hand-arm system in the horizontal reach. Similarly, Chapter 4.2 presents data on operational output (torque strength) and muscle strain associated with inward and outward rotations of the arm which have to be obeyed for a suitable working technique. The main objective of this study was to quantify the influence of hand preference and rotatory movement direction, i.e., inward and outward rotation of the arm on torque strength as well as physiological costs of the main muscles involved. As for screwing in, a right-handed person normally uses supinations (outward rotations) of the dominant right arm which are weaker than pronations of the subdominant left hand, it would be advisable rather to apply pronations of the subdominant hand. For unscrewing, in any case, the dominant right hand guarantees that a tightened screw can be loosened with less effort. Chapter 5 deals with conventional and ergonomic keyboards as main computer input devices. Chapter 5.1 provides basic information on the degree of muscle strain of the hand-arm-shoulder system (by standardized Electromyographic Activity sEA [%] of 8 muscles) during alternatingly typing at conventional and ergonomic keyboards in long-lasting working tests. On the one hand, the study delivered consistent results which give statistically reliable insight into the time-varying degree of strain of the various muscles during typing. On the other hand, the results enable an objective evaluation of the keyboards. These were in favor of the ergonomic split keyboard which has been designed with slightly angled keys and a pantile-like inclination of the two keyboard halves to reduce ulnar deviation of the wrist and pronation of the forearm. The positive effects on muscle strain associated with the test keyboard, however, were not as strong as the results shown in Chapter 5.2, i.e., the estimated and experienced subjective assessments via specifically designed questionnaires given to the test subjects prior to and after the working tests. Chapters 6 and 7 demonstrate that an armrest or also a wrist rest can reduce or even prevent physical complaints which often arise while typing at keyboards that requires longer periods of time. Continuously measured electromyographic activity (EA) of the most important muscles, as indicator of physiological costs, was substantially lower when using the armrest or the wrist rest. Relating EA values without the working aid to those with the working aid, shows that working without the working aid is far more strenuous than working with it. For instance, muscular strain of the descendent part of the trapezius, which keeps the shoulder in position and which always is a bottleneck muscle for sedentary work, is around twice as high as with the two working aids. As a safe sign of a rapidly beginning fatigue, strain of this muscle exhibits an increasing tendency within the 10-min blocks of continuous typing and from one block to another. For the three functional parts of the deltoid muscle which are involved in forward and backward moving, and in abducting of the upper arm, muscle strain is even up to 4 times higher without the armrest. For the wrist rest which reduces muscle strain of the upper arm and shoulder generally less effectively, nevertheless, the effects are statistically significant. This means this working aid also helps to save a lot of physiological costs which otherwise has to be paid by the muscles when working without it. The subjective assessment after the tests under the impression of the own working experience corresponds well with the objectively measured physiological data.

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Preface

vii

Chapter 8 reports on a detailed study during which an ergonomically designed handle of a mason’s trowel was tested in comparison with two standard types. Under well-controlled conditions, physiological costs of muscles associated with mixing and throwing of mortar onto a vertical wall, translatory carrying and depositing of sand on a horizontal wall, rotatory scoping movements with and without an external load of the trowel, and static holding of the tool in different working postures were measured. The ergonomic quality of the handles was rated by means of a questionnaire. The specific relief of strain, e.g., in the grip musculature and the ulnar deviation muscles, when using the ergonomic model, though significantly proven, is much less in scale than was expected from subjective assessments. This makes clear that a numerical quantification of the ergonomic quality based only on subjective rating data of the subjects would have led to an essential overestimation of the ergonomic handle. This handle, no doubt, proved to be better than the standard models; however it was not found to be several times as good as is suggested by the results from subjective rating. Chapter 9 reports on a similar study focussing on the assessment of the ergonomic quality of file handles. Due to substantial differences between electromyographic, i.e., objective data and the subjective evaluation, inferences have to be drawn that only the combination of subjective surveys and objective measurements represent the opportunely to assess the ergonomic quality of working tools adequately. Chapter 10 provides useful and important information on screwdrivers, i.e., the most widely used tool which can be found in every toolbox, oftentimes even in several sizes. Although a detailed description of ergonomically optimal screwdriver handles exists for quite a long time, models on the market do not always exhibit, e.g., the shape and dimensions that would follow from the hand’s anatomy. As a result, complaints, muscle pain, and blisters oftentimes occur. Chapter 10.1 describes the results of a comprehensive study in which the ergonomic quality of 11 professional-grade tools was tested in terms of maximum achievable torque, physiological strain, and subjective rating of various design criteria and complaints, e.g., pressure marks and blisters in the palm by experienced test persons. Maximum exertable torques and associated muscle strain were not only measured in a power grip during pronation and supination but also when the tool’s surface was altered due to practical working conditions from a clean to an oil-contaminated handle. The results of the study, which reflect the advantages and shortcomings of the different models’ specific design, were used by several manufacturers to improve their products. The study described in Chapter 10.2 was carried out with a limited set of test models of the preceding chapter as a follow-up investigation into maximum exertable torques and physiological costs. It enabled testing the reliability of methods applied. In the studies described in Chapters 10.3 and 10.4 independent parameters were gender of user, handle (4 and 5 commercially available screwdrivers, respectively) and blade length. The dependent parameters were the maximum supination torque in a static task, physiological responses of the outward rotator of the arm and the grip musculature, and a discomfort rating for the upper extremity under a dynamic task. Amongst other results, it could be shown that blade length is not significantly related to any dependent measure. Chapter 11 deals with the product-ergonomic evaluation of diagonal cutter handles, i.e., a typical two-legged tool which has to be operated dynamically. The results of the study were gained in the “status nascendi” of a new tool and, therefore, could be used by the manufacturer for improving his product. Chapter 12 propagates the concept of “snap-on-handles” matched with the proper hand size with a fixed hacksaw blade. The ergonomically designed hacksaw handles were tested/compared with conventional/market handles, in terms of performance or productively, muscular effort, and subjective scores. The experimental results conclusively proved that the ergonomically designed handles were significantly better than the other handles in terms of the stated criteria. The objective of the studies described in Chapters 13 through 15 was to assess the ergonomic quality of hand tools and working devices which demand bi-manual working in a closed kinematic chain. Since, additionally, several control elements were attached to the tested electrically-powered hedge-clippers, fire fighting nozzles and ambulance cots, this caused complex scheduled test sequences during which the various ways of handling the tools and operating the controls were tested by

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Preface

extensive subjective ratings and work-physiological measurements. Comfort and discomfort as well as considerable details in the design and arrangement of levers, knobs and handlebars could not be evaluated by work-physiological methods but were duly reflected in subjective ratings at bipolar 4-step scales provided for the items of structured interviews. While comparing the results, interestingly, fire nozzles and ambulance cots of producers which pretended to have been designed ergonomically and were even more expensive than others, did not live up to their promise. The studies in these final chapters are examples of how misleading the term “ergonomic” for a tool can be when the producer does not have a comprehensive understanding of ergonomics. It is not enough to call a hand-held product “ergonomic” when ergonomic considerations, as it is all too common today, are limited to the proper coupling of hand and tool, as it was the case for a pistol grip in a fire fighting nozzle. Ergonomics far exceeds traditional measurements of body parts and involves at the very least an understanding of dynamic situations with respect to the interaction of the user with the product in a comprehensive working environment. To perceive ergonomics as nothing more than an anthropometric consideration is to miss the major part of the point. Instead of this, the term has to be defined as the “study of the efficiency of persons in their working environment”. This amounts to considerably more than percentiles of limb segments, so that the transatlantic term “human factors engineering” seems to be more appropriate. In the long run, it would be detrimental to the science of ergonomics if the label “ergonomic” would be carelessly handed out by designers or solely on the basis of very popular paper and pencil tests or checklists (as they produce clear yes/no decisions) or based only on relatively simple subjective ratings. Results from subjective ratings, as shown in several chapters of this book cannot substitute fully objective measurements, are not free of bias and uncontrollable transfer effects. Therefore, they must be corroborated, validated, and possibly related via objective measurements, e.g., performance and electromyographic registrations. Only via a multidimensional approach can a more consistent result in the evaluation of a hand tool be reached. For efficient working with tools, purchasing decisions should not almost exclusively be based on monetary considerations but in the long run, also in due form “physiological costs” which must be “paid” by the operator and subjectively felt complaints have to be considered.

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I wish to extend my sincere thanks to all authors and to Dr. Hartmut Irle who did a great job in improving the colored figures and in formatting the book. The cooperation of IOS Press and in particular Dr. Einar H. Fredriksson, Publisher, IOS Press is duly acknowledged. Prof. Dr.-Ing. habil. Helmut Strasser

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Siegen, 2007

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Contents Preface...................................................................................................................................................... v

Chapter 1

Design Principles for Hand-Held Tools and Hand-Operated Controls

A Systematic Approach for the Analysis and Ergonomic Design of Hand-Held Tools and Control Actuators – Visualized by some Real-Life Examples ........................................................ 1 H. Strasser and H.-J. Bullinger Chapter 2

Design Principles and Evaluation / Testing of Hand Tools

Ergonomics Evaluation, Design and Testing of Hand Tools ............................................................... 23 B. Das Chapter 3

Electromyography – Physiological Costs of Work

A Knowledge-Based System for Utilizing Electromyographic Methods for the Measurement of Physiological Costs Associated with Operating Hand-Held Tools and Computer Input Devices ................................................................................................................ 41 H. Strasser, K. Kluth and E. Keller Chapter 4.1

Favorable Translatory Movements of the Hand-Arm System

Electromyographically Determined Muscle Strain Associated with the Direction of Manual Movements in the Horizontal Reach ....................................................................................... 57 H. Strasser and K.-W. Müller

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Chapter 4.2

Hand Preference and Rotatory Movements of the Arm

Effects of Hand Preference and Direction of Rotation on Screwdriver Torque Strength and Physiological Costs of Muscles Involved in Arm Pronation and Supination ...................................... 67 H. Strasser and B. Wang Chapter 5.1

Keyboards

Muscle Strain of the Hand-Arm-Shoulder System During Typing at Conventional and Ergonomic Keyboards .......................................................................................................................... 75 H. Strasser, R. Fleischer and E. Keller Chapter 5.2

Keyboards

Estimated and Experienced Subjective Assessment of the Ergonomic Quality of a Keyboard ........... 89 E. Keller, R. Fleischer and H. Strasser Chapter 6

Armrests

Ergonomic Evaluation of an Armrest for Typing .............................................................................. 101 E. Keller and H. Strasser

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Contents

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Chapter 7

Wrist Rests

Electromyographic and Subjective Evaluation of a Wrist Rest ......................................................... 111 E. Keller and H. Strasser Chapter 8

Masons’ Trowels

Evaluation of the Ergonomic Quality of Masons’ Trowels ............................................................... 127 H. Strasser, B. Wang and A. Hoffmann Chapter 9

File Handles

Assessment of the Ergonomic Quality of File Handles ..................................................................... 143 K. Kluth, H.G. Kellermann and H. Strasser Chapter 10.1

Screwdrivers

Ergonomic Quality and Design Criteria of Professional-Grade Screwdrivers ................................... 153 K. Kluth, H.-C. Chung and H. Strasser Chapter 10.2

Screwdrivers

Maximum Torque and Muscle Strain While Using Screwdrivers with Clean and Contaminated Surfaces in Bi-Directional Use ................................................................................... 173 E. Keller, T. Özalp and H. Strasser Chapter 10.3

Screwdrivers

Torque Levels, Subjective Discomfort, and Muscle Activity Associated with Four Commercially Available Screwdrivers Under Static and Dynamic Work Conditions ...................... 183 M.-J.J. Wang, C.-L. Lin, Y.-C. Shih, H.-C. Chung and H. Strasser

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Chapter 10.4

Screwdrivers

The Effect of Screwdriver Handle Design and Blade Length on Muscle Activity and Torque MVC ............................................................................................................................... 191 M.-J.J. Wang, C.-L. Lin, Y.-C. Shih and H. Strasser Chapter 11

Pliers Handles

Product-Ergonomic Evaluation of Diagonal Cutter Handles ............................................................. 197 K. Kluth, D. Zühlke and H. Strasser Chapter 12

Hacksaws

Ergonomic Snap-On-Handles for a Hand-Powered Hacksaw ........................................................... 207 B. Das Chapter 13

Hedge-Clippers

Handle Design of Hedge-Clippers Assessed by Means of Electromyography and Subjective Rating ......................................................................................................................... 227 J. Böhlemann, K. Kluth, K. Kotzbauer and H. Strasser

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Contents

Chapter 14

xi

Fire Nozzles

Assessment of the Ergonomic Quality of Fire Nozzles ..................................................................... 239 K. Kluth, O. Pauly, E. Keller and H. Strasser Chapter 15.1

Ambulance Cots

Ergonomics in the Rescue Service – Part 1: Strain-Oriented Evaluation of Ambulance Cots .......... 255 K. Kluth, E. Keller and H. Strasser Chapter 15.2

Ambulance Cots

Ergonomics in the Rescue Service – Part 2: Subjective Evaluation of Ambulance Cots .................. 267 K. Kluth and H. Strasser

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Author Index ...................................................................................................................................... 281

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Assessment of the Ergonomic Quality of Hand-Held Tools and Computer Input Devices, IOS Press, Incorporated, 2007. ProQuest

Assessment of the Ergonomic Quality of Hand-Held Tools and Computer Input Devices H. Strasser (Ed.) IOS Press, 2007 © 2007 The authors and IOS Press. All rights reserved.

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Chapter 1

A Systematic Approach for the Analysis and Ergonomic Design of Hand-Held Tools and Control Actuators – Visualized by some Real-Life Examples H. Strasser and H.-J. Bullinger 0. Summary To this day, handymen and employees use tools and controls, i.e., hand-operated tools that are freely movable or in a fixed position. In order to avoid cumulative trauma disorders, work-related illnesses, or even occupational diseases especially during repetitive use, the tools must satisfy the equation “suitable for the human body = suitable for the hand.” That is, the aspect of compatibility in the basic ergonomic design must take the characteristics of the human hand-arm system, e.g., the motion ranges as well as the limits of the various joints, into consideration. Accordingly, a systematic ergonomic layout of the hand side of tools with respect to shape, dimensions, materials, and surface must be preceded by a thorough analysis that, for example, examines what needs to be performed with the tool under what conditions and how and where and with which type of grip and coupling it needs to be performed. The analysis as well as the subsequent design must always strive for holistic – rather than sectoral – goals in a systematic fashion and must consider interdependencies between the various design criteria. Several real-life examples in conjunction with evaluation studies to test the ergonomic quality of hand-held tools with electromyographic and subjective methods demonstrate the usefulness of such a systematic approach. It is furthermore helpful for the appropriate selection of truly ergonomically designed hand tools from an assortment of several variants.

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1. On the importance of hand-held tools over time Hand-held tools have a long tradition. For centuries, their use provided the solid foundation for craftsmen and thus large parts of the population. During the prime time of craftsmanship, they offered an amazingly high level of user-friendliness since they were manufactured based on the human body’s dimensions. Journeymen took their high-quality and sometimes lovingly adorned precious tools with them from job to job when they took to the road. Contrary to today’s practice in an industrial setting, such tools were not intended for multiple uses and the individual design measures were dominated by “popular standards” and “rules of thumb.” Extremities were truly determining the dimensions as well as the shape of hand-held tools. For example, the handle of an axe had the appropriate length if it stretched from the axle to the fingertips, if it had the width of 6 “hands,” if it was equivalent to a user’s 3 handspans or 2 feet. Unfortunately, today’s mass production does not allow for such individualization, and the shelves of hardware and home improvement stores are typically dominated by standard sizes according to the metric system. Poorly shaped tool handles that are not customized lead to losses in the transmission of power, to an unbalanced pressure distribution in the palm and fingers with resulting blister formation, and thus to adverse effects on the tactile feedback. If hand-held tools are not manufactured according to the hand and the hand-arm system with their diverse anatomic and physiological characteristics, substantial detrimental effects on work efficiency on the one hand and the human body on the other hand can be expected. This, of course, is especially true for cutting tools with an increased risk of injury. Repetitive use of poorly designed tools can even lead to work-related illnesses and occupational diseases.

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H. Strasser and H.-J. Bullinger / Analysis and ergonomic design of hand-held tools and control actuators

It is not uncommon nowadays that workers themselves try to improve on the abysmal ergonomic quality of machines and instruments by applying band-aids and retrofitting rubber sleeves to pipes and handgrips that do not deserve to be called “handles.” On the other hand, advertisers hastily and frivolously use the term “ergonomic” for a product even if a critical examination would not reach such a conclusion since only selected design approaches are realized. Casual advertising slogans in the form of rhetorical questions such as “Will this tool (the hand) last a lifetime?” with the tagline “That all depends on this tool” for the photograph of a newly developed product should be scrutinized. “Ergonomically designed” means more than a designer-created new shape of a product. 2. Compatibility as primary guideline of ergonomic work design

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Work design should always take a holistic and systematic approach that coordinates – as shown in the top part of Fig. 1 – all elements of the control loop “work.” For example, the spatial workplace design must be based on the human body’s dimensions and targets must be set for workphysiologically justifiable work courses and appealing work content that must not be simply secondary to technical requirements, simplistic, or possibly even futile.

Figure 1:

Visualization of ergonomics’ main tasks for the matching of technologically designable system elements of work to “human factors” with the human operator as control unit in a human-machine system (top) and compatibility in the matching of technical devices to human characteristics in the sensory and motor interface as well as unambiguous attribution of cause and effect or display’s and control’s effect (bottom)

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H. Strasser and H.-J. Bullinger / Analysis and ergonomic design of hand-held tools and control actuators

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The human operator as control unit cannot be expected to deliver constant output around the clock like a technical servo element. The minimization of negative influences from the physical work environment and the optimal work tool design in the sensory and motor interface must be priorities. The overall interrelationships – both with respect to the entire control loop as well as its elements – must always be critically examined. Similarly, interdependencies between the various design goals must be considered in order to achieve holistic rather than only sectoral work conditions for human beings. Improvements of details would possibly not be much more than just a cosmetic repair on a work system that is conceptually inadequate. The guideline for design measures must always be the principle of compatibility. Technical design specifications must always be based on anatomical-physiological characteristics of the human body, e.g., the shape of joints and the range of motion of the hand with its dimensions. An effort must be made in the sensory interface of a human-machine system to limit the required adaptation, accommodation, and fixation effort for the eyes in devices, such as displays and monitors, that deliver information. Appropriate design measures must be applied for optimal information reception to achieve the goal of stimulus-stimulus compatibility (cp. bottom part of Fig. 1). Analogously, achieving response-response compatibility must be the guiding principle in the design of the motor interface (cp. STRASSER 1993). That is, in meeting the equation “suitable for the human body = suitable for the hand,” it must be ensured that (response-response) compatible opportunities to intervene are created for the hand.

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3. Systematics for the design of the hand side of tools While hand-operated controls are attached in a fixed position to machines and consoles, hand-held tools are freely movable. Two-handle tools (e.g., pliers or scissors) usually require finger-dynamic motion whose range is quite limited due to the fingers’ anatomy. Tools with one handle, e.g., screwdrivers, mason trowels, files, carpet knives and cutting knives as well as saws or drills, are usually held and moved in a power or pinch grip and only occasionally require finger motion (e.g., for locking or switching). On the other hand, wide-ranging straight translation motions and rotatory turning motions must sometimes be executed by parts or all of the hand-arm-shoulder system with less precision, but more force. In addition to various design criteria of the hand side, which will be discussed in more detail in the following chapters, criteria for the work side such as the desirable direction of function and force, the resistance that must be overcome, potential precision and speed of work execution with the tool are partially determined by the design of the hand side. The systematics that were already developed by BULLINGER and SOLF in the late 1970s (cp. BULLINGER and SOLF 1979a; 1979b; BULLINGER 1994) are optimally suited for the ergonomic design of hand-held tools and hand-operated controls. As shown in Fig. 2, the actual design of the hand side of tools with respect to the correct shape and suitable dimensions, material, and surface should always be preceded by both a general and detailed analysis with various sub-items in order to scrutinize all constraints that can potentially impact on the design. If the tool or element is used to overcome high levels of resistance (force or torque), the only truly viable option is a power grip. For small repetitive forces, the contact grip is appropriate instead. For example, a computer mouse that is moved around in the power grip is not suitable for the finecoordinative work on the computer that requires a high degree of precision even if the mouse design is claimed to avert carpal tunnel syndrome and electromyographically determined relief for the shoulder muscles is meant to demonstrate its alleged ergonomic quality (cp. N.N. 2003). If both quick and precise work is required, friction coupling that allows for quick adjustments to the handle of the tool is superior to positive coupling even if the deliverable torque is only sub-optimal. Especially for outdoor work with unfavorable environmental factors (heat or cold) and conditions that are conducive to sweat on the hand, blank metal surfaces on handles should at least be covered by a layer of plastic to reduce the possibility of overheating or heat loss of the hand. Alternatively, gloves could be provided. But the use of them may require an increase in the tool’s dimensions.

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H. Strasser and H.-J. Bullinger / Analysis and ergonomic design of hand-held tools and control actuators

Figure 2:

Flow chart for the design of the hand side of tools

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3.1. General analysis In the analysis of the “work task,” a crucial question concerns what must be accomplished with the tool how, where, and under what kind of work and environmental conditions (e.g., dry or “oily” handle surfaces). A number of criteria matter in this context that have an important impact on the “blocks” in the flow chart of Fig. 2. The questions in Table 1 show topics that must be considered in a systematic work analysis. Not only because of the general need for prudence deriving from occupational medicine and ergonomics, but also because of legal-normative requirements (cp., among others, BetrSichV; BGV A1; GPSG; DIN EN 294; DIN EN 894-3; VDI 2242), safety considerations for the layout and use of hand-held tools are a top priority – even if they are not listed until the end of Table 1. The block “Position and posture of the human body” stands for body positions and postures that are required during work. Body postures are variants of the basic positions “sitting” and “standing.” Unfavorable body postures such as a twisted upper torso or a humpback lead to premature fatigue and cause long-term problems for the body’s support structure, hence they should be avoided via appropriate designs. The same applies to constrained postures of the hand-arm system, the upper torso, or the legs, which are necessitated by ergonomically poorly designed tools or difficult accessibility of the work location and tool. They involve static muscle work, which reduces blood circulation. As a result, the amounts of required and provided energy become unbalanced. Especially during two-handed tasks in a closed kinematic chain, the design should allow for preferred directions of movements during work relative to the frontal body plane, e.g., 60° during filing with a vise. For repetitive movements of the hand-arm systems (as an open kinematic chain) in the horizontal plane, an angle of 30° between frontal plane and the direction of movement was found to be optimal. Arm movements in any other direction require more effort. For an angle of 90°, for example, i.e., in the sagittal or median plane, the strain on the upper arm and shoulder musculature is approximately twice as high (cp. STRASSER and MÜLLER 1999).

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Table 1: Criteria of a systematic analysis of the work task

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For the unaided holding of heavy hand tools, solutions must be found that, e.g., permit the optimal force potential at an elbow angle of approx. 100°. Both substantially smaller and larger angles between forearm and upper arm do not allow the biceps as strong flexor of the forearm to reach its full potential. If a wide range of motion is required in conjunction with high force, the body position “standing” is generally preferable. If there is an emphasis on fine-coordinative tasks, “sitting” is always advantageous, which also tends to lower fatigue. However, backrests, footrests, and armrests are necessary to avoid fatigue if the work is carried out over extended periods of time. The analysis of the “Motion range of the hand-arm system” is an elementary chapter. Knowledge of the various joints of the hand-arm-shoulder system with their ranges of motion (cp., among others, SOMMER 1984) is essential. The individual phalanges and the bones of the forearm and upper arm are connected through many different types of joints with differing degrees of freedom. Hinge joints and pivot joints each have 1 degree of freedom, saddle joints have 2, and ball and socket joints have 3 (rotatory) degrees of freedom. If the limits of motion of peripheral joints are exceeded, the joints that are one step closer to the torso must necessarily become active. This can ultimately lead to movements of large body parts or even the torso itself, which are both cumbersome and unnecessary. As can be seen from the information in the top middle part of Fig. 2, the wrist as the possibly most important joint allows substantial translatory movements towards the back of the hand (in dorsal direction) and towards the palm (in volar direction). Nonetheless, it will be shown in a case study that even a range of 60° can be limiting for some tasks and results in constrained postures. The hand’s range of motion that is limited to approx. 30° in ulnar direction is especially problematic, however. If the hand must be moved finger-dynamically repetitively in such a position, ailments and ultimately tendovaginitis can result. As shown in Fig. 3, consideration should be given in a tool’s design to the advantages and disadvantages of a horizontal versus a vertical grip axis. For example, the hand’s limiting horizontal range of motion is not a factor with a socket wrench (cp. right part of Fig. 3) or a check lever on a lathe with vertical grip axis. Slippage of the grips is not an issue when the breakaway torque of a stuck screw must be overcome because the socket wrench is held in a positive coupling. Since joints allow straight or rotatory movements (which, e.g., are reduced from 180° with an outstretched arm to 120° with a bent arm) based on their type, attention must be paid to the compatibility of functional and anatomic joints. That is, rotatory movements should be carried out by a joint with a rotatory degree of freedom. The same applies to translatory movements with, e.g., hinge joints and ellipsoid joints.

Figure 3:

Horizontal grip axes on an operating device of a hand-knitting machine and on a socket wrench that result in restricted range of motion due to the hand’s limited ulnar and radial range (top) and vertical grip axes (bottom) that permit wide-ranging movements in the elbow and the shoulder joint using a power grip with positive coupling in a normal hand posture

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Such facts will be discussed in the block “Motion alignment of handtool and hand-arm system.” Representative for the various cases in Fig. 2, the misalignment of the saddle joint of the thumb relative to the interphalangeal (hinge) joints of the fingers in the design of scissors cannot be without consequences. A case study will discuss this further.

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3.2. Detailed analysis A detailed analysis of the “Type of gripping” must take the specific advantages and disadvantages of a contact, pinch, or power grip into consideration along with the different types of grip that the hand, in principle, can realize. While a contact grip allows for quick reactions, the fingers are not very resilient. A tool can be led with high precision using a pinch grip, but such precision is lost with a power grip in exchange for substantially higher forces and torque that can be transmitted. With power grips in which the distal, middle, and proximal phalanges of at least one finger as well as the thumb’s distal and proximal phalanges – possibly together with the entire palm – touch the handle, the coupling area is always largest and, assuming an ergonomic shape of the handle (see below), the pressure distribution in the hand is fairly even. Both positive and friction coupling, however, require a suitable dimensioning for a favorable bio-mechanical transmission of force (also see below). With pinch grips on plier-like two-handle tools, for example, all fingers touch the two shanks (legs). It must be possible for the legs to slide from the distal phalanx via the intermediate phalanx to the proximal phalanx when the tool is operated. The exerted force is, of course, minimal, when due to a rather large distance between the two legs only the weak distal phalanges couple. An absolutely minimum force occurs if even only the little finger’s distal phalanx is involved. The maximum force can be achieved when the proximal phalanges are also able to reach the handle legs. Conversely, high precision and sensitivity can be expected when 2 or 3 distal phalanges couple in the two-finger grip when tweezers are used or with a dynamic three-finger pinch grip to hold a pen. When only the fingertips or the finger pads couple with the surface of, e.g., keyboards or a pushbutton switch in a contact grip, quick reactions and input are possible that are time-phased by the mobility of the various fingers or the hand. The thumb as the strongest finger can be used for even higher resistance, but it has deficits with regard to speed. The analysis considers whether the expected force when making a fist in normal “Hand posture” (when hand and forearm are aligned) can actually be mobilized or whether the hand’s forced dorsal or volar posture results in drastic losses in force due to “tendon insufficiency.” For example, the activation (contraction) of the musculature on the inner side of the forearm (m. flexor digitorum superficialis) during volar flexion of the hand and thus the force of the muscle has little effect on the phalanges since the tendons in their sheaths are too long in this position – similar to a bowden cable between actuator and actuating element. Dorsal extension of the hand is unfavorable as well since it leads to input tension of the system of muscles, tendons, and phalanges. An unfavorable posture (ulnar deviation) can sometimes be the result of the geometry of a tool’s work side, e.g., the triangular shape of mason trowel handles, which may have its advantages on other criteria. Repetitive finger movements in ulnar deviated hands (limited to a mean value of approx. 30°) pose the risk of tendovaginitis since the friction-reducing fluid at the deviating points in the tendon sheaths is displaced, thus resulting in friction when the tendons are moved. The “keyboard” case study will provide more information. Finally, how the hand’s force is transferred to the tool is of crucial importance. The displayed hacksaw in Fig. 2’s block on the “Type of coupling” exhibits the less favorable friction coupling as well as positive coupling for both hands. As shown in Fig. 4, the only indirect transmission of force of the friction coupling is unfavorable for the finger musculature since higher forces are required than with positive coupling. Furthermore, positive coupling should provide for a forward-slanted grip surface similar to a handgun’s handle. It is also advantageous when the directions of force and function are aligned since tilting effects can be avoided.

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Figure 4:

Friction coupling (top right) and positive coupling (compatible with thrust) on a metal hacksaw (left) and positive-coupling pistol grip on drill tools (bottom)

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3.3. Design of the hand side of tools As far as the design of the hand side of tools is concerned, all details and characteristics that result from the general and detailed analyses must be coordinated and considered, which oftentimes results in compromises. For each type of grip and coupling, a highly distinct “shape” of the tool’s handle can result, although a number of common criteria must be realized. For example, it is advantageous to have shapes for finger or thumb contact grips on controls and on keyboards as well as on freely movable tools that exhibit concave cavities (e.g., finger depressions) that are suitable for the dimensions of the coupling phalanges (cp. the section on “dimensions”). This eliminates slippage on pushbutton switches, toggle switches, and rocker switches as well as on pivot levers. Also, a change in direction of an actuation, e.g., with a throttle, does not require a time-consuming re-grasping (cp. top part of Fig. 5). With 2-finger pinch grips (e.g., on industrial tweezers and keys), the shapes that are illustrated in the bottom part of Fig. 5 are found to be advantageous. Fan-like broadenings in the longitudinal contour of tweezers facilitate the handling. For picking up plane parts off a work surface, a tapering of the tweezers’ hand side from the “work side” is helpful and allows for a normal hand posture. Writing utensils that are held with a 3-finger pinch grip should not have the round cross section that is tapered towards the tip that is commonly found on pens. Round or hexagonal pencils are not appropriate, either, and can lead to tension in the finger musculature. The only cross section that is compatible with a 3-finger pinch grip has the rounded triangular shape that is shown in the bottom right part of Fig. 5. It provides suitable support for the thumb’s distal phalanx, the index finger pad, and the inside of the slightly lateral positioned middle finger. Rubber sheathing or slightly pressure-anthropomorphic (elastic) surfaces further help to avoid punctiform pressure. In addition to a cross section of a certain minimum size and a minimum length of the writing utensil, relaxed writing requires that the end rests on the wrinkle between the thumb and the index finger. Engraving and dental tools oftentimes benefit from a triangular design as well.

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Figure 5:

9

Basic shapes of controls and hand-held tools that are used with contact grip (top) and pinch grip (bottom), with and without compatible cross-sectional area and surface contour

In the determination of the longitudinal contour of pliers’ handles (cp. bottom left of Fig. 2), a spherical shape should always be considered since it follows the hand shape and ensures that all fingers touch the handle in a pinch grip. This also applies to a power grip for single-handle tools. High finger forces can lead to very painful outcomes with the cambered handles that conform to a standard because it is not possible for all fingers to touch the handle surface without deformation of the hand. Additionally, the fingers get pressed against each other when force is applied. “Standardization does not prevent folly,” and strictly following “Technical Rules” (cp. N.N. 1973) can sometimes lead to versions of tools that are questionable from an ergonomics point of view. For the cross section of handles on pliers, knives, and paring tools that must be held with a 4-finger pinch grip with the distal, intermediate, and proximal phalanges, the length of the middle phalanx must determine the thickness. Handles that are too thin can lead to contractions while shanks and handles that are too thick do not provide a good “fit.” The optimal design for pliers’ handles and wire cutters has a slightly rounded trapezoid cross section (cp., among others, KLUTH et al. 1999). As far as the “dimensions” are concerned, they must be based on the hand with fingers and phalanges. While the German standard DIN 33402 nowadays offers current data regarding the dimensions of the hand and fingers, percentile information for the phalanges are still missing. Even if it will not be possible for practical and economic reasons to have sizes similar to clothing, at least three size groupings for large, medium-sized, and small hands would be desirable. For scissors, that is actually absolutely essential since large eyes would lead to unacceptably poor coupling conditions for users with small thumbs and fingers. Figure 6 makes it clear that if handles, e.g., in pliers, do not have a spherical longitudinal contour for the pinch grip or for all tools that must be used with a power grip, large areas of the palm are unable to couple with the tool’s hand side. As a consequence, losses in the transmission of force or unnatural deformations of the hand occur.

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Figure 6:

Coupling area of the hand using tools with straight and curved longitudinal contour (with cylindrical and double-cone-shaped hand side) that require a power grip

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Decades ago, BULLINGER and SOLF (1979a) already recommended practical size gradations for the length and the diameter of handles. They referred to power grips and were suitable for rotatory as well as translatory motions in both frictional and direct, positive force transmission (cp. Fig. 7). For positive transmission for translatory motions, the diameters D1 and D2 could be approx. 4-6 mm smaller.

Figure 7:

Dimensions for hand power grips that are suitable for large, medium-sized, and small hands during rotatory activities with frictional coupling (top) as well as transferable torque with frictional coupling for supination (outward rotation, left) and pronation (inward rotation, right) with wooden and plastic handles of different diameters

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Pronation (inward rotation of the hand) with frictional force transmission generally, but especially with larger handle diameters permit substantially higher torque than supination (outward rotation). The reason is that the thenar is part of the coupling area during pronation. Studies with various models of screwdrivers (cp. STRASSER and WANG 1998) confirmed that the results clearly depend on the direction of the rotation, as visualized in the bottom part of Fig. 7. Additionally, it was shown that the differences in maximum torque between inward rotation of the dominant (typically the right) hand and outward rotation of the sub-dominant (typically the left) hand are in excess of 50 %. The higher operational output during pronation occurs even at lower levels of muscle strain, especially in the musculature for grasping. This is of practical importance for the sparing use of physical force during manual labor since even the sub-dominant hand during pronation is “stronger” than the dominant hand during the typical supination when screws are fastened. The “material” of choice for handles is nowadays plastic rather than wood. A wide range of possible materials exists. Plastic handles are typically acid-resistant and shatterproof and injection molding permits the simple and inexpensive production in a large variety of shapes that are suitable for the hand. Care must be taken to differentiate between greatly differing materials and “surfaces” in the context of the coefficient of friction. Since the surface and the material determine the coefficients of friction, they must be carefully chosen for all handles that are used with friction coupling. Cellulose acetate is slightly more expensive than polypropylene, but has a substantially better coefficient of friction than the latter due to its slightly wax-like surface. If coefficients of friction are too low, e.g., on brick trowels with lacquered wooden handles, slippage may occur during rotatory pivot movements. At the same time, coefficients of friction can be undesirably high. Chiseled handles and surfaces with a rough texture carry the risk of high surface pressure for the hand and contractions of the skin with resulting blistering. Smooth, micro-textured surfaces result in the biggest coupling areas, which is optimal for the transmission of force. For similar reasons, fine profiles are always preferable to rough profiles. Specification of coefficients of friction in dependence of material and surface can be found, e.g., in BULLINGER (1994) or, in more detail, in BULLINGER et al. (1979). Comparative studies (cp., among others, KLUTH et al. 2004a) have found multi-component surfaces to be particularly advantageous. Deviations from the ideal alignment of handle and hand, in particular during fast work, can be compensated via elastic, pressure-anthropomorphic surface parts. At the same time, the quasi-positive coupling leads to an increase in exertable force due to the elastic deformation. For areas on a handle with sliding coupling, hard micro-textured plastic was found to be more suitable. Pronounced malleable nubs, e.g., on socalled “power screwdrivers,” are less suitable for the hand’s surface, however. It will later be shown in a real-life example that the various design criteria must be coordinated. Selection of the wrong material can lead to worse outcomes even with the ergonomically appropriate shape. Especially with a handle that is used with friction coupling, pressure-anthropomorphic materials with soft, possibly slightly hollowed-out surfaces that conform to the hand and the convex phalanges are favorable. 4. Real-life examples 4.1. Ergonomic design of hairdresser scissors This first case study (cp. Fig. 8) is used to show that ergonomically designed tools that are highly compatible with the anatomic characteristics of the hand-arm system not only provide immediate positive effects on the hand-arm system itself but, surprisingly, also have positive indirect effects on other body regions. A study that had been commissioned some time ago by the Federal Institute for Occupational Safety and Health (the German NIOSH) showed that barbers exhibited above-average rates of work-related illnesses in the form of varices in the calves. While extensive standing with the expected increased static stress is part of the barber profession, it may still be surprising that the use of comb and scissors would lead to varicose veins that are related to the use of these tools.

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Figure 8:

Anatomic characteristics and necessary functional consequences for the compatible design of hairdresser scissors

BULLINGER and SOLF (1978) used work analyses to identify the hand’s range of motion, which was limited to approximately 60° in a dorsal direction, as the main cause, which may be surprising at first glance. In order to avoid horizontal lines in the neck area, a barber must bring the scissors’ cutting surfaces to a vertical position. Due to the hand’s limited range of motion in a dorsal direction, the barber usually must lean backwards to maintain a constant angle of the elbow. As a consequence, almost all of the body weight is carried on one leg, which leads to reduced blood circulation in the calf area due to the static stress. In the long run, the increased static pressure in this body region is detrimental to the flow of blood through the veins as well as the shape of the veins so that varices seem a plausible consequence. The anatomic characteristics that are shown in excerpts in Fig. 8 were instrumental in the redesign of hairdresser scissors that was compatible with those characteristics. For example, the redesign obeyed that the saddle joint of the thumb is crossed relative to the knuckles. It also allows for the thumb’s natural motion towards the middle finger rather than the index finger or the little finger. If a barber grasps the scissors with thumb and ring finger of the hand that is facing the customer and mainly uses the thumb in a dynamic fashion in the process, it follows that the scissors’ blade that is used with the thumb must be shortened accordingly in order to allow for the natural movement pattern towards the middle finger. Additionally, the shape and dimension of the eye for the ring finger and thumb should be based on those fingers’ dimensions. The main axis of the elliptical thumb eye should be offset by the appropriate angle to achieve compatibility between technical and anatomic joints. Furthermore, it would be evidence of a lack of understanding of ergonomic principles if the size of the scissors’ eyes would be based on the well-known motto to “base interior dimensions on the largest users (i.e., following 95th-percentile measurements) and exterior dimensions on the smallest users (i.e., 5th-percentile measurements).” While basing the scissors’ eyes on 95th-percentile measurements of fingers would allow all hairdressers with more slender fingers to use the scissors, the scissor handles would be too large for most of them. At least three different sizes are a necessity. The improvements in the details as described above can be expected to make the use of scissors more comfortable and to reduce deformities of the fingers of barbers. The crucial modification in this tool, however, is the preset positioning of the cutting surface relative to the side of the hand such that the rotation of the cutting surfaces by approximately 30° relative to the handle would eliminate the previous unnatural backward-leaning posture without the drawbacks for barbers’ work when the scissors are used horizontally on the head.

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4.2. Handle design in tools with vibration stress An unnatural hand position on vibrating handles can have grave consequences. Mechanical vibrations that are caused by shaking, rocking, and vibrating tools that impact on the hand-arm system not only lead to premature fatigue and detrimental effects on performance over time, but can also pose substantial health risks. In particular, when compressed-air powered tampers, chisels, or demolition hammers cause excitation frequencies in the frequency range of the hand-arm system, degenerative changes and damage to the bone and joint system can ultimately not be excluded. This is especially true if conventional (horizontally oriented) handles, as shown in the top left of Fig. 9, require a more or less pronounced ulnar abduction, i.e., deviation of the wrist towards the elbow, which is limited to approximately 30° because of anatomical-physiological conditions.

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Figure 9:

Ulnar deviation of the wrist and normal hand posture with different types of jackhammer handles as well as effects on the contact pressure on the tool’s work side resulting from both arm forces and flow of force in the carpal bones

If pressure is exerted on the tool in this hand position via the arms, the contact pressure on the work side results from vectors of force from the hand side in an obtuse angle (of oftentimes up to 90°) via the forearms. The ulnar abduction in the wrist, in turn, causes the entire force to run from the forearms to the hand via the lunate (one of 8 carpal bones), as shown in the bottom left part of Fig. 9. As a result, the lunate bone can over time literally be pulverized if the contact pressure and vibration stress are sufficiently high since it is not designed to withstand such stress. In addition to reduced vibration, such an abrasion of the lunate along with the associated risk of a stiff wrist (recognized as an occupational disease (BK 2103)) can be lessened if the handle can be held in a “normal” hand posture (with no ulnar abduction). This necessitates tapering of the handles downwards and towards the worker, as shown in the top right part of Fig. 9. Additionally, matching the handle’s width to the worker’s shoulders is advantageous. In this hand posture, the forces run via a substantially larger coupling area (consisting of the lunate and the substantially larger scaphoid bone) to the ulna and the radius (bones of the forearms) so that the surface pressure is substantially reduced even if the contact pressure and vibration stress are unchanged. If the handle’s width is matched to the shoulders’, the angle between the two vectors of force between the two arms is reduced. As a consequence, the resultant on the work side with constant arm forces is increased, or, alternatively, less force is required from the worker to overcome the same resistance. As a result, the strain on the worker is reduced and the risk of abrasion of the carpal bones is mitigated. Finally, it is safer to use a tool that is compatible with a normal hand posture. The risk of injuries is reduced and a positive impact on work safety is achieved even if the causative vibration stress cannot be sufficiently reduced via primary protection measures.

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4.3. Keyboards

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In addition to arthrosis deformans (BK 2103) or the vibration-induced vasospastic syndrome (BK 2104) as a result of hand-arm vibrations during finger-static work with hand-held tools (see DUPUIS 1999 and HARTMANN 2006, among others, for details), persistent problems can also develop due to the finger-dynamic operation of controls, i.e., tools that are in a fixed spatial position (cp. G 46). For example, the use of traditional keyboards may lead to static stiffness of the entire hand-arm-shoulder system as well as tendovaginitis especially in the wrists. This is in no small part due to the spatial orientation of the keys in four horizontal rows that was chosen more than a hundred years ago. Even though this layout is actually incompatible with the hand-arm system (cp. bottom left part of Fig. 10) and, in addition, results in higher strain for the left than the right hand (cp. KELLER et al. 1991), it is still the standard design of the QWERTY keyboard.

Figure 10: Standard QUERTZ keyboard according to German standards and models of keyboards with advertised ergonomic layout of keys (cp. KELLER et al. 1991)

Tendovaginitis no longer plays the prominent role as the only occupational disease in an office setting (BK 2101) as it did in the 1960s, which is mainly due to the introduction of electrical typewriters and keyboards, which require contact pressure for the fingers on the keys that is approximately only one tenth of the pressure that was required for mechanical machines. Nonetheless, traditional keyboards still lead to problems, which are due to static stiffness (myogelosis). Before going into a more detailed discussion of the illustrations in Fig. 10, it will be useful to discuss some of the hand’s characteristics. As already mentioned in the context of the systematics of tool design and Fig. 2, the hand’s possibilities for translative movements towards the back of the hand (dorsal extension) of approximately 60° and towards the cupped hand (volar flexion) of approximately 75° are relatively extensive. Contrary to that, the range of motion in ulnar direction is only approximately 30° and it is even only 15° in radial direction. Furthermore, the outstretched arm can be rotated approximately 180° around its longitudinal axis so that pronation and supination, i.e., inward and outward rotation of almost 90° is possible. This high axial ability to rotate is made possible by the ball and socket joint that

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connects the upper arm and the shoulder. The ability to rotate is reduced by 1/3 to approximately 120° when the rotation must be carried out with the lower arm when the arm is bent. The normal position of the hands can be described as having the palms face the hips when the arms are dangling. If the forearms are held at an angle of 90° relative to the upper arms – as it is typical for the typing position – then typing on a typewriter or a PC keyboard requires unnatural displacements of the handarm system. Because of the forearm’s limited range of rotation, the elbow inevitably must be abducted in order to accomplish the necessary inward rotation. This causes the forearm-hand axis to run towards the keys at an angle, however, so that additional bending of the hand is necessary for proper usage. While it is possible to work in this position, it is certainly not comfortable for extended periods of time, especially since hand and arm must be held in a static fashion. It becomes clear why the solely static optimization of numerically-based workplace design, i.e., when the human body as a support structure is only used as “support for hand and eye,” increasingly leads to stiffness not only in the hand, but also in higher-up muscle sections. Similarly, the fingerdynamic typing results in friction in the tendons, especially in the area of the deviating points in the wrist. In an attempt to at least alleviate these complaints, work-physiological (electromyographic) studies on the strain in arm muscles in dependence on the joints’ angles were carried out as early as the 1980s (cp., among others, ZIPP et al. 1981). The studies showed that strain increases disproportionally with an increase in the angle between the natural starting position and the necessary working position. Consequently, a first step must be to split a traditional keyboard into two halves that are separated by an angle (cp. top part of Fig. 10). Additionally, the two halves must exhibit a slight inclination to resemble a tiled roof. Even an angle between the two halves of only 25° to reduce the hand’s ulnar deviation and an angle of inclination of only 10° along with a corresponding reduction in the arm’s pronation were shown to substantially reduce the strain of those muscle groups that experience static strain during the angling of the hand and the pronation of the forearm. The keyboards that are shown in the middle part of Fig. 10 follow such design rules and thus represent preventive measures of occupational safety as tools that are compatible with anatomic characteristics of the hand-arm system. The one-hand chord keyboard shown in Fig. 10, bottom right, has similar features (cp. KELLER et al. 1991). By now, manifold results from electromyographic and goniometric as well as subjective studies exist that document that ergonomic keyboards provide relief for the hand-arm system (cp., among others, the bibliographies by KROEMER 2001 and MARKLIN and SIMONEAU 1997 as well as more recent studies by KELLER and STRASSER 2004 and STRASSER et al. 2004).

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4.4. Mason trowels Assuming that mason trowels are indeed meant to be tailored to the mason’s hand, i.e., that they have handles that are compatible with the hand-arm system, then a shape such as the one shown in the top right part of Fig. 11 with its straight longitudinal contour or cylindrical handle shape should not exist. Only a convex longitudinal contour allows all fingers to touch the handle without necessitating unnatural deformations of the palm. In addition to translative movements with positive coupling with such a tool, friction-coupling rotating movements, e.g., when mortar is applied to a vertical wall, are required. Therefore, a somewhat square-edged cross section is preferable to a round one because it reduces the tendency of slippage inside the fist. A rounded trapezoid-shaped cross section (as sketched on the left in Fig. 11 and shown in a photograph on the bottom right) allows a reduction in grasp force. A support for the stretched-out thumb ensures that the wrist’s motion ability is less restricted than it would be if the thumb, as the strongest finger, were integrated into the fist. Details on the critical analysis and ergonomic design of hand- and work-facing ends of tools with dimensioning can be found in BULLINGER and SOLF (1979b). Comparative studies under realistic working conditions (STRASSER et al. 1996) demonstrate the dominance of the ergonomic quality of the newly developed handle relative to standard models with and without gooseneck.

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Figure 11: Handles that are compatible with the hand longitudinally and cross-sectionally (left) and traditional handles of mason trowels according to DIN 6440 (right)

4.5. File handles

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A handle according to a German standard (DIN) is not necessarily a compatible solution for a flat or a square file (cp. Fig. 12). The reason is that the work usually involves plane surfaces so that the “feel” (by proprioceptive feedback) for the parallel positioning of the file relative to the work surface requires a certain design support. Contrary to a rotation-symmetric rounded cross section according to DIN 395, a rounded square handle as suggested by SOLF (1977) allows just that. Moreover, compatibility with the hand necessitates a large diameter at the hand’s widest part, but this is not required by the standard.

Figure 12: File handles according to DIN 395 and according to ergonomic considerations with advisable prepositioning of the vise on a workbench

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Filing typically requires a closed kinematic chain including both hands and the tool. The ideal natural preferred work direction according to BULLINGER and SOLF (1979b, 1979c) between frontal plane and work direction is approximately 60°. Because it should be possible to complete other tasks within the physiological maximum reach when the worker is facing the workbench, it can be advantageous to set up the vise at a 30° angle, especially in a training facility. File handles are available in three different sizes, which is desirable. Comparative evaluating studies on file handles (cp. KLUTH et al. 2004b) show slight advantages for the ergonomic model relative to two other models in terms of the measured strain. The electromyographically determined strain of a total of 9 involved muscle groups showed slightly favorable results for the preferred direction of 60° (between frontal surface and work direction). In the subjective rating, however, the test subjects favored a filing direction of 30°. 4.6. Screwdriver handles

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A concluding case study once more illustrates the meaning of compatibility of the tool’s hand side in the sense of the slogan “suitable for the human body = suitable for the hand” and the expected consequences for human work input. Screwdrivers are the most widely used tool and can be found even in every do-it-yourselfer’s toolbox, oftentimes even in several sizes. Although BULLINGER and SOLF (1979d) provided a description of ergonomically optimal screwdriver handles quite some time ago, current models do not always exhibit the shape and dimensions that would follow from the hand’s anatomy. As a result, complaints, muscle pain, and blisters oftentimes occur. Handles with a straight longitudinal contour and round cross section, as shown in the right part of Fig. 13, which often also exhibit a grooved, rough surface, are neither compatible with the hand’s shape for the power grip to tighten a screw or to provide necessary breakaway torque, nor are they compatible with the hand’s shape for the pinch grip to quickly tighten or loosen a loosely fitting screw.

Figure 13: Shapes of screwdriver handles that are suitable for the power grip and pinch grip with respect to longitudinal contour and cross section (left) and standard models that have the tendency to slip (right)

The only handles that truly fit the hand well are the ones that exhibit a convex longitudinal contour that is compatible with the hand’s shape. That is, they should exhibit a double cone surface and should provide a large contact surface with a suitable cross-sectional profile to support friction coupling for torque delivery. Because distal, middle, and proximal phalanges as well as the thumbs distal and proximal phalanges together with the thumb-forefinger fold create a polygon shape, favorable coupling

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conditions can be expected from a handle with such a cross section assuming that the tool is meant for rotatory use. For the introduction of axial forces, a rounded handle head turns out to be suitable for the carpal area. Flattened areas at the handle’s end that faces the blade (outside the hand’s coupling area) ensure that the screwdriver cannot roll away. The biomechanical transmission of power to develop torque greatly depends on the handle’s volume and its surface as well as its material. A smooth surface with slightly concave cavities for the convex phalanges and materials with a suitable friction provide the best grip.

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5. Test methods to rate ergonomic quality The system of work tool design can also successfully be used for the rating of ergonomic quality and the selection of tools. The advantages and disadvantages of different shapes, dimensions, surfaces, and materials must mirror the body’s characteristics. Additionally, tests that measure, for example, deliverable torques or forces under real-life conditions (cp. the monograph by STRASSER 2000) are always practical. Measuring “physiological costs,” i.e., what the muscles must “pay” for the use of the hand-held tool is much more demandable (cp., among others, details on the analysis of electromyographic time series with filtering, standardization, and extraction of ergonomically relevant parameters such as static and dynamic components in STRASSER 2002; 2004; 2006). This additional effort should be invested, however, in the “certification” by test methods. Subjective evaluations of various design criteria after standardized work tests by experienced users can easily be implemented, but the resulting data are less concrete and reliable. For example, the objectively (via electromyography) determined physiological costs of work with two models (A and B) of a tool may differ by approximately 20 %. Subjective ratings may be 1.5 for model A and 3.0 for model B on a unipolar scale from “0” to “4,” i.e., they differ by a factor of 2 or 100 %. The interpretation that work with model B is twice as strenuous as model A would not be justified, however. Thus, the possibilities of quantifying strain this way are somewhat limited and such procedures should not be used as the only method. Given the large variety of handles and hand-operated controls, e.g., on ambulance roll-in cot systems (cp. KLUTH and STRASSER 2006), however, subjective ratings are indeed useful for the rating of existing models and the derivation of design recommendations for improvement measures especially when the required effort is considered. It is oftentimes possible to subjectively “feel” things that cannot be measured using technology. Screwdrivers will be used as an example (cp. Fig. 14) to show that handles that conform to the hand’s characteristics allow higher forces and torques. Reversely, a certain level of work can be accomplished with less effort. Work studies that used specially developed product-ergonomic rating methods to test various commercially available handles found that the standard handle is the least suitable variant (cp. top right part of Fig. 14). The maximum torque that could be achieved was only approximately half as high as with the handle that satisfied ergonomic requirements particularly well (cp. top left part of Fig. 14). It is noteworthy that such operational output was achieved with almost identical physiological effort, i.e., with physiological costs that were measured in electromyographic values of the flexor digitorum as grasping muscle and the biceps as supinator. With the set point of equal sub-maximal stress of 20 % or 40 % of the individual maximum torque for 20 or 10 seconds, the determined electromyographic data showed that handles without ergonomic design require substantially more physiological resources (cp. bottom part of Fig. 14), i.e., that substantially more muscle force is required if the tool is not designed to match human dimensions. As can be seen in the left part of Fig. 15, the number of test subjects (in %) who, on a specially designed questionnaire, indicated any kind of complaint with the cupped hand, thumb, and fingers after the completion of work with the standard model is substantially higher than with an ergonomic model. The results were similar for the perceived strength of an unfavorable distribution of pressure, as shown in the right part of Fig. 15. The preventive aspect of the ergonomic solution in the sense of reducing human effort as well as avoiding pressure marks and blisters should be apparent (cp. STRASSER 2007).

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Figure 14: Maximum exertable torque (means over several test subjects) with an ergonomically designed screwdriver handle (left) and a standard model (right) with strain values of the grip and rotation musculature at constant work (delivery of 20 % and 40 % of maximum torque for 20 s and 10 s) (cp. STRASSER et al. 1990)

Figure 15: Subjective rating of ailments in various sections of the inner hand after work with an ergonomically optimized screwdriver (bottom) and a standard model (top) (means over 12 test subjects; cp. KLUTH et al. 2004)

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6. References BULLINGER, H.-J. and SOLF, J.J. (1978) Produktergonomie hilft Berufskrankheiten vermeiden. Exemplarische Darstellung der Ursachenanalyse und der Problemlösung. REFA-Nachrichten 31 (1) 17-21 BULLINGER, H.-J. and SOLF, J.J. (1979a) Ergonomische Arbeitsmittelgestaltung I – Systematik. Forschungsbericht Nr. 196 der Bundesanstalt für Arbeitsschutz und Unfallforschung, Wirtschaftsverlag NW, Verlag für neue Wissenschaft GmbH, Bremerhaven BULLINGER, H.-J. and SOLF, J.J. (1979b) Ergonomische Arbeitsmittelgestaltung II – Handgeführte Werkzeuge – Fallstudien. Forschungsbericht Nr. 197 der Bundesanstalt für Arbeitsschutz und Unfallforschung, Wirtschaftsverlag NW, Verlag für neue Wissenschaft GmbH, Bremerhaven BULLINGER, H.-J. and SOLF, J.J. (1979c) Werkzeuge – Feilenhefte. 4/79. Arbeitswissenschaftliche Erkenntnisse – Handlungsanleitung für die Praxis. Bundesanstalt für Arbeitsschutz und Arbeitsmedizin (Hrsg.), Wirtschaftsverlag NW, Verlag für neue Wissenschaft GmbH, Bremerhaven BULLINGER, H.-J. and SOLF, J.J. (1979d) Werkzeuge – Schraubendreherhefte. 3/79. Arbeitswissenschaftliche Erkenntnisse – Handlungsanleitung für die Praxis. Hrsg. Bundesanstalt für Arbeitsschutz und Arbeitsmedizin, Wirtschaftsverlag NW, Verlag für neue Wissenschaft GmbH, Bremerhaven BULLINGER, H.-J.; KERN, P. and SOLF, J.J. (1979). Reibung zwischen Hand und Griff. Der Einfluss von Material und Oberfläche auf das Reibungsverhalten zwischen Hand und Arbeitsmittelseite. Forschungsbericht Nr. 213 der Bundesanstalt für Arbeitsschutz und Unfallforschung, Wirtschaftsverlag NW, Verlag für neue Wissenschaft, Bremerhaven BULLINGER, H.-J. (1994) Ergonomie – Produkt- und Arbeitsplatzgestaltung. B.G. Teubner Verlag, Stuttgart DUPUIS, H. (1999) Erkrankungen durch Hand-Arm-Schwingungen. Handbuch der Arbeitsmedizin. Kap. IV – 3.4.1.22. Ergänzungslieferung. Ecomed-Verlag, Landsberg/Lech GRANDJEAN, E.; NAKASEKO, M.; HÜNTING, W. und LÄUBLI, Th. (1981) Ergonomische Untersuchungen zur Entwicklung einer neuen Tastatur für Büromaschinen. Z.Arb.wiss. 35 (7 NF) 4, 211-226 HARTMANN, B. (2006) Gelenkerkrankungen durch Erschütterungen bei der Arbeit – epidemiologische Grundlagen der Berufskrankheit Nr. 2103 BKV. Zentralblatt für Arbeitsmedizin, Arbeitsschutz und Ergonomie 56 (7) 184-193 ILG, R. (1987) Ergonomic keyboard design. Behaviour and Information Technology 6 (3) 303-309 KELLER, E.; BECKER, E. and STRASSER, H. (1991) Objektivierung des Anlernverhaltens einer Einhand-AkkordTastatur für Texteingabe. Z.Arb.wiss. 45 (17 NF) 1, 1-10

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KELLER, E. and STRASSER, H. (2004) Estimated and experienced subjective assessment of the ergonomic quality of a keyboard. Occupational Ergonomics 4 (2) 121-131 KLUTH, K.; ZÜHLKE, D. and STRASSER, H. (1999) Untersuchungen zur Produkt-Ergonomie von Seitenschneidern mittels elektromyographischer und subjektiver Methoden. Z.Arb.wiss. 53 (25 NF) 2, 120-130 KLUTH, K.; CHUNG, H.-C. and STRASSER, H. (2004a) Verfahren und Methoden zur Prüfung der ergonomischen Qualität von handgeführten Arbeitsmitteln – Professionelle Schraubendreher im Test. Schriftenreihe Ergo-Med., Band 5, Dr. Curt Haefner-Verlag GmbH, Heidelberg KLUTH, K.; KELLERMANN, H.G. and STRASSER, H. (2004b) Assessment of the ergonomic quality of file handles using electromyographic and subjective methods. Occupational Ergonomics 4 (2) 133-142 KLUTH, K. and STRASSER, H. (2006) Ergonomics in the rescue service – ergonomic evaluation of ambulance cots. Int. Journal of Industrial Ergonomics 36, 247-256 KROEMER, K.H.E. (2001) Keybords and keying. An annotated bibliography of the literature from 1878 to 1999. Universal Access in the Information Society 1 (2) 99-160 MARKLIN, R.W. and SIMONEAU, G.G. (1997) An ergonomics study of alternative keyboard designs. Final Performance Report submitted to NIOSH, CDC and NIH, 173 pp. N.N. (1973) Werkzeugnormen. Hand-Werkzeuge. DIN-Taschenbuch Nr. 42. Beuth Verlag, Berlin N.N. (2003) Schmerztherapie für den Nervus medianus. Revolutionäre 3M-Ergonomie-Maus beugt dem Karpaltunnelsyndrom vor. ErgoMed 6, 201

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SOLF, J.J. (1977) Ergonomische Gestaltung eines Feilenheftes. Technische Zeitschrift für praktische Metallbearbeitung 71 (1) 33-35 SOMMER, K. (1984) Der Mensch – Anatomie – Physiologie – Ontogenie. Volkseigener Verlag, Berlin STRASSER, H.; LAUBER, M. und KOCH, W. (1990) Produkt-ergonomische Beurteilungsmethoden für handbetätigte Arbeitsmittel. Leistungsdaten und Beanspruchung des Hand-Arm-Systems beim Test verschiedener Schraubendrehergriffe. Z.Arb.wiss. 44 (16 NF) 4, 205-213 STRASSER, H. (1993) Kompatibilität. Kap. 2.4.5. In: HETTINGER, Th. and WOBBE, G. (Eds.) Kompendium der Arbeitswissenschaft. Kiel-Verlag, Ludwigshafen/Rhein, pp. 274-288 STRASSER, H.; WANG, B. and HOFFMANN, A. (1996) Electromyographic and subjective evaluation of hand tools: The example of mason’s trowels. Int. Journal of Industrial Ergonomics 18 (1) 91-106 STRASSER, H. and WANG, B. (1998) Screwdriver torque strength and physiological cost of muscles dependent on hand preference and direction of rotation. Occupational Ergonomics 1 (1) 13-22 STRASSER, H. and MÜLLER, K.-W. (1999) Favorable movements of the hand-arm system in the horizontal plane assessed by electromyographic investigations and subjective rating. Int. Journal of Industrial Ergonomics 23, 339-347 STRASSER, H. (2000) Ergonomische Qualität handgeführter Arbeitsmittel – Elektromyographische und subjektive Beanspruchungsermittlung. ERGON-Verlag GmbH, Stuttgart STRASSER, H. (2002) Elektromyographie in der Arbeitsmedizin – Einsatzmöglichkeiten und Grenzen. In: NOWAK, D. and PRAML, G. (Eds.) Dokumentationsband über die 42. Jahrestagung der Deutschen Gesellschaft für Arbeitsmedizin und Umweltmedizin e.V., Rindt-Druck, Fulda, pp. 240-247 STRASSER, H. (2004) Elektromyographie in der Arbeitsphysiologie. 2004 letztmalig aktualisierte Leitlinie der DGAUM, Link: www.dgaum.med.uni-rostock.de/leitlinien/EMG.htm STRASSER, H. (2006) Electromyography: methods and techniques. In: KARWOWSKI, W. (Ed.) International Encyclopedia of Ergonomics and Human Factors Vol. III. Methods and Techniques. 2nd Edition, Taylor and Francis, London / New York, pp. 3115-3118 STRASSER, H. (2007) Handwerkzeuge. In: LANDAU, K. (Ed.) Lexikon Arbeitsgestaltung. Gentner Verlag, Stuttgart, pp. 641-650 STRASSER, H.; FLEISCHER, R. and KELLER, E. (2004) Muscle strain of the hand-arm-shoulder system during typing at conventional and ergonomic keyboards. Occupational Ergonomics 4 (2) 105-119

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ZIPP, P.; HAIDER, E.; HALPERN, N.; MAINZER, J. and ROHMERT, W. (1981) Untersuchungen zur ergonomischen Gestaltung von Tastaturen. Zentralblatt für Arbeitsmedizin, Arbeitsschutz und Ergonomie 31 (8) 326-330 Standards, guidelines, regulations BetrSichV (2002) German regulation governing plant safety: Occupational safety and health act on the provision and use of work equipment, on safety precautions for operating industrial plants that require monitoring and on the organization of employment protection. Betriebssicherheitsverordnung: Verordnung über Sicherheit und Gesundheitsschutz bei der Bereitstellung von Arbeitsmitteln und deren Benutzung bei der Arbeit, über Sicherheit beim Betrieb überwachungsbedürftiger Anlagen und über die Organisation des betrieblichen Arbeitsschutzes. Carl Heymanns Verlag KG, Köln BGV A1 (2004) Accident-prevention regulation: Prevention principles. Grundsätze der Prävention. Carl Heymanns Verlag KG, Köln GPSG (2004) German equipment and product safety act. Geräte- und Produktsicherheitsgesetz. Carl Heymanns Verlag KG, Köln G 46 (2005) Berufsgenossenschaftlicher Grundsatz: Belastungen des Muskel- und Skelettsystems. Bearbeitung: Ausschuss Arbeitsmedizin, Arbeitskreis 2.2 „Belastungen des Muskel- und Skelettsystems“, Berufsgenossenschaft Metall Süd, Mainz. Arbeitsmedizin – Sozialmedizin – Umweltmedizin 40 (8) 429-440 DIN 395 (1968-12) File handles. German Institute for Standardization, Beuth Verlag, Berlin DIN 2137-2 (2003-09) Text and office systems - Keyboards - Part 2: German keyboard for data and text processing; Key arrangement and allocation of graphic characters to keys. German Institute for Standardization, Beuth Verlag, Berlin

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DIN 2137-6 (2003-09) Text and office systems - Keyboards - Part 6: German keyboard for data and text processing as well as for typewriters; Key arrangement and allocation of functions to keys. German Institute for Standardization, Beuth Verlag, Berlin DIN 2137-10 (2003-09) Text and office systems - Keyboards - Part 10: German keyboard for data and text processing as well as for typewriters; Arrangement of key positions and key distances. German Institute for Standardization, Beuth Verlag, Berlin DIN 2137-13 (1995-04) Text and office systems – Alphanumeric keyboards - Part 13: German keyboard for data and text processing; Key arrangement and allocation for split and tilted keyboards. German Institute for Standardization, Beuth Verlag, Berlin DIN 6440 (1988-05) Masons trowels. German Institute for Standardization, Beuth Verlag, Berlin DIN 33402-2 (2005-12) Ergonomics - Human body dimensions - Part 2: Values. German Institute for Standardization, Beuth Verlag, Berlin DIN EN 294 (1992-08) Safety of machinery; safety distances to prevent danger zones from being reached by the upper limbs. German Institute for Standardization, Beuth Verlag, Berlin DIN EN 894-3 (2000-06) Safety of machinery - Ergonomic requirements for the design of displays and control actuators - Part 3: Control actuators. German Institute for Standardization, Beuth Verlag, Berlin VDI 2242-1 (1986-04) Engineering design of products in accordance with ergonomics; fundamentals and procedures. Association of German Engineers, Beuth Verlag, Berlin

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VDI 2242-2 (1986-04) Engineering design of products in accordance with ergonomics; work aids and relevant literature. Association of German Engineers, Beuth Verlag, Berlin

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Chapter 2

Ergonomics Evaluation, Design and Testing of Hand Tools B. Das

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0. Summary A systematic approach to ergonomics evaluation, design and testing of hand tools with particular emphasis on hand-powered tools is presented. Basically, ergonomics evaluation and testing processes used for hand tools are synonymous. However, for the purpose of this chapter, ergonomics evaluation of a hand tool deals with an existing (non-ergonomically designed) hand tool of a particular kind to identify the shortcomings or deficiencies from an ergonomics viewpoint for designing or redesigning an ergonomically sound hand tool of the selected kind. The ergonomics testing of a hand tool deals with an ergonomically designed hand tool, in terms of selected criteria. The ergonomics evaluation of hand tool deals with the characteristics of the tool and the task and the effects of the tool on the operator. The ergonomics evaluation of hand tool is concerned with the mechanical output of the tool, tool mass and centre of gravity, tool dimension and grip, use of different grips and tool surface. In the performance of the hand tool task, the demands on force, precision and task duration must be given due consideration. The impact of the tool on the operator is evaluated in terms of working posture, wrist flexion/deviation angles, machine load and fatigue, type of grip used, local hand pressure and injury risk. In the ergonomics design of hand tool special emphasis is given to the physiology and anthropometric characteristics of the hand and hand grip. In the design of hand-powered hand tool and in particular the design of the tool handle, the relevant factors are: length, size (diameter), shape, material, angulations, snap-on-handles, mechanical output, centre of gravity and weight. For the design of power hand tool, the main issue of concern is vibration. Before conducting ergonomics testing of hand tool, it is necessary to obtain relevant information concerning (tool) functional requirement, work method or motions involved in task performance and working posture through direct observation and/or input from experienced users. The ergonomics testing of an ergonomically designed or redesigned hand tool is performed for functional effectiveness, work performance or productivity, physiological or muscular stress and subjective acceptance or comfort. It should be understood that the details presented under ergonomics evaluation, design and testing are not applicable in each and every situation, only relevant factors need to be considered in a particular situation. The characteristics of hand-powered (hammers, shovels, knives, saws, pliers and screwdrivers) and power (power drills and nutrunners) tools commonly used in industry are presented.

1. Introduction Hand tools when poorly designed and used excessively would cause increased incidence of cumulative trauma of the hand, wrist and forearm (cp. ARMSTRONG 1983; AGHAZADEH and MITAL 1987). For the development of cumulative trauma disorders (CTD), there are four major work-related factors: (1) use of excessive force, (2) extreme or awkward joint motions, (3) high degree of repetition of the same movement, and (4) lack of adequate rest for the traumatized joint to recover (cp. PUTZANDERSON 1988). Furthermore, short-term fatigues and discomfort are considered as risk factors and they are related to handle and work orientation in hammering task (cp. SCHOENMARKLIN and MARRAS 1989a; 1989b) and to tool shape and work height in work performed with screwdrivers (cp. ULIN et al. 1990; ULIN and ARMSTRONG 1991). Poor design of tool grip also leads to higher grip forces (cp. COCHRAN and RILEY 1986a; KILBOM and EKHOLM 1991) and to extreme wrist deviations (cp. MEAGHER 1986).

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B. Das / Ergonomics evaluation, design and testing of hand tools

Hand tool injuries in the US comprised about 9 % of all work-related injuries and 9 % of those were disabling injuries. About 80 % of hand tool injuries were caused by hand-powered hand tools. The medical cost alone associated with these injuries is about $400 million per year (cp. AGHAZADEH and MITAL 1987). These estimates are believed to be low because injury produced by cumulative trauma has been recognized only recently for compensation and thus for reporting purposes. To reduce the risk of injury associated with the use of hand tool, more attention is needed to the design of the hand tools. This can be achieved by incorporating ergonomics principles and data in the design of hand tools and in particular by incorporating anthropometric measurement of hands of the user population and dealing with the biomechanical and physiological stresses involved in performing industrial tasks with the tool. A major ergonomic concern is the proper selection, evaluation and use of hand tools. As stated earlier 80 % of all hand tool injuries which are caused by hand-powered hand tools are knives (44.3 %), hammers (10 %), wrenches (8.9 %) and screwdrivers (5.7 %) (cp. AGHAZADEH and MITAL 1987). Powered saws, drills, grinders, and hammers are mostly involved in injuries than any other hand tools. From an ergonomics viewpoint, the focus has been on hand-powered hand tools, considering the injury level and the interface of such hand tools with the human hand. Consequently, the design of tool handle is of major importance to ergonomists. There is a trade-off between manual and powered tool use. Based on the force capacity and greater fatiguability of humans compared to machines, the use of power tools has been advocated. However, power tools, whether powered electrically or pneumatically, produce some vibration. Typically, vibration damping requires either an increase in the inertial mass at the cost of increasing the weight of the tool and increasing the fatigue of the user or vibration-absorbing systems. Such systems cause a “stop” in the hand/handle interface that absorbs the vibration, resulting in a loss of tool control (cp. FREIVALDS 1996). The evaluation of hand tools must give due consideration to the functional properties, quality and reliability and at the same time deal with the user’s expectations and apprehensions (cp. KADEFORS et al. 1993). The biomechanical and physiological stresses involved in performing working tasks with the hand tool are of major concern. MEAGHER (1987) has pointed out essential criteria for the design of hand tools: size, shape, texture, purpose, ease of operations, shock absorption, and weight. PUTZANDERSON (1988) has stated the indicators for faulty hand tools: (1) static loading of arm and shoulder muscles, (2) awkward hand position, especially wrist deviation, (3) excessive or continuous pressure on the palm and fingers, (4) exposure to vibration and cold from power tools, (5) pinch points with double handle tools and (6) handles requiring stretching of the hand to grip or high force to hold. Hand tool shape must be given major importance to avoid wrist deviation, shoulder abduction and to assist grip (cp. CHAFFIN et al. 1999). The frequency of activation and the related muscle fatigue are also important (cp. GREENBERG and CHAFFIN 1977). The testing of a hand tool often refers to the evaluation of a redesigned hand tool. The evaluation process often deals with an existing hand tool. Stated otherwise, the testing process basically means assessment of a redesigned hand tool in comparison to the existing hand tool. In effect, testing is also an evaluation process. The objective of this chapter is to present a systematic ergonomics approach to evaluation, design and testing of hand tools with a major emphasis on hand-powered tools. The terms, evaluation and testing of hand tools are especially used for the purpose of this chapter and the necessary clarification has already been made. Characteristics of some common industrial hand and power tools are also presented. 2. Ergonomics evaluation of hand tool An efficient hand tool must fulfill some basis requirements. It must perform effectively the function for which it is intended. It must be designed proportional to the body dimension of the operator. It must be designed to match the strength and work capacity of the operator. Due allowances have to be made for the gender, age, training and physical fitness of the operator. It should not cause undue fatigue resulting from unusual posture or practices. It must provide sensory feedback. It’s capital and maintenance costs should be reasonable (cp. DRILLS 1963).

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In the evaluation of hand tool due consideration must be given to: (1) static muscle loading, (2) awkward wrist position, (3) tissue compression, (4) gender, (5) handiness, (6) posture and (7) repetitive finger motion (cp. FREIVALDS 1996). Static muscle loading results when tools are used while the arms must be elevated or they have to be held for extended periods. In such a situation, the arms and hands may be loaded statically, resulting in fatigue, reduced work capacity and soreness. During an awkward wrist position, the wrist is moved from its neutral position, which results in a loss of strength. TERRELL and PURSELL (1976) found that starting from a neutral position, pronation decreases grip strength by 12 %, flexion/extension by 25 % and radial/ulnar deviation by 15 %. During the operation of hand tools, tissue compression will often result, when considerable force is applied by the hand. Considerable compression force on the palm of the hand or the fingers will result from such actions. Gender plays an important part in terms of grip strength, while operating hand tools. Typically, female grip strength ranges from 50 to 67 % of male strength (cp. CHAFFIN et al. 1999). The posture during task performance of hand tools affects torque exertion capability (cp. MITAL 1986). With increasing reach distance, the torque exertion capability had decreased linearly. When the index finger is involved in repetitive finger action for operating triggers, symptoms of trigger finger develops (cp. N.N. 1983). A hand tool evaluation process deals mainly with the characteristics of the tool and the effects of the tool on the operator. In addition to the tool characteristics, the characteristics of the task also have a profound impact on the operator. The task characteristics deal with demands on force, precision and task duration. Often the modification of the task characteristics is the only possible solution in a difficult work situation (cp. KADEFORS et al. 1993). FREIVALDS (1996) has provided detailed hand tool evaluation checklist in terms of: basic principles, anatomical concerns, handles and miscellaneous and general consideration. It should be realized an exhaustive evaluation process is not possible in most of the real life work situations. However, such a checklist gives an overview of the evaluation process to assist in identifying the relevant issues involved in a particular hand tool.

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2.1. Characteristics of the tool The characteristics of the tool include: mechanical output of the tool (force, torque, acceleration), tool mass and centre of gravity, tool dimension and grip characteristics, possibility of using different grips or two handed grip and grip surface characteristics (cp. KADEFORS et al. 1983). In the evaluation process of hand tool, the appropriate measurement methods used include: physical, physiological, or psychophysical. However at the present time no quantitative recommendations are available that will permit acceptability or non-acceptability of a working situation for a particular tool usage or application. It is hoped that in the future, a normative standard framework will be available. In the absence of normative framework, comparison of tools for a certain application is desirable to satisfy the user requirement. It is desirable to: (1) inspect the tool, (2) observe the sequence of work and (3) make measurement on the tool or on the workpiece. 2.1.1. Mechanical output of the tool Mechanical output of the tool consists of forces and torques in six degrees of freedom: three translations and three relations. However, in real life situation, it is sufficient to measure in one or two degrees of freedom out of six. For example, in the case of a screwdriver, axial force and axial rotation would be adequate (cp. Fig. 1). Measurement of this type may be performed on the workpiece or on the tool itself. For practical situations, the measurement of tool output and force demand employing a strain gauge transducer approach may be complicated and expensive.

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Figure 1:

Assessment of external forces: Axial force and axial torque of a screwdriver (from KADEFORS et al. 1993)

2.1.2. Tool mass and centre of gravity The centre of gravity of the tool should be as close to the centre of the hand as possible. Before the evaluation experiment takes place, this information is noted. 2.1.3. Tool dimension and grip characteristics The handle characteristics are duly noted with regard to the dimensions corresponding to the grip type of grip applied (cp. KADEFORS et al. 1993). The grip characteristics should consider whether the hand tool can be operated by left hand persons. 2.1.4. Use of different grips or two-handed grip The grip characteristics should consider whether different grips can be applied and the tool design allows for operation with two hands to generate high force levels. Before conducting the use evaluation, these aspects are duly noted. 2.1.5. Tool surface characteristics The tool surface characteristics with regard to sharp edges or corners, material and type of structure are duly noted to assess surface friction. It is not always desirable to have a high friction between hand surface and tool, especially when frequent changing of grip is involved.

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2.2. Effects of the tool on the operator The effects of the tool on the operator include: working posture, wrist flexion/deviation angles, muscular load and fatigue, type of grip employed, local pressure in the hand and risk of injury. 2.2.1. Working posture (whole body) The tool design will have a major impact on the manner of tool usage and working posture. The working posture for a standardized task can be assessed by video filming and subsequent analysis. This analysis is generally based on posture classification schemes (cp. KARHU et al. 1977) or on automatic postural analysis using reflecting markers. Also, the body-mapping method which provides information on occurrence and localization of strain and discomfort can be used in some instances (cp. CORLETT and BISHOP 1976). 2.2.2. Wrist flexion/deviation angles An important aspect of the use of hand tools is the load on the wrist and especially the occurrence of extreme positions. Video observation techniques are not suitable for the purpose. Instead, a goniometer has been used advantageously for the purpose. 2.2.3. Muscular load and fatigue Physiological assessment of muscular load can be measured by myoelectric (electromyography or EMG) signals from muscles that are particularly strained in the operation of the hand tools (cp.

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BASMAJIAN and DE LUCA 1985; STRASSER 1996). Prime movers of the fingers and wrist are selected for the purpose. The muscles are largely located in the forearm and they include: flexor digitorum, extensor digitorum communis, flexor carpi ulnaris, and extensor carpi radialis (cp. KILBOM et al. 1991). The high doses of muscular involvement causes localized muscle fatigue (cp. CHAFFIN 1973). The EMG method can be supplemented by using psychophysical methods based on the body map principle (cp. CORLETT and BISHOP 1976) A hand map has been developed for the purpose, on which the subject can identify areas of discomfort and pain immediately after the task completion (cp. Fig. 2).

Figure 2:

Hand map for subjective rating of pain and discomfort (from KADEFORS et al. 1993)

2.2.4. Type of grip employed The properties of different grips vary with regard to force and precision. The type of grip employed can be documented by video filming. 2.2.5. Local pressure in the hand In the interface between tool and hand, the combinations of high contact force and small contact area create high local pressures in the hand (cp. FRANSSON and KILBOM 1991). In most cases pain in the forearm relates to muscular exertion and pain in the wrist related to high internal forces, whereas pain in the hand itself relates to high local pressure (cp. KADEFORS et al. 1993; KLUTH et al. 2007). 2.2.6. Risk of injury

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The injury in typical use of the tool may be caused either by the tool itself or by the workpiece. A subsequent risk analysis may suggest the use of gloves or protective glasses. 3. Ergonomics design of hand tool From an ergonomics viewpoint, a hand tool must satisfy functional properties, quality and reliability aspects of a particular tool. However, actual tool design involving mechanical part(s), and tool material requires expertise in mechanical and metallurgical engineering. An ergonomist should define the functional requirements of a hand tool but the design and construction or manufacture of the tool belong to mechanical and manufacturing engineers. The main contribution of the ergonomist is in the design of tool handle, which interfaces with the human hand. Consequently, in the design of hand tools, especially hand-powered hand tools due emphasis must be given to the physiology, anthropometric characteristics of the hand and hand grip. In the design of the tool handle due consideration must be given to the following factors: length, size, shape, materials, angulation, snapon-handles, mechanical output, centre of gravity and weight. For powered hand tools, vibration is an important consideration. 3.1. Hand physiology Functional capabilities of the hands are of great importance for work efficiency of humans. Hands, as part of the human body, require a certain level of strength and precision, depending on the type of task being performed. The right combination of strength and precision in handling the task involves a

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delicate management of the sensory system of the hand. The consideration of anatomical and physiological characteristics of the hand-arm-system based on the equation “in conformity with human = in conformity with hand” is an absolutely necessary principle when designing working hand tools (cp. STRASSER 1991). The knowledge of the hand is of great essence to facilitate the process of designing the hand tool. Several characteristics of hand physiology in designing hand tools deserve special attention, which include the connective tissues and flexor muscles in the hand, fat distribution in the hand, as well as location and sensitivity of various nerves and arteries embedded in the hand. The human hand is a complex structure of bones, arteries, nerves, ligaments and tendons (cp. Fig. 3). Extensor carpi and flexor carpi muscles in the forearm control the movements of the fingers. The muscles are connected to the fingers by tendons, which pass through a channel in the wrist formed by the bones of the back of the hand on one side and the transverse carpal ligament on the other. Various arteries and nerves pass through this channel, known as carpal tunnel. The bones of the wrist connect to two long bones in the forearm, the ulna and the radius. The radius connects to the thumb side of the wrist, and the ulna connects to the little finger of the wrist. The orientation of the wrist joint permits movement in only two planes, each at 90° to the other. The first gives rise to palmar flexion and dorsification or extension. The second movement plane gives ulnar and radial deviation.

Figure 3:

Anatomy of hand (from FREIVALDS 1996)

TICHAUER and GAGE (1977) suggested three areas of hand considered as pressure sensitive, which are palmar arch, the ulnar nerve in the heel of the hand and the mid palmar area. The arteries, the median nerve, and the sysorium of the finger flexor tendons in the centre of the palm make the region highly vulnerable to repeated force exertions. Thus the hand tool handle should be designed to be broader in the region where it presses against the heel of the hand to minimize stresses in this region. The tool handle should also be designed, so that it extends beyond the palm to avoid stress concentration in the mid palmar region. 3.2. Hand anthropometry The basic principle of ergonomics is that the design should include the entire male and female populations, so that the task can be performed in an optimum manner. In the design of hand tools, the hand anthropometry should include the various parts of the hand for both the male and female populations. Table 1 presents the relevant hand dimensions for the 5th, 50th and 95th percentiles male and female populations.

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Table 1. Relevant anthropometric measurements (cm) of hand for the design of hand tool handles (from WOODSON et al. 1992; KONZ 1995)

The hand dimensions of male and female adults differ not only in size but also in strength. FRANSSON and WINKEL (1991) found that 35 % of the sex difference in hand strength was due to hand size differences. However, PHEASANT (1986) pointed out that there is little evidence to suggest the need of a special hand tool for female operators. The interaction of handle size and shape with the kinematics and anthropometry of the hand have a significant effect on hand posture and grip strength (cp. BUCHHOLZ 1989). Consequently, different sizes of handles should be provided to accommodate the different anthropometry of the workers’ hands. BOBJER (1989) found in his study of ergonomic knives for meat packing and processing industry that there was a clear need for at least two sizes of handles. The larger handle to fit 50th to 95th percentile of the males and a smaller handle to fit the 5th and 95th percentile females. The preferred hand (dominant hand) is the right hand for about 90 % of the population and the percentage appears constant across cultures and for both sexes. Non-preferred hand tends to have 94 % of the grip strength of the preferred hand (cp. KONZ 1995). Individual fingers on the non-dominant hand are weaker than the corresponding fingers on the dominant hand, and also the error (accuracy) is worse with the non-preferred hand. Thus, a hand tool usable in either hand has two benefits. The first is for the 10 % of the population left out, and the second benefit is that the non-preferred hand can be used when the preferred hand is otherwise engaged or resting. Substantially larger differences in screwdriver torque strength of the (dominant) right and left hands during pronation and supination are shown by STRASSER and WANG (1998).

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3.3. Hand grip There are many types of grips used when holding objects. The type of grip used depends on the hand tool design and the posture assumed when using the hand tool. One common grip is the power grip, with the fingers wrapped around the object and the thumb placed over the first finger, as one might hold a hammer. Likewise, as is the case of holding a hacksaw and a medium to large size screwdriver. This type of grip is used especially for power. The recommended weight of the tool supported by the users is about 1.1 kg but no more than 2.3 kg (cp. MITAL and KILBOM 1992). In the lateral grip, the object is held between the thumb and the side of the first finger. A key is held in this way. Tip pinch is the grip where the tips of thumb and first finger are placed together and the object is held between the two fingers, as one would use when threading a needle. This type of grip is used for precision. 3.4. Hand-powered hand tool design considerations In the design of the hand-powered hand tool and in particular the design of the tool handle due consideration must be given to the following factors: length, size, shape, materials, angulations, mechanical output and centre of gravity. 3.4.1. Handle length Length of handle depends on the type of grip used and hand size of the user population. KONZ (1995) provides guidelines for handle lengths for a power grip, with all four fingers making the contact. A minimum handle length of 10 cm is recommended, however, 12.5 cm would be more

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comfortable. Use of 10 cm length is considered minimum for external precision grip. For an internal precision grip, the tool handle must extend beyond the tender palm but not so far as to hit the wrist. SELAN (1994) has suggested to lengthen handles, so that they do not end in the palm. The handle should be 12.5 cm long and if the worker is wearing gloves, an additional length of 1.25 cm ought to be added. EASTMAN KODAK (cp. N.N. 1983) recommended a length of 12.0 cm. PHEASANT and O’NEIL (1975) maintain that hand/handle contact area should be maximized in hand tool design, as this will minimize shear stress on the skin and thus reduce abrasion. Since increase of friction and the reduction of abrasion are not comparable objectives, some compromise must be reached. 3.4.2. Handle size (diameter) Size of tool handle is of major importance in the design of hand tools. If the hand (diameter) is too large that force must be applied with the tip of fingers, resultant tendon forces can be two to three times larger than the forces applied with the base of the fingers (cp. KROEMER et al. 1994). As a consequence, a considerably more force has to be produced by the muscle, which will lead to muscle fatigue in a short period of time. Overexertion of muscle has been linked to many CTDs, as stated earlier. If the handle size is too small, finger flexor muscles are shortened to the point that they cannot produce enough tension. Thus, much force cannot be exerted, resulting in large local tissue pressures (cp. KROEMER et al. 1994). COCHRAN and RILEY (1986a) found the greatest thrust forces in handles of about 4.1 cm equivalent circular diameter (based on their 13.0 cm circumference) for both males and females. EASTMAN KODAK (cp. N.N. 1983), based on company experience, recommends 3.04.0 cm with an optimum of 4.0 cm for power grips and 0.8-1.6 cm with an optimum of 1.2 cm for precision grips.

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3.4.3. Handle shape RUBARTH (1928) investigated handle shape and found that for a power grip, one should design for maximum surface contact so as to minimize unit pressure of the hand. Thus, a tool with a circular cross-section was found to give maximum torque. But due to several experimental results for a screwdriver handle used in a power grip with friction coupling, a rounded hexagonal cross-section is most compatible with the hand (cp. STRASSER 1995) and permits highest working efficiency. In most circumstances the tool should not rotate in the user’s hand. A counter-torque in the hand can prevent rotation. KONZ (1995) has recommended by improving the moment arm (by providing a longer moment arm) or by having good bearing surface to minimize slippage. Furthermore, the shape’s crosssection should vary over the length of the handle. A rectangular cross-section also allows tactile orientation of the tool which may be decisive for using a flat file properly (cp. KLUTH et al. 2004). Another strategy is to improve the coefficient of friction of the handle. If the rotation of the tool is neither good nor bad, then a circular cross-section is more forgiving on the hand since there are no sharp edges. A change in cross-section reduces movement of the tool forward and backward in the hand, permits greater force to be exerted along the tool axis due to the better bearing surface and can also act as a shield if placed at the front. A flange at the end of the handle prevents the hand from sliding off the handle (cp. KROEMER et al. 1994). For tasks that have a predominance of both orthogonal push and pull activities together, rectangular handles with a width to height ratio of about 1 to 1.25 appears to be the best compromise. If the task involves much more orthogonal push than pull, then circular handles only and circular handles with two flat sides are preferred (cp. COCHRAN and RILEY 1986a). SELAN (1994) recommends that the tool handles should be curved such that the concave surface formed by the fingers and the convex surface formed by the heel of the palm and thumb are accommodated. Finger shaped handles should be avoided unless customized to a specific individual. KONZ (1995) maintains that some degree of bend is desirable for power grips when the tool extends from the top of the hand. The best angle probably depends upon the exact tool and the tool-task orientation. However, both the tool designers and tool purchasers should consider handles that are not straight. A hand tool’s shape directly affects a person’s ability to firmly grasp it. The major muscles which flex the fingers to provide grip force are located in the forearm. There is a long tendon, which spans the

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wrist joint. This means that the position of the wrist affects the grip capability of the fingers. This is especially true when the wrist is flexed or abducted, as is often the case when a tool or workplace design requires a person to insert a tool into a confined area (cp. GREENBERG and CHAFFIN 1977). Thus a well designed hand tool should keep the wrist in the neutral (handshake) position. When the wrist is not in the neutral position, not only the grip strength will be less, but also the risk of acquiring CTD will increase (cp. KONZ 1995). Wrist neutrality can be achieved by bending the tool. Bend the tool, not the wrist. 3.4.4

Handle material

The type of material used for the handle is important, since it will determine the surface friction property and subsequently the ability to grasp and manipulate the hand tool. The frictional characteristics of the tool surface vary with the pressure exerted by the hand, the smoothness and porosity of the surface, and the type of contamination. Sweat increases the coefficient of friction, whereas oil reduces it. Compressive grip materials such as rubber, compressible plastic or wood, are better for the hand than hard plastic or metal. KONZ (1995) states that non-conductivity materials (e.g. wood, rubber) are good for two reasons. First, they release heat to the hand more slowly and so they can be held for a longer time before injury occurs. Second they gain heat more slowly and so are unlikely to reach a high temperature.

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3.4.5. Handle angulations Angulations of handles may be necessary for tools, such as power tools, to maintain a straight wrist. The handle should reflect the axis of the grasp, which is about 78° from the horizontal, and should be oriented in order that the eventual tool axis is in line with the index finger (cp. SILVERSTEIN et al. 1987). Optimum tool angle depends on the posture. The amount of torque and force that can be exerted also depends on the working posture. Repetitive screw driving should not be done on a horizontal surface above the elbow (cp. KONZ 1995). Pistol grip tools should be avoided, if elevated work must be done on horizontal surfaces. Push or pull should be done in the direction of the forearm, with the handle directly in front of it (cp. KROEMER et al. 1994). Muscle fatigue studies of the shoulder muscles have indicated that the shoulder abduction angle directly affects the rate of fatigue onset. The greater the angle of abduction, the shorter is the time to reach significance in muscle fatigue. The fatigue rate increases when the arm is abducted more than 30°. This is due to the increased shoulder torque as the arm is raised, and the shortening of the active deltoid muscle, which in turn reduces its physiological efficiency to produce tension for a given level of metabolism (cp. BROOKS et al. 1996). Thus, when using hacksaw, the posture assumed should be such that the angle of abduction of both the right and left arms are as small as possible ( 1, the effect of the wrist rest is a positive one. Values < 1, i.e., the columns remaining below the reference line, suggest that working conditions without the device are more favorable. This way of forming a relative value also eliminates, or at least reduces, the work time-dependent effect of fatigue. This can be clearly seen for the trapezius muscle, which does not now show a tendency of increasing values within the 10-min blocks. When quantifying the multiple of the muscle strain of the work without the wrist rest, it can be seen that all muscles monitored had to invest higher muscle forces for the positioning of the hand-arm system without the device. During the 3 sections of entering text, all the 1-min values for all the three functional parts of the deltoid muscle – which are involved in moving and stabilizing the upper arm – are clearly above 1 and, with values between 2 and 3 on average, indicate that work without the wrist rest is about at least twice as strenuous than with the working aid. In these relative figures, the positive effect of the wrist rest on the trapezius seems to be not especially high. But it should be mentioned that it is more ergonomically important, when – as proven here – a working aid leads to a workphysiologically more acceptable strain level, rather than when the wrist rest results in considerable relief in muscles that are anyhow not subject to high levels of strain. The responses of the trapezius to working without the wrist rest – though not very high in numbers and figures – could be proven at a high level of significance. The ratios from sEA values without and with the working aid for the trapezius during mouse handling are partly smaller than 1 and, therefore, speak against the wrist rest, yet the negative effects are not very strong. As already mentioned, for the biceps, the work without the wrist rest (the three 10min blocks) was indeed about 4 times more exerting on average. But this muscle is not subjected to high strain, as is the case for the trapezius. The effect of the wrist rest on the three other muscles acting on the hand – as expected – is very limited, but at no time in any of the working sections was it negative. 5. Discussion and Conclusions Knowledge of practical methods of appropriate recording and processing of myoelectric data (i.e., objective assessment) and also subjective assessment approaches has been fed into a computer-based system which has already been reported for the first time some years ago (cp. STRASSER et al. 1999). In addition to conventional averaging of electromyographic activity, EA(t), which represents the envelope of amplified, filtered, and rectified myoelectric signals, and which predominantly is applied for the evaluation of static muscle work, treatment of electromyographic data includes optional smoothing of the “rough” EA-time series, standardization, and splitting-up of sEA into static and dynamic components (cp. STRASSER 1996). As a prerequisite for interpreting work-related electromyographic responses as indicators of muscle strain – defined by the demanded amount of the total individual capacity – a standardization via EAmax-values (associated with maximum voluntary contractions) and EA0 values (the resting activity) of a muscle in working posture is needed. Furthermore, during dynamic muscle work, the immediately succeeding work-related maximum and minimum values of an EA-time series, which reflect muscle contractions and relaxations, have to be determined. Based on averaging a preset number of physiological responses to the correspondent working cycles, or averaging EA data over a pre-fixed duration of work, the ergonomically significant characteristic

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“static” and “dynamic” components of muscle strain can be calculated. Additional software in the knowledge-based system also allows the graphic representation of the results. With respect to the example shown in section 4, it has to be pointed out that even carefully registered and evaluated data, which has been averaged over a group of 10 Ss, i.e., time series of 1-min standardized electromyographic activity values should not be averaged over larger periods of time, since they often fluctuate within time. Short periods of test time in standardized working tests (some minutes without a re-test) cannot be expected to yield reliable and valid results which can be associated with the object under test. On the other hand, even within 10 min of continuous working with an effective working aid or a hand tool, fatigue effects can become apparent in steadily increasing sEA values, at least in bottleneck muscles (e.g., the trapezius). When relating these values to corresponding values from sections working without this aid or a standard tool, this information gets lost, but via this procedure extra physiological cost can be quantified very clearly in numbers and figures. However, these numbers and figures, i.e., multiples of physiological cost as shown in the right part of Fig. 13, must be interpreted with caution, i.e., always in the context of the absolute muscle strain (sEA values), in order as not to overestimate the results, as it would be the case with the results of the biceps. With reservations, this accumulated knowledge of procedures which have been further developed during the last decade represents the state-of-the-art of electromyographic methods for measuring and assessing local muscle strain. Until now, this could only be shown in extracts in books and papers. The system also contains a set of investigations into the ergonomic evaluation of keyboards, armrests (KELLER and STRASSER 1996), and wrist rests, as well as various hand-held tools already carried-out and published (STRASSER et al. 1996; KLUTH et al. 1997; STRASSER and WANG 1998) which may be useful for planning new studies. The knowledge-based system for utilizing advanced electromyographic and subjective methods for the evaluation of the ergonomic quality of hand-held tools and computer input devices (Fig. 14) is available on a CD (up to now only available in German) (cp. STRASSER 2000).

Figure 14: Computer-based system for the evaluation of the ergonomic quality of hand-held tools and computer input devices

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H. Strasser et al. / A knowledge-based system for utilizing electromyographic methods

Using these specifically developed and well-proven software packages, which can be loaded into virtually all PCs for recording, analyzing, and standardizing electromyographic data. Thus a user who is less familiar with these tasks will, hopefully, gain access to electromyography and subjective assessments as efficient and important ergonomic methods applicable with reasonable expenditure both in the laboratory and in the field. 6. References ARMSTRONG, T.J. (1983) An ergonomics guide to carpal tunnel syndrome. American Industrial Hygiene Association, Akron, OH BENKTZON, M. (1993) Designing for our future selves: The Swedish experience. Applied Ergonomics 24 (1) 19-27 BÖHLEMANN, J.; KLUTH, K.; KOTZBAUER, K. and STRASSER, H. (1994) Ergonomic assessment of handle design by means of electromyography and subjective rating. Applied Ergonomics 25 (6) 346-354 EKLUND, J. and FREIVALDS, A. (1993) Hand tools for the 1990s – An Applied Ergonomics special issue based on presentations at the Symposium on Hand Tools and Hand-Held Machines, 21. August 1990, University of Technology, Linköping, Sweden. Applied Ergonomics 24 (3) 146-147 FELLOWS, G.L. and FREIVALDS, A. (1989) The use of force sensing resistors in ergonomic tool design. Proceedings of the 33rd Annual Meeting of the Human Factors Society, pp. 713-717 FELLOWS, G.L. and FREIVALDS, A. (1991) Ergonomics evaluation of a foam rubber grip for tool handles. Applied Ergonomics 22 (4) 225-230 GILAD, I. (1999) Editorial in memoriam of Prof. Erwin R. Tichauer 1918-1996. Special issue of International Journal of Industrial Ergonomics 23, 251-253 KELLER, E. and STRASSER, H. (1996) Ergonomic evaluation of an armrest for typing via electromyographic and subjective assessment. In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M. and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I. ISOES, Cincinnati/Ohio, USA, pp. 838-845 KELLER, E. and STRASSER, H. (1998) Electromyographic and subjective evaluation of a wrist rest for VDU operators. Occupational Ergonomics I (4) 239-257

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KLUTH, K.; KELLERMANN, H. and STRASSER, H. (1997) Electromyographic and subjective methods for the assessment of the ergonomic quality of file handles. In: SEPPÄLÄ, P.; LUOPAJÄRVI, T.; NYGARD, C.-H. and MATTILA, M. (Eds.) From Experience to Innovation. Proceedings of the XIIIth Congress of the International Ergonomics Association. Vol. 4, Finnish Institute of Occupational Health, Helsinki/Finland, pp. 515-517 KUMAR, S. (1995) Electromyography of spinal and abdominal muscles during garden raking with two rakes and rake handles. Ergonomics 38 (9) 1793-1804 KUMAR, S. and MITAL, A. (Eds.) (1996) Electromyography in Ergonomics. Taylor and Francis, London LEWIS, W.G. and NARAYAN, C.V. (1993) Design and sizing of ergonomic handles for hand tools. Applied Ergonomics 24 (5), 351-356 MATTILA, M. and LANDAU, K. (1999/2000) Guest-editorial. Special issue of Occupational Ergonomics 2 (3) 135 MCGORRY; R.W.; DEMPSEY, P.G. and LEAMON, T.B. (2003) The effect of technique and shaft configuration in snow shoveling on physiologic, kinematic, kinetic and productivity variables. Applied Ergonomics 34, 225-231 SMOLANDER, J.; LOUHEVAARA, V.; AHONEN, E.; POLARI, J. and KLEN, T. (1995) Energy expenditure and clearing snow: A comparison of shovel and snow pusher. Ergonomics 34 (4) 749-753 STRASSER, H. (1996) Electromyography of upper extremity muscles and ergonomic applications. In: KUMAR, S. and MITAL, A. (Eds.) Electromyography in Ergonomics, Taylor and Francis, London, pp. 183-226 STRASSER, H.; WANG, B. and HOFFMANN, A. (1996) Electromyographic and subjective evaluation of hand tools: The example of mason’s trowels. International Journal of Industrial Ergonomics 18 (1) 91-106 STRASSER, H. and WANG, B. (1998) Screwdriver torque strength and physiological cost of muscles dependent on hand preference and direction of rotation. Occupational Ergonomics 1 (1) 13-22

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STRASSER, H. and MÜLLER, K.-W. (1999) Favorable movements of the hand-arm system in the horizontal plane assessed by electromyographic investigations and subjective rating. International Journal of Industrial Ergonomics 23, 339-347 STRASSER, H.; KLUTH, K. and KELLER, E. (1999) Multi-channel electromyography and subjective methods for the evaluation of the ergonomic quality of hand-held tools and computer input devices. In: LEE, G.C.H. (Ed.) Advances in Occupational Ergonomics and Safety III. IOS Press, Ohmsha, Amsterdam / Berlin / Oxford / Tokyo / Washington DC, pp. 347-352 STRASSER, H. (2000) Ergonomische Qualität handgeführter Arbeitsmittel – Elektromyographische und subjektive Beanspruchungsermittlung. 154 Seiten, 94 Abb., 7 Tab., 121 Literaturstellen, mit CD-ROM, erstellt von KLUTH, K., Ergon Verlag GmbH, Stuttgart STRASSER, H. (2006) Electromyography: methods and techniques. In: KARWOWSKI, W. (Ed.) International Encyclopedia of Ergonomics and Human Factors Vol. III. Methods and Techniques. 2nd Edition, Taylor and Francis, London / New York, pp. 3115-3118 STRASSER, H.; FLEISCHER, R. and KELLER, E. (2004) Muscle strain of the hand-arm-shoulder system during typing at conventional and ergonomic keyboards. Occupational Ergonomics 4 (2) 105-119 TICHAUER, E.R. (1978) The Biomechanical Basis of Ergonomics: Anatomy Applied to the Design of Work Station. Wiley, New York

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WU, S.-P. and HSIEH, C.-S. (2002) Ergonomics study on the handle length and lift angle for the culinary spatula. Applied Ergonomics 33, 493-501

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Chapter 4.1

Electromyographically Determined Muscle Strain Associated with the Direction of Manual Movements in the Horizontal Reach H. Strasser and K.-W. Müller

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0. Summary A study was carried out in order to analyze the influence of different movement directions on muscular strain of the hand-arm system when handling light weights. 11 female subjects had to perform a one-handed lifting task in the horizontal plane, moving repetitively objects of approximately 0 kg and of 1 kg on a table. Thirteen different directions in the frontal area had been provided. Electromyographic activity (EA) was continuously recorded from 8 muscles of the left hand-armshoulder system and the upper trunk. All data were standardized by the activity arising under maximum voluntary contractions (MVC). The EA values were separated into static and dynamic components. Before the test, the subjects had to assess their preference of each direction. After each working direction the actually felt strain was rated on a scale. Both static and dynamic components of the muscular activity show a strong dependence on the moving direction. The directions around 30° (measured from the body plane) cause less than half of the muscular load in comparison with directions between 90° and 160°, which are often found in real work situations. When moving the weight of 1 kg EA values up to 30 % of the maximum EA were found. According to these findings the strain in the relevant muscle groups dependent on the working direction is not neglectable. When moving the nearly weightless object, the recorded strain was also essential. The pure arm movement seems to be already responsible for a relatively high strain during this kind of work. The subjective ratings before the tests differed considerably from the physiological findings. The ratings after each test, however, corresponded with the measured strain in some important muscles which represent bottlenecks in repetitive manual movements. The repetitive handling of light weights is often found in industrial workplaces, as for example in supermarket checkouts and in assembly lines. The layout of these workplaces normally refers to static anthropometric data and not to the dynamics of the movements. Especially the direction of the movements is an important parameter for the muscular strain. Almost half of the static strain and considerable part of the dynamic strain of some relevant muscles can be reduced if the workplaces are reconstructed according to the findings of this study.

1. Introduction For several decades electromyography proved to be an excellent method to measure static muscle load (e.g., TICHAUER 1978) and in a limited sense, to also calculate the risks of injuries caused by moving heavy weights. But also the handling of light weights can affect health, especially in combination with long-lasting repetitive movements and awkward body postures. Modern recording devices and the development of powerful computer-based techniques (cp. amongst others STRASSER et al. 1992; AARAS and RO 1996; KUMAR and MITAL 1996) now enable to analyze the muscular load during light physical work, too. Such “light” physical work is often found in supermarket checkouts and in assembly lines of the mechanical or electronics industry. Even if the workplaces are designed appropriate to ergonomic guidelines, a lot of workers complain of musculosceletal pain or even suffer after a long working time

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from injuries. A possible reason may be that the layout of such workplaces refers only to anthropometric data, that means to a static basis. The load caused by body movements and especially by repetitive movements of the hand-arm-shoulder system is mostly neglected. On the other hand the movements of body parts are the basis of the predetermined motion time systems (e.g., MTM, WF), which are often used to determine the working speed. These systems consider some factors like the motion distance, the moved weight, or the accuracy of the motion. For example, MTM (Methods Time Measurement) regards manual movements of an external load above 1 kg demanding more time but does not differ between 0 and 1 kg. Other factors like the position of the weight and especially the direction of the movements are not taken into account, even if they may be responsible for a different muscular load. That is why an identical (forced) working speed can result in different body strain. In order to clarify if there exist some movement directions inducing lower load than others, and therefore, reducing the risk of muscular complaints, a laboratory study was carried out. The load of the hand-arm-shoulder system was recorded by means of multi-channel electromyography and by subjective ratings. 2. Methods 2.1 Subjects and task

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It can be expected that a lot of the relevant workplaces are occupied by female workers. Therefore, the study was focussed on female subjects. 11 subjects – aged between 19 and 27 years (with an average body weight of about 60 kg and a height of 168 cm) – participated in the test session. As shown in the upper left part of Fig. 1, a one-handed lifting task in the horizontal plane was performed during seated work.

Figure 1:

Assessment and evaluation of physiological cost of repetitive horizontal materials handling in different working directions by means of electromyographic methods

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The subjects had to handle articles from varying starting points (S) to a fixed point (Z) near the body. According to the ergonomic guidelines for the optimal reach the lifting distance was determined to 38 cm. Cube-shaped objects weighing 1 kg had to be lifted repetitively with the left arm at a frequency of 24 cycles per minute. These parameters were chosen to simulate a task which can be found at supermarket checkouts or assembly lines. The working period lasted 3 minutes. Within this time a stable reaction of the EMG-data was found. The subjects had to move the weight in 13 different directions within an angle area of 10° to 250° to the body plane. In order to also measure the physiological cost associated with unloaded arm movements, the same task had to be performed by handling an almost weightless article (in the following called 0 kg).

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2.2. Electromyographic procedures Electromyographic activity, which means rectified, integrated, and smoothed myoelectric signals picked up by surface electrodes, was registered continuously and evaluated by means of computeraided methods which are described in details in STRASSER (1996). In the following they will be explained in extracts. The chart in the lower right part of Fig. 1 represents an example of a typical time course of electromyographic activity during an evaluation period (30 s) of dynamic muscle work reflecting phases of contractions and relaxations during the work cycles. The minima normally refer to the permanent muscle contraction level which cannot be fallen below during the work cycles. The maxima were determined by the movements and contractions. By calculating a mean maximum and a mean minimum activity in a well proven procedure (see STRASSER et al. 1989) – in addition to the mean EA – splitting-up of EA into static and dynamic components associated with dynamic work was accomplished. Because it is impossible to compare EMG amplitude data of different muscles and from different subjects – due to uncontrollable recording conditions as, e.g., different lead placements or muscles varying in size – all myoelectric data were normalized. Therefore, maximum EA from (isometric) maximum voluntary contractions preceding all the tasks was registered for each subject and each muscle. For this reason, a device for measuring pulling and pushing as well as lifting muscle strength in three dimensions at different places in the horizontal working area was used (see upper middle part of Fig. 1). Actual electromyographic activity EA of each muscle during all the tasks was related to its maximum EAmax after having subtracted the resting activity, EA0. Regarding the fact that biomechanical prerequisites (e.g., coupling conditions and angles between the limbs), determine muscle forces, it must be stressed that the global maximum EAmax of several measuring conditions was chosen in order to establish a truly reliable reference basis. By means of these procedures, the degree of muscular strain – not just in mean values of the electromyographic activity but also divided into static and dynamic components – could be expressed in figures and numbers. Finally, the results were plotted in circle diagrams (see upper right part of Fig. 1) compatible to the working directions demanded in the test design (see upper left part of Fig. 1). 2.3. Muscles monitored During material handling in a horizontal plane from an ergonomics point of view, the following movements must be regarded as essential (see Fig. 2).

• An anteversion, i.e., a forward movement of the arm, which (among other muscles) is brought about by the • pectoralis, i.e., the big chest muscle.

• A retroversion, a backward movement of the arm, which is effected by the • spinal part, i.e., the posterior part of the deltoid and some parts of the • latissimus dorsi, the long back muscle.

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Figure 2:

Muscles continuously monitored by surface electromyography and some elementary arm and shoulder movements in which they are involved

• A lateral lifting, an abduction of the upper arm, mainly accomplished by the • acromial, i.e., the middle part of the deltoid.

• A medial rotation, an inward rotation of the arm towards the body, in which in any case • the pectoralis major and • the latissimus dorsi are involved.

• The adduction of the arm, effected by the same muscle groups • pectoralis major and • latissimus dorsi.

• The elevation and retraction of the scapula, which were brought about by

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• different parts of the trapezius. Besides these movement elements of the upper arm and shoulder during manual handling processes, further movement parts which are possibly influenced by the direction of work include

• the flexion of the forearm, in which the biceps is involved, and • the extension of the fingers. Therefore, in order to quantify more or less favorable movement directions of the arm by means of electromyographic methods, the 8 muscle groups which were already mentioned and are shown in Fig. 3 again were monitored by electrodes in a bipolar arrangement attached on top of the muscle parts. 3. Results 3.1. Directions of manual movements evaluated via electromyography The charts of Fig. 4 represent the results of all the electromyographic investigations visualized in circle diagrams arranged locally compatible to the 8 muscle groups shown in Fig. 3. Static and dynamic components (the black and yellow areas) of the standardized electromyographic activity sEA[%] of the two parts of the trapezius and the deltoid and of the pectoralis major, the latissimus dorsi, the biceps and the extensor digitorum were shown for all working directions in each graph.

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Figure 3:

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Location of the muscles monitored with placement of the electrodes

Two circle diagrams of muscle strain for arm movements in the different directions without (0 kg) and with an external load of 1 kg were arranged together in each graph. The following results should be stressed: Due to the different size and shape of the diagrams as well as to the two working conditions (0 kg, 1 kg), it can be concluded that essential differences in intermuscular strain do exist. But there are quite consistent results with respect to the 13 directions of work. For all muscles showing a clear dependency of strain on the working direction, 150° turned out to be a most unfavorable direction. The area of about 30° revealed to be optimal for almost all muscle groups. Here, strain is less than half as high as in the least favorable direction. This means that almost half of the physiological cost can be spared if favorable movements in horizontal handling of materials are made possible by a corresponding work design. Especially for handling a real external load, total strain – indicated by the outer circle lines – encompasses 30 % and more in the two parts of the trapezius and the deltoid as well as in the biceps and the extensor digitorum. Relatively strong muscle groups – as, for example, the middle part of the deltoid and the trapezius – do not show a significant higher level of strain when an external load has to be manipulated. But 1 kg in addition to the weight of the forearm and the upper arm is enough to significantly increase the workload for the relatively small muscle groups of the spinal part of the deltoid. Similar results were yielded by the relatively strong biceps and by the extensor digitorum as an antagonist of the m. flexor digitorum. The static component of muscular strain (i.e., the black area in the circle diagrams) varies from very low values of less than 5 % in the biceps and the big chest muscle to relatively high values of about 10 - 15 % in the deltoid and trapezius. Generally speaking, the influence of the external load does not seem to be decisive for the static component. This means that an increase of strain induced by material handling is almost solely restricted to the dynamic component, if the frequency is not varied. A relatively small degree of strain in both static and dynamic components was found in muscles acting on the shoulder joint, i.e., the latissimus dorsi and the pectoralis. One reason for that may be the fact that these muscles are not as involved in manual work as all the others acting on the hand and forearm, the upper arm and the shoulder.

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Figure 4:

H. Strasser and K.-W. Müller / Muscle strain associated with the direction of manual movements

Static and dynamic components (inner and outer parts of the circle diagrams) of myoelectric activity of 8 muscle groups. Standardized values sEA associated with arm movements in different working directions without (0 kg) and with an external load (1 kg) (lifted at a frequency of 24 cycles/min). (Means of 11 female Ss). The red lines represent mean EA.

Static strain in these trunk muscles is almost negligible and, also, the dynamic part reaches up to only about 20 % so that, indeed, most of the workload from an external load seems to be already absorbed by the physiological cost of the hand-arm-shoulder muscles. But the low values are certainly also due to the physiological property of those multipennate muscles with wide-feathered fibres from which only a small part can be monitored and assessed by electromyographic methods, even in surface EMG. In congruence with the results of another study (see STRASSER and ERNST 1992) with directions of about 30°, which turned out to be favorable to arm movements, the relatively highest physiological cost of the big chest muscle was measured. But this effect is due to the physiological destination of this muscle group which, indeed, is more actively involved when arm movements in front of the trunk with medial rotation of the arm are dominant.

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3.2. Subjective assessment of the different movement directions The results of the subjective assessments are shown in Fig. 5. On the left side the means of the 11 subjects who had rated the 13 working directions before the tests show that, surprisingly, the subjects themselves expected the working direction of about 90° to be optimal. This result does not coincide at all with the electromyographic data. But subjectively felt strain – assessed after finishing work in each of the working directions at a bipolar 4-step scale – reveals to be relatively congruent with the physiological data, at least with the strain of some “guide muscles.” The most pleasant feeling was associated with the working direction of 30°, and the most unpleasant experiences with 150°. This impression must have been strongly influenced by the level of strain measured, e.g., in the deltoid.

Figure 5:

Subjective evaluation of the working directions at two rating scales: a: Assessment of the 13 working directions before working at a 13-step scale b: Assessment after each of the 13 working directions at a bipolar 4-step scale (means of 11 subjects)

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4. Discussion Due to trends in occupational health and safety regulations in Europe (see BIENECK 1995; DOMPKE 1996), manufacturers are more and more obliged to construct machines and working tools as well as workplaces in congruence with ergonomic demands. Therefore, a reliable and valid inventory of methods for the assessment of the ergonomic quality of workplaces and tools will be needed. In this respect, physiological measurements, subjective and expert ratings as well as methods of video recording and image processing (see BUBB and GEUSS 1996) are helpful. Yet, as already has been shown in several multidisciplinary studies, an isolated use of the different methods sometimes can lead to incorrect conclusions (see, e.g., STRASSER et al. 1996). Whereas the method of video recording seems to be appropriate to recognize awkward body postures, subjective ratings are necessary to evaluate long-time effects of insufficient ergonomic design. Expert ratings can help to evaluate the importance of the findings based on different methods. Objective physiological measurements, however, allow to identify and quantify an overload of singular body parts, but also a strain below the threshold of perception. Static and dynamic muscular strain with and even without an external load strongly depend on the direction of arm movements. Especially, the regions of the forearm, upper arm and shoulder are affected. Whereas the external load of 1 kg is low compared to the maximum arm force, in some directions values up to 30 % of the maximum EA were found. This proves that even in light work conditions the pure arm movement is a factor to be taken into consideration. However, the breast and back muscles showed a general low activation and should not be monitored in further studies.

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H. Strasser and K.-W. Müller / Muscle strain associated with the direction of manual movements

The movement area of about 30° revealed to be optimal for the relevant muscle groups. The measured strain was less than half of that in the directions between 90° and 160°. These directions, however, are most common in assembly workplaces in industry. Therefore, it is not sufficient when the design of workplaces concentrates only on static conditions like the optimum reaching area of the arms. Almost half of the physiological cost can be spared if the workplaces are redesigned according to the dynamic aspects found in this study. Applications in checkout design of self-service shops are shown e.g. by STRASSER (1990) and STRASSER et al. (1991). The subjective assessment before the test series differed considerably from the physiological findings. But the ratings after the test show that the subjects got a differentiated feeling of the strain during work. The preferences of the working directions experienced during the course of the tests are consistent with the strain profiles of the most important muscles representing bottlenecks in repetitive manual movements. Therefore, it would not be sufficient to rely on subjective data or expert ratings without work experience. On the other hand, energy expenditure and work pulses are not able to determine the local strain of the hand-arm system (see STRASSER and ERNST 1992). Multi-channel electromyography, however, proved to be an appropriate method to get important data for the ergonomic evaluation and design of tools (compare amongst others GILAD and HAREL 1996; MÜLLER 1996; KELLER and STRASSER, 1996). The same is true for the assessment of movement-based workplaces. 5. References AARAS, A. and RO, O. (1996) Electromyography (EMG). Methodology and application in occupational health. In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M. and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I. ISOES, Cincinnati, pp. 511-516 BIENECK, H.-J. (1995) Auswirkungen der Rechtsentwicklung auf die Anwendung ergonomischer Erkenntnisse – Perspektiven für den präventiven Arbeitsschutz. In: STRASSER, H. (Ed.) Arbeitswissenschaftliche Beurteilung von Umgebungsbelastungen – Anspruch und Wirklichkeit des präventiven Arbeitsschutzes. Ecomed-Verlag, Landsberg/Lech, pp. 121-127

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BUBB, H. and GEUSS, H. (1996) Possibilities of ergonomic modelling of products and workplaces. In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M. and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I. ISOES, Cincinnati, pp. 566-571 DOMPKE, M. (1996) Arbeitswissenschaft im Zuge der europäischen Neuerungen – Betrieblicher Bedarf und Forschungsgegenstand. In: STRASSER, H. (Ed.) Beanspruchungsgerechte Planung und Gestaltung manueller Tätigkeiten – Elektromyographie im Dienst der menschengerechten Arbeitsgestaltung. Ecomed-Verlag, Landsberg/Lech, pp. 187-197 GILAD, I. and HAREL, S. (1996) Geometric variables in keyboard operations. In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M. and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I. ISOES, Cincinnati, pp. 496-501 KELLER, E. and STRASSER, H. (1996) Ergonomic evaluation of an armrest for typing via electromyographic and subjective assessment. In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M. and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I. ISOES, Cincinnati, pp. 838-845 KUMAR, S. and MITAL, A. (1996) Electromyography in Ergonomics. Taylor and Francis, London / New York MÜLLER, K.-W. (1996) Effects of ergonomic keyboards on static and dynamic components of electromyographic activity. In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M. and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I. ISOES, Cincinnati, pp. 502-507 STRASSER, H.; MÜLLER, K.-W.; ERNST, J. and KELLER, E. (1989) Local muscular strain dependent on the direction of horizontal arm movements. Ergonomics 32 (7) 899-910 STRASSER, H. (1990) Evaluation of a supermarket twin-checkout involving forward and backward operation. Applied Ergonomics 21 (1) 7-14

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STRASSER, H.; GROß, E. and KELLER, E. (1991) Electromyographic evaluation of the physical load of the left handarm-shoulder system during simulated work at eight different cash register arrangements. In: KARWOWSKI, W. and YATES, J.W. (Eds.) Advances in Industrial Ergonomics and Safety III. Taylor and Francis, London / New York, pp. 457-463 STRASSER, H. and ERNST, J. (1992) Physiological cost of horizontal materials handling while seated. International Journal of Industrial Ergonomics 9, 303-313 STRASSER, H.; ERNST, J. and MÜLLER, K.-W. (1992) Elektromyographische Untersuchungen zu günstigen Bewegungen des Hand-Arm-Systems als Grundlage ergonomischer Arbeitsgestaltung. Band 11, Schriftenreihe Zentralblatt für Arbeitsmedizin, Dr. Curt Haefner Verlag, Heidelberg STRASSER, H., (1996) Electromyography of upper extremity muscles and ergonomic applications. In: KUMAR, S. and MITAL, A. (Eds.) Electromyography in Ergonomics. Taylor and Francis, London, pp. 183-226 STRASSER, H.; WANG, B. and HOFFMANN, A. (1996) Electromyographic and subjective evaluation of hand tools: The example of masons’ trowels. International Journal of Industrial Ergonomics 18, 91-106

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TICHAUER, E.R. (1978) The Biomechanical Basis of Ergonomics – Anatomy Applied to the Design of Work Situations. John Wiley & Sons, New York

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Chapter 4.2

Effects of Hand Preference and Direction of Rotation on Screwdriver Torque Strength and Physiological Costs of Muscles Involved in Arm Pronation and Supination H. Strasser and B. Wang 0. Summary

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The focus of this research was to investigate how maximum torque and muscle forces were affected by pronation and supination, i.e., inward and outward rotation of the forearm in a series of screwdriver tests with 6 varied handles. Consecutively, maximum torque for pronation and supination was determined, submaximum isometric levels of torque were demanded, and, finally, an equal dynamic screwing work for all subjects was simulated. Physiological cost of performance was simultaneously measured by registrations of electromyographic activities (EA) from 4 muscles, which were expected to be involved intensively in screwing tasks. Significant and essential differences between maximum torque values produced by pronation and supination of the right and the left arm of the mainly righthanded subjects were found. For clockwise work, as it is necessary e.g., for driving in screws, inward rotations (pronations) of the nondominant hand are at least as strong as outward rotations of the dominant hand. Differences of about 8 % even in favor of pronations were found. Yet, for counter clockwise work involved, e.g., in removing a tightened screw, inward rotations of the dominant hand yielded a much more stronger torque strength than outward rotations of the nondominant hand. Differences of more than 50 % in favor of pronation of the right-handed subjects were measured. Also, EA values of the 4 muscles monitored on the right arm differed significantly. Systematically operational and physiological differences due to the varied screwdriver handles, as results of investigations which were not the main objective of the study, corresponded well with the findings of prior studies.

1. Introduction For product ergonomics approaches and the design of working tools, the consideration of anatomical and physiological characteristics of the hand-arm system based on the equation “conformity with human equals conformity with hand” is an absolutely necessary principle. The quality of a handle therefore depends on the degree of ergonomic requirements realized in combination with design and aesthetics. Different ergonomics design solutions must, however, be subject to an objective assessment, taking into account the real effects during work. Furthermore, it has to be taken into account that the right arm or the left arm can be applied for clockwise or counterclockwise rotations, and it must also be considered whether appropriate muscle groups and proper coupling of the hand and handle are used. For instance, driving in screws is normally performed by outward rotations (supinations) of the right arm. Yet, inward rotations (pronations) of the left arm can be applied for the same effect. When a tightened screw has to be unscrewed, the reverse operations are performed. From an ergonomics point of view, differences in torque strength and muscular effort are to be expected from persons with a dominant use of the left or right hand; the result that pronation is superior to supination is to be expected as well. Of course, shape, thickness, and length of a screwdriver handle also influence both operational performance and physiological strain. Therefore, the objective of this study was to

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H. Strasser and B. Wang / Screwdriver torque strength and physiological costs of muscles

investigate how maximum torque and muscle force are affected by pronation and supination of the forearm and to what extent performance and physiological cost are influenced by the different handles of the screwdrivers used. 2. Methods and materials 2.1. Subjects and procedures A group of 10 male unpaid subjects (Ss) which, as shown in Table 1, was relatively homogeneous with respect to age (37.3 ±4.8 years), body weight (69.1 ±7.6 kg) and height (172.6 ±5.2 cm), width and length of hand (8.7 ±0.5 cm and 18.6 ±0.7 cm), and elbow height (107.6 ±4.1 cm) participated in a series of screwdriver tests (with 6 varied handles). Seven Ss were typical right-handed persons, 1 person normally preferred using the left hand, and 2 Ss were not in a position to state a left or right hand preference when performing physically demanding arm work.

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Table 1: Physical characteristics of the 10 male subjects

According to the schedule of the test program (see Fig. 1) within a total test duration of 4 hours which was interrupted by rest pauses, consecutively, maximum torque strength (for pronation and supination) was determined, submaximum isometric levels of torque were demanded, and, finally, a dynamic screwing task – the same one for all Ss – was simulated. For the determination of the recovery periods between the different operational measurements a formula developed for static muscle work in 1960 by Rohmert (cp. ROHMERT and LAURIG 1993) was used. According to this formula for maximum voluntary contractions (MVC) with a duration of at least 3 s a recovery time of 1660 % of 3 s results. A static muscle load of 50 % of MVC over 0.5 min would still result in 400 % recovery time, i.e., 2 min. Therefore, regarding worst case conditions and in order to avoid any effect of cumulative stress over the 4-hour period of each experiment and transfer effects from the different test sections, 4 min or at least 2 min intervals were provided after all tests where maximum or submaximum torques had to be exerted. For the relatively high dynamic muscle work in the last test section, 3 min recovery periods were regarded as necessary for similar reasons.

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Figure 1:

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Schedule of test program

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2.2. Experimental set-up Figure 2 shows the test set-up for measuring the screwing performance and muscle responses during the simulated work process. A torquemeter which had been used already before (cp. STRASSER 1991) was altered in such a way that, additionally, axial forces could also be measured during maximum torque exertions. In addition to a cylindrical roller which was used to simulate constant dynamic screwing work, a conical roller was also applied to increase the workload continuously. The chosen diameters guaranteed that on the whole an identical dynamic muscle work was performed. Yet, it was expected that by such an increasing demand more differentiated effects of the various screwdriver handles could be measured. For both set-ups screwing upwards a weight of 70 N within a period of 10 s resulted in an average output of 6.3 Watt demanded from the Ss. The job height of the screwing device had been adjusted to individual elbow height in such a way that the longitudinal axis of the screwdrivers was aligned with the forearm. As driving in screws sometimes demands the use of both arms, and as for such an operation with a two-handed kinematic chain an angle of 60° between the frontal plane and the main active hand is favorable, the Ss were brought into line with the test device in a corresponding way (see right part of Fig. 2.). During all the simulation processes, the Ss were instructed not to apply a power grip – which generally would enable the utmost maximum exertable torque strength – but to work in a five-finger pinch coupling. Postural variations of the Ss which would have had significant effects on the exertable torques were avoided via an exactly indicated working position.

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Figure 2:

Test devices and recording equipment for simulating static and dynamic screwing tasks and for simultaneous measuring muscle responses

2.3. Muscles monitored via electromyographic recordings The physiological cost was measured simultaneously with the operational performance tests by registering the Electromyographic Activities (EA) of 4 muscles (compare also left part of Fig. 2) which were expected to be intensively involved in different screwing tasks: • • • •

the flexor digitorum as grip musculature, the biceps brachii as powerful supinator, the brachioradialis both as supinator and pronator, and the anterior part of the deltoid, mainly in action when axial forces have to be exerted.

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The EA of the 4 muscles was integrated over predetermined test durations and digitally displayed; it was also continuously plotted by a polygraph and A/D-converted for further evaluation by a workstation. 3. Results 3.1. Torque strength Figure 3 shows the results of the measurements of maximum pinch torque strength based on means of all 10 Ss and all the 6 screwdriver handles. As can be seen, for inward rotation (pronation) the right hand (of predominantly right-handed Ss) is superior to the left hand, but when calculating the ratio between 4.6 Nm and 4.1 Nm, the right hand is superior only to a small extent of 12 %. Yet, with regard to outward rotations (i.e., supination) the difference between 3.8 Nm and 3.0 Nm rises up to 27 %. When analyzing the question as to the degree of lateralization of muscular strength, the values of the left-handed person and of the 2 Ss without an a priori “right-left” preference have to be shifted to the “right” side. As a consequence of this decision, highly significant differences result at least for supination with a mean of 35 % (3.9 Nm / 2.9 Nm), because the 2 Ss without preference proved posterior to be left-handed persons. The only slight differences of merely 12 % in pronation between the stronger and the weaker side remain almost at the same level. With regard to the workload during the screwing process, the relation of pronation left/supination right is more important. The same is valid for the torque demanded for the task of unscrewing via pronation of the right arm or via supination of the left arm.

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Figure 3:

71

Maximum torques exerted by the left hand and the right hand during supination and pronation

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By calculating the ratio 4.1 Nm / 3.8 Nm (or 4.1 Nm / 3.9 Nm for the individual laterality of the Ss) for driving in screws and the ratio 4.6 Nm / 3.0 Nm (or 4.6 Nm / 2.9 Nm) for removing screws, “hand preference” or the laterality of muscular strength turned out to be very important even for the same rotation direction. Differences of only 8 % (5 %) between pronation of the left hand and supination of the right hand, but 53 % (59 %) between pronation right hand and supination left hand, show that – at least for removing a tightened screw – the right choice of the rotation direction and the hand to be used can highly influence the work efficiency. According to Fig. 4, for the 10 mainly right-handed Ss the supination is a less favorable rotation direction for all the different screwdrivers used. Significant differences between corresponding values of maximum torque for supination and pronation of both hands can be observed. Furthermore, significant differences occur with respect to the maximum torques which can be exerted via the help of the various screwdrivers.

Figure 4:

Relative height of maximum torques exertable by the left and right hand during supination and pronation

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H. Strasser and B. Wang / Screwdriver torque strength and physiological costs of muscles

Regarding the maximum torque strength values for driving in screws (clockwise) and unscrewing (counter clockwise), as presented in Fig. 5, it can be concluded that right-handed persons have to use their left hand and not their right hand for clockwise work. For this direction, the “weaker” side is even stronger. But in order to exceed the torque of unscrewing in counter clockwise direction, the right hand is always significantly superior to the left hand as a consequence of the pronation. For left-handed persons, of course, quite the opposite is true.

Figure 5:

Relative maximum torques arranged for clockwise and counter clockwise operation of the left hand and right hand

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3.2. Physiological cost of pronation and supination Figure 6 shows physiological cost determined by standardized EA in % of individual maximum EA values of the monitored 4 muscles which are involved in pronation and supination of the right hand during the measuring of maximum torque. As expected, no essential systematic differences occurred between the EA values attributable to the different screwdrivers used during maximum voluntary contractions, but a highly significant reaction to pronation and supination was measured. The activity of the biceps as a dominant supinator (outward rotator) during pronation is nearly at the resting level. The activity of the grip musculature of the hand had to be increased during supination by about 10 % with the effect that only an even significantly lower maximum torque could be produced during supination. During pronation, the main strength is exerted by the brachioradialis, which is an adequate muscle both for pronation and supination. As a result of an increase of activity at the range of 10 % during pronation, a better performance with regard to a corresponding higher torque can be observed. The anterior part of the delta muscle shows a considerably higher degree of activity during pronation, i.e., this muscle group supports inward rotations effectively. Differences of about 25 % for the general level of strain make evident that the 20 % higher torque values for pronation (4.6 Nm / 3.8 Nm) result from this higher activity of the deltoid.

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Figure 6:

73

Relative height of EA of 4 muscle groups involved in pronation (red columns) and supination (green columns) of the right hand when exerting maximum torque

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4. Discussion The following inferences can be drawn from the results of this study. Firstly, it may be emphasized that systematically operational and physiological differences due to the various screwdriver handles, as results of investigations which were not the main objective of this study, proved to correspond well to the findings of prior studies (cp. STRASSER et al. 1990; STRASSER 1991). Concerning the different torque strength for the left and the right hand, especially for supination, and also in accordance with SCHMAUDER and SOLF (1992) or with KONZ (1990), the laterality of muscular strength has to be regarded to be of higher importance than generally expected. According to KONZ (1990), we have to consider dexterity of the non-preferred hand declining more in general grip strength than a mere 10 % when handtools are used. For instance, a 69 % rate for turning a screw in and out with a screwdriver is reported by LOWDEN (1977). Differences in the “hand preference” which normally are taxed at about 10 %, e.g., for the muscular strength exertable for closing the fist (cp. LAURIG 1976), can be confirmed for pronation but not for supination. Following the results mentioned above, remarkably larger differences for this preference have to be taken into consideration for supination. Furthermore, if differences caused by dexterity and by direction of rotation are listed, a variance of even more than 50 % must be taken into account. Differences in the exertion of torques during pronation and supination for power grip and pinch coupling, measured by BULLINGER and SOLF (1979), were classed until now at the range of only 10 %. Finally, the biceps as a flexor and outward rotator apparently supports the rule that, according to the general view of physiology textbooks, it seems to be easier for a right-handed person to drive in a right-handed thread. Yet, based on the results of this study with considerably diverging operational

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maximum values for performance and an also very different distribution of physiological cost, this opinion can be seen as ambiguous and possibly doubtful, at least with regard to the task of driving screws. 5. Conclusions The main objective of this study was not to identify the ergonomic quality of the various screwdrivers which, e.g., depends on the shape, diameter, length, volume, surface and material of the handle but to quantify the influence of the dominant and subdominant hand, and the direction (i.e., inward versus outward) rotation of the arm on torque strength as well as physiological cost of the main muscles involved. It can be shown that through various biomechanical coupling conditions represented by the different handles, consistent and significant differences of more than 50 % exist between pronation of the dominant and supination of the subdominant arm whereby the much stronger pronation of the dominant right hand results from an even more favorable muscle load. As for screwing in, a right-handed person normally uses supinations of the right hand which are weaker than pronations of the left hand, it would be advisable to apply pronations of the nondominant hand. For unscrewing, the dominant right hand guarantees that a tightened screw can be loosened with relatively low effort. But there are still some questions to be answered, before generalizing the results with respect to hand preference. Firstly, some findings of this study need strengthening and confirmation by further investigations into typical left-handed Ss. Secondly, since until now only muscle strain of the right hand-arm system has been measured via electromyography, simultaneous recordings on both sides will have to be made in order to get an insight into the muscle strain of the subdominant hand and possible contra-lateral co-contractions. 6. References BULLINGER, H.-J. und SOLF, J.J. (1979) Ergonomische Arbeitsmittelgestaltung I. Systematik. Wirtschaftsverlag NW, Verlag für Neue Wissenschaft GmbH, Bremerhaven KONZ, S. (1990) Work Design: Industrial Ergonomics, Chapter 16: Handtools. 3rd edition, Publishing Horizons, Inc., Scottsdale, Arizona, pp. 237-258

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LAURIG, W. (1976) Ergonomische Probleme standardisierter Verfahren zur Ermittlung von Erholungszeiten. Dargestellt an Beispielen einseitig dynamischer Muskelarbeit. Fortschrittliche Betriebsführung und Industrial Engineering 25 (6) 365-373 LOWDEN, K. (1977) Manual dexterity, dominant vs. nondominant hand with pliers, screwdriver and wrench. Internal report of Dept. of Industrial Eng., Kansas State University (cited in KONZ 1990) ROHMERT, W. und LAURIG, W. (1993) Physische Beanspruchung durch muskuläre Belastungen. In: SCHMIDTKE, H. (Ed.) Ergonomie. Hanser Verlag, München, pp. 121-143 SCHMAUDER, M. und SOLF, J.J. (1992) Kräfte im Hand-Arm-System – Vergleichsuntersuchung zur Problematik der Linkshändigkeit. Dokumentation Arbeitswissenschaft 33, Dr. O. Schmidt Verlag, Köln, p. 36 STRASSER, H.; LAUBER, M. und KOCH, W. (1990) Produkt-ergonomische Beurteilungsmethoden für handbetätigte Arbeitsmittel. Zeitschrift für Arbeitswissenschaft 44 (16 NF) 4, 205-213 STRASSER, H. (1991) Different grips of screwdrivers evaluated by means of measuring maximum torque, subjective rating and by registering electromyographic data during static and dynamic test work. In: KARWOWSKI, W. and YATES, J.W. (Eds.) Advances in Industrial Ergonomics and Safety III. Taylor and Francis, London / New York / Philadelphia, pp. 413-420

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Chapter 5.1

Muscle Strain of the Hand-Arm-Shoulder System During Typing at Conventional and Ergonomic Keyboards H. Strasser, R. Fleischer and E. Keller 0. Summary Manufacturers of ergonomic split keyboards promise maximum effectiveness and comfort as well as a reduction of physical complaints. In order to determine the positive effects claimed, a study was carried out during which 10 male subjects (Ss) participated in standardized working tests. They entered text into a PC, alternatingly using a conventional keyboard and an ergonomic keyboard. Electromyographic activity (EA) of 8 muscle groups was simultaneously recorded during altogether 6 working phases with a duration of 10 min, each. Measurements of the maximum activity, EAmax, via maximum voluntary contractions of the 8 muscles – which were necessary for calculating standardized electromyographic activity (sEA) used to represent muscle strain as a percentage – were always taken at the end of the experiment. Muscle strain varied from muscle to muscle but the level of the sEA values for the different muscles was reproducible and stable. Also, activation of most muscles acting on the shoulder, upper arm, forearm, and the hand showed differences which, though small in amount, could be statistically secured and associated with the keyboard type. The ergonomic design of the tested keyboard led to objectively verifiable and plausible reductions of muscle strain.

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1. Introduction It is an undisputed fact that the utilization of modern technology can lead to a considerable increase in productivity; however, working at modern VDU-workstations, for example, is by no means an “easy” task. Increased use of computers and long periods of typing in the course of word processing or manually entering text may lead to pain in muscles, tendons, and joints. This can often be attributed to the mostly static posture assumed when conventional keyboards are used. This posture, which is mainly determined by the keyboard, is characterized by an abducted upper arm and a statically held, bent and pronated forearm with an ulnar deviation and dorsal extension of the hand. Over longer periods of text input, a more or less high strain of the musculature of the whole hand-arm-shoulder system results and it appears that the deviated wrist and forearm posture (ulnar deviation and pronation), dictated by the design of the conventional computer keyboard, is implicated in the etiology of upper extremity work-related musculoskeletal disorders. Due to the results of a well-controlled laboratory study (cp. SERINA et al. 1999) via continuous electrogoniometric measurements over longer periods of time, wrist and forearm postures during typing on a standardized workstation adjusted to the subjects’ body dimensions were sustained at non-neutral angles (ulnar deviation between 15° and 20°, mean dorsal wrist extension approximately 20°, well beyond the 8° slope of the keyboard that was used, pronation ~80°). Furthermore, wrist motions during typing were rapid and were similar in magnitude to wrist motions of industrial workers performing jobs having a high risk for developing cumulative trauma disorders. The efforts of Dvorak and Klockenberg at the beginning of the 20th century, as well as solutions offered by KROEMER (1972) in the 1970s, did not result in any modification to the traditional keyboard design (cp. KELLER et al. 1991). Typists rejected proposed changes. Scientific and empirical evidence

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was insufficiently strong to justify the costs of retraining and obtaining new equipment (cp. ZECEVIC et al. 2000). Nevertheless, the legacy of early innovators (cp. KROEMER 2001) is reflected in various attempts to apply segmentation of the alphanumeric zone to new keyboard designs. Manufacturers of ergonomic split and tilted keyboards with a frontal angle between the two halves and a pantile-like inclination, a lateral tilt of the two halves, promise maximum effectiveness and comfort as well as a reduction of physical complaints. However, few data are available to substantiate whether these new keyboard designs are actually effective in reducing discomfort and musculoskeletal problems in users (cp. SWANSON et al. 1997). Therefore, the objective of this study was to assess the ergonomic quality of a test keyboard from a prominent producer (cp. N.N. 1994) in comparison with a conventional standard keyboard (Fig. 1). Standardized work experiments were carried out during which hypothetically expected muscle relief in the hand-arm-shoulder system was measured via electromyographic methods. This workphysiological objective method was supplemented by a subjective evaluation based on the work experiences of a group of workers (cp. KELLER et al. 2004). 2. Methods The right part of Fig. 1 shows the test object, which was used alternatively to a reference keyboard (left part of Fig. 1). For detailed characteristics of the keyboards see Fig. 2. The split-field keyboard with lateral sloping promised to reduce ulnar deviation and pronation of the forearm, thus allowing a more natural position of the hand-arm system. The reference keyboard was, as may be seen in the left part of Fig. 2, a standard QWERTZ-keyboard (German version of the QWERTY “101” style keyboard).

Test keyboard (right part) and standard keyboard (left part)

Figure 2:

Characteristics of the test keyboard in comparison with standard DIN 2137-13 (right part) and characteristics of the reference keyboard (left part)

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Figure 1:

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2.1. Test set-up

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The keyboards were integrated into an ergonomically designed VDU workplace (cp. STRASSER 1995), as can be seen in Fig. 3, and tests were carried out while alternatingly typing at the standard and the test keyboard. The workplace consisted of a divided table for the optimal adjustment of the monitor with respect to the relaxed visual axis and a keyboard work surface that was adjustable to an ergonomically acceptable height of 72 cm. General recommendations for a comfortable screen location relative to the eyes are difficult to make, since interindividual preferences vary in a wide range (cp. JASCHINSKI et al. 1999). But due to physiological plausibility and practical experience (cp. JASCHINSKIKRUZA 1991; KROEMER and HILL 1986) and having regard to ISO 9241-5, it was expected that, for a viewing distance of 70 cm, an angle of vertical gaze inclination of 40° may provide an ergonomic solution for many users. The chosen placement of the screen should induce less demands on the accommodation and convergence system, and hence less eyestrain, than shorter distances, and raised gaze angles recommended very often because of limited space on conventional tables. A chair which had compatible allocation of functional and anatomical joints according to the synchronous technique was used (cp. STRASSER 1995). In order to simulate manual tasks at a VDU workstation which would lead to at least hypothetical effects of the test object, the test subjects repetitively had to “blindly” enter the character combinations shown in the middle part of Fig. 3 using the 10-finger (touch) system on a traditional QWERTZ-keyboard (left part of Fig. 3) and on the test keyboard (right part of Fig. 3).

Figure 3:

VDU workplace (dimensions and adjustment ranges in mm) with standard and test keyboard chosen character combination for text input that ensured equal stress on the left and right hand

and

2.2. Test subjects Since single tests cannot lead to representative results, 10 test subjects (Ss) were chosen. They were all male, their average age was 33, and their average height was 185 cm. Other essential anthropometric data and characteristics of the Ss relevant for the ergonomics design are shown in Table 1. 2.3. Test procedure Each of the 10 Ss completed a work session which was performed over a period of altogether 245 min, i.e., more than 4 hours (cp. Fig. 4). After the Ss had been instructed and introduced to the experiments to be performed, a questionnaire asking for characteristics of the subject concerned had to be filled out.

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Table 1: Characteristics of the subjects 1 through 10 with work-relevant anthropometric dimensions

Figure 4:

Schedule of the test program with subjective rating and continuous measurement of electromyographic activities taken simultaneously to text input

It took approximately 30 minutes to carefully and properly attach the electrodes for the electromyographic measurements onto the various muscle groups and to allow the electrode paste to take effect. Thereupon, the Ss had to fill out the parts of the questionnaire concerning the • assessment of the subjective well-being and • subjective assessment of the workplace.

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After completion of the electrical alignment of the EMG-amplifiers, the test task was practiced, the measuring units were inspected and checked again for correctness, and, finally, in a 10-min break, measurements of resting activity, EA0, for all muscles in working posture were made. After this, the actual work experiments were started with • alternatingly entering text with the standard and the test keyboard in three 10-min blocks , and separated by blocks of 5-min recovery time. Simultaneously, the continuous recording of the electromyographic activity, EA, of 8 muscles of the hand-arm-shoulder area took place. Prior to proceeding to a second experiment block of alternatingly and , the Ss had to fill out the part of the entering text with the test and standard keyboard , questionnaire which referred to the • subjective assessment of the workplace and • subjective assessment of the effects of work with the standard keyboard,

.

A corresponding part of the questionnaire after completion of the second block was provided to gain information again on the • subjective assessment of the workplace and • subjective assessment of the effects of work with the test keyboard,

.

Additionally, the Ss had to rate details of the test keyboard, , under the impression of their own working experience. Since maximum voluntary contractions (MVC) of a muscle possibly provoke fatigue over an uncontrolled period of time, which could lead to cumulative stress and transfer effects, the maximum electromyographic activity, EAmax, of each muscle was measured at the end of the experiment.

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2.4. Explanations to the electromyographic measurements As shown in Fig. 5, myoelectric signals from a S were recorded via a multi-channel EAmeasurement unit (which consists of differential-amplifiers, rectifiers, and integrators). The data was then saved using a portable data-recorder. Upon completion of a test, data was transferred via an A/Dconverter to a PC for further processing and for final graphical representation of the results. Finally, specially developed plotting programs allowed an extensive visual workup of the results (cp. STRASSER et al. 2004). The upper left part of Fig. 5 shows the muscles of the right hand-arm-shoulder system which have been monitored. Since muscles of the lower and upper arm and the shoulder, in particular, are subjected to load by the working posture when typing at a keyboard, the following 8 muscle groups, which are also shown in Fig. 6, were monitored in the experiments: • the flexor carpi ulnaris which brings about ulnar deviation of the hand; • the pronator teres as inward rotator (pronator) of the forearm; and • the extensor digitorum (lower part of Fig. 6) which brings about dorsal extension, and which as antagonist of the flexor muscles is also involved in typing operations; • the caput longum, the long head of the biceps as flexor and supinator of the lower arm (middle part of Fig. 6); • the frontal (clavicular) part, the middle (acromial), and the rear part (spinal part) of the deltoid (upper part of Fig. 6) which are involved in • forward moving of the arm via the pars clavicularis, • spreading, i.e., abducting of the upper arm (pars acromialis), and • backward moving (retroversion) of the arm (pars spinalis). • Furthermore, the descendent part of the trapezius muscle (m. trapezius pars descendens) (upper part of Fig. 6), which is responsible for the drawing up or keeping the shoulder in position, and which normally has to be regarded as a bottle-neck muscle in sedentary work, was monitored.

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Registration and processing of electromyographic data via a portable data-recorder and a personal computer

Figure 6:

Selected muscles acting on the forearm and the hand (lower part), the long head of the biceps (middle part), and selected muscles acting on the upper arm and shoulder (upper part)

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Figure 5:

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Figure 7 shows the positioning of the silver-silver-chloride electrodes on the different muscles of a subject. Since electromyographic amplitude values cannot simply be interpreted as strain data, they had to be standardized via electromyographic activities (EAmax) arising under maximum voluntary contractions (MVCs). From the actual electromyographic activity, EA, (shown in the lower right row of Fig. 8), which was measured continuously during all the phases of work (i.e., the several 10-min blocks), standardized electromyographic activity, sEA [%], (shown in the upper right column of Fig. 8) was calculated via a widely accepted formula. After subtracting the resting activity, EA0, the actual EA values were related to the electromyographic activity, EAmax, which occurred under maximum voluntary contractions of a muscle. The standardized values, sEA, indicate muscle strain in percent. Muscle strain with the conventional keyboard and with the test object, together with the subjective assessments, should allow the evaluation of the test object’s ergonomic quality in figures and numbers. The results of the subjective ratings will be reported on in Chapter 5.2.

Positioning of the Ag-AgCl electrodes on the descendent part of the trapezius (left), the deltoid (middle) and the muscles acting on the hand (right)

Figure 8:

Methods for the calculation of standardized electromyographic activities sEA [%] from EA values on-the-job, resting activity EA0 values, and EAmax values

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Figure 7:

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3. Results Figure 9 shows the time series of the standardized electromyographic activity, sEA [%], of the 4 muscles acting on the forearm and the hand (lower block) and of the 4 muscles acting on the upper arm and shoulder (upper block) while alternatingly entering text with the standard keyboard, , and the test keyboard, , during altogether six 10-min blocks. The black and white parts of the columns associated with static and dynamic muscle strain represent 1-min mean values from 10 Ss. At first it has to be stressed that muscle strain varied considerably from muscle to muscle. This is true for the total electromyographic activity and, especially, the static part (black columns in Fig. 9). The height of the bar graphs for some muscles also may be associated with utilizing the two types of keyboards. But apparently these effects are not very strong. This seems to be true for all muscles acting on the forearm and the hand (lower block of Fig. 9) as well as the muscles acting on the upper arm and shoulder (upper block of Fig. 9). Working was associated with total sEA values of about 25 % for the descendent part of the trapezius (upper row of upper block) and very low values of about 5 % for the spinal part of the deltoid (lower row of upper block). The generally high static part of the m. trapezius pars descendens, which almost regularly exceeds the 15 % threshold of the endurance level with both keyboards, seems noteworthy. Therefore, it is not surprising that muscle fatigue is already detectable in increasing sEA values within the relatively short working phases of 10 min. Once again it can be shown very clearly that the descendent part of the trapezius muscle must generally be considered a bottleneck muscle during text input. Physiological responses to utilizing the different keyboards can scarcely be detected in this presentation of the results. In order to quantify potentially immanent effects, i.e., on the one hand the possible muscle relief brought about by the ergonomic test keyboard and on the other hand the additional physiological cost associated with the standard keyboard, in a first step all (total) sEA values from working with the two keyboards have been related to the overall mean values stemming from working with the test keyboard. For all single 1-min average values of the 10-min blocks of work, the quotient [sEA(t)–sEAT1+T2+T3] / [sEAT1+T2+T3] was calculated. This way of forming a value as a percentage promises to reveal much better increases or reductions of muscle strain due to the type of keyboard. Hypothetically, for the columns associated with the test keyboard in the sections , , and , this form of relating the sEA values, which have been visualized in Fig. 9, should result in only small deviations from zero. On the other hand, an increase in muscle strain – represented in the columns above the zero-line – should be the result of typing at the standard keyboard during the sections , , and . As can be seen in Fig. 10, this expected pattern indeed occurs, but it is not consistent and always true for the muscles acting on the shoulder and the upper arm (upper block). The descendent part of the trapezius, at least sometimes, reveals higher physiological cost for the sections , , and and almost a zero level during the sections , and . Yet for the two parts of the deltoid (p. acromialis and spinalis), which are involved in abduction and retroversion of the upper arm, no systematic effects were found which could be attributed to the keyboards. The pattern of changes in the percentage values due to the type of keyboard is more pronounced and consistent for the frontal (clavicular) part of the deltoid, which usually is involved in stabilizing the upper arm in a somewhat forward-moved position. About 25 % extra physiological cost or even more during has to be paid when typing at the standard keyboard, or can be saved when utilizing the test keyboard. The lower block of Fig. 10 exhibits clearer physiological responses of all the muscles acting on the forearm and the hand, which can be well associated with the type of the keyboard. During all the phases when the 10 Ss worked with the test keyboard , and , only marginal increases or decreases in the relative values, i.e., variations around the 0-line, occur. On the other hand, the systematic increase of strain associated with the standard keyboard during the sections , , and – represented in columns above “0” – indicate that, e.g., about 25 % of the physiological cost of the biceps which has to be paid by this muscle when working with the conventional keyboard can be saved while typing at the ergonomic keyboard. The effect can – as shown for and – be verified in a second and third measurement.

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Figure 9:

83

Time series of static components (black part of columns) and dynamic components (white part of columns) of the standardized electromyographic activity sEA [%] (1-min means of 10 Ss) of 4 muscles acting on the forearm and the hand (lower block) and of 4 muscles acting on the upper arm and shoulder (upper block) over the 6, 10-min work phases while alternatingly entering text with the standard keyboard and with the test keyboard

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Figure 10: Increase or reduction of physiological cost of the 8 muscle groups acting on the forearm and the hand (lower block) and on the upper arm and shoulder (upper block). Standardized electromyographic activity and test keyboard related to overall sEA from all 6 working phases with the standard keyboard mean sEA from + + . Significance level indicated by the symbols *, **, and *** (one-sided t-test)

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Since the total sEA values from the biceps as flexor and supinator (outward rotator) of the forearm are very low (Fig. 9), from a work-physiological point of view, the very clear and (in two retests) reproducible effects due to the type of keyboard are less important than those of the other 3 muscles acting on the forearm and the hand. Regarding the flexor carpi ulnaris, which brings about ulnar deviation of the hand, and the extensor digitorum, involved both in dorsal extension of the hand and in typing operations (as antagonist of the flexor digitorum), the physiological cost is increased by about 10 % while typing at the conventional (standard) device. Using the split keyboard with a pantile-like inclination, which is more compatible with the normal position of the hand and arm while typing (cp. STRASSER 1995), saves about 25 % of the physiological cost which has to be paid by the pronator teres as inward rotator of the forearm when operating the conventional (standard) keyboard. When relating the sEA values of a test phase to those of the immediately succeeding test phase, i.e., the ten 1-min values from and , and , and and of Fig. 9, the extra-physiological cost which has to be paid by the muscles involved in using one of the keyboards can be quantified in an alternative way. When, for example, the ratio / would exceed “1”, the physiological cost would be higher for the standard keyboard and vice versa. Figure 11 shows the results of these calculations and, together with the number of stars ( ), also indicates the level of significance. According to the results, 6 out of the 8 muscles monitored, at least for some time of the three 10-min test phases when involved in utilizing the standard keyboard, are associated with significantly higher physiological cost. Whereas for phases and muscles with ratios lower than “1” (especially the middle and spinal part of the deltoid), the significance level never was exceeded. That means, at no time could a significant increase of muscle strain be attributed to utilizing the test keyboard.

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4. Discussion As has been shown in Fig. 9, large variations between the muscles exist, but the level of the sEA values for the different muscles is reproducible and quite stable. The highest strain was measured for the descendent part of the trapezius, with sEA values of 25 %, which increased from minute to minute within the 10-min phases. This is a certain sign of fatigue in the musculature which is responsible for pulling up the shoulder and keeping it in position. Also, activation of most muscles acting on the shoulder, upper arm, forearm, and the hand showed differences which, though small in amount, could be statistically secured and associated with the type of keyboard. When the results from immediately following working phases are compared, a substantial reduction in muscle strain can be associated with the ergonomic keyboard. The frontal, the middle, and the spinal parts of the deltoid, which are involved in forward motion, abduction, and retroversion of the upper arm revealed total strain levels of slightly more than 10 % (p. clavicularis) and approximately 5 % (for the others). The static part amounts to about 50 %, i.e., half of the total sEA value which, as expected, does not cause fatigue. Reductions of these sEA values in the range of 25 % (upper block in Fig. 10), which could be proven statistically significant for the frontal (clavicular) part of the deltoid during all 10-min blocks of work with the ergonomic keyboard, should not be overestimated, because of the relatively low level of strain, even when working with the standard keyboard. For the other two parts of the deltoid, no systematic effects which could be attributed to the keyboards were found. The biceps as flexor and supinator of the forearm, the flexor carpi ulnaris which brings about ulnar deviation of the hand, the pronator teres as inward rotator of the forearm, and the extensor digitorum, involved both in dorsal extension of the hand and in typing operations (as antagonist of the flexor digitorum), yielded consistent results, which can be ascribed to the type of the keyboard used. Regarding the flexor carpi ulnaris and the extensor digitorum, the mean sEA values of about 20 to 25 % and 15 to 20 % (cp. lower block of Fig. 9) can be reduced by at least 10 % with the ergonomic test device (lower block of Fig. 10). For the static component of muscle strain, the relief is somewhat clearer.

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Figure 11: Extra physiological cost for the 8 muscle groups acting on the shoulder and upper arm, the forearm and hand while typing with the standard keyboard in the course of 3 successive working phases (sEA[%]standard keyboard/sEA[%]test keyboard) (e.g., “2.0” would mean that physiological cost of utilizing the standard keyboard is twice as high as physiological cost of the test keyboard). Significance level indicated by the symbols *, **, and *** (one-sided t-test)

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Using the ergonomic keyboard saves more than 25 % of the physiological cost which otherwise has to be paid from the pronator teres and the biceps when operating the standard keyboard. From a workphysiological point of view, these very clear and (in two retests) reproducible effects are less important for the biceps – which has to bring only 5 % of its capacity into work when working at the conventional keyboard – than for the other muscles acting on the forearm and the hand with a higher level of activity. In contrast to previous studies (cp. FERNSTRÖM et al. 1994), which failed to show a significant influence of 20°-angled prototype keyboards, as well as positive effects of a palm rest, or where only small positive effects of an angled keyboard use were found (cp. GERARD et al. 1994; MÜLLER 1996), the ergonomic design of the split fixed-angle keyboard tested in this study led to objectively verifiable, substantial, and plausible reductions of muscle strain, especially in the muscles acting on the forearm and the hand. The physiological cost which, in comparison with a standard keyboard, could be saved by the test keyboard, can be regarded as a strong correlate of a more neutral posture of the hand and forearm, which has been demonstrated when carrying out a comprehensive biomechanical investigation into wrist, forearm and finger angles of 90 professional touch typists using three alternative computer keyboards and a conventional keyboard (cp. MARKLIN and SIMONEAU 1997). Using a goniometric method, it was shown that a more neutral forearm and hand position is achieved through the use of the test keyboard than when the standard keyboard is used (cp. SMUTZ et al. 1994; REMPEL et al. 1995). The effects in the 8 muscles monitored were not as strong as was to be expected from simultaneously recorded subjective assessments (cp. Chapter 5.2). Subjective questioning revealed that the Ss did not at all favor the conventional (standard) keyboard layout; they preferred the new and innovative ergonomic layout very clearly. Especially after having experienced the test device, they felt that work with it was much more comfortable and less strenuous than with the conventional keyboard.

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5. Conclusions Despite the fact that this study delivered consistent electromyographic results in favor of the ergonomic keyboard, it may not be neglected that a computer keyboard today should not be evaluated in isolation. In this context, it was pointed out that, ironically, new “ergonomic” keyboards that have been designed with angled keys to reduce ulnar deviation by splitting or widening the orientation of keys, may encourage pointing device placement to a position that abducts the arm (cp. HARVEY and PEPER 1997). This type of “wider” keyboard, while reducing the risk of wrist and hand injury, may unintentionally lead to a significant increase in trapezius muscle tension during mouse use. It may be mentioned that this can be compensated by an armrest (cp. KELLER and STRASSER 1996) or a wrist rest (cp. KELLER and STRASSER 1998). Subjective reports of muscle tension in the study of HARVEY and PEPER (1997) did not correlate with surface electromyography. Therefore, their results also support the general conclusion that it is always advisable to utilize both objective and subjective methods for avoiding overestimation or underestimation of experimental results, i.e., for an adequate and correct evaluation of the ergonomic quality of working tools. 6. References FERNSTRÖM, E.; ERICSON, M.O. and MALKER, H. (1994) Electromyographic activity during typewriter and keyboard use. Ergonomics 37 (3) 477-484 GERARD, M.J.; JONES, S.K.; SMITH, L.A.; THOMAS, R.T. and WANG, T. (1994) An ergonomic evaluation of the Kinesis ergonomic computer keyboard. Ergonomics 37 (10) 1661-1668 HARVEY, R. and PEPER, E. (1997) Surface electromyography and mouse use position. Ergonomics 40 (8) 781-789 JASCHINSKI-KRUZA, W. (1991) Eyestrain in VDU users: viewing distance and the resting position of ocular muscles. Human Factors 33, 69-83

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JASCHINSKI, W.; HEUER, H. and KYLIAN, H. (1999) A procedure to determine the individually comfortable position of visual displays relative to the eyes. Ergonomics 42 (4) 535-549 KELLER, E.; BECKER, E. und STRASSER, H. (1991) Objektivierung des Anlernverhaltens einer Einhand-Akkordtastatur für Texteingabe. Z.Arb.wiss. 45 (17 NF) 1, 1-10 KELLER, E. and STRASSER, H. (1996) Ergonomic evaluation of an armrest for typing via electromyographic and subjective assessment. In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M. and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I. IOS Press, Amsterdam, pp. 838-845 KELLER, E. and STRASSER, H. (1998) Electromyographic and subjective evaluation of a wrist rest for VDT operators. Occupational Ergonomics 1 (4) 239-257 KELLER, E.; FLEISCHER, R. and STRASSER, H. (2004) Estimated and experienced subjective assessment of the ergonomic quality of a keyboard. Occupational Ergonomics 4 (2) 121-131 KROEMER, K.H.E. (1972) Human engineering the keyboard. Human Factors 14 (1) 51-63 KROEMER, K.H.E. and HILL, S.G. (1986) Preferred line of sight angle. Ergonomics 29, 1129-1134 KROEMER, K.H.E. (2001) Keyboards and keying. An annotated bibliography of the literature from 1878 to 1999. Universal Access in the Information Society 1 (2) 99-160 MARKLIN, R.W. and SIMONEAU, G.G. (1997) An ergonomics study of alternative keyboard designs. Final performance report, submitted to NIOSH, CDC and NIH, 173 pp. MÜLLER, K.-W. (1996) Effects of ergonomic keyboards on static and dynamic components of electromyographic activity. In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M. and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I. IOS Press, Amsterdam, pp. 502-507 N.N. (1994) User’s Guide – Microsoft Natural Keyboard. Microsoft Corporation, Redmond, USA REMPEL, D.; HONAN, M.; SERINA, E.R. and TAL, R. (1995) Wrist postures while typing on a standard and a split keyboard. In: BITTNER, A.C. and CHAMPNEY, P.C. (Eds.) Advances in Industrial Ergonomics & Safety VII. Proceedings of the 10th Annual International Industrial Ergonomics and Safety Conference, Seattle, Washington, Taylor and Francis, pp. 619-622 SERINA, E.R.; TAL, R. and REMPEL, D. (1999) Wrist and forearm postures and motions during typing. Ergonomics 42 (7) 938-951 SMUTZ, P.; SERINA, E.R. and REMPEL, D. (1994) A system for evaluating the effect of keyboard design on force, posture, comfort and productivity. Ergonomics 37 (10) 1649-1660

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STRASSER, H. (1995) Ergonomic efforts aiming at compatibility in work design for realizing preventive occupational health and safety. International Journal of Industrial Ergonomics 16 (3) 211-235 STRASSER, H.; KLUTH, K. and KELLER, E. (2004) A computer-based system for the use of electromyographic methods for the measurement of physiological costs associated with operating hand-held tools and computer-input devices. Occupational Ergonomics 4 (2) 73-87 SWANSON, N.G.; GALINSKY, T.L.; COLE, L.L.; PAU, C.S. and SAUTER, S.L. (1997) The impact of keyboard design on comfort and productivity in a text-entry task. Applied Ergonomics 28 (1) 9-16 ZECEVIC, A.; MILLER, D.I. and HARBURN, K. (2000) An evaluation of the ergonomics of three computer keyboards. Ergonomics 43 (1) 55-72 Standards, guidelines, regulations DIN 2137-13 (1995-04) Text and office systems - Alphanumeric keyboards - Part 13: German keyboard for data and text processing; Key arrangement and allocation for split and tilted keyboards. Draft, German Institute for Standardization, Beuth Verlag, Berlin ISO 9241-5 (1998-10) Ergonomic requirements for office work with visual display terminals (VDTs) - Part 5: Workstation layout and postural requirements. International Organization for Standardization

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.

Chapter 5.2

Estimated and Experienced Subjective Assessment of the Ergonomic Quality of a Keyboard E. Keller, R. Fleischer and H. Strasser 0. Summary At an ergonomically optimized VDU-workstation, a group of 10 test subjects (Ss) carried out a typing task both at the test keyboard and a reference computer keyboard. The participants’ personal subjective assessments concerning the working conditions with the two keyboards and the layout of the ergonomic model were recorded via specially designed questionnaires. These were given prior to, and after, the working tests and, therefore, reflected ratings without and with working experience. The posture of the hand, the lower and the upper arm, and the shoulder during text input were evaluated quite differently as a consequence of using the two keyboards. The working posture associated with the conventional keyboard was never assessed positively. The ratings always reached positive values when operating the ergonomic model. The handling of the keyboard and the overall impression were also in favor of the test keyboard as a result of the working tests. Comfort and effort while typing at the keyboards also differed substantially and were in favor of the test keyboard. The same was true for altogether 10 items of the questionnaire, aiming at evaluation of details of the ergonomic keyboard and its overall appearance.

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1. Introduction The main goal of the design of man-machine systems and human-computer interaction must be the compatibility between the characteristics of the functional principles of the human organism and the adaptable technical components of the working system. In the past, ergonomic knowledge also has been applied more and more in the office and administration sectors in order to avoid detrimental psychological and physical stress. This should hopefully result in human capital which is involved and motivated at work, rather than one which simply fulfills the bare minimum of what is expected. In this respect, ergonomic keyboards for VDU-workplaces, which enable a less constrained posture of the hand-arm-shoulder system at work, are important devices. 2. Methods Taking work-physiological aspects into consideration, a test layout was developed which was capable of identifying the effects of the ergonomic design of a keyboard (cp. DIN 2137-13) in comparison with a standard device (cp. ISO 9241-4) via electromyographic and subjective methods (cp. Chapter 5.1 and STRASSER et al. 2004). At an ergonomically optimized VDU-workstation, a group of 10 test subjects (Ss), aged between 24 and 41, with a body height between 178 and 194 cm, carried out a task, i.e., typing both at the test keyboard and a reference keyboard. In addition to the multi-channel electromyography, characteristics of the test keyboard were rated, and subjective assessments of muscle strain associated with typing at a conventional and the test keyboard were made, in order to evaluate the ergonomic quality of the keyboards. According to Fig. 1, the workplace, the posture of the hand, forearm, upper arm, and shoulder during typing, and overall impression of both the standard and the test keyboard were evaluated by the Ss using a well-tried 4-

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step bipolar scale (cp. KELLER and STRASSER 1998). The Ss’ options ranged from “-4” (extremely unfavorable) to “+4” (extremely favorable).

Figure 1:

Questionnaire for the subjective assessment of the workplace

They also evaluated the comfort and the effort during text input for both keyboards using a verbally explained unipolar scale with values from “0” (i.e., no comfort or no effort) to “4” (very high). The questionnaire shown in Fig. 2 was used prior to and after the experiments with the reference keyboard and the test keyboard. While the test Ss filled out such a questionnaire to evaluate comfort, posture, and working conditions after every test, it was deemed sufficient to record specific perceptions with respect to the test keyboard immediately after the last text input. For this purpose, the questionnaire shown in Fig. 3, which is similar to the previously utilized bipolar scale was developed. The questionnaire, on a 9-step scale, allows for a general evaluation, as well as a classification of ergonomic design parameters, e.g., splitting of the keyboard into two halves, the angle between the two keyboard halves, and the frontal as well as the lateral slope, and the classification of functional features, e.g., positioning and size of keys, surface of the keyboard, and release pressure. As can be seen in Fig. 4, the Ss also were asked to categorize the physical strain – which resulted from work with the different keyboards – felt on specific sections of the right hand-arm-shoulder area, the neck, and the back on a discrete scale of “0” (no noticeable strain) to “4” (very heavy strain). As a reference, and in order to evaluate the test subjects’ suitability, impairments – such as muscle soreness due to previous exercise – were registered before the tests.

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Questionnaire for the subjective assessment of comfort and effort associated with utilizing the two keyboards

Figure 3:

Bipolar questionnaire for the assessment of details of the test keyboard

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Figure 2:

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Figure 4:

Questionnaire for the subjective assessment of the effects of work (visualization of body regions where complaints associated with work could be specified on a scale from 0 to 4)

3. Results

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The results (averaged over all 10 subjects) of the Ss’ evaluation of the VDU workstation before and after the tests are shown in Fig. 5. On average, the test subjects’ assessments of • • • •

the screen (I1), the distance between the eyes and the screen (I2), the individually adjusted desktop height (I3), and the position of the chair (I4)

were about the same before and after the tests, i.e., without and with working experience.

Figure 5:

Evaluation of the VDU workstation by the test subjects before and after the working experiments

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Generally, the conditions of the workstation were rated favorably by the Ss. The values ranged from “1” (slightly favorable) to “2” (fairly favorable). Differences in the evaluations before beginning work and after completing the working phases with both keyboards were not expected. Figure 6 illustrates the results of the repeated interrogations with respect to different criteria of the working posture. The posture of the hand, the forearm, and the upper arm and shoulder during text input with the standard keyboard (I5, I7, I9), was not evaluated positively, while the working posture when using the ergonomically designed keyboard was rated significantly higher (I6, I8, I10). The relief which was expected, i.e., estimated by the Ss for the forearm-hand region, could be confirmed in the experiment.

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Figure 6:

Pre and post test values of the subjects’ assessment of the working posture (position of the hand, forearm, upper arm and shoulder) associated with utilizing the test keyboard and the standard keyboard (Level of significance of differences according to the results of a one-tailed Wilcoxon test)

But while the great differences in the evaluation of the working posture of these body regions before the tests were fairly uniform (statistically proven at a significance level of at least 95 %), the evaluation after text input with both keyboards was less homogeneous, i.e., only significant for the hand at a lower level. On average, both the ergonomic and the conventional keyboards were evaluated more positively after completion of the tests, whereby the increase of the rating was higher for the standard keyboard. However, the hypothesis “the working posture when using the test keyboard is better than when using the reference keyboard” for the questioning after the tests can only be confirmed at a 95 % significance level for the hands. The item referring to the working posture of the upper arm and shoulder showed that the expectations of the keyboard were clearly verified through the working experience. The ergonomic keyboard was evaluated more favorably after working experience than before (with 0.9 versus 0.5) and the reference keyboard was rated less favorably (with -0.3 versus 0.0); thus, the advantage of the test keyboard (in the individual evaluations) is proven at a higher significance level (99.5 %) than before the test (95 %). Furthermore, the test Ss were instructed to rate the handling of the standard keyboard (I11) and the test keyboard (I12) in various working situations, such as text or number entry, and use of editing and function keys as well as special keys. As can be seen in Fig. 7, both keyboards were rated equally well

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before the test phases, whereas the ratings after the test clearly differed. Work with the standard keyboard was evaluated as worse than expected; use of the test keyboard was evaluated better than expected. Since the assessments of the test Ss did not tend to be uniform, the difference in the evaluation is not significant. Figure 7 also gives the overall impression the Ss had of the utilized working devices.

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Figure 7:

Assessment of the handling and overall impression of the working devices

The global assessment of the test keyboard was better than that of the standard keyboard both before and after the test series. The Ss suspected already in advance that the text input with the new keyboard would be more comfortable and less strenuous. Their assumptions were confirmed by their own practical working experiences. Finally, the positive assessments of the ergonomic keyboard after the tests were even more uniform and considerably more distinct than before. This statement can be statistically secured. The comfort during typing at the standard keyboard was assessed poorly at a unipolar 4-step scale both with and without working experience (left part of Fig. 8). At the same time, the Ss evaluated the work with the test keyboard as (slightly to) fairly comfortable. Significant differences after the tests were also revealed in the assessment of the effort. According to the ratings shown in the right part of Fig. 8, the Ss felt the text entry clearly less exhausting with the test keyboard than with a keyboard according to standards (e.g., DIN 2137-2; ISO 9241-4). The greater comfort and lower effort which were expected were also reflected in the rather representative assessments, which were supported by working experience. Using the questionnaire described already in Fig. 3, the test participants rated the new keyboard which was utilized in three test series. According to Fig. 9, typical design features of the ergonomic keyboard (I20) and their actual realization by the producer (I21 through I23) were positively evaluated at a bipolar 4-step scale. The ratings mostly ranged from “1” (somewhat favorable) to “2” (fairly favorable). The Ss were satisfied with the arrangement of the keys (I24 through I25), and it is noticeable that the space bar (which is not in accordance with standards) was especially well-liked. The design of the surface of the keys was evaluated very well (2.3), and the release pressure was also rated well (1.9). The overall appearance, with a mean value of 2.2, turned out rather well.

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Figure 8:

Estimated and experienced subjects’ assessment (pre and post test values) of comfort and effort while typing with the test keyboard and the standard keyboard

Figure 9:

Subjects’ assessment of characteristics of the test keyboard. Means of 10 Ss on a bipolar 4-step scale

Figure 10 shows the effects of the work as evaluated by the test participants, whereby the typical problem areas for typing are illustrated. The strain resulting from typing with the two different

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keyboards was evaluated as practically equal for the lumbar vertebra section (LV), the upper arm (UA), and the wrist (W). For most of the Ss, the additional strain felt in the shoulder-neck region (FS, RS, and N) when using the test keyboard can partially be attributed to the unaccustomed arrangement of the keys. Therefore, most of the participants were forced to look (often steeply) down and to displace their arms even more. However, the ergonomic keyboard did bring about a certain relief for the forearm (FA) and back (B). Due to the aforementioned habitual-physiological reasons, a greater effort was felt in the fingers (F) when using the new keyboard. Also, the participants’ psychological state, which got progressively worse over the test duration – the test Ss had already been subjected to tests involving text entry for several hours – could have negatively influenced their assessments of the test keyboard. But it should be mentioned that complaints associated with utilizing the two keyboards in an already optimized workplace from an ergonomics point of view were generally very low. Consequently, no large differences between the standard and the test keyboard could be expected.

Figure 10: Assessment of complaints in various body regions associated with work when utilizing the standard keyboard and the test keyboard. Means of 10 Ss, on a scale from “0” (no complaints) to “4” (very heavy strain)

4. Discussion Research results regarding the impact of keyboard design on comfort and productivity are very heterogeneous and inconsistent. The results of a research (cp. SWANSON et al. 1997), which explored the effects of design features (such as splitting the keyboard in half, and laterally inclining the halves) on performance and self-report measures of discomfort and fatigue in a text-entry task, suggest a minimal impact, at least after two days of exposure. But physiological strain measurements and posture assessments of the hand-arm system carried out already in the early eighties were in favor of the alternative prototypes of that time (cp. ZIPP et al. 1983; NAKASEKO et al. 1985). For the most part, the test keyboard conforms with ergonomic recommendations. The splitting of the keyboard into two halves and the lateral sloping of the key blocks represent significant improvements to the traditional working device. Using a goniometric method researchers were able to show that a more neutral forearm and hand position is achieved through the use of the test keyboard

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than when the standard keyboard is used (cp. SMUTZ et al. 1994; REMPEL et al. 1995). ZECEVIC et al. (2000) analyzed hand position, typing productivity and keyboard preference by comparing the standard keyboard with the same model as was tested in this study and an extreme version of a segmented alternative keyboard with a marked lateral inclination of ~ 40°. They were able to show that the test keyboard kept the hand in a neutral posture most of the time, avoiding extreme dorsal extension and ulnar deviation, but, compared with typing on the standard keyboard, productivity was reduced by approximately 10 %. Taking into consideration all elements of the evaluation (hand position, typing productivity and keyboard preference), and recognizing that the study was limited to an analysis of wrist of the dominant hand during a short typing period, they concluded that the alternative (ergonomic) design has the potential to improve hand posture substantially. This would reduce the risk of developing cumulative trauma disorders of the wrist due to keyboard use. This study, in addition to posture assessments, was able to identify relief effects in muscle strain. Characteristic values were quantified via a computer-aided, standardized method for the processing of myoelectric data from time series and a practice-relevant standardization. The ergonomically designed keyboard was not able to reduce the muscle strain in the upper arm-shoulder region substantially, except to the frontal part of the deltoid. In this context, it should be mentioned that an individually adjustable armrest is a suitable means of limiting the prevalence of usually unavoidable static tension of the hand-arm-shoulder area (cp. KELLER and STRASSER 1996). Yet in the forearm-hand region, a lower general strain results, especially for the m. pronator teres and the m. extensor digitorum. This is mainly due to the reduction of the static characteristic values. The lower static strain of the m. flexor carpi ulnaris is also noteworthy. Such a reduction of strain always can be interpreted as favorable in the sense of a better load distribution within the muscle. Subjective questioning revealed that the test Ss did not favor the traditional keyboard layout; they preferred the new and innovative ergonomic layout. The participants also felt that work with the test keyboard was much more comfortable and less strenuous than with the traditional keyboard. However, muscle relief during typing at the test keyboard, which was measured via electromyography, was not as high as expected subjectively. Previous investigations into operational performance showed that there were no significant differences between the two keyboards with respect to typing performance and number of mistakes. An earlier study (cp. ROSENBLAD et al. 1995) pertaining to the typing of Swedish texts revealed the easy applicability to the input of continuous texts in the German language. After an adequately long training period, experiments carried out with similarly designed ergonomic keyboards yielded corresponding results (cp. METKER und TRÄNKLE 1991; MÜLLER 1996; MARKLIN and SIMONEAU 1997). Hence, the keyboard is in accordance with ISO 9241-4. However, the keys “G,” “H,” “N,” “T,” “6,” and “7” and the space bar, as well as the arrangement of the function keys, are not in accordance with the German standard draft for ergonomic keyboards (cp. N.N. 1994). The results of the study presented here are valid for the desirable “blind” touch-typing. They are not necessarily valid for two-finger-typing. Hunt-and-peck typists, people who use fewer than 10 fingers and look at the keyboard while typing, found the ergonomic keyboard, the same tested in this study, more difficult to use than a standard QWERTY keyboard (cp. THUM e al. 1997). Also the results of two 45 minute typing sessions, one with each keyboard, indicate that there were significant decreases in the effectiveness (words per minute), and the means of EMG values of the flexor and extensor muscles of the forearm (5-min means from 5 Ss) were generally higher for the ergonomic keyboard as compared to the standard keyboard. Yet for touch typing, as several test Ss of this study stated, particularly the splitting of the keyboard was conducive to learning the professional typing method. Hence, this ergo-keyboard proves itself to be an alternative to the standard keyboard for beginning typists (cp. HERTTING-THOMASIUS und STEIDEL 1990). Especially in the professional sector, typical injuries and complaints in the forearm and hand could be avoided via utilization of the test keyboard. Together with an armrest, it is a recommended working device for VDU workstations. However, this result is only a part of that which must be considered when designing a workplace. Other influencing factors, such as the table-chair system, lighting, or the acoustic and climatic stress, must also be optimized.

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5. Conclusions The Ss’s assessment of the VDU-workstation was quite positive on a bipolar 4-step scale and did not vary in the pre and post assessments. This can be regarded as an indication of a given test-retest reliability when applying the questionnaire. The posture of the hand, the lower and the upper arm, and the shoulder during text input were evaluated quite differently as a consequence of using the two keyboards. The working posture associated with the conventional keyboard was never assessed positively. The ratings always reached positive values on average (in the range between 0.5 and 1.5 for the various items) when operating the ergonomic model, and the differences became clearer with working experience. The handling of the keyboard and the overall impression were also in favor of the test model with drastically increasing and decreasing values for the two keyboards as a result of the working experience. Comfort and effort while typing at the keyboards also differed very clearly and were in favor of the test keyboard. The same was true for altogether 10 items of the questionnaire aimed at evaluating details of the ergonomic keyboard and its overall appearance. Despite the very clear subjectively felt positive characteristics of the ergonomic keyboard and the more comfortable posture resulting from use of the keyboard, complaints associated with the working tests, which altogether lasted about 4 hours, did not vary systematically between the two keyboards. Possibly due to the ergonomic layout of the VDU-workplace, the complaints in various body regions (finger, wrist, forearm, upper arm, shoulder, neck, back, and lumbar vertebra section) were very low and never exceeded the intensity of “1” on a unipolar 4-step scale. When comparing the results of the subjective assessments of the ergonomic quality of a keyboard with the objectively measured muscle strain of the hand-arm-shoulder system, one may conclude that subjective and objective differences cannot be interpreted in the same way and compared on a one-toone ratio. Relatively strongly felt subjective effects in the range of more than “1” and even up to “2” on a bipolar 4-step scale correspond with small muscle relief in the range of only 10-25 %, which has been quantified via multi-channel electromyographic methods. 6. References HERTTING-THOMASIUS, R. und STEIDEL, F. (1990) Zur Problematik der Einführung alternativer Tastaturbauformen. Informationsschrift. Technische Universität Berlin, Institut für Arbeitswissenschaft, Berlin

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KELLER, E. and STRASSER, H. (1996) Ergonomic evaluation of an armrest for typing via electromyographic and subjective assessment. In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M. and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I. IOS Press, Amsterdam, pp. 838-845 KELLER, E. and STRASSER, H. (1998) Electromyographic and subjective evaluation of a wrist rest for VDT operators. Occupational Ergonomics 1 (4) 239-257 MARKLIN, R.W. and SIMONEAU, G.G. (1997) An ergonomics study of alternative keyboard designs. Final performance report, submitted to NIOSH, CDC and NIH, 173 pp. METKER, T. und TRÄNKLE, U. (1991) Tastschreiben auf konventionellen und ergonomisch optimierten Tastaturen – Untersuchungen zum Neulernen und Umlernen. Z.Arb.wiss. 45 (17 NF) 1, 11-19 MÜLLER, K.-W. (1996) Effects of ergonomic keyboards on static and dynamic components of electromyographic activity. In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M. and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I. IOS Press, Amsterdam, pp. 502-507 N.N. (1994) User’s guide – Microsoft Natural Keyboard. Microsoft Corporation, Redmond, USA NAKASEKO, M.; GRANDJEAN, E.; HÜNTING, W. and GIERER, R. (1985) Studies on ergonomically designed alphanumeric keyboards. Human Factors 27, 175-187 REMPEL, D.; HONAN, M.; SERINA, E.R. and TAL, R. (1995) Wrist postures while typing on a standard and a split keyboard. In: BITTNER, A.C. and CHAMPNEY, P.C. (Eds.) Advances in Industrial Ergonomics & Safety VII. Proceedings of the 10th Annual International Industrial Ergonomics and Safety Conference, Seattle, Washington, Taylor & Francis, 619-622

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ROSENBLAD, T.; CLARIDGE, N. and SYNNERMAN, S. (1995) Conformance test of the microsoft natural keyboard against ISO 9241, Part 4.2 (draft). Nomos Management AB, Stockholm SMUTZ, P.; SERINA, E.R. and REMPEL, D. (1994) A system for evaluating the effect of keyboard design on force, posture, comfort and productivity. Ergonomics 37 (10) 1649-1660 STRASSER, H.; FLEISCHER, R. and KELLER, E. (2004) Muscle strain of the hand-arm-shoulder system during typing at conventional and ergonomic keyboards. Occupational Ergonomics 4 (2) 105-119 SWANSON, N.G.; GALINSKY, T.L.; COLE, L.L.; PAU, C.S. and SAUTER, S.L. (1997) The impact of keyboard design on comfort and productivity in a text-entry task. Applied Ergonomics 28 (1) 9-16 THUM, J.E.; DAWKINS, E.; KRUEGER, T. and FREDERICKS, T.K. (1997) A practical evaluation of ergonomic keyboards. In: DAS, B. and KARWOWSKI, W. (Eds.) Advances in Occupational Ergonomics and Safety II. IOS Press and Ohmsha; Amsterdam, pp. 425-428 ZECEVIC, A.; MILLER, D.I. and HARBURN, K. (2000) An evaluation of the ergonomics of three computer keyboards. Ergonomics 43 (1) 55-72 ZIPP, P.; HAIDER, E.; HALPERN, N. and ROHMERT, W. (1983) Keyboard design through physiological strain measurements. Applied Ergonomics 14, 117-122 Standards, guidelines, regulations DIN 2137-2 (1999-02) Text and office systems - Keyboards - Part 2: German keyboard for data and text processing; Key arrangement and allocation of graphic characters to keys. Draft, German Institute for Standardization, Beuth Verlag, Berlin DIN 2137-13 (1995-04) Text and office systems - Alphanumeric keyboards - Part 13: German keyboard for data and text processing; Key arrangement and allocation for split and tilted keyboards. Draft, German Institute for Standardization, Beuth Verlag, Berlin

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ISO 9241-4 (1998-08) Ergonomic requirements for office work with visual display terminals (VDTs) - Part 4: Keyboard requirements. International Organization for Standardization

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Chapter 6

Ergonomic Evaluation of an Armrest for Typing E. Keller and H. Strasser 0. Summary Even when utilizing ergonomically designed flat keyboards, longer lasting periods of work with a word processor or typewriter can cause physical complaints. In this context, a specially developed armrest is said to enable more comfortable working and to prevent physical complaints. Therefore, the objective of this study was to assess the effects of this armrest via an experimental investigation. Ten male subjects (Ss) participated in tests at an already ergonomically optimized VDU workplace where continuous input of the same text was demanded in sections with and without the armrest. Before and after the tests, the subjects had to subjectively rate important criteria of the armrest. During the tests, muscular strain associated with working was measured continuously via electromyographic activities (EA) of 5 muscle groups. These data – as indicators of “physiological cost” – were essentially lower when using the armrest. On the contrary, by relating EA values without the armrest to those with the working aid, it could be shown that working without the armrest is far more strenuous than working with it. Subjective assessments after the tests, as opposed to prior to the tests, corresponded well with the objectively measured physiological data.

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1. Introduction The ergonomic design of VDU workplaces for the office and administration area may enhance work performance and meet with workers’ subjective expectations. But despite fitting the task to man, longer lasting periods of work with a word processor or typewriter, even when utilizing ergonomically designed flat keyboards, can cause physical complaints. These complaints can be related to the sitting posture, where the human body acts statically – like a tripod for hand and eye – rather than dynamically, which is preferable from a work physiological point of view. In this context, a specially developed armrest (cp. N.N. 1995) is said to enable more comfortable working and to prevent physical complaints. Therefore, the objective of this study was to assess the effects of this armrest via an experimental investigation. Ten male subjects (Ss) between the ages of 26 and 40 years with a mean body height of 182 cm participated in the tests. 2. Methods and materials 2.1. Device being tested and test set-up According to the manufacturer’s instructions and as illustrated in the upper left part of Fig. 1, the armrest was attached to the table top. Its front edge was positioned at a distance of approximately 100 mm to the keypad, and the tabulator key and the surface of the armrest were aligned at equal height. This is to ensure that the support takes place suitably in the region of the second third of the forearm. The armrest is guaranteed to be capable of supporting a maximum load of 30 kg when mounted in conformity with the instructions. As also shown in Fig. 1, the device was integrated into an ergonomically designed VDU workstation (cp. STRASSER 1993), and experiments were carried out with and without the armrest.

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Figure 1:

E. Keller and H. Strasser / Ergonomic evaluation of an armrest for typing

Features of the device being tested and VDU workstation realized for work experiments with and without the armrest

The right part of Fig. 1 shows the dimensions of the work area selected from an ergonomics point of view. A monitor placed on top of the rear section of a table which consisted of two parts was arranged so that – at a viewing distance of 700 mm which was chosen for visual reasons (instead of 500 mm as postulated by N.N. 1981) – an orthogonal view on the monitor screen was possible, thus obtaining a relaxed visual axis of 40°. The height of the desktop for the manual operation of the keyboard and the height of the seat were individually selectable. To obtain a work-physiologically optimal working posture on a chair with synchronous technique (cp. among others STRASSER 1995a), a “working-planeto-seating-plane” distance of 280 ±20 mm had to be kept. As is generally known, this distance is of high ergonomical significance. On the one hand, choosing a distance smaller than this range would cause an arched back resulting in corresponding complaints in the lumbar vertebra region. On the other hand, unnecessary static tension would be imposed on the upper arm/shoulder zone musculature when choosing an excessive distance (cp. STRASSER 1990). To provide for constant climatic and acoustical conditions, the workplace was set up in a corresponding test room. In order to simulate manual activities at a VDU workstation which would be principally suited to create effects in conjunction with an armrest, the Ss had to repeatedly touch-type the character combination shown in the middle part of Fig. 1, using the traditional QWERTZ keyboard. Thereby, they had to complete several uninterrupted working phases of 10 min each. Given a continually equal “manual workload,” it was assumed that there would be noticeable differences between the working phases with and without the armrest in the subjective experience and in the objective measurements.

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2.2. Test variables 2.2.1. Subjective methods of assessment Using specifically developed questionnaires, the Ss had to rate the physical strain on certain body regions (sections of the right hand/shoulder region, neck, and back) on a scale ranging from “0” (no perceptible strain) up to “4” (severe strain). Furthermore, a bipolar questionnaire showing a scale of faces which had been applied successfully in several previous cases (cp. STRASSER et al. 1991; STRASSER et al. 1994) was used to assess the workplace layout, specific features of the armrest, the position and the accessibility of the keyboard, and the comfort or the physical effort made when entering the text with and without the armrest. The checking of boxes in the columns headed by the corresponding face could be transferred for quantifying the assessment in digits ranging from “-4” (extremely unfavorable) to “+4” (extremely favorable). 2.2.2. Electromyographic measurements

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Continuous electromyographic derivations were provided during the work experiments in order to obtain an objective quantification of the different muscle strains to be expected. A special EA measuring system, as shown in Fig. 2, was used to record myoelectric signals from the Ss via bipolar electrodes. Upon completion of a test, data was transferred via an A/D-converter integrated into the portable recorder to a personal computer for further processing and for final graphical representation of the results.

Figure 2:

Diagram showing the flow of recording and processing of electromyographic data by means of a portable data-recorder and a personal computer

Since muscles of the upper arm and shoulder in particular are subjected to load by the working posture at a VDU workstation, the following 5 muscle groups were included in the experiments, as shown in the left upper part of Fig. 2:

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• the biceps as flexor of the lower arm, • the frontal, middle, and spinal parts of the deltoid which are involved in • forward moving of the arm (m. deltoideus pars clavicularis), • spreading, i.e., abducting of the upper arm (m. deltoideus pars acromialis), and • backward moving of the arm (m. deltoideus pars spinalis). • Furthermore, the descending part of the trapezius muscle, which normally has to be regarded as a bottle neck in sedentary work, was monitored. Since electromyographic amplitude values cannot be interpreted as strain data (cp. KLUTH et al. 1994; STRASSER 1995b), maximum voluntary contractions (MVCs) were used to measure the electromyographic activities (EAmax) arising under these conditions. Based upon these data, and after recording the resting activity (EA0) in the working posture (compare lower right part of Fig. 2), it was possible to calculate standardized electromyographic activities sEA for all the phases of the work (10-min blocks).

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2.3. Test procedure Each of the 10 Ss completed a work session which was performed over a period of more than 4 hours. After the Ss had been introduced to the experiments to be performed, they filled out a questionnaire for characterizing the subject concerned. Thereupon they had to fill out that part of the questionnaire which concerned the subjective assessment of the workplace with and without the armrest (without own working experience). After completion of the electrical alignment of the 5 measuring amplifiers, the manual activity was learned, the measuring chain was inspected and checked again for correctness, and, finally, in a 10-min break, the resting activity EA0 was recorded for all muscles in working posture. After that, the work experiments were started with an uninterrupted text input using the armrest. They comprised 4 blocks, each having a duration of 10 min and being separated by a 5-min break. Simultaneously, the continuous recording of the EA of the 5 groups of muscles took place. Before proceeding to a second experiment block and entering the text using the same pattern but without the armrest, the Ss had to fill out that part of the questionnaire which referred to the effects of work with the armrest. A corresponding part of the questionnaire after completion of the second block was provided to give information on the effects of work without the armrest. Finally, the Ss had to rate details of the workplace and of the armrest – this time, however, under the impression of their own working experiences – using a similar questionnaire as that one applied at the beginning. Since MVCs possibly provoke fatigue of a muscle over an uncontrolled prolonged period of time, which could lead to misinterpretations, the maximum activity EAmax of each muscle was measured at the end of the experiment. 3. Results 3.1. Influence of the armrest on muscle strain Figure 3 shows the electromyographic results of the 5 muscle groups being tested (in % of the individual muscles’ EAmax values). The bargraphs in the left-hand section (obtained from 4 working phases, each having a duration of 10 min) are the results of tests with the armrest. The values in the middle part were measured during work without the armrest. Comparison of the corresponding bars points to a distinct systematical effect of the armrest. For all measured values (each representing ten 1-min average values of the standardized electromyographic activities), this device brought about a visible relief of the muscular system, which, indeed, is not always equally high but appears to be rather consistent. The biceps and the front part of the deltoid (pars clavicularis), i.e., those muscles which are involved predominantly in the (static) keeping of the lower part of the arm or in moving the hand-arm system slightly forwards in a writing posture, are considerably relieved. Strain values of almost 5 % or around 5 % (without the armrest) are reduced to marginal values in the vicinity of approximately 1 % of the maximum EA.

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E. Keller and H. Strasser / Ergonomic evaluation of an armrest for typing

Figure 3:

Standardized electromyographic activities (sEA [%]) of the 5 muscles being tested over 4 working phases of 10 min each, with and without the armrest (left and middle part), and additional expenditure in physiological cost when working without the working aid (right part) (means of 10 Ss)

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Again, these results of the measurements confirm that the trapezius muscle (in this case the functional part which is responsible for drawing up or keeping the shoulders in position) is already relatively heavily strained only by the writing posture in the sitting position. Values of sEA between 5 % and approximately 10 % which have an increasing tendency with increasing working duration both within the 10-min blocks and from one block to another – a safe sign of a rapidly beginning fatigue – can also be significantly reduced by the armrest. The positive effect is smaller for the rear and front parts of the deltoid which basically are subjected only to minimum strain and which, in the writing posture (also without the armrest), are functionally involved in the positioning of the hand-arm system only to a small extent. The right part of Fig. 3 illustrates the additional effort straining the muscles being tested when writing activities were performed without the armrest. In this graph, all single 1-min average values of a 10-min block of work from the sections without the armrest were referred to the corresponding l-min values obtained when working with the armrest. This way of forming a relative value inevitably eliminates or at least reduces the work time-dependent effect of fatigue, because the values l through 10 (i.e., the average value of the first up to the tenth min) of each section are referred to each other. As a result, within the 10-min blocks, but also from one block to another, increasing values are rather unlikely to be expected unless an over-proportional fatigue over the time exists in the activity without the armrest. However, it should be possible for all the ten 1-min values of a block to quantify the multiple of the muscle strain of the work without the armrest. The results – represented as bars of different heights – confirm that the armrest helps to save “a lot of physiological cost” for each of the maximally possible 40 comparisons (four times ten 1-min average values, each obtained from 10 Ss). For the biceps, the work without the armrest is about 4 to 5 times more exerting. The same applies to the front (clavicular) part of the deltoid. Also, the trapezius and the medium (acromial) part of the deltoid are evidently subjected to quite a strain, in fact twice or even about three times as much. It is only for the rear (spinal) part of the deltoid that the strain without the armrest is not greatly increased. Yet, even there, the influence of the armrest can definitely be objectified in figures. Especially in the case of extended periods of activities at a terminal or a typewriter, this fact must be stressed as highly significant in preventing or at least limiting the formation of myogeloses and physical complaints.

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3.2. Subjective assessment of the working conditions with and without the armrest The upper part of Fig. 4 illustrates the rating of various criteria of the armrest (Items I5 – I10, and I11 – I14) given by the Ss in the two parts of the questionnaire. It is clear that in most cases the working experiences entailed a reinforcement of the basically positively assessed technical details of the working means. In particular, the position (I8) and the depth of the armrest (I10), i.e., circumstances which the Ss did regard positively but a bit reservedly prior to the experiments, were rated rather positively – with values between “+2” and “+2.5” on a scale whose upper limit was “+4” – after the subjects’ own experiences in experiments that lasted several hours. The right upper part of Fig. 4 shows the assessment of the fabric cover added to the already padded armrest with respect to certain quality features (I11 – I14), as an armrest with a smooth plastic surface may have the drawback that – when working with bare lower arms – it is less comfortable than an armrest with a leather cover. In this respect, an additional antistatic fabric cover which prevents sticking of the skin and, in addition, provides a soft and easy feel would be generally desirable. The white bars for the items I11 – I14, representing the anticipated attitude towards this criterion, are not completely congruent with the ratings after the subsequent working experiences which are symbolized by the gray bars, whereby one may proceed on the assumption that the questioning that took place after the experiments (i.e., with working experience) is more likely to give true results. Hence, the fabric cover evidently offers a certain increase in desired adhesiveness, and it even offers comfort and contributes to the prevention of an undesired hardness. Grave differences between the assessment of the already padded armrest with and without the additional fabric cover were indeed not to be expected.

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Figure 4:

Subjective assessment of various criteria of the armrest and qualities of the VDU workstation prior to and after the subjects’ own working experience

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The middle and lower parts of Fig. 4 depict the assessment of the workplace layout with and without the armrest plotted in the form of black and gray bars, whereby workstation details had to be rated with regard to the items I15 to I18. Before the subjects’ own work experience, the position of the keyboard was not considered to be negatively affected by the armrest. With the armrest in use, the Ss found the position of the keyboard to be quite satisfactory. Keyboard accessibility with the armrest, i.e., the lower positive value of item I16, was also given a higher rating after work experience. Furthermore, the Ss found the text input with the device to be more comfortable and less stressing than without it. The subjects’ experience confirmed their subjective assessments. The positive ratings in favor of the armrest are substantially clearer after the completion of the experiments; work without the armrest was more negatively assessed after the working experience. Thus, the differences in the subjective assessment of the working conditions with and without the armrest become apparent. Figure 5 gives an overview of physical regions in which problems from typing activities typically arise. The armrest, whose effects are represented by the height of the black columns, is obviously greatly helpful in attenuating the negative physical sensations in the front and rear shoulder region, in the neck, and in the entire hand-arm system. In the region of the lumbar vertebra and of the back, where – supposedly because of the already favorable ergonomical layout of the table-chair system – the degree of complaints was only slightly distinct, hardly any further improvements are to be observed.

Figure 5:

Subjects’ assessment of complaints associated with work using the armrest and without the armrest. Average values for certain body regions from 10 subjects on a scale ranging from “0” (none) to “4” (severe strain)

4. Conclusions Based upon comprehensive experiments and tests, it can be certified that the tested armrest is indeed a working aid which results in an objectively detectable significant reduction of the physiological cost in manual text input. From the subjective data which are less “solid” in comparison with the reductions in muscular strain that are quantifiable in numbers and figures, it becomes evident that the objectively verifiable advantages are also obvious in the subjective sphere.

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5. References KLUTH, K.; BÖHLEMANN, J. and STRASSER, H. (1994) A system for a strain-oriented analysis of the layout of assembly workplaces. Ergonomics 37 (9) 1441-1448 N.N. (1981) Sicherheitsregeln für Bildschirmarbeitsplätze im Bürobereich. Hauptverband der gewerblichen Berufsgenossenschaften – Zentralstelle für Unfallverhütung and Arbeitsmedizin. Zbl. 1/618, Bonn N.N. (1995) TOPTEC Armauflage. Produktbeschreibung and Montageanleitung der Fa. Schmidt GmbH & Co KG, Nisterau STRASSER. H. (1990) Ergonomie im Bürobereich. Zentralblatt für Arbeitsmedizin, Arbeitsschutz, Prophylaxe und Ergonomie 40 (5) 130-147 STRASSER, H.; GROSS, E. and KELLER, E. (1991) Electromyographic evaluation of the physical load of the left handarm-shoulder system during simulated work at eight different cash register arrangements. In: KARWOWSKI, W. and YATES, J.W. (Eds.) Advances in Industrial Ergonomics and Safety III. Taylor and Francis, London / New York / Philadelphia, pp. 457-463 STRASSER, H. (1993) Ergonomie – Arbeitsplatz. Kap. 2.4.4. Bildschirmarbeitsplätze. In: HETTINGER, Th. and WOBBE, G. (Eds.) Kompendium der Arbeitswissenschaft. Kiehl, Ludwigshafen/Rhein, pp. 217-227 STRASSER, H.; WANG, B. and HOFFMANN, A. (1994) Electromyographic and subjective assessment of mason’s trowels equipped with different handles. In: AGHAZADEH, F. (Ed.) Advances in Industrial Ergonomics and Safety VI. Taylor and Francis, London / New York / Philadelphia, pp. 553-560 STRASSER, H. (1995a) Ergonomic efforts aiming at compatibility in work design for realizing preventive occupational health and safety. Int. J. Industrial Ergonomics 16, 211-235

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STRASSER, H. (1995b) Electromyography of upper extremity muscles and ergonomic applications. In: KUMAR. S. and MITAL, A. (Eds.) Electromyography in Ergonomics: Fundamentals, Applications and Case Studies. Taylor and Francis, London, pp. 183-226

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Chapter 7

Electromyographic and Subjective Evaluation of a Wrist Rest E. Keller and H. Strasser 0. Summary This study assessed the effects of a wrist rest for VDT operators via an experimental investigation. Muscular strain associated with the working posture was measured continuously via electromyographic activities (EA) of 8 muscle groups which were involved in the working tasks. The electromyographic measurements yielded a clear systematical effect of the wrist rest while entering text using the 10-finger touch system. Values of the EA – as an indicator of “physiological cost” – were essentially lower with the wrist rest. Working without the wrist rest is at least two times more strenuous than working with it. The differences are statistically highly significant. Less positive and less consistent results were found while working on the prefixed mousepad of the wrist rest. Considering the influence of the working aid on entering text, the results of the subjective assessment after the tests corresponded well with the objectively measured physiological data. Whereas the working aid for entering text was assessed quite positively overall, the integrated mousepad earned a clearly negative rating.

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1. Introduction In the beginning of the nineties of the last century visual aspects like character sizes, viewing distances, gaze inclination and screen technology of visual display terminals (VDTs) were questioned in ergonomics journals and conference proceedings (cp. LUCZAK et al. 1993), whereas thereafter the effects of the design and arrangement of manual input devices, like keyboards and mice, and arm supports were analyzed (see REMPEL 1997; SALVENDY et al. 1997). Concerns have arisen that the keyboard or the mouse are causal factors in the development of work-related musculoskeletal disorders among VDT operators (cp. HAGBERG 1995). Yet, self-report measures of discomfort and fatigue as well as performance measures (outcome) of a study carried out by SWANSON et al. (1997) suggested only a minimal impact of the keyboard design features (5 alternative keyboards versus standard keyboard). A study of BARR et al. (1997) indicated that risk factors for cumulative trauma disorders (CTDs) of the forearm and wrist can be attributed to mouse operation per se. Therefore, improved mouse design can contribute to the reduction of such risk. Also mouse use position seems to be decisive for muscle strain and comfort (cp. HARVEY and PEPER 1997). Even when flat keyboards or ergonomic mice (cp. SMITH et al. 1997) that comply with national or international ergonomics standards are used and office furniture that satisfies ergonomic and safety requirements is provided, extended periods of data input and text processing during VDT work can cause fatigue and tensed muscles. Office furniture even may include monitors with positive polarity that are adjusted in height and angle to enable an orthogonal top view, tables designed according to national and international standards for office equipment (cp. DIN EN 527-1), and optimized office chairs which allow dynamic sitting. Fatigue and tensed muscles can occur solely from continued holding of the hand-arm-shoulder system in writing position. This may be problematic for certain muscles from a work-physiological point of view due to the articulated kinematic chain’s own weight. Consequently, a workplace which has been optimized with respect to static criteria is not necessarily optimal or completely acceptable from an ergonomics viewpoint. Insofar it seems that all

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devices which promise relief for the worker should be desirable. However, a reliable evaluation of the ergonomic quality of such a device should not be based on single experts` opinions and prejudices. A new gel-filled wrist rest with a soft, comfortable surface that is mounted on a plate which also holds the keyboard promises soothing support and greater comfort when working with a keyboard and a mouse. According to the manufacturer, this wrist rest fits most standard keyboards, and its height can be adjusted by roughly 1 cm simply by turning the top part. The objective of this study was to assess the quality of this new device which is from the line of ergonomic products created by an international company (cp. N.N. 1995). Standardized work experiments carried out according to well-tried electromyographic methods (cp. Chapter 3) were utilized in measuring hypothetically expected muscle relief in the hand-arm-shoulder system. The objective evaluation of the ergonomic quality of the new product was supplemented by a subjective assessment based on the work experiences of a group of workers. 2. Method 2.1. Test object

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The upper right part of Fig. 1 shows the available models of the gel-filled wrist rest. The integrated keyboard and mouse version was used in the experiments. The wrist rest is also available for keyboard only or for a mouse with an integrated mousepad. The keyboard can be placed in such a way that a mousepad, which is connected to the device, can be used on either the left side or the right side. Lifting and rotating of the gelpad allows height adjustment of the wrist rest from 2.54 to 3.49 cm. In all the tests, the wrist rest was in the normal position (2.54 cm).

Figure 1:

Test set-up of a VDT workplace with and without wrist rest, and dimensions and adjustment possibilities of the ergonomically designed desk-chair system

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2.2. Test set-up

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The test object was integrated into an ergonomically designed VDT workplace (cp. STRASSER 1993a; STRASSER 1993b), as can be seen in the lower part of Fig. 1. The tests were carried out with and without the wrist rest. The position of the keyboard and the mousepad was not varied between the different test conditions. A divided table for the optimal adjustment of the monitor with respect to the relaxed visual axis was used. The keyboard workplace was adjustable to an ergonomically acceptable height in order to prevent effects of varying anthropometric dimensions. It was coordinated with the chair which had compatible allocation of functional and anatomical joints according to the synchronous technique (cp. STRASSER 1995). This workplace was located in an isolated laboratory in order to ensure constant optic, acoustic, and climatic conditions. To simulate manual tasks at a VDT workstation which would lead to at least hypothetical effects of the test object, the test subjects (Ss) repetitively had to “blindly” enter a character combination on a traditional QWERTZ-keyboard using the 10-finger (touch) system. This is shown in the upper part of Fig. 2. Several continuous phases of 10 min, each, had to be completed. Thereafter, the Ss had to utilize the mouse to select text according to a given program, as can be seen in the lower part of Fig. 2. In a given text portion, e.g., the Ss had to select text passages in “Word for Windows” and set them in bold, cut and paste, or delete text. The lower part of Fig. 2 also shows the necessary movements and the corresponding clicking of the mouse buttons.

Figure 2:

Chosen character combination for text input that ensured equal stress on the left and the right hand (left part) and example of a task with mouse (right part)

The manual work with keyboard and mouse was carried out without the wrist rest. The muscle strain was compared to the values from the same work with the wrist rest. 2.3. Test subjects Since single tests cannot lead to representative results, 10 Ss were chosen. They were all male, their average age was 32 years (±5.9 years), and their average height was 185 cm (±5.3 cm). All of them had experience in computer work (typing and mouse handling). The essential characteristics of the subjects are shown in Table 1.

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Table 1: Characteristics of the Ss 1 through 10 with work-relevant anthropometric measurements

2.4. Test variables

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2.4.1. Subjective evaluation methods Specific questionnaires for the subjective evaluation of the work with and without the wrist rest as well as for the evaluation of the quality of the device were developed. The Ss were asked to categorize the physical strain they felt on certain body regions (sections of the hand-arm-shoulder area, the neck, and the back) on a scale from 0 (no noticeable strain) to 4 (very heavy strain). The layout of the workplace, certain criteria of the wrist rest, the position and accessibility of the keyboard, and the comfort or the physical effort of text input with and without the wrist rest were evaluated with a principally well-tried 4-step bipolar scale (cp. Chapters 5.2 and 6). Analogously, the integrated mousepad of the wrist rest was subjectively evaluated. The Ss’ options ranged from “-4” (extremely unfavorable) to “+4” (extremely favorable). 2.4.2. Electromyographic measurements Continuous electromyographic derivations were provided according to proven methods (cp.Chapters 3 and 5.1) during the work experiments to obtain an objective quantification of the different muscle strains. The measuring system described in details in Chapters 3 and 5.1 was used to record myoelectric signals. Since muscles of the lower and upper arm and the shoulder, in particular, are subjected to load by the working posture at a VDU workstation, the following 8 muscle groups were monitored in the experiments: • • • •

the flexor carpi ulnaris which brings about ulnar deviation of the hand; the pronator teres as inward rotator (pronator) of the forearm; and the extensor digitorum as antagonist of the flexor muscles involved in typing operations; the caput longum, the long head of the biceps as flexor and supinator of the lower arm;

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• the frontal (clavicular), the middle (acromial), and the rear parts (spinal part) of the deltoid which are involved in • forward moving of the arm via the pars clavicularis, • spreading, i.e., abducting of the upper arm (pars acromialis), and • backward moving (retroversion) of the arm (pars spinalis); • furthermore, the upper part of the trapezius (m. trapezius pars descendens), which is responsible for the drawing up or keeping the shoulder in position, and which normally has to be regarded as a bottle-neck in sedentary work was monitored. Electromyographic amplitude values cannot simply be interpreted as strain data, therefore they had to be normalized (standardized). A comprehensive review of normalization of surface EMG amplitude (from the upper trapezius muscle taken as an example) as well as key elements of the normalization procedure are provided by MATHIASSEN et al. (1995). Very often maximum electromyographic activity arising under maximum voluntary contractions (MVCs) is used as reference. Based upon this activity EAmax during Maximum Voluntary Contractions, and regarding the resting activity (EA0) in the working posture, via an often used formula (cp. MARRAS 1990; MIRKA 1991; BÖHLEMANN et al. 1994; KLUTH et al. 1994; STRASSER et al. 1996), it was possible to calculate standardized activities sEA from the actual electromyographic activity EA, which was measured continuously during all the phases of work (i.e., the several 10-min and 3-min blocks). The standardized values sEA indicate muscle strain in percent. Muscle strain with and without the test object together with the subjective assessments should allow the evaluation of the wrist rest’s ergonomic quality in figures and numbers. 2.5. Test procedure

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Each of the 10 Ss completed a work session which was performed over a period of more than 4 hours (Fig. 3). After the Ss had been introduced to the experiments to be performed, a questionnaire for characterizing the S concerned had to be filled out. It took approximately 30 min to carefully and properly attach the electrodes onto the skin covering various muscle groups and to allow the electrode paste to take effect. Next, the Ss had to fill out the part of the questionnaire concerning the subjective assessment of the workplace with and without the wrist rest (without own working experience). After completion of the electrical alignment of the 8 measuring amplifiers, the manual activity was learned, the measuring chain was inspected and checked again for correctness. Finally, in a 10-min break, the resting activity EA0 was recorded for all muscles in working posture. After that, the actual work experiments were started with: • uninterrupted text input using the wrist rest in 3, 10-min blocks separated by 5-min breaks and • utilization of the mouse in 2, 3-min blocks separated by a 4-min break. Simultaneously, the continuous recording of the EA of the 8 muscle groups took place. Prior to proceeding to a second experiment block of entering the text and utilizing the mouse using the same pattern but without the wrist rest, the Ss had to fill out the part of the questionnaire which referred to the subjective rating of the effects of work with the wrist rest. A corresponding part of the questionnaire after completion of the second block was provided to gain information on the rating of the effects of work without the wrist rest. Finally, the Ss had to rate details of the workplace and of the wrist rest. This time, however, under the impression of their own working experiences, using a similar questionnaire as the one applied at the beginning of the experiment. Maximum voluntary contractions possibly provoke fatigue of a muscle over an uncontrolled period of time. This could lead to transfer effects, and so the maximum activity EAmax of each muscle was measured at the end of the experiment.

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Figure 3:

Schedule of the test program with subjective rating and continuous measurement of electromyographic activities taken simultaneously to text input and mouse handling

3. Results 3.1. Influence of the wrist rest on muscle strain Figure 4 shows the time series of the standardized electromyographic activity of 4 muscles acting on the forearm and the hand (lower block) and of 4 muscles acting on the upper arm and the shoulder (upper block) averaged over the 10 Ss while entering and editing text with and without the wrist rest. The bar graphs in the left-hand section of the figure show the muscle strain associated with the 3, 10min blocks of uninterrupted text input and the 2, 3-min blocks during utilization of the mouse with the wrist rest. The results of test phases without the wrist rest – under otherwise identical conditions – can be seen in the same arrangement on the right side of the figure.

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E. Keller and H. Strasser / Electromyographic and subjective evaluation of a wrist rest

Figure 4:

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Time series of standardized electromyographic activity (sEA [%]) of 4 muscles acting on the upper arm and shoulder (upper block) and of 4 muscles acting on the forearm and the hand (lower block)

It must be stressed that muscle strain varied from muscle to muscle. Working without the wrist rest, e.g., was associated with sEA values of about 15 % for the extensor digitorum and relatively low values of only about 5 % for the biceps (see lower block of Fig. 4). Even a quick glance at the bar graphs for the 4 muscles in the left part of the figure reveals that working with the wrist rest brought a relief or had at least no negative effects on the muscles acting on the forearm and the hand. The positive effect seems to be most evident in the biceps, a muscle which is involved predominantly in the static holding of the forearm, with marginal sEA values in the vicinity of approximately 1 % of the maximum EA. The effect seems to be very limited in the other muscles acting on the forearm and the hand.

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Similar is true for the time series of standardized electromyographic activity of the 4 muscles acting on the upper arm and shoulder (upper block of Fig. 4). Yet, the sEA values without the wrist rest range from a level of less than 5 % to about 25 % for the descendent part of the trapezius, i.e., the functional part which is responsible for drawing up or keeping the shoulder in position. These relatively high values have a distinct increasing tendency with increasing working duration both within the 10-min blocks and from one block to another. This is a safe sign of a rapidly beginning fatigue, which can be reduced essentially by the wrist rest. To quantify the muscle relief brought about by the wrist rest, all the measured EA values from working with and without the wrist rest have been related to each other and the results have been plotted in Fig. 5.

Figure 5:

Illustration of the additional “physiological cost” for the 8 muscles being tested via referring the EA [%] values from single 1-min average values from 10 Ss working without the wrist rest to the values obtained with the wrist rest. Significance level for the additional effort when working without the wrist rest (onetailed t-test) indicated by the number of stars “”

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For all single 1-min average values of the 10-min and 3-min blocks of work, the quotient of sEA without and sEA with the wrist rest was calculated (sEA [%] ({) / sEA [%] (z)). Herewith the additional expenditure in physiological cost of work without the wrist rest can be explained. When the numerical value of the quotient is > 1, the effect of the wrist rest is a positive one. Values < 1 would suggest that working conditions without the device would be more favorable. This way of forming a relative value also eliminates or at least reduces the work time-dependent effect of fatigue. This can be clearly seen for the trapezius muscle (upper row of the upper block of Fig. 5), which does not show a tendency of increasing values within the 10-min blocks now. When quantifying the multiple of the muscle strain of the work without the wrist rest it can be seen that all muscles had to invest more muscle power for the positioning of the hand-arm system without the device. During the 3 sections of entering text all the 1-min values for all the three functional parts of the deltoid muscle, which are involved in moving and stabilizing the upper arm, are clearly above 1 and with values between 2 and 3 on average, indicate that work without the wrist rest is about at least twice as strenuous than with the working aid. In these relative figures, the positive effect of the wrist rest on the trapezius seems to be not especially high but it is more ergonomically important, when as proven here, a working aid leads to a work-physiologically more acceptable strain level rather than when the wrist rest results in considerable relief in muscles that are not subject to high levels of strain anyhow. As can be seen also from the upper row of Fig. 5, the responses of the trapezius to working without the wrist rest, though not very high in numbers and figures, can be proven at a high level of significance at least during two 10-min blocks. The ratios from sEA values without and with the working aid for the trapezius during mouse handling are partly smaller than 1 and, therefore, speak against the wrist rest, yet, the negative effects are not very strong. As already mentioned, for the biceps (lower block of Fig. 5) the work without the wrist rest (see the 3, 10-min blocks) was even about 4 times more exerting on the average. But this muscle is not subjected to high strain as is the case for the trapezius. The effect of the wrist rest on the three other muscles acting on the hand, as expected, is very limited but for all the working sections the calculated ratio sEA [%] ({) / sEA [%] (z) was > 1.

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3.2. Subjective assessment of the wrist rest, the mousepad, and the working conditions with and without the device being tested The subjective assessment of the VDT workstation prior to and after the standardized work experiments (not shown graphically here), which was carried out on a bipolar 4-step scale in the Items I1 through I4, was a clearly positive one, whereby there were no great differences between the “before” and “after” ratings. That was to be expected insofar as the Ss had to evaluate some details of an already mainly ergonomically-designed VDT workstation. Neither anticipated nor experienced effects of the wrist rest had an influence on the Ss’ assessment because they had to integrally rate the working conditions with and without the wrist rest. Figure 6 illustrates the Ss’ rating of various criteria of the wrist rest. It is remarkable that the working experiences lead to somewhat of a reduction rather than an increase of the initial basically very positive assessment of some technical details. Rather favorable ratings (with values around or above “+2”), in particular, the position (I8), the (desired) friction (I11), and especially the “softness” of the surface (I12) achieved by the gel filling, were reduced by approximately 1 unit. These ratings, however, were still clearly in the positive range. Working experiences resulted in a more realistic (rather than euphoric) assessment of the wrist rest. The width and depth of the wrist rest (items I9 and I10) were rated as neither markedly positive nor negative. Items such as mounting facilities (I5) and handling of the wrist rest (I7) were included in the questionnaire but should be considered irrelevant since during the working tests no variations were possible with respect to these items. This is confirmed by the “neither/nor” ratings. A priori, the fact that it is not possible to stepwise adjust the wrist rest was rated negatively by the Ss. However, after working with the device, the Ss found that proper height adjustment of the keyboard can compensate for that.

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Figure 6:

E. Keller and H. Strasser / Electromyographic and subjective evaluation of a wrist rest

Subjective assessment of various criteria (items I5 through I12) of the wrist rest prior to and after the subjects’ own working experience

The two diagrams in the upper part of Fig. 7 illustrate the assessment of the VDT workstation with and without the wrist rest (black and white bars, respectively); details such as “position” and “accessibility” of the keyboard and “comfort” and “effort” when entering text were rated. The top and bottom parts of the figure show the Ss’ assessment prior to and after the tests, respectively. Prior to the tests, the position of the keyboard (I15) was evaluated more positively with the wrist rest. The accessibility (I16) was evaluated more positively without wrist rest. However, even with the device, still favorable conditions were expected. These expectations were confirmed for the most part by the Ss’ own working experiences. The Ss expected more comfort and less effort with the wrist rest than without it. The representation of item I18 on a bipolar 4-step scale may be confusing at first glance. A positive value does not indicate effort, but rather stands for relief. The Ss’ work experience confirmed these expectations and showed that work without the wrist rest is not comfortable. Rather, it leads to negative evaluations, as can be seen in items I17 and I18. While the wrist rest was assessed quite positively overall, the mousepad earned a clearly negative rating. All 4 items shown in the two diagrams in the lower part of Fig. 7, i.e., position and size of the mousepad as well as comfort and effort when editing text with the mouse, received a negative rating prior to as well as after the tests. Differences between the ratings for working with and without the mousepad are mostly high significant. Finally, Fig. 8 illustrates the subjects’ physical complaints in various body regions. When working without the wrist rest, the Ss generally experienced greater negative effects. However, on a scale from “0” to “4,” relatively low intensities of strain were reported for the front shoulder region (FS) and the whole hand-arm system, starting with the fingers (F) over the wrist (W), the fore-arm (FA), and the upper arm (UA). The same is true for the neck (N), the rear shoulder region (RS), as well as the back (B) and the lumbar vertebra section (LV). Despite the Ss’ general tendency not to complain, it is remarkable that the wrist rest, which clearly helps to attenuate negative physical sensations, had an overall systematically positive effect. As expected, the rating of the positive effects was not particularly high due to the relatively low level of the profile of complaints of the Ss. Yet, the effects of the wrist rest on the front and rear shoulder region, the neck, and upper arm are statistically significant.

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E. Keller and H. Strasser / Electromyographic and subjective evaluation of a wrist rest

Figure 7:

Subjective assessment of the VDT workstation with and without the wrist rest (black and white bars, respectively) (items I15 - I18), and subjective assessment of the integrated mousepad (items I19 - I22)

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Figure 8:

Subjects’ assessment of complaints associated with work with and without the wrist rest. Average values for certain body regions from 10 Ss on a scale ranging from “0” (none) to “4” (severe strain)

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4. Discussion During recent years several types of arm supports have been the topic of a lot of experimental investigations. The results of the studies by GARCIA et al. (1997), FENG et al. (1997), AARAS et al. (1997) and WELLS et al. (1997) indicated that not all the different types of working aids had significant effects on comfort, effort required or subjectively rated perceived exertions. Arm supports had no effects at all on absolute heart rate (cp. GARCIA et al. 1997), because only small parts of the whole body weight can be supported. Yet, there should be an essential decrease in static muscle strain of distinct muscles which can be assessed via electromyographic methods. But recording time must not last only seconds, as was the case in the studies of FENG et al. (1997). Based on standardized work experiments each lasting several hours with a group of 10 Ss, it was objectively proven in figures and numbers that the test object helps to save a lot of physiological cost associated with manual text input using the 10-finger (touch) system. Via multi-channel electromyographic methods, it was also proven that a certain relief in some muscles was achieved when utilizing the side-integrated mousepad. However, the results which are usually statistically highly significant for text input are not statistically significant for all muscles at all times during mouse handling. The level of muscle strain in all parts of the test series, i.e., with and without the wrist rest, was generally higher than in a comparable study in which the ergonomic quality of an armrest (cp. KELLER and STRASSER 1996a and Chapter 6 of this book) was quantified. This systematic difference is

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especially noticeable for the descending part of the trapezius muscle, which is considered the most important bottleneck muscle in sedentary work (cp. WESTGAARD et al. 1996). The strain on this muscle with values up to 15 % with the wrist rest and even up to 25 % without the device is almost twice that of the previous study. The strain level of practically all muscles already in the first part of the test was higher in comparison with that of another group of Ss participating in the armrest study (cp. KELLER and STRASSER 1996a). This can be attributed to the fact that the wrist rest apparently does not support the considerable weight (several kilograms) of the hand-arm system in the same manner as a device which offers support near the elbow. This ensures lower torque at the elbow and shoulder joints. A wrist rest, however, lends support even ahead of the wrist joint and well ahead of the elbow joint. Thus for the stabilization of the shoulder and upper arm as well as the support of “weighty” other parts of the kinematic chain of the hand-arm system, the respective muscles closest to the joints must be activated more intensely. This is especially true for the descending part of the trapezius muscle which is responsible for holding up the shoulder as well as for the 3 interactive parts of the delta muscle. They are more or less responsible for the fixation of the upper arm and are subjected to even greater strain. Only the biceps, responsible for flexing the lower arm as well as for supination (with no gravitational influence worth mentioning), shows an only slightly higher level of strain. The second part of the test also showed differences in muscle strain between the experiment without the armrest and that without the wrist rest, which objectively were carried out under identical working conditions. This must be resulting from so-called transfer effects from the first part of the test. Once used to the respective work conditions, the organism may have learned a certain economy of muscle cooperation. It can be assumed that such a learning process took place on different levels due to the different devices that were no longer used in the second part of the test. Then the deterioration does not necessarily have to lead to an identical strain level with an accuracy in per cent which had not been experienced up until then. Inter- and intra-individual behavior, also with respect to the “setting in operation” or the posture of the hand-arm system, can never completely be excluded. Despite that, positive effects for the device in this study must be mentioned, even for the statistically seen uncorrelated samples. The strain values with the wrist rest in muscles such as the biceps or the front part of the delta muscle are still distinctly lower than the ones for work without the arm rest in the comparative study. In uncorrelated samples, systematic, uncontrollable influences can never be completely avoided. Other sources of bias are possibly different EAmax values which are not totally free from the respective motivational influences on maximum voluntary contractions of the test subjects. The objectively proven positive effects of the wrist rest are at least for text input principally confirmed subjectively, albeit on a different level. This does not hold true for the work with the mousepad. Although limited positive effects for the mousepad were proven with the help of electromyographic methods, the fact that the mousepad is not freely movable but fixed to the wrist rest is subjectively considered a negative limitation and does not lead to an improvement of the work conditions. Similar to the need to stretch one’s legs from time to time, there is the desire to stretch the upper extremity to varying degrees while working with the mouse. This is not possible with the compact version of the wrist rest. Therefore, separate wrist rests should be used for the work with the keyboard and the mouse. 5. Conclusions The following conclusions can be drawn from this research: 1. 2.

Muscles of the lower and upper arm and the shoulder are subjected to quite different muscle load between ca. 5 % and 25 % by the working posture during uninterrupted text input at a VDT workstation. Muscle strain of the descendent part of the trapezius, i.e., the functional part which is responsible for drawing up and keeping the shoulder in position, is maximum and shows a distinct increasing tendency with working time even

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3.

4. 5.

6.

within 10-min blocks. This is a safe sign of a rapidly beginning fatigue in this “bottleneck” muscle. A wrist rest helps to save a lot of physiological cost which otherwise has to be paid by the muscles when working without it. Referring the sEA [%] values resulting from working without the wrist rest to the sEA [%] values associated with the wrist rest during text input revealed that working without the wrist rest is at least two times more strenuous for the muscles acting on the upper arm and shoulder. The quantified muscle relief brought about by the wrist rest is maximum for the biceps as flexor and supinator of the lower arm. Yet, this result is not important because this muscle already without a wrist rest is subjected to only low strain of 5 %. Utilizing the mouse on the side-integrated mousepad produced inconsistent results in the electromyographic measurements. The subjective assessments after the tests (under the impression of own working experience) corresponded well with the objectively measured physiological data, albeit on a different level. This was not the case for the assessments prior to the tests. Whereas the working aid for entering text was assessed quite positively overall, the integrated mousepad earned a clearly negative rating. Therefore, separate wrist rests should be used for the work with a keyboard and a mouse.

6. References AARAS, A.; FOSTERVOLD, K.I.; RO, O.; THORESEN, M. and LARSEN, S. (1997) Postural load during VDU work: a comparison between various work postures. Ergonomics 40 (11) 1255-1268 BARR, A.E.; ÖZKAYA, N. and NORDIN, M. (1997) Effect of computer mouse design on CTD risk factors of the forearm and wrist among skilled and novice users. In: REMPEL, D. (Ed.) Marconi Research Conference 1997. Conference Proceedings, University of California, Richmond, 12, pp. 1-4 BÖHLEMANN, J.; KLUTH, K.; KOTZBAUER, K. and STRASSER, H. (1994) Ergonomic assessment of handle design by means of electromyography and subjective rating. Applied Ergonomics 25 (6) 346-354

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FENG, Y.; GROOTEN, W.; WRETENBERG, P. and ARBORELIUS, U.P. (1997) Effects of arm support on shoulder and arm muscle activity during sedentary work. Ergonomics 40 (8) 834-848 GARCIA, D.T.; FERNANDEZ, J.E. and AGARWAL. R.K. (1997) Implementation of arm supports as an aid to computer work in the office. In: DAS, B. and KARWOWSKI, W. (Eds.) Advances in Occupational Ergonomics and Safety II. ISO Press Ohmsha, Amsterdam, pp. 429-432 HAGBERG, M. (1995) The “mouse-arm” syndrome – Concurrence of musculoskeletal symptoms and possible pathogenesis among VDU operators. In: GRIECO, A.; MOLTINI, G.; PICCOLI, B. and OCCHIPINTI, E. (Eds) Work with Display Units 1994. Elsevier Science, Amsterdam, pp. 23-24 HARVEY, R. and PEPER, E. (1997) Surface electromyography and mouse use position. Ergonomics 40 (8) 781-789 KELLER, E. and STRASSER, H. (1996a) Ergonomic evaluation of an armrest for typing via electromyographic and subjective assessment. In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M. and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I, IOS Press, Amsterdam, pp. 838-845 KELLER, E. and STRASSER, H. (1996b) Elektromyographische und subjektive Methoden zur ergonomischen Beurteilung einer Armauflage für manuelle Texteingabe. Z.Arb.wiss. 50 (22 NF) 3, 162-169 KLUTH, K.; BÖHLEMANN, J. and STRASSER, H. (1994) A system for a strain-oriented analysis of the layout of assembly workplaces. Ergonomics 37 (9) 1441-1448 LUCZAK, H.; CAKIR, A. and CAKIR, G. (Eds.) (1993) Work with Display Units. Proceedings of the 3rd International Scientific Conference on Work with Display Units. North-Holland, Amsterdam MARRAS, W.S. (1990) Guidelines industrial electromyography (EMG). International Journal of Industrial Ergonomics 6, 89-93

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MATHIASSEN, S.E.; WINKEL, J. and HÄGG, G.M. (1995) Normalization of surface EMG amplitude from the upper trapezius muscle in ergonomic studies – A review. J. Electromyogr. Kinesiol. 5 (4) 197-226 MIRKA, G. (1991) The quantification of EMG normalization error. Ergonomics 34 (3) 343-352 N.N. (1995) Set-up and usage guide for 3M gel-filled adjustable wrist rest – 3M Office Ergonomics. 3M Commercial Office Supply Division, St. Paul, MN, USA REMPEL, D. (Ed.) (1997) Marconi Research Conference 1997. Conference Proceedings, University of California, Richmond SALVENDY, G.; SMITH, M.J. and KOUBEK, R.J. (1997) Design of Computing Systems, Advances in Human Factors/Ergonomics. 21A, Proceedings of the 7th International Conference on Human-Computer Interaction, San Francisco/California, Elsevier, Amsterdam SMITH, W.; EDMISTON, B. and CRONIN, D. (1997) Ergonomic Test of Two Hand-Contoured Mice. Global Ergonomics Technologies Inc., Palo Alto, California STRASSER, H. (1993a) Ergonomie – Arbeitsplatz, Kap. 2.4.3. Arbeitsgestaltung im Büro- und Verwaltungsbereich. In: HETTINGER, Th. und WOBBE, G. (Eds.) Kompendium der Arbeitswissenschaft. Kiehl-Verlag, Ludwigshafen/Rhein, pp. 192-216 STRASSER, H. (1993b) Ergonomie – Arbeitsplatz, Kap. 2.4.4. Bildschirmarbeitsplätze. In: HETTINGER, Th. und WOBBE, G. (Eds.) Kompendium der Arbeitswissenschaft. Kiehl-Verlag, Ludwigshafen/Rhein, pp. 217-227 STRASSER, H. (1995) Ergonomic efforts aiming at compatibility in work design for realizing preventive occupational health and safety. International Journal of Industrial Ergonomics 16 (3) 211-235 SWANSON, N.G.; GALINSKY, T.L.; COLE, L.L.; PAN, C.S. and SAUTER, S. (1997) The impact of keyboard design on comfort and productivity in a text-entry task. Applied Ergonomics 28 (1) 9-16 WELLS, R.; LEE, I.H. and BAO, S. (1997) Investigations of the optimal upper limb support conditions for mouse use. In: REMPEL, D. (Ed.) Marconi Research Conference 1997. University of California, Richmond, 4, pp. 1-3 WESTGAARD, R.W.; JANSEN, T. and JENSEN, C. (1996) EMG of neck and shoulder muscles: The relationship between muscle activity and muscle pain in occupational settings. In: KUMAR, S. and MITAL, A. (Eds.) Electromyography in Ergonomics. Taylor and Francis, London, pp. 227-258 Standards, guidelines, regulations

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DIN EN 527-1 (2000-07) Office furniture - Work tables and desks - Part 1: Dimensions. German Institute for Standardization, Beuth Verlag, Berlin

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Chapter 8

Evaluation of the Ergonomic Quality of Masons’ Trowels H. Strasser, B. Wang and A. Hoffmann

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0. Summary The design and attractive outfit of products and tools very often are cues which encourage somebody to buy them. Yet, the first impression and the succeeding working experience sometimes diverge considerably. Furthermore, stereotypical behavior and traditional habits as well as the lack of numbers and figures indicating the ergonomic quality and the utility values of a product or working tool are obstacles to the introduction of actually innovative items. The same is true for the handle of a mason’s trowel, which has been developed some years ago but has not achieved general acceptance in bricklaying. Therefore, the objective of this study was to evaluate the ergonomically designed handle of a mason’s trowel in comparison with two standard types, both with a round cross-section of the handle and either a straight neck or a swan’s neck. All the tools were equipped with the same blade. Jobspecific dynamic and static working elements were performed by 10 subjects in a laboratory. Under well-controlled conditions, physiological cost associated with mixing and throwing of mortar onto a vertical wall, translatory carrying and depositing of sand on a horizontal wall, rotatory scooping movements (supination and pronation of the forearm) with and without an external load of the trowel, and static holding of the tool in different working postures were measured. Electromyographic activity (EA) of the biceps brachii, pronator teres, flexor digitorum, and extensor carpi ulnaris was registered continuously and summed up during all of the test sessions lasting 30 or 45 s, each. All data were standardized by means of maximum EA resulting from preceding job-specific maximum voluntary contractions. Before and after the working sessions, which lasted about 4 h for each subject, the ergonomic quality of the handles had to be rated by means of a questionnaire with 9 items on a bipolar 4-step scale. In accordance with the hypothesis that the ergonomically designed handle should enable a specific relief of the strain in the grip musculature and the ulnar deviation muscles, significantly lower EA values were measured with this model during most of the test phases. But the effect was much less in scale than was expected from the subjective assessment before the tests. Also, subjective rating data after the working sessions differed clearly between the three handles mostly corresponding with the pretest assessment.

1. Introduction Besides the cost/benefit ratio and the advertised effectiveness, the design and attractive appearance of products and tools very often influence the customer’s decision to buy them. However, there sometimes exists a discrepancy between the first impression of a product and consequent working experience. Furthermore, stereotypical behavior and traditional habits as well as the lack of numbers and figures indicating the ergonomic quality and the utility values of a product or working tool are often obstacles to the introduction of actually innovative items (see e.g., bent-handled tools like pliers or plate-shears, developed in order to reduce wrist deviation and therefore long-term tenosynovitis or cumulative trauma disorders (cp., e.g., LEWIS and NARAYAN 1993; DEMPSEY and LEAMON 1994)). The same is true for the handle of a mason’s trowel, which was developed already some years ago. Despite the fact that thorough and painstaking investigations into the necessary hand-side and efficiency functions were carried out by BULLINGER and SOLF (1979a), and despite its pleasing appearance at first glance in correspondence with an ergonomic design (with respect to shape, dimensions, and material), this handle has not achieved general acceptance in the trade. Therefore, the objective of this study was to evaluate this mason's trowel in comparison with two standard types.

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2. Methods and Materials 2.1. Test objects

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As illustrated in Fig. 1 in original view, side elevation, and top view, 3 mason’s trowels (H1, H2, H3) each having a similar trapeziform blade, but a different design of the hand side were used. Both the handle shown in the middle as well as the model on the left-hand side were designed according to German standard proportions (cp. DIN 6640) and manufactured out of wood with a smooth, varnished surface; the tool in the middle (H2) was additionally outfitted with a swan’s neck-like handle, with the effect that the distance between the gravity center of the blade G and the center of the handle C was shortened by about 10 mm. This reduction of lever arm and the thus expected relief of the wrist was even stronger with the handle on the right-hand side (H3). Furthermore, the hand side of the ergonomically designed tool, which is made out of crack- and acid-resistant synthetic material that grips well, forms a smaller angle of only 19° towards the blade.

Figure 1:

Original view, side elevation, and top view of the mason’s trowels H1, H2, and H3 with dimensions in mm

Besides this, the cross section of the handle is not rotationally symmetrically round as are the standard models 1 and 2, but has a rounded trapeziform figure and a clearly more ballistic form in the longitudinal contour which fits well into the palm of the hand. As an additional feature, the ergonomically designed handle has a concave indentation supporting the thumb as the strongest finger. In this way it is possible to stretch the thumb (without integrating it into the fist formed by the other fingers), providing a better flexibility of the wrist. The tools were of almost equal weight, ranging from 0.320 kg (H2), over 0.325 kg (H1) to 0.360 kg (H3). 2.2. Subjects and procedures 10 male subjects between the ages of 23 and 43 years (29.3 ± 5.1 years) with a hand width of 9.5 ± 0.5 cm and with further characteristics represented in Table 1 participated in the experiments. Their first task consisted of a subjective assessment of the 3 mason’s trowels (without own working

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experience). By means of a 9-item questionnaire, the handles had to be rated on a bipolar 4-step scale. The next step was to electromyographically evaluate whether and how the tools with the different handles affect different muscular strain during typical working situations. Therefore, the trowels were arranged at random for all participants during the series of experiments. Table 1: Characteristics of the subjects

2.3. Experimental set-up for electromyographic recordings

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As shown in Fig. 2, four muscle groups were monitored. The electromyographic activity (EA) was integrated continuously during registrations over a fixed time span or over a number of working cycles, respectively, for each subject and each handle in the different phases of the test. By means of the resting activity in working position EA0 as well as the maxima of electromyographic activity EAmax during typical measurements of maximum voluntary contractions and the actual electromyographic activity EA, standardized values sEA were calculated indicating muscle strain in percent, dependent on the tasks and handles.

Figure 2:

Experimental set-up and evaluation procedure for the electromyographic data

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Since translatory as well as rotatory movements with bendings in the elbow-joint as well as supinations and pronations of the forearm, i.e., outward and inward rotations, occur in principle while working with a trowel, the biceps as flexor and outward rotating muscle and the pronator teres as inward rotating muscle of the arm were expected to show work-specific reactions. The varying degrees of prehensile strength of the muscles involved in flexing the fingers during a power grip on the 3 handle types should be reflected in the varying myoelectric activity of the flexor digitorum. The different ulnar deviations of the hand should be expressed in different activities of the extensor carpi ulnaris. 2.4. Simulation of working elements and test procedure As can be seen from the schedule of the test program (Table 2) and also on the left-hand side of Fig. 3, the mixing of mortar in a hod, which was adjusted at the same reference level according to the elbow height of each subject (E - 200 mm), was simulated in session 1. With the help of a guideway rail which was bent into a figure of eight, a reproducible “parcour” of movements was created. This rail had to be followed several times with each tool, whereby the blade had to be dipped 80 mm into the sand (i.e., about half of the maximum). The left loop had to be followed clockwise, the right one counter-clockwise. Alternatively, supinations and pronations, superimposed by abductions and adductions of the arm, respectively, had to be produced. Electromyographic activity was integrated over 30 s so that 4 complete cycles with a duration of 6.25 s each, triggered and synchronized by a metronome, could be covered in any case.

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Table 2: Schedule of test program

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Figure 3:

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“Mixing of mortar” (left-hand side) and “translatory carrying and depositing of sand onto a horizontal surface” (right-hand side)

The task in session 2 was – as illustrated on the right-hand side of Fig. 3 – to take the working material out of hod I, whereby the blade had to be inserted at an angle into the sand in 30 specific areas, and then to transport the material to point H of hod II, where the sand after pronation of the right arm and the blade fell through a grid into hod II. Integration of electromyographic activity started after work in the third field was completed, and it comprised a registration time of 45 s, i.e., 20 of altogether 30 cycles in the middle of the test session. One single cycle took 2.22 s. In session 3 according to Fig. 4, the subjects had to hold the trowel with a horizontally aligned blade both with an external load of 0.9 kg and without any load for 30 s each. While doing this, they had to stand erect and hold their right upper arm in an abduction of 30° with horizontally aligned forearm. After a break of 120 s, several trials followed during which again with and without load a static position of 90° pronation and supination, respectively, was demanded. An external load of 0.9 kg (in the form of a magnetic metal plate) in addition to the weight of the tools (ranging from 0.32 kg to 0.36 kg) was chosen in order to simulate true working conditions with a maximum amount of mortar on the blade. According to BULLINGER and SOLF (1979a), the given total load on the wrist is about 13 Newton. As the strain induced by the working elements with a duration of 30 s in all parts of the session was expected to be less than 50 %, resulting from maximum voluntary contractions, a break of 120 s between the test trials was considered to be appropriate to avoid transfer effects from fatigue. If, indeed, 50 % of the maximum strength was demanded for 30 s during static workload, that stress according to ROHMERT and LAURIG (1993) would require a break of 400 %, i.e., a recovery time of 120 s. In session 4 (see also Fig. 4) rotational motions of up to 90° each via pronation and supination were demanded. After the subjects were accustomed to the motion rhythm with a cycle time of 1.2 s given by a metronome, the electromyographic registration period of 30 s started.

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Figure 4:

H. Strasser et al. / Evaluation of the ergonomic quality of masons’ trowels

Static holding and rotating movements of the mason’s trowel with horizontal alignment of the blade, with pronation and supination of the forearm up to 90° (all trials with and without an external load)

Session 5 required scooping movements on a trajectory according to the left-hand side of Fig. 5 for an exact simulation of throwing mortar onto a vertical wall. As in the other test sessions, position traces or points on a marking board parallel to the frontal plane of the subjects were utilized as guide marks for the starting and the finishing point as well as for the trajectory of the movements. These movements were again carried out both with and without an external load of 0.9 kg on the blade, whereby they started with a supination in point A, continued by an accelerating pronation on the trajectory from point B to point C, and were always accompanied by an abduction of the arm. With a cycle duration of 2.22 s and a registration period of 30 s, the EA data again were averaged over a relatively high number of working cycles. In the final session (session 6) – as illustrated on the right-hand side of Fig. 5 – the subjects had to stand laterally and parallel to the mortar container and had to take the sand out of 30 indicated areas and throw it, with a rotatory scooping movement, onto a vertical wall within a specified range of height. This motion had to be repeated 30 times, again starting with a supination and then continuing with an accelerating pronation of the forearm. The motion was simultaneously accompanied by an adduction and abduction of the upper arm. The duration of a cycle was 2.22 s. The EA data were summed up over a 45-s period, i.e., the results were associated with 20 cycles in the middle of the test course, starting with sand field number 4. Finally, after each of the 4-h test series, the mason’s trowels again had to be subjectively assessed via the same questionnaire already used at the beginning of the test series.

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Figure 5:

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“Simulation of rotatory scooping movements” (left) and “throwing of mortar at a vertical wall” (right)

3. Results 3.1. Subjective evaluation of the tools prior to the tests

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Figure 6 shows the subjective assessment of the mason’s trowels before the tests. The columns represent mean values from the 10 subjects regarding the classification criteria (items I): • • • •

combination of material and surface (I1) avoidance of pressure marks (I2) general handiness (I3) through the assessment of the angle between the skeleton-line of the handle and the blade (I9).

It is remarkable that the ergonomic handle – represented in the black columns – was constantly rated best by far in all 9 criteria on a bipolar 4-step scale, namely with results of about +2 or “rather good.” The differences between handle 3 and the two standard handles 1 and 2 are so clear and uniform, that a numerical calculation of statistical significance was not necessary. In the subjective evaluation, handle 1, the handle without a swan’s neck, was generally assessed as the poorest, sometimes even as negative, but it is not significantly poorer than handle 2.

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Figure 6:

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Subjects’ assessment of the mason’s trowels prior to the test on a bipolar 4-step scale with 9 items (means of 10 subjects rating 3 handles each)

3.2. Muscle strain in percent dependent on tasks and tools

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With regard to these clear subjective assessments and expectations, the objective workphysiological results are of particular importance. The columns in Fig. 7 represent the strain of the 4 monitored muscles, expressed in percent of the maximum activity EAmax for the task “mixing of mortar.” At first glance, almost no noticeable results associated with the three handles can be recognized in a general level of muscle strain ranging from 20 to more than 30 %.

Figure 7:

Relative height of the strain of 4 muscle groups (standardized electromyographic activity sEA in % of EAmax from MVC) involved in mixing mortar (means of 10 subjects working with the conventional handles H1 and H2 and with the ergonomic handle H3)

In the musculature for supination and pronation (i.e., the biceps and pronator teres), differences are not necessarily to be expected, particularly as the task “mixing of mortar” requires less rotation or fewer rotatory movements of the arm with a friction resistance coupling than translatory movements on a trajectory, whereby a close contact form coupling was provided for each handle. The same is true for the flexor musculature of the hand, which would scarcely be stressed differently by the tools. Nevertheless, lower EA values of the biceps, significant at p < 0.05 (one-tailed t-test), were measured when working with the ergonomic trowel 3 than when working with trowel 1.

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During the movements at the guideway rail bent into a figure of eight, ulnar and radial deviations of the hand with varying strength depending on the angle between the handle and blade were inevitably demanded. These deviations are lower for the ergonomic handle H3, and in contrast to handle H1, they are essentially associated with significantly lower EA values of the extensor carpi ulnaris (p < 0.05). Figure 8 illustrates that the 4 muscles are variously involved in executing translatory arm movements, such as lifting up, transporting, and putting down the mortar and sand, respectively, on a horizontal surface. Compared with the relatively strong biceps as flexor of the arm, which is little strained at 15 %, the musculature which is, e.g., responsible for the ulnar deviation of the hand is loaded rather strongly, as indicated by standardized EA values of almost 40 %. Therefore, longer working periods in the same job would be more likely to cause wrist problems than the actually expected fatigue of the upper arm. Expected differences between the 3 handles are only marginal, and though trowel 3, in comparison with trowel 1, causes smaller, even significantly smaller strain of the m. extensor carpi ulnaris as well as the m. flexor digitorum, these effects, which are only minor in extent, are of no particular importance.

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Figure 8:

Relative height of the strain of 4 muscle groups (standardized electromyographic activity sEA in % of EAmax from MVC) involved in depositing mortar onto a horizontal surface (means of 10 subjects working with the conventional handles H1 and H2 and with the ergonomic handle H3)

Figure 9 shows the results of the comprehensive tests demanding different types of static holding of the mason’s trowels. As shown before, the height of the columns represent the results of actual EA in percent of the maximum EA during maximum voluntary contractions of the muscles, whereby the black and the light columns stand for the results from the conditions with and without external load. An external load of 0.9 kg placed on the trowel blade leads to an expected and obvious increase of strain for all blade alignments and for all 3 handles. The corresponding differences in standardized electromyographic activity are highly significant for all muscles. But for the abduction musculature (extensor carpi ulnaris) in the case of a horizontally aligned blade the results were divergent due to the external load. Supination of 90°, induced by the biceps, causes a relatively high strain of the musculature flexing the fingers, while the abduction muscle of the hand is used at its lowest possible level. Pronation on the other hand, i.e., a 90° inward rotation of the arm, demands not only a high exertion of the pronator teres responsible for this movement, but also a massive strain of the abduction musculature of the hand (i.e., the extensor carpi ulnaris). More than 40 % sEA values were measured during the task. As already shown in other studies (e.g., WANG and STRASSER 1993), the flexor musculature of the hand is positively supported by the pronation posture, since – compared to supination – considerably lower strength has to be exerted. In view of the actual objective of the investigation, namely possible differences in physiological cost associated with the 3 versions of the trowels, it has to be emphasized that favorable results were yielded in most of the cases with trowel 3, the ergonomic trowel. The most uniform responses were found in the muscle involved in ulnar deviations of the hand, which shows significantly low EA values while holding the blade horizontally (p < 0.001) and in the pronation position (p < 0.01). Significant differences supporting the hypothesis of the investigation concerning the musculature closing the hand (i.e., the flexor digitorum) are also proven sometimes, but one must not overinterpret them as the degree of the effect is relatively low.

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Clearly lower EA values for the pronator teres while working with trowel 3 may be mentioned, especially as this muscle is specifically involved in pronation. The differences between sEA values associated with the handles H1 and H2 on the one side and H3 on the other side are significant at p < 0.001 for an unloaded tool. In contrast to that, supination and a horizontally aligned blade are to be seen as non-specific tasks for this muscle, whereby it is only exerted at a very low level, and consequential differences caused by the tools can be neglected. But with the weight of 0.9 kg, differences concerning the handle type during pronation obviously do not come to fruition at a generally very high level of strain.

Figure 9:

Relative height of the strain of 4 muscle groups (standardized electromyographic activity sEA in % of EAmax from MVC) during static holding of the mason’s trowel. 90° supination, normal alignment, and 90° pronation of the right arm (all trials with and without an external load). Means of 10 subjects working with the conventional handles H1 and H2 and with the ergonomic handle H3

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Conversely, no clear differences regarding the type of handle can be proven for the muscular strain of the biceps during pronation and during holding the blade horizontally. On the other hand, the ergonomic handle causes a higher level of strain of the biceps during muscle-specific supination, which is significant at least without external load (p < 0.01). As can be deduced from Fig. 10, these muscle responses are corroborated to a large extent by those responses resulting from dynamic rotation movements around the longitudinal axis of the arm.

Figure 10: Relative height of the strain of 4 muscle groups (standardized electromyographic activity sEA in % of EAmax from MVC) involved in rotary movements of the mason’s trowel with supination up to 90°, and pronation up to 90°, starting from the horizontally aligned blade (each trial with and without an external load). Means of 10 subjects working with the conventional handles H1 and H2 and with the ergonomic handle H3

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For supination, the ergonomic trowel H3 again demands highest EA values of the biceps, which in comparison with the two other handles, do not completely reach the level of significance (p > 0.05) even in a one-sided test. Concerning the remaining 3 muscles, the ergonomic trowel H3 turned out to be more favorable, especially for inward rotations of the blade, i.e., pronations. Highly significant differences (p < 0.001) can be revealed for the muscle involved in ulnar deviations of the hand (i.e., the extensor carpi ulnaris) for all 8 possible comparisons between trowel 3 and the two other tools. Figure 11 illustrates the relative height of strain of the 4 muscle groups involved in the exact simulation of scooping movements and movements involved in throwing mortar and sand, respectively, onto a vertical wall. As the upper part of the figure shows, the simulated movements, carried out with an empty and a loaded blade, caused lower EA values when the ergonomic trowel was used, especially in the musculature flexing the fingers and the musculature involved in ulnar deviations of the hand, but also in the biceps. But the differences are not especially large and not always significant. Surprisingly, no systematic differences can be found for the pronator teres, which is specifically involved in this movement.

Figure 11: Relative height of the strain of 4 muscle groups (standardized electromyographic activity sEA in % of EAmax from MVC) involved in simulation of throwing mortar at a vertical wall by means of rotary scooping movements with and without an external load (upper part) and involved in throwing sand at a vertical wall (lower part). Means of 10 subjects working with the conventional handles H1 and H2 and with the ergonomic handle H3

Finally, the results from the tasks in session 6 (in the lower part of Fig. 11), which were more practical, do not lead to conclusions which can definitely support the experimental hypothesis. Therefore, it can be stated that during tests with objective methods, the ergonomic handle does not always turn out to be favorable. Reduced physiological cost could not be found in every respect and to the extent subjectively expected. Therefore, the impressive differences in the subjective assessment cannot be fully confirmed and have to be seen in relative terms to a high degree. 3.3. Subjective evaluation of the tools after the tests (with own working experience) As the results in Fig. 12 (which contains the assessment of the 3 handles by experienced subjects after the test series) illustrate, differences on the basis of opinions and experiences cannot always be

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taken at face value. Even if the differences between the trowels turn out to be smaller when assessed by the subjects after the tests, and as a consequence correlate better with the work-physiological results, then, at the same time, the still clear values in favor of the ergonomic trowel by any criteria do not finally mean an adequate reduction of physiological cost during work.

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Figure 12: Subjects’ assessment of the mason's trowels after the test on a bipolar 4-step scale with 9 items (means of 10 subjects rating 3 handles each)

4. Discussion Technical standards and rules are an indispensable prerequisite for mutual adaptation and interchangeability of components and system elements of machines and tools. They are also an important means for guaranteeing a distinct quality level of technical products. When a human operator has to make use of his hand in the fullest sense of the word for working with controls and hand tools, then additional to technical requirements also human-related criteria in the design must be obeyed in order to achieve compatibility not only in terms of technology but also between the anatomical and physiological properties of the hand-arm systems and the technical lay-out (see among others N.N. 1988; N.N. n.d.; GRANDJEAN 1988, SCHMIDTKE and RÜHMANN 1993, STRASSER 1995). Mason’s trowels designed only according to national technical standards cannot represent the optimum in the hand-side of tool design, which may be achieveable even without considerable extra cost. BULLINGER and SOLF (1979a) could convincingly demonstrate already many years ago that a mason’s trowel which normally has to be handled both in a form and friction coupling and during translatory as well as rotatory movements of the arm, must not have a simple handle with a circular cross-section and a straight longitudinal contour according to standard requirements. With the help of a specially developed systematical procedure (cp. Chapter 1 of this book and BULLINGER and SOLF 1979b) based on the analysis of what has to be done with the tool under which

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working and environmental conditions, and taking the specific facilities and capacities of the hand-arm system into consideration, very clear design alternatives resulted with respect to shape, dimensions, material, and surface of the hand-side of the tool. But by all means, the advantages of such an ergonomic design should also be countable in figures and numbers from simulations of as realistic working trials as possible. Despite the generally high physical workload in the building trade (cp. ADELMANN et al. 1994) where nowadays as in the past masons have to work with hammers, chisels, and trowels, occupational diseases or work-related injuries related to the trowel can hardly be expected as with unhandy tools in unnatural hand postures during tayloristic work design with repetitive manual movements (see e.g., EKLUND and FREIVALDS 1993; TICHAUER 1978). Anyway, long-term effects such as CTD, RSI, or other musculoskeletal disorders could be tackled more properly by retrospective epidemiological investigations than merely by trends which perhaps can be determined in laboratory tests, which may only last a relatively short time. Nevertheless, a trustworthy assessment of the ergonomic quality of a tool should be possible via working tests which must include both subjective ratings which, no doubt, are of some importance, and objective work-physiological measurements which, in any case, are necessary. When the anatomical and muscle-physiological characteristics of the hand-arm system are taken into consideration, compatible interfaces for manual activities with hand tools must exist; this should lead to reductions in the demands of physical exertions. In this sense, appropriate and well-selected electromyographic registrations during realistic laboratory working tests can objectively indicate physiological cost associated with both movement-based work design as well as with a more or less ergonomic tool design (cp. STRASSER and ERNST 1992, STRASSER 1996). In the future, a reliable and valid inventory of methodical approaches including physical and workphysiological measurements as well as psycho-physiological assessments usually incorporating subjective or expert rating (see e.g., KADEFORS et al. 1993) will be needed even more urgently, as this is explicitly requested by recent trends in occupational health and safety regulations in Europe (cp. BIENECK and RÜCKERT 1994). Nowadays, it is viewed as insufficient to merely protect employees from accidents and occupational diseases. Rather, in an extended definition of work science tasks, manufacturers meanwhile came under the obligation to construct machines, technical devices, and working tools in congruence with ergonomic demands. In the long run, it would be detrimental to the science of ergonomics if the label “ergonomic” would be somewhat carelessly handed out by designers or solely on the basis of the at present very popular checklists (as they produce clear yes/no decisions) or based only on relatively simple subjective ratings. Methods of a Swedish approach for the classification of work with hand tools and the formulation of functional requirements, which can be considered circumspect and yet easy to work with (cp. SPERLING et al. 1993), still cannot substitute fully objective measurements which must already be applied in the “status nascendi” of a hand tool. In spite of the simple and schematic observation of various criteria from checklists and data gained from “paper and pencil tests” which often seem quite unrelated and incompatible, the knotted interrelations between varying design objectives must always be seriously considered in work scientific analyses. As is well-known from different studies (see, e.g., KENDALL et al. 1994) and has already been demonstrated in investigations into other working tools (cp. STRASSER 1991; BÖHLEMANN et al. 1993; BÖHLEMANN et al. 1994), the results from subjective ratings, which are not free of bias and uncontrollable transfer effects, must be corroborated, validated, and possibly related via objective measurements, e.g., electromyographic registrations. Only via a multidimensional approach, can a more consistent result in the evaluation of a hand tool be reached. In summing up the results of this investigation, it must again be mentioned that the differences in ergonomic quality between the three mason’s trowels can be objectively stated via electromyography. Yet, a numerical quantification of that quality based only on the subjective rating data of the subjects would have led to an essential overestimation of the ergonomic handle. This handle no doubt proved to be better than the standard models; however, it was not found to be two or even three times as good as is suggested by the results from several items of the questionnaire.

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5. References ADELMANN, M.; WAKULA, J.; BUNK, W.; SCHILDGE, B.; LINKE-KAISER, G. and ROHMERT, W. (1994) Fliesen-, Platten- und Mosaikleger – Arbeitsmedizinische und arbeitswissenschaftliche Studie der Belastungen und Beanspruchungen. Schriftenreihe des Zentralblattes für Arbeitsmedizin, Band 14, C. Haefner Verlag, Heidelberg BIENECK, H.-J. and RÜCKERT, A. (1994) Neue Herausforderungen für die Arbeitswissenschaft – Konsequenzen aus den EG-Richtlinien. Zeitschrift für Arbeitswissenschaft 48 (20 NF) 1, 1-4 BÖHLEMANN, J.; KLUTH, K.; KOTZBAUER, K. and STRASSER, H. (1993) Electromyographic investigations into different handles of electrical hedge-clippers. In: MARRAS, W.S.; KARWOWSKI, W.; SMITH, J.L. and PACHOLSKI, L. (Eds.) The Ergonomics of Manual Work. Taylor and Francis, London / Washington DC, pp. 167-170 BÖHLEMANN, J.; KLUTH, K.; KOTZBAUER, K. and STRASSER, H. (1994) Ergonomic assessment of handle design by means of electromyography and subjective rating. Applied Ergonomics 25 (6) 346-354 BULLINGER, H.-J. and SOLF, J.J. (1979a). Fallstudie Maurerkelle. Umsetzung arbeitswissenschaftlicher Erkenntnisse in die Handwerkspraxis. Humane Produktion - Humane Arbeitsplätze 5: 16-18 und 6: 16-18 BULLINGER, H.-J. and SOLF, J.J. (1979b) Ergonomische Arbeitsmittelgestaltung I - Systematik. Wirtschaftsverlag NW, Verlag für Neue Wissenschaft GmbH, Bremerhaven DEMPSEY, P.G. and LEAMON, T.B. (1994) Bent-handled pliers and range of motion: Theoretical versus empirical concerns. In: AGHAZADEH, F. (Ed.) Advances in Industrial Ergonomics and Safety VI. Taylor and Francis, London, pp. 533-538 EKLUND, J. and FREIVALDS, A. (1993) Hand tools for the 1990s. Applied Ergonomics 24 (3) 146-147 GRANDJEAN, E. (1988) Fitting the Task to the Man - A Textbook of Occupational Ergonomics. 4th edition, Taylor and Francis, London / New York / Philadelphia KADEFORS, R.; ARESKOUG, A.; DAHLMAN, S.; KILBOHM, A.; SPERLING, L.; WIKSTRÖM, L. and Öster, J. (1993) An approach to ergonomics evaluation of hand tools. Applied Ergonomics 24 (3) 203-211 KENDALL, C.B.; SCHOENMARKLIN, R.W. and HARRIS, G.F. (1994) A method for the evaluation of an ergonomic hand tool. In: AGHAZADEH, F. (Ed.) Advances in Industrial Ergonomics and Safety VI. Taylor and Francis, London, pp. 539-545 LEWIS, W.G. and NARAYAN, C.V. (1993) Design and sizing of ergonomic handles for hand tools. Applied Ergonomics 24 (5) 351-356

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N.N. (1988) Ergonomic Tools in our Time. An Atlas Copco Know-how Publication, Stockholm N.N. (n.d.) Vom Einfluß der Ergonomie auf das Design von Elektrowerkzeugen. Robert Bosch GmbH, ZVI, Stuttgart ROHMERT, W. and LAURIG, W. (1993) Physische Beanspruchung durch muskuläre Belastungen. Kap. 3.3. In: SCHMIDTKE, H. (Ed.) Ergonomie. 3. Auflage, Hanser Verlag, München / Wien, pp. 121-143 SCHMIDTKE, H. and RÜHMANN, H.-P. (1993) Betriebsmittel-Gestaltung, Kap. 6.6. In: SCHMIDTKE, H. (Ed.) Ergonomie. 3. Auflage, Hanser Verlag, München / Wien, pp. 521-554 SPERLING, L.; DAHLMAN, S.; WIKSTRÖM, L.; KILBOHM, A. and KADEFORS, R. (1993) A cube model for the classification of work with hand tools and the formulation of functional requirements. Applied Ergonomics 24 (3) 212-220 STRASSER, H. (1991) Different grips of screwdrivers evaluated by means of measuring maximum torque, subjective rating and by registering electromyographic data during static and dynamic test work. In: KARWOWSKI, W. and YATES, J.W. (Eds.) Advances in Industrial Ergonomics and Safety III. Taylor and Francis, London, pp. 413-420 STRASSER, H. and ERNST, J. (1992) Physiological cost of horizontal materials handling while seated. International Journal of Industrial Ergonomics 9, 303-313 STRASSER, H. (1995) Ergonomic efforts aiming at compatibility in work design for realizing preventive occupational health and safety. International Journal of Industrial Ergonomics 16, 211-235

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STRASSER, H. (1996) Electromyography of upper extremity muscles and ergonomic applications. In: KUMAR, S. and MITAL, A. (Eds.): Electromyography in Ergonomics: Fundamentals, Applications and Case Studies. Taylor and Francis, London, pp. 183-226 TICHAUER, E.R. (1978) The Biomechanical Basis of Ergonomics – Anatomy Applied to the Design of Work Situations. A Wiley-Interscience Publication, John Wiley & Sons, New York / Chichester / Brisbane / Toronto WANG, B. and STRASSER, H. (1993) Left- and right-handed screwdriver torque strength and physiological cost of muscles involved in arm pronation and supination. In: MARRAS, W.S.; KARWOWSKI, W.; SMITH, J.J. and PACHOLSKI, L. (Eds.) The Ergonomics of Manual Work. Taylor and Francis, London / Washington DC, pp. 223-226 Standards, guidelines, regulations

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DIN 6440 (1988-05) Masons trowels. German Institute for Standardization, Beuth Verlag, Berlin

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Chapter 9

Assessment of the Ergonomic Quality of File Handles K. Kluth, H.G. Kellermann and H. Strasser 0. Summary In a comparative investigation 3 file handles were evaluated with regard to their ergonomic quality. By means of a mobile measuring system for the recording of peripheral-physiological data, the muscle strain of 9 muscles of the left and right hand-arm-shoulder system was quantified with surface electromyography. A special laboratory device facilitated the standardized execution of the tests. A specific, bipolar questionnaire had enabled the subjective assessment of the design, surface material, general usability, avoidance of pressure marks and blisters, suitability for the exertion of the necessary pressure and pushing forces, and the suitability during hand perspiration. Also, the assessment of more or less favorable body positions to various filing directions was made. From substantial differences in the objective data and the subjective evaluation, the inference has to be drawn that only the combination of subjective surveys and objective measurements represent the opportunity to assess the ergonomic quality of working tools adequately.

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1. Introduction Anatomical and physiological characteristics of the hand-arm-shoulder system are a guiding principle that must not be neglected in the design of working tools according to the principle “fitting the hand equals fitting the person”. The quality of a tool handle depends on whether ergonomic demands dominate over the ideas of designers or perhaps also over historical forms. Ergonomic variations of the design should be based not only on theoretical considerations, but they must also be quantified through their actual effects at work (cp. STRASSER 1991). Paying attention to the ergonomic quality of products and jobs and their documentation became mandatory through changes in the legal foundation of occupational health and safety. Since 1997, the observance of ergonomic principles has not only turned into an ethical-moral obligation in the sense of preventive health-protection, but rather to a legally “anchored” demand (Occupational Health and Safety Act). This investigation would show that the evidence for an ergonomical design demanded by the legislator may not be produced just through various expert ratings and subjective assessment methods on the utility value of a product. Statements on the physical strain produced by a working tool can also be based on objective measuring methods. Up to now, costly laboratory tests and extensive technical equipment have been necessary to obtain these statements. This research will show, on the basis of a well-known example of design, that laboratory tests and the rarely found current field investigations under real working conditions do not need to be stationary and no longer have a high demand in terms of technical equipment and staff. The ambidextrous processing of a workpiece with a file is only possible through the application of (sometimes) high gripping, pulling, pushing and pressing forces. For such a chain of motions, the file handle is an important interface for these applied forces. Therefore, ergonomic quality can be decisive both for performance and workload. Consequently, an example taken from the metal and wood working sector for such physically strenuous manual work in a closed kinematic chain, and which has so far been of little importance in the scientific literature will clarify product-ergonomical assessment methods through a methodical study.

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2. Methods

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As illustrated in Fig. 1, the test subjects had to work on a metal workpiece in standardized working posture with a double-cut file. The workpiece was attached to a support so that the handling of the file was possible only in one direction, led by guide rails. Five positions for working were prescribed, among them also the ergonomically favorable alignment of the operator to the vise at an angle of 60° between the frontal plane and the filing direction, as recommended in the literature and textbooks (cp. BULLINGER und SOLF 1979). Additionally, the alignment of the person to the vise could be chosen freely in a sixth series of experiments. On the floor, the foot-position for the right foot was marked. Thus the supervisor of the experiment was able to control the observance of the angle-position. Furthermore, the upper edge of the workpiece was set up individually according to the elbow-height of each test subject. Nine male subjects (Ss) with previous experience in filing were recruited to participate in the study. The experimental protocol was explained and demonstrated, and all Ss gave informed consent. Their age ranged from 22 to 42 years.

Figure 1:

Experimental set-up for 5 predetermined and 1 optional working position

In all 6 angle-positions, the double-cut file used for the metal processing was equipped, as shown in Fig. 2, with 3 different handles: a wooden file handle according to a German standard (cp. DIN 395) (c), a plastic handle with a rounded rectangular cross-section (d), or a handle designed from an ergonomics point of view (e) (cp. BULLINGER und SOLF 1979), also made of plastic. For the handle variants (d) and (e), available in three different sizes, a medium size was chosen, which corresponded to the hand-size of all test subjects and to the size of the wooden handle (c). As can be seen from the schedule of the test program (Table 1), each of the Ss completed the whole series of experiments, which lasted a total of nearly 5 h. After the instruction of a test subject about the tests to be completed, a questionnaire was to be filled in to document individual characteristics of the respective subject.

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Figure 2:

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Characteristics of the three analyzed file handles

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Table 1: Schedule of test program

Prior to the first practical test, the Ss assessed the file handles and the different angle-positions to the workpiece by means of a specified 9-item questionnaire with a bipolar 4-step scale. For all test parameters introduced, continuous measurements of muscle strain over 3 minutes in each case were carried out by means of surface electromyography. After these tests, in altogether 6 sessions with a randomized sequence of the three file handles, standardization of the electromyograhic activity of altogether 9 muscle groups of the hand-arm-shoulder system via maximum voluntary contractions took place (cp. MARRAS 1987; MÜLLER et al. 1989; MIRKA 1991). Finally, the Ss filled in questionnaires for the subjective assessment of the file handles and the different angle-positions to the workpiece.

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The central part of the measurement equipment for the assessment of physical strain was the surface electromyography (cp. KUMAR and MITAL 1996). In order to analyze local muscle strain through electromyographic activity, multidimensional polygraphic recording devices were used (cp. STRASSER 1996). The recording and visualization of the electrical potential variations in the electromyogram were performed by picking up the myoelectric signals through miniature surface electrodes. The influences of interference and attenuation were largely eliminated by screened cables, and by the use of advanced EA-preamplifiers using SMD-technology, which preamplified, rectified, smoothed and integrated the bio-signal (cp. KELLER and STRASSER 1996). The resulting electromyographic activities were A/D-converted and stored on a portable data recorder. The mobile data-recording system used is illustrated in Fig. 3. It enabled a systematic and practical measurement of physical strain and its assessment with a high precision of technical evaluation, completely outside the laboratory area. The amplitude of the bio-signal, dependent on the respective working condition, could be optimally adjusted to the area of the input voltage of the data recording unit. Several muscle groups of the hand-arm-shoulder system are involved in working in a closed kinematic chain such as filing. After several pretests, it became obvious that the following 9 muscles had to be chosen. The lower part of Fig. 3 shows also the anatomic position of the muscles monitored:

Figure 3:

Portable data recording and evaluation system for the analysis of peripheral-physiological parameters and selected muscles of the left and right hand-arm-shoulder system

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• the part of the trapezius muscle which is responsible for the retraction of the shoulder, • the three parts of the delta muscle (p. clavicularis, p. acromialis, p. spinalis) which lead the arm, • the muscles of the forearm for the palmar flexion • and for the ulnar deviation of the wrist, • for the inward rotation of the forearm and, finally, • the muscle responsible for the flexion of the fingers. The further processing of the “muscle strain data,” recorded with the mobile equipment, such as a detailed analysis of the time-series and the visualization of the results, could be executed later with existing computer programs on a portable data recording and evaluation system (cp. Chapter 3 of this book). 3. Results 3.1. Muscle strain dependent on the various file handles

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The upper columns in Fig. 4 represent – expressed in percent of the maximum EA-activity which was measured during maximum voluntary contractions – the local muscular strain in the form of the standardized electromyographic activity (sEA) of the 9 monitored muscles associated with filing, utilizing the 3 file handles in the 60° posture. There are clear differences in muscle strain between the left and right hand-arm-shoulder system. The angle-position of 60° of the body to the direction of filing has already been favored in past test series (cp. BULLINGER und SOLF 1979).

Figure 4:

Standardized electromyographic activity (sEA) of 4 muscles of the left (left part) and 5 muscles of the right hand-arm-shoulder system (right part). The upper columns represent measured sEA values associated with utilizing the 3 various handles. The lower columns are abrasion-adjusted means of 9 Ss

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The present test has again yielded the lowest strain values for this position. As the objective of this investigation was primarily to illustrate the ergonomic quality of the handles, not all the results of the whole measurement of this well-known angle problem can be presented. Looking at the upper part of Fig. 4, surprisingly no systematic differences in sEA between the 3 handles can be seen. But through checking the performance of the Ss in the various tests, a significant influencing parameter became obvious. Despite advising the Ss to work with the same intensity during all tests when utilizing the three various handles, a systematically higher abrasion was measured when the Ss used handle (e) in the 60° and 30° angle. Regarding the abrasion during the filing process and building abrasion-adjusted sEA values, yielded the results shown in the lower columns of Fig. 4. These results clearly can be ascribed to the handle types. The ergonomically designed file handle (e) – with a rounded square cross-section and a ballistic longitudinal contour – is now associated with lowest values of all muscles. The handle with the rounded rectangular cross-section (d)caused in all muscles the highest strain values. The differences between file handle (e) and the other two handles were so clear and uniform that a numerical calculation of statistical significance was not necessary. 3.2. Subjective assessment of the file handles prior to, and after the tests

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Figure 5 shows the subjective assessment of the file handles prior to, and after the tests. It was very surprising that the working tool (c), i.e., the standard model, was clearly preferred by the Ss in the subjective assessment of almost all criteria, since it fared poorly in the sEA values. Most surprisingly, the ergonomic handle (e) was rated especially poorly with respect to surface structure and material. In most of the criteria in the subjective rating, it was evaluated even worse than the file handle with the rounded rectangular cross-section (d), which was associated with highest sEA values. These evaluations were not only given prior to the test series, but they were repeated after the tests, sometimes even more strongly (Fig. 5, e.g., suitability during work causing perspiration of the hands). These discrepancies between subjective and objective assessments of the ergonomic quality of a hand tool clearly show that the design of the ergonomic tool still needs to be improved, at least in some details, such as surface and material.

Figure 5:

Subjects´ assessment of the file handles prior to (back row) and after the tests (front row) on a bipolar 4-step scale (means of 9 Ss)

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The subjective assessment of the body position to the filing direction also yielded unexpected results. The test subjects did not prefer the angle-position of 60°, proven to be physiologically most favorable in the objective measurements. The light grey columns in Fig. 6 stand for the assessment of different filing aspects with respect to the body position prior to the tests, the dark grey ones show that these assessments were confirmed after the tests. An angle-position of 30° is given a very clear subjective preference. This is true for the results prior to the tests (black columns) as well as afterwards (white columns).

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Figure 6:

Subjects´ assessment of the working directions 60° (front row) and 30° (back row) prior to (left columns) and after the tests (right columns) on a bipolar 4-step scale (means of 9 Ss)

Figure 7 illustrates EMG data associated with handle (e), whereby the results for the objectively most favorable and the subjectively preferred angle-position were arranged together. All data show a higher physiological strain associated with the subjectively favored body position. In conclusion, it can be seen that an ergonomic assessment of working tools, only related to a subjective assessment, need not always correspond to an advantageous physical strain or vice versa.

Figure 7:

Standardized electromyographic activity (sEA) of 4 muscles of the left (left part), and 5 muscles of the right hand-arm-shoulder system (right part) associated with 2 working directions (abrasion-adjusted means of 9 Ss)

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4. Discussion Technical standards and rules have the aim and objective to guarantee exchangeability of devices, tools and system-elements. This should certify a certain standard in the quality of technical products. As soon as the hand gets involved in the workplace, then anatomical criteria also must be pursued besides technical aspects. Not only compatibility in a technical sense should be reached, but also compatibility between the technical layout and anatomical-physiological laws of the hand-armshoulder system. A higher degree of compatibility will be indicated by lower physiological costs of the muscles. Long-term effects, such as cumulative trauma disorders, repetitive strain injuries or musculoskeletal problems, are certainly only verifiable by retrospective long-term studies and not through short series of laboratory or field tests. Nevertheless, a certain assessment of the ergonomic quality of a working tool should be made possible through standardized working tests. One might wonder about the result shown in the lower part of Fig. 4, i.e., the fact that all 9 muscles of the left and right hand-arm-shoulder system were associated with lowest sEA values when working with the handle (e). Such clear and consistent results after the abrasion-adjustment of the sEA data seem not to be plausible or, at least, not very reasonable. Indeed it is probably the outcome of an overestimation by applying the procedure of abrasion-adjustment to all muscles. Probably, it would have been more correct to limit abrasion-adjustment to just those muscles which are directly involved in producing pressure on the workpiece and pushing the file forward, i.e., to the palmaris longus and the clavicular part of the deltoid muscle of the right hand-arm system. No doubt, at least the musculature closing the hand is also co-contracted when involved in these movements. Undoubtedly, subjective assessments are important in any case. But this investigation has shown that they never should be used alone for an ergonomic assessment. Only in combination can subjective surveys and suitable objective measurements represent an important indication for a more or less acceptable ergonomical design of working tools.

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5. Conclusions From an ergonomics point of view, the use of a flat file or square file equipped with a file handle according to the German standard (cp. DIN 395) does not make very much sense. The main task consists in producing plane surfaces. Therefore, a special design can be helpful in supporting the "feeling" of a parallel position of the file blade to the work piece surface. Creating this feeling is nearly impossible with a circular cross-section of the handle, but much easier with the help of a rounded 4edge. Additionally, the largest diameter of the handle must correspond to the largest hand curvature, as well as the shortest diameter to the smallest hand curvature. Compatibility to the hand cannot be achieved by file handles according to standard solutions, which dictate that the largest diameter of the handle should correspond to the smallest hand curvature. The advantages of an ergonomically designed handle curvature probably cannot always be felt by the workers, but, as shown in this study, other criteria, such as surface and material of the handle, also influence the subjective assessment (with preferences for the wooden handle (c) when working in a high intensity and with sweating hands). No doubt that the handle for which the lowest physiological costs have to be paid will be advantageous in the long run. The same is true for the filing postures which have been analyzed with respect to subjective assessments and the physiological cost of the muscles involved in working. During the filing process, a closed kinematic chain created by both hands and the tool is normally required for efficient working, during which – according to BULLINGER and SOLF (1979) – a naturally preferred working direction exists at about 60° between the frontal plane and the horizontal movement directions. There are arguments in favor of the posture of 60° between the filing direction and the frontal plane and a prepositioning of the vise by 30° so that a frontal orientation of the worker to the work bench is possible. This can also be confirmed by the electromyographic data of this study. But subjective assessments, in contrast to objectively measured muscle strain, were in favor of a 30° angle between the filing direction and the frontal plane. Furthermore, filing in a non-prescribed working direction (in test

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series 6) was associated with the overall lowest muscle strain. Since also with respect to this outcome the discrepancies between subjective and objective results are much larger, recommendations of a working direction should be given with caution. It must again be mentioned that the results from the subjective ratings, which are often not free of bias and uncontrollable transfer effects, must be evaluated in relation to objective measurements, e.g., electromyographic results. This discrepancy, however, is not new, but has already been observed in many studies (cp. KENDALL et al. 1994) and has been demonstrated in investigations into other working tools (cp. BÖHLEMANN et al. 1996; STRASSER 2000). Nevertheless, the objective measurements should also never be considered independently. Only the multidimensional approach (cp. STRASSER 2002/2003) can ensure more consistent results in the evaluation of a working tool, especially of a hand tool. 6. References BULLINGER, H.J. and SOLF, J.J. (1979) Ergonomische Arbeitsmittelgestaltung I, II – Systematik und Beispiele. Wirtschaftsverlag NW, Verlag für Neue Wissenschaft GmbH, Bremerhaven BÖHLEMANN, J.; KLUTH, K.; KOTZBAUER, K. and STRASSER, H. (1996) Ergonomic assessment of handle design by means of electromyography and subjective rating. Applied Ergonomics 25 (6) 346-354 KELLER, E. and STRASSER, H. (1996) Ergonomic evaluation of an armrest for typing via electromyographic and subjective assessment. In: MITAL, A.; KRUEGER, H.; KUMAR, S.; MENOZZI, M. and FERNANDEZ, J.E. (Eds.) Advances in Occupational Ergonomics and Safety I. IOS Press, Amsterdam, pp. 838-845 KENDALL, C.B.; SCHOENMARKLIN, R.W. and HARRIS, G.F. (1994) A method for the evaluation of an ergonomic hand tool. In: AGHAZADEH, F. (Ed.) Advances in Industrial Ergonomics and Safety VI. Taylor and Francis, London, pp. 539-545 KUMAR, S. and MITAL, A. (Eds.) (1996) Electromyography in Ergonomics. Taylor and Francis, London MARRAS, W.S. (1987) The preparation, recording, and analysis of the EMG signal. In: ASFOUR, S. (Ed.) Trends in Ergonomics. Human Factors IV. Elsevier, Amsterdam, pp. 701-707 MIRKA, G.A. (1991) Quantification of EMG normalization error. Ergonomics 34 (3) 343-352

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MÜLLER, K.-W.; ERNST, J. and STRASSER, H. (1989) Ein Normierungsverfahren der Elektromyographischen Aktivität zur Beurteilung dynamischer Muskelbeanspruchung. Z.Arb.wiss. 43 (15NF) 4, 129-135 STRASSER, H. (1991) Different grips of screwdrivers evaluated by means of measuring maximum torque, subjective rating and by registering electromyographic data during static and dynamic test work. In: KARWOWSKI, W. and YATES, J.W. (Eds.) Advances in Industrial Ergonomics and Safety III. Taylor and Francis, London, pp. 413-420 STRASSER, H. (1996) Electromyography of upper extremity muscles and ergonomic applications. In: KUMAR, S. and MITAL, A. (Eds.) Electromyography in Ergonomics. Taylor and Francis, London, pp. 183-226 STRASSER, H. (2000) Ergonomische Qualität handgeführter Arbeitsmittel – Elektromyographische und subjektive Beanspruchungsermittlung. Ergon Verlag GmbH, Stuttgart STRASSER, H. (2002/2003) Work physiology and ergonomics in Germany: From the past to future challenges. Occupational Ergonomics 3 (1) 19-44 Standards, guidelines, regulations DIN 395 (1968-12) File handles. German Institute for Standardization, Beuth Verlag, Berlin Occupational Health and Safety Act (1998) (Arbeitsschutzgesetzt – Gesetz über die Durchführung von Maßnahmen des Arbeitsschutzes zur Verbesserung der Sicherheit und des Gesundheitsschutzes vom 07.08.1996, zuletzt geändert am 19.12.1998). BGBI I, p. 2843

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Chapter 10.1

Ergonomic Quality and Design Criteria of Professional-Grade Screwdrivers K. Kluth, H.-C. Chung and H. Strasser 0. Summary

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In a comparative ergonomic study, which comprised both objective and subjective assessments, 11 types of currently available screwdrivers were tested with respect to their ergonomic quality. In order to objectively record performance (maximum exertable torque) and physical strain during the use of the professional-grade screwdrivers, various test series were carried out (consisting of static torque measurements and dynamic tests of screwdriver use). Twelve male right-handed test subjects (Ss) who were between 22 and 30 years of age participated in the test series. The muscle activity of 4 muscles was recorded via surface electrodes in order to quantify objectively measurable differences in strain during the various screwdriving tests. Each of the Ss had to complete the test series with all screwdrivers under identical, controlled working conditions. The Ss also assessed in detail the design of the handles via a specifically developed questionnaire. The handles exhibited substantial differences with respect to the 4 most important design aspects: “shape,” “dimensions,” “material,” and “surface.” Items such as working efficiency with clean and oilcovered handles as well as the general design, among others, were also part of the questionnaire. In a special block of items of the questionnaire, the Ss were asked to express also potential physical complaints with respect to intensity, duration, and occurrence in the fingers, hand, and forearm. The same was true for various detailed areas of the palm. The detailed subjective evaluation of the handles by means of approximately 30 questions offered a differentiated view of the impacts of the work situation and reflected the advantages and shortcomings of the different models’ specified design characteristics. Additionally, the objective measurements showed that especially an ergonomic shape in conjunction with favorable dimensions and the proper design of the material’s friction coefficient of a pressure-anthropomorphic material can contribute to the success of a product.

1. Introduction Screwdrivers are a type of hand-held tools, which are limited in their range of application only by the user’s imagination. They are part of the group of single-legged tools that are characterized by a finger-dynamic mode of operation during fine-mechanical and electronic tasks as well as by a fingerstatic mode of operation during more physical work. Past scientific studies have repeatedly examined the ergonomic quality of working tools such as screwdrivers in order to allow their systematic evaluation and, frequently, subsequent improvements (cp., e.g., SHIH and WANG 1996; HABES and GRANT 1997; STRASSER and WANG 1998; MCGORRY 2001; KLUTH et al. 2004; KLUTH and STRASSER 2005). Screwdrivers continue to be a standard tool in all metal and wood processing professions and are also widely used for home improvement and even leisure tasks. Some are downright peculiar, and they do not always correspond well with the classical image of a screwdriver. However, such tools are very rarely used by professionals. Additionally, electrical aids are available. But, there will always remain work situations or tasks especially in the production of furniture section and, e.g., in garages that do not allow the use of electrical screwdrivers. Therefore, manual screwdrivers are still widely used nowadays.

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With exclusively manual labor, the physical work required to carry out the task is of primary importance. In this context, it is decisive that the task can be carried out under conditions that are favorable for output deployment. Conversely, a minimization of physiological costs during the handling of the tool is, of course, advantageous (cp. STRASSER et al. 1992). The necessary “energy transmission” from human to work piece in both cases occurs at the following two interfaces: • human (hand) – tool (hand-facing end) and • tool (work-facing end) – work piece.

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To achieve optimal efficiency of human work, both interfaces must be designed such that they allow a low-loss energy transmission at a high degree of efficiency. In the context of necessary product developments, possibilities for the evaluation of screwdrivers are of elementary interest to the manufacturers. In light of the European as well as the international competition, it is imperative for a manufacturer to increase an existing competitive advantage in order to clearly differentiate the own products from those of competitors. In today’s world, the necessary mean to that end is the systematic design of handles according to ergonomic requirements. Oftentimes, however, screwdrivers with design shortcomings lead to substantial problems for the user. For example, quick fatigue of the hand-arm-shoulder system during work and occurrence of pressure spots or blisters on the hand may result. While this may be mainly a nuisance, it can be detrimental to the quality and quantity of work that can be carried out. In other cases, e.g., if the handle is prone to slippage, more serious injuries can result. Standards can help to establish ergonomic quality. But the German standard DIN 5268-2 (cp. Fig. 1) provides only minimal specifications with respect to the length-to-volume ratio. Ergonomics is more advanced in this regard. As described in Chapter 1 of this book, it has been known for some time that an ergonomically designed screwdriver should not have a round grooved shape with a cylindrical longitudinal contour, a smooth surface and should not only be made of cellulose acetate. Instead, its profile should have a six-edged shape, rounded edges with a longitudinal concave double-cone cylindrical surface. A one-component design (e.g., cellulose acetate) can be chosen, but modern handles are made of polyamides and thermoplastic elastomers in a multi-component design. The soft, pressure-anthropomorphic parts of the screwdriver handle provide an increase of the maximum possible torque. The surface should be task-related, primarily micro rough.

Figure 1:

Dimensions of plastic screwdrivers according to the German Standard DIN 5268-2

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In this context, it was tested to what extent the technical design characteristics of currently available screwdrivers are based on the characteristics and needs of the human body (cp. Fig. 2). The objectification of strain during manual work with screwdrivers via ergonomic evaluations, physiological strain measurements, and subjective interviews made it possible to relate specific strain to identifiable design criteria. Additionally, favorable strain situations can also be associated with corresponding design criteria.

Figure 2:

Project planning of the product-ergonomic evaluation of manual screwdrivers

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2. Methods In the evaluation of products and work equipment that needs to be “handled,” the electromyographically determined physiological costs (the muscle strain) during the handling of working tools in standardized work tests are of crucial importance, as shown in relevant studies (cp. STRASSER 2000, among others). Additionally, structured interviewing of the participating test subjects about their subjective assessment based on their work experiences provides valuable clues concerning ergonomic quality. Thus, a test layout was developed for the product-ergonomic assessment of the handles of 11 professional-grade screwdrivers from various manufacturers (cp. Fig. 3). The screwdriver handles exhibited differences – some of them substantial – with respect to the 4 most important design aspects “shape,” “dimensions,” “material,” and “surface,” but the blade was identical for all screwdrivers. In order to ensure comparability, tools that have the same use were selected for the examination of handle design. It was decided to limit the study to the so-called workshop edition of screwdrivers whose handle and blade axis are aligned and which require friction locking. Models with pistol grip or a T-grip with form coupling were not considered for this study. Two screwdrivers were tested whose design resembles the specifications of the German standard. It was the single-colored green one with a hard and spare-edged handle surface (SD 9) and the screwdriver with a coarse-textured handle surface (SD 11). The manufacturer of the screwdriver SD 11 wrote in his advertising, that this one is one of the top screwdrivers with respect to the ergonomic quality. The test was designed to analyze potential

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advantages and shortcomings of each handle type in a setting that simulated real working conditions while taking the anatomical and physiological characteristics of the human hand-arm-shoulder system into consideration.

Figure 3:

Handles of professional-grade screwdrivers (SD 1 through SD 11) from various manufacturers

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Various test series (consisting of static torque measurements and dynamic screw-in and screw-out tests, e.g. using a wooden wall) were carried out with the 11 types of new screwdrivers and a group of 12 male right-handed test subjects of average height and weight (who were between 22 and 30 years of age; cp. Table 1). The mean value of the test subjects’ hands corresponded to the statistically average German hand (cp. Table 2). Hand and finger dimensions were determined with high precision, using a grid and a photocopier. Each of the test subjects had to complete all test series with all screwdrivers under identical, controlled working conditions. For all tests, the coupling conditions with a pinch or power grip in a fixed body posture were specified (cp. Fig. 4). Table 1: Characteristics of the test subjects (N=12)

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Table 2: Specific data of the subjects’ right hand (means and standard deviations of specific dimensions, N = 12)

Figure 4:

Body and foot postures during the recording of the maximum torque when applying a pinch grip (left) and a power grip (right)

In order to detect possible model-specific differences in muscle strain that can be objectified, the electromyographic activities (EA) of the flexor digitorum, biceps, brachioradialis, and the clavicular part of the deltoid of the right hand-arm-shoulder system (cp. right part of Fig. 5) were recorded continuously via stationary measuring devices. The left part of Fig. 5 shows a test subject with the experimental equipment for static torque measurements. Subsequently, all data were standardized by means of maximum EA (cp. Chapter 3 of this book) resulting from preceding work-specific maximum voluntary contractions (MVC) and were evaluated.

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Figure 5:

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Test subject with the experimental equipment for static torque measurements and quantification of the physiological costs of the muscles involved in work (left) and placement of the surface electrodes for electromyographic activity measurements of 4 muscles (right)

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In addition to the objective evaluation via electromyographic measurements (cp. KLUTH et al. 2004; KLUTH and STRASSER 2005), the test objects were subjectively assessed by the test subjects. Prior to the beginning of the hands-on test series, the test subjects were asked about their physical condition. Large parts of the questionnaire – which consisted of 30 items overall and was specifically developed for this application – had to be answered during and after the tests in order to capture the work experiences with the test objects. The results were presented to reflect the advantages and shortcomings of the various models and to possibly provide guidance in future designs. Questions about the handles’ shape, dimensions, material and surface design, color, manufacturing quality, working efficiency with clean and oil-covered handles, as well as the general design, were also part of the questionnaire. After working with each screwdriver, the Ss were asked to express potential physical complaints with respect to intensity and occurrence in the fingers, palm, hand, and forearm. Figure 6 gives an overview.

Figure 6:

Selected topics of the questionnaire for the assessment of design aspects and complaints

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3. Results 3.1. Objective assessment of maximum torque and physiological costs while using various screwdriver handles

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As already described, the study focused on measuring performance of various screwdriver handles under real-life working conditions (with different hand’s rotational directions as well as clean and oilcovered hands) via maximum torque measurements and measurements of the associated muscle strain of muscles involved in screwing tasks. Maximum achievable torque values and sEA values for both supination of the right arm for screwing in a screw and pronation for unscrewing a screw (cp. illustration in the left part of Fig. 7) were determined. The right part of Figure 7 shows the results of the operational output measurements. The pink and the green bars correspond with the maximum possible torque for the 11 screwdrivers while operating the tools in a power grip during supination and pronation. There are some noticeable differences in the results for the various screwdrivers. Screwdriver SD 9, whose design resembles the specifications of the German standard DIN 5268-2, appears especially weak. SD 11, another screwdriver with a very sleek contour is only marginally better. Conversely, some of the test objects performed quite favorably, for example SD 2 and SD 4. By the way, the possible maximum torque that a person can achieve can be measured when the person uses a handle like a T-grip, which, of course, means that a positive fit (a form fit) is used (not shown here), rather than a frictional fit. In addition to the effects of the handle type, Fig. 7 illustrates also results from tests that required rotation in both directions. Even a casual glance reveals that the green bars, i.e., gripping the tools with a power grip during pronation, are somewhat higher. An examination of the difference between supination and pronation confirms the pronation’s well-known advantage (cp. Chapter 4.2 of this book). However, the extent of the advantage varies and depends on the screwdriver that is used.

Figure 7:

Maximum achievable torque while applying a power grip during supination and pronation, using various screwdriver handles (means of 12 test subjects)

Figure 8 shows the maximum torque values achievable with clean hands and while gripping the tool after a predefined amount of oil had been applied to the handle’s surface with a cloth. This was done to simulate an oil-covered tool in a garage. While lower torque values can be observed for all types of handles due to a reduced friction coefficient, the effects are not equally large. The two screwdrivers SD 3 and SD 4 – and with some restrictions SD 8, too – show the best results under the modified conditions.

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Figure 8:

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Maximum achievable torque while gripping the tool with oil-covered and clean hand (in a pinch grip) during supination, using various screwdriver handles (means of 12 test subjects)

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Due to the partially rather large microfiber insets in the area of the palm (cp. left part of Fig. 10), the torque remains highest with limited performance reductions (cp. Fig. 9) of only approximately -15 % (SD 3/SD 4) or -25 % (SD 8), respectively. Interestingly, also a micro-textured one-component handle (shown in the right part of Fig. 10) can yield respectable results, as demonstrated by the handle of SD 2, which exhibited a performance reduction of only -20 % (cp. Fig. 9).

Figure 9:

Losses in maximum achievable torque while gripping the tool with oil-covered hand compared to clean hand (in a pinch grip) during supination, using various screwdriver handles (means of 12 test subjects)

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Figure 10: Screwdriver SD 4 with microfiber insets in the area of the palm (left) and screwdriver SD 2 with a micro-textured one-component handle (right) to limit the performance reductions of an oil-covered tool

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It seems difficult to achieve effective power transmission without the microfiber insets or a microfine surface structure. The handles of SD 5 and SD 9 with a hard and only slightly textured or smooth surface, respectively, perform most poorly as a consequence. Elastomer insets in the handles of SD 5, SD 6, and SD 11 do not offer any support. Similarly, handles with a geometry that is molded to the palm and with soft rubber-like material (SD 1, SD 7, and SD 10) seemed to provide substantial grip before the application of the oil. This subjective assessment was confirmed objectively by the measured values. However, once the oil had been applied, the results for those handles were only marginally superior to those of the previously mentioned handle types of SD 5, SD 6, and SD 9. Somewhat different results resulted from an additional test, which involved sub-maximum torque. The test subjects had to maintain 40 % of the maximum torque which had been achieved with screwdriver SD 11 for 10 s. It can be concluded from the muscular strain shown in Fig. 11 that this task was more easily accomplished with voluminous, skid-proof handles that are molded to the palm (SD 2, SD 3, SD 4, and SD 7).

Figure 11: Standardized electromyographic activity sEA [%] (indicating physiological costs) of 4 muscles during static holding of 40 % of the maximum achievable torque with screwdriver SD 11 for 10 s (in a pinch grip) during supination, using various screwdriver handles (means of 12 test subjects)

Due to the smaller handle size on SD 8 and SD 10, the grip musculature and the m. brachioradialis exhibit a slight increase in strain. A substantially larger increase in the required effort is noticeable for the handle of SD 5 (with its rather smooth, hard, and thus rather slippery surface) and for the handle of

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SD 6 (with its shape that is not molded to the palm). The weak performance of the slim handle of SD 9 and SD 11 is easily identifiable in this graph. Even for this (physical) task, which is easily manageable with the other handle types, the required effort shows a marked increase. In addition to the static torque measurements, dynamic tests of screw driving were carried out, whereby screws had to be driven into a wooden wall. The time course of the electromyographic activities of all 4 muscles – visualized in Fig. 12 – reflects an increase in work intensity the more the screw is driven into the wood.

Figure 12: Exemplary representation of the standardized electromyographic activity curves of 4 muscles while screwing in 4 screws into a wooden wall with clean hand (in a pinch grip)

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Screwdriver SD 9 – as expected and illustrated in Fig. 13 – again exhibits slightly higher sEA values. However, the values for all other screwdrivers show only marginal differences.

Figure 13: Standardized electromyographic activity sEA [%] of 4 muscles while screwing in 5x25 wood screws into a wooden wall with clean hands (in a pinch grip). Means of 12 test subjects

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3.2. Subjective assessment of physical complaints associated with using various screwdrivers

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A first complex of questions was provided to assess subjectively experienced physical effects of the different static and dynamic screwdriving tasks in order to identify possible relationships between handle design and potentially existing physical effects. The test subjects reported their physical complaints due to work with the different handle models with respect to their frequency and the extent of the complaint. It was found out that working with 9 of the 11 screwdrivers caused no or only low strain in the front part of the shoulder, the upper arm and forearm (cp. upper part of Fig. 14 for SD 1). The recorded complaints concerning the wrist are minor and do not play a major role in everyday professional or leisure life or further analyses. It can be assumed that the identified strain is possibly exclusively due to the standardized body posture and the large number of torque measurements in the test situation in the laboratory. For screwdriver SD 9 (cp. lower part of Fig. 14) and also SD 11, differences in strain that are attributable to the specified design characteristics of the screwdriver handles do occur.

Figure 14: Subjective assessment of physical complaints after the tests associated with SD 1 and SD 9 (relative frequency (in %) and intensity of complaints in different body regions, N = 12)

As far as the palm of the hand and the fingers are concerned, differences in strain that are attributable to the specified design characteristics of the screwdriver handles are substantial. An unfavorable design of the handle’s curvature and cross-section, a negatively assessed material hardness and surface design as well as a low suitability for the transmission of body forces result – either as individual parameter or in conjunction with other factors – in a high risk of pressure spots during work with screwdrivers. Figure 15 exemplifies the results with respect to the frequency and intensity of the subjectively experienced physical complaints for 3 screwdriver handles.

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Figure 15: Subjective assessment of complaints in the hand after the tests. Relative frequency (in % of 12 Ss) of subjects who had any complaints in the visualized hand region while using SD 3 (top), SD 7 (middle) and SD 9 (bottom) as well as intensity of complaints (means and standard deviations of 12 Ss)

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For the three models that were chosen as examples in Fig. 15, it can be noted that the 3-component handle SD 3 would be on top of a ranking that assesses the “avoidance of pressure spots.” Shape, material choice, and surface design together with a sufficiently voluminous handle prevent any problem areas for the palm of the hand or the fingers. The strain for the palm and the fingers that is due to the 2-component handle SD 7 can be characterized very succinctly: a very uniform pressure distribution at a low level. Only the sizable hole (that is used to hang the screwdriver or to apply a lever) is noticeable for the distal phalanx of the little finger. The results for the 1-component handle SD 9 that conforms to the Swiss VSM-standard 35610 are quite different. While the documented strain – with the exception of the palm of the hand – is not particularly high and mostly is in the range of the other screwdriver handles, the frequency with which the test subjects negatively indicated this type of handle clearly makes it distinct from the other handles. Unfortunately, the smooth, hard surface with its numerous edges clearly “leaves its mark” on the interior of the hand. Summarizing, it can be stated that most handles – both in terms of technical and design aspects – have reached such a high level of quality that their everyday use should not cause any substantial complaints. Under normal working conditions, all of them are suitable working tools, with the exception of screwdrivers SD 9 and SD 11.

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3.3. Subjective assessment of different design aspects In order to analyze the working conditions when using the 11 examined screwdrivers as comprehensively as possible, the integral work-scientific analysis of the test objects also included each test subject’s personal statement regarding various design criteria of the handles as well as a personal overall judgement. Those took place in addition to the objectification of stress and strain via real-life tests of screwdriving and the subjective assessment of the associated physical complaints. The goal of including such personal statements was to discover additional strengths, but also heretofore unnoticed weaknesses of the screwdriver handles that could not be identified with the sensation of complaints alone. The following figures show the results of the subjective assessment of various design aspects of the screwdrivers during and after the screwing tests in the form of bar charts. Figure 16 shows a profile of good, mediocre, and poor results that was also found for many of the other design details. It also closely resembles the profile for the overall assessment of the handles’ design in Fig. 23. While the reasons that lead to the detailed results for the assessment of each handle vary greatly, a screwdriver that is rated well overall typically also fares well in the individual aspects. Similarly, a poorly designed screwdriver may exhibit tolerable results in some aspects, but that has only a limited impact on the overall negative assessment. Some details according to the longitudinal contour of the handles: A double-cone voluminous handle (especially SD 7, but restrictedly also SD 1 and SD 2) has led to an assessment ranging from “rather favorable” to “favorable”. Yet, cylindrical contours proved to prevent the transmission of high torque. They were rated “somewhat unfavorable” (SD 11) to “very unfavorable” (SD 9). In order to make the best use of the physical potential of a user, the cross-section line should not be continuous as in handles of SD 1 and SD 2 and by no means circular round, as in screwdrivers SD 9 and SD 11 (cp. Fig. 17). The ideal design is that of a rounded cross-section in form of a six-edged shape and visible contours for the fingers in the longitudinal section, as in handles of SD 7 and SD 10. It can be seen from Fig. 18 – which shows the overall assessment of the handles’ shape – that the handles of screwdrivers SD 7 and SD 10 that have the identical shape, albeit of different dimensions, were favored by the test subjects. The least favored screwdriver handles were SD 9 and SD 11. With regard to the screwdriver blades, the handle size only corresponds to the blade size. Ideally, there should be three handle sizes for each blade size. This means that complete satisfaction can never be reached because of the different hand sizes of the individual users. Yet, some producers are evidently able to convince the test subjects of their products more than other producers. But this is only the case for voluminous handles.

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Figure 16: Subjective assessment of the handles’ shape (I) (means and standard deviations of 12 Ss)

Figure 17: Subjective assessment of the handles’ shape (II) (means and standard deviations of 12 Ss)

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Figure 18: Overall subjective assessment of the handles’ shape (means and standard deviations of 12 Ss)

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This means – as can be seen in Fig. 19 – that the biggest diameter for the handles of SD 1 to SD 4 as well as SD 6 and SD 7 can be accepted without any changes. Yet, none of the handles met the ideal concept of all test subjects.

Figure 19: Subjective assessment of the handles’ maximum diameter (means and assessment in % of 12 Ss)

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Nevertheless, the means shows that they are seen as only marginally “too large” or “too small”, which means that a change in design is not justified. Screwdriver SD 5 creates an ambivalent impression. One third of the test subjects assessed the handle diameter as “rather too large”, whereas the rest found it “somewhat too small”. Handles of SD 8 and SD 10 were seen as too small. Handles of SD 9 and SD 11 met little or no acceptance among the users, having the smallest diameter. They simply offer too small a contact area for transmitting torque. As to the assessment of the handles’ length, the screwdrivers are nearly divided into two groups. Figure 20 shows that again handles of SD 1 and SD 2 and for the first time also the handle of SD 11 were favored by the test subjects with regard to the handle length. Most of the users rated SD 3 and SD 4 as “just right” but the handles seemed to be a little bit too long. Handles of SD 5, SD 7 and SD 8, however, were rated a little bit too short, whereas handles of SD 6, SD 9 and SD 10 were rejected due to their present handle length.

Figure 20: Subjective assessment of the handles’ length (means and assessment in % of 12 Ss)

The results of the interviews regarding the respective handle’s material as well as the partitioning of softer and harder surface materials in multi-component handles consequently led to the overall assessment with respect to the choice and distribution of the handle material (shown in Fig. 21). Given the overall state of development, it is not surprising that the old 1-component mass product SD 9 finishes a distant last. The other hard 1-component handle (SD 2) or all multi-component handles with some hard components (2-component handles of SD 1, SD 4, SD 5, SD 6, SD 7, SD 10, and SD 11; 3-component handles of SD 3 and SD 8) are rated from “neither favorable nor unfavorable” to “somewhat favorable” and are thus considered fairly positive, yet are not truly satisfactory to the user. Often, screwdrivers are used in a dirty, wet, or oily environment. Some of the tested screwdrivers are specifically advertised claiming their good performance especially under these conditions. As it turns out, such claims are well justified. The test subjects had much less difficulty to rate the individual handles as “very good” or “very bad” once the friction coefficient of the coupling area has been reduced with oil. The good impression left by SD 7 during the working with clean hands disappears when the hands are oily (cp. Fig. 22).

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Figure 21: Overall subjective assessment of the handles’ material (means and standard deviations of 12 Ss)

Figure 22: Subjective assessment of the handles’ surface: working with an oily hand (means and standard deviations of 12 Ss)

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Under such conditions, the subjective assessments mirror the objective measurements (cp. KLUTH et al. 2004; KLUTH and STRASSER 2005). Only the handle of SD 2 with its micro-fine surface texture and the models that have micro-fiber insets (SD 3, SD 4 and – with some limitations – SD 8) can convince the users. Large parts of these screwdrivers’ surface have microfiber inlays in the area of the palm, which substantially reduce slipping of the oily hand during forceful driving. However, the result of SD 2 shows that such an elaborate design is not even necessary. The micro-textured surface structure that is created during the production of the handle by the molding tool proves to be sufficient. It’s only a 1-component handle. 3.4. Overall assessment of the handles’ design

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The mass-produced SD 9 that conforms to the Swiss VSM-standard 35610 receives a downright scathing overall assessment (cp. Fig. 23). This was to be expected given that the other models in the test are the result of years of additional development. SD 9 may be sufficient for some hobbyists who only occasionally have to manually screw in or unscrew screws. It should not be used in a professional setting, however.

Figure 23: Overall assessment of the general design of 11 screwdriver handles (means and standard deviations of 12 Ss)

Similarly, screwdriver SD 11 does not live up to the claim of “ergonomic” design that is made by its manufacturer. It may be easy to handle it during quick, low-load screwdriving tasks, but based on the provided sizes of blade tips for this type of handle, frequent situations that require more force can be assumed. The handle of SD 11 is not suited for such tasks. Handles of SD 5 and SD 8 are inconspicuous and receive neither positive, nor negative overall assessments. This result is mainly due to the somewhat over-pronounced shape that is uncomfortable for the hand and the distinctive contours that are made from a hard material in the relatively short SD 5. Similarly, it is due to details in the design of model SD 8 that result in a lower assessment. The flange is too square-edged and the hole is somewhat too big. These two details lead to a distinctly noticeable, sometimes even painful impression both at the tip of the thumb and the little finger during forceful work.

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The differences in results between the remaining handles are too small to determine a clear winner of this comparison test. While the handle of SD 7 achieved the best overall assessment, other screwdriver handles delivered convincing results in certain individual criteria. The best recommendation seems to be to select a screwdriver handle from these models that is best suited for a specific purpose. 4. Discussion Regarding the presented results of the subjective assessments as well as the exertable maximum torque and physiological costs of muscles involved in static and dynamic work, a spherical longitudinal contour (cp. left part of Fig. 24) and a rounded hexagonal cross-section (cp. right part of Fig. 24) apparently are the characteristics of a basic ergonomic design. Such a shape ensures that the antrum of the palm (and thus the hand’s coupling area during a power grip as well as a pinch grip) as well as the length of the phalanges are taken into consideration.

Figure 24: Spherical longitudinal contour (left) and rounded hexagonal cross-section (right) of SD 7

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An ergonomically appropriate shape, however, also requires favorable dimensions of a tool. Similar to the available choices in clothing sizes, tools should also be offered with at least three different handle sizes (cp. Fig. 25) – instead of just one standard size – from an ergonomics point of view, in order to offer workers with small, medium-sized, and large hands appropriate coupling conditions. Such customization is desirable with respect to the handle length, the course of the lengthwise contour, the handle cross-section, the thumb rest, and the slip-guard.

Figure 25: Three different handle sizes for small, medium-sized, and large hands

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Another important factor for the appropriate design is the material’s friction coefficient between the skin and the handle, which varies with load conditions and surface factors. A pressureanthropomorphic material with ideal hardness offers a compromise between sufficient friction and adhesion of the hand to the handle. Regardless of the surface (profiled or smooth), the friction coefficient varies under the influence of varying forces. Modern multi-component composite materials facilitate such a compromise of an ideal cooperation of material and surface design. 5. References HABES, D.J. and GRANT, K.A. (1997) An electromyographic study of maximum torques and upper extremity muscle activity in simulated screwdriving tasks. International Journal of Industrial Ergonomics 20, 339-346 MCGORRY, R.W. (2001) A system for the measurement of grip forces and applied moments during hand tool use. Applied Ergonomics 32, 271-279 KLUTH, K.; CHUNG, H.-C. and STRASSER, H. (2004) Verfahren und Methoden zur Prüfung der ergonomischen Qualität von handgeführten Arbeitsmitteln – Professionelle Schraubendreher im Test. Curt Haefner-Verlag, Heidelberg KLUTH, K. and STRASSER, H. (2005) Assessment of the ergonomic quality of professional-grade screwdrivers using objective methods. In: LOCKHART, T. and FERNANDEZ, J.E. (Eds.) Proceedings of the XIX Annual Conference of the International Society for Occupational Ergonomics & Safety (ISOES), Las Vegas/USA, pp. 189-194 SHIH, Y.-C. and WANG, M.-J.J. (1996) Hand/tool interface effects on human torque capacity. International Journal of Industrial Ergonomics 18, 205-213 STRASSER, H.; ERNST, J. and MÜLLER, K.-W. (1992) Günstige Bewegungen für die ergonomische Arbeitsgestaltung – Elektromyographische Untersuchungen des Hand-Arm-Systems. Schriftenreihe “Arbeitsmedizin – Arbeitsschutz – Prophylaxe und Ergonomie”, Band 11, C. Haefner Verlag, Heidelberg STRASSER, H. and WANG, B. (1998) Screwdriver torque strength and physiological cost of muscles dependent on hand preference and direction of rotation. Occupational Ergonomics 1 (1) 13-22 STRASSER, H. (2000) Ergonomische Qualität handgeführter Arbeitsmittel – Elektromyographische und subjektive Beanspruchungsermittlung. Ergon Verlag, Stuttgart Standards

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DIN 5268-2 (1973-08) Plastic handles for screwdrivers. German Institute for Standardization, Beuth Verlag, Berlin VSM-Standard 35610 (1973) Handles for screwdrivers. Swiss Institute for Standardization, Winterthur

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Chapter 10.2

Maximum Torque and Muscle Strain While Using Screwdrivers with Clean and Contaminated Surfaces in Bi-Directional Use E. Keller, T. Özalp and H. Strasser 0. Summary The use of manual screwdrivers is still an important part of the work of, e.g., car mechanics or in the furniture section. Tightening or loosening screws often requires high torque strength. Therefore, the ergonomic design of the screwdriver handle helps to fulfill the working task by reducing physiological costs for the muscles of the upper extremity as well as complaints like blisters and pressure marks at the fingers and in the palm of the hand. In a series of screwdriver tests with 5 various handles (4 ergonomically designed and one “old-fashioned” handle as reference), bi-directional exertions with clean and oil-contaminated hands were demanded to simulate typical work tasks. Twelve male subjects (Ss), all right-handed, between the ages of 15 and 32, participated in standardized working tests during which the maximum achievable torque was determined. Simultaneously, electromyographic activities (EA) of 4 muscles involved in the working tasks were recorded, processed, and standardized. Significant differences between the standardized electromyographic activities of the muscles investigated were obtained depending on the direction of rotation. Also substantial differences between maximum torque strength were determined for pronation and supination, and for clean and contaminated contact surfaces. The results of this study were mostly consistent with those of the study described in Chapter 10.1 and, therefore, enable testing the reliability of methods applied.

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1. Introduction In many work places, hand-held tools are important elements of daily work. Musculoskeletal disorders are common among workers who use repetitively hand-held tools in industrial settings. Among others, AGHAZADEH and MITAL (1987) as well as MITAL (1991) have shown that tool design may play an important role in the development of work-related injuries in the upper extremities. Working with improperly designed hand tools frequently or for extended periods of time can cause unnecessary physiological costs or long-term cumulative trauma disorders (CTD’s) and repetitive strain injuries (RSI), i.e., progressive damage to the tendons, tendon sheaths, and nerves of the hand, wrist, elbow, and arm. These harmful effects can be the reason of reduced performance and increased absenteeism at work that are related to the use of unsuitable tools. Other risk factors which were identified for upper extremity CTD’s are unnatural postures during the variety of working tasks (cp. ARMSTRONG and CHAFFIN 1979) as well as excessive muscular forces and high rates of manual repetition (cp. SILVERSTEIN et al. 1986). With respect to hand and wrist problems, risk factors for carpal tunnel syndrome include combined wrist flexion and high muscular force, which increases the pressure in the median nerve channel (cp. REMPEL 1994). This problem may occur in industrial work while using improperly designed hand-held tools (cp. SILVERSTEIN et al. 1986; STRASSER 1996; HÄGG et al. 1997).

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It is likely that all these injuries could be mitigated if the hand-facing end of these tools were ergonomically well designed with emphasis on user comfort and safety (cp. LEWIS and NARAYAN 1993; STRASSER 1996; KLUTH et al. 2004). Shape, thickness, length, volume, surface quality, and material of the handles influence both operational performance and physiological strain. The handle size is one of the criteria in tool design which is decisive for the transfer of maximum torque or force. The techniques for evaluating the hand-facing end of screwdriver handles from an ergonomics point of view according to Chapter 10.1 of this book consist of • expert evaluations, • maximum achievable torque measurements, • physiological strain measurements of muscles involved in the tasks, and • subjective assessments of design aspects and possible complaints by experienced test persons. The objective of the study carried out as a follow-up investigation of the study of Chapter 10.1 was to measure 1. maximum exertable torque dependent on • handle design, • rotation direction, i.e. outward rotation (supination) and inward rotation (pronation) of the right arm, and • clean or oil-contaminated surface; 2. electromyographically muscular load of 4 hypothetically varying muscles of the handarm-shoulder system dependent on pronation and supination as well as clean or oilcontaminated surface. Muscle load, however, should remain unchanged by the type of the handle, since for Maximum Voluntary Contractions (MVC) the same maximum physiological input has to be invested by the subjects. 2. Methods and materials A group of 12 male right-handed test subjects (Ss) – which, as shown in Table 1, was relatively homogeneous with respect to age (25.3 ±4.3 years), body weight (82.9 ±17.7 kg), height (181 ±5.0 cm), and elbow height (115.3 ±4.3 cm) – participated in a series of screwdriver handle tests.

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Table 1: Specific data of the 12 male test subjects

From the wide choice of commercially available professional-grade screwdrivers, five models with different handles (4 ergonomically designed and one “old-fashioned” handle as reference) were selected for the study (cp. Fig. 1). The chosen Torx® screwdrivers with an identical star-shaped blade tip are used mostly in the automotive, appliance, and electronics sectors. Torx® screwdrivers have the advantage that during a screwdriving task high torques associated with simultaneously less axial pressing forces are possible.

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Figure 1:

175

Longitudinal contour and cross-sectional shape of 5 professional-grade screwdrivers from various manufacturers with identical blade tip but different handles

Four muscles from the right hand-arm-shoulder system which were expected to be strongly involved in different screwdriving tasks were monitored. They are listed in Table 2 together with their musculoskeletal functions.

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Table 2: Functions of the 4 muscles from the upper extremity

According to the work schedule of the test program within a total test duration of one-and-a-half hours, which was interrupted by rest pauses, maximum torque values for pronation and supination of the right arm – due to loosening (unscrewing) or tightening (screwing in) of screws (cp. Fig. 2) – were determined. The Ss were asked to apply a power grip.

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Figure 2:

Rotational direction of pronation and supination. Screwdriver held in a power grip

Each single experiment lasted about 5 s and was repeated twice by the test subjects with clean and with oil-contaminated hands. For maximum torque strength measurements, a torque-meter (cp. KLUTH et al. 2004) was used. The working height of the device had been adjusted to individual elbow height in such a way that the longitudinal axis of the screwdrivers was aligned with the forearm (concerning the test set-up cp. Chapter 10.1 of this book). Since posture changes of the Ss would have had significant effects on the maximum torque that could be exerted, the Ss were asked to maintain the indicated working position. Simultaneously with the maximum torque, the Electromyographic Activities (EA) of the 4 muscles selected from the right hand-arm-shoulder system mentioned above were measured by means of a computer-based multi-channel recording device. Surface electrodes were placed over the bellies of the selected muscles, parallel to the longitudinal axis of these muscle fibers as recommended by ZIPP (1982). The data was A/D converted for further evaluation by a workstation. A software package providing appropriate recording and processing of myoelectric data was fed into the program system in order to evaluate physiological costs in the standardized Electromyographic Activity (sEA) values. For further information according to recording, processing, and analyzing myoelectric data see, among others, STRASSER (1991) and Chapter 3 of this book. To assess the influence of real working conditions, e.g. using an oil-contaminated tool in a garage, the maximum achievable torque measurements during supination and pronation were carried out twice, with clean hands and after a predefined amount of oil had been applied to the handle’s surface with a cloth (cp. Fig. 3).

Figure 3:

Moistening of the handle surface with a definite amount of oil on a cloth

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3. Results 3.1. Results of torque strength measurements

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Figure 4 shows the results with respect to the maximum achievable torque during supination and pronation while gripping the tool both with clean and oily hands. While lower maximum achievable torques were measured due to a reduced friction coefficient with oil-contaminated hands for the handles of SD B to SD E, screwdriver SD A surprisingly shows the best results under the modified conditions. Due to the micro-textured surface of the one-component handle, even a slight increase of performance can be noticed with oil-contaminated hands. A three-component handle with – partially rather large – microfiber insets in the area of the palm can also yield respectable results, as demonstrated by the handle of SD B, which exhibited a performance reduction of only around 10 %. The maximum torque during supination with a clean hand is higher than maximum torque exerted with an oily hand for all screwdrivers used (including the screwdriver SD E with the old-fashioned handle). While the highest torque values for supination were obtained for SD B, the lowest values were obtained for SD E. SD B has the longest handle among the 5 SDs used in this study.

Figure 4:

Maximum achievable torque while gripping the tool with oil-contaminated and clean hands (in a power grip) during both supination and pronation, using 5 various screwdrivers (means of 12 Ss)

Maximum torque values obtained during pronation by all screwdrivers used with a clean hand were greater than the pronation values obtained with an oily hand, except for the values for SD A. From Fig. 4, it can also be noticed that the handle of SD D resembles the one of SD C in terms of its shape. SD C and SD D, with their rounded six-edged cross-section and a longitudinal double-cone cylindrical surface, match well with the curvature of the hand. But SD D, with its non-sliding surface, exhibits higher torque values than SD C. Therefore, it may be concluded that the surface quality of a screwdriver handle is very important with respect to a tight grip. SD E is made of cellulose acetate and its surface is slippery. Thus, it is very difficult to grip the handle tightly even with a clean hand. SD E also has the shortest length and smallest cross-section of all tested handles. Thus, SD A, SD B, SD C, and SD D provide room for finger placement while the old-fashioned handle SD E has no proper finger-place provision. Additionally, the significance levels of the maximum torque values of the two working conditions “clean/oily hand” during “supination/pronation” with the various screwdrivers are given in the Figure.

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3.2. Physiological costs of supination and pronation

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Figures 5 through 8 present the physiological costs determined by standardized Electromyographic Activities sEA [%] of the biceps, the flexor digitorum, the clavicular part of the deltoid, and the brachioradialis as well as the statistical significances for differences in the physiological cost measurements. The mean physiological cost values obtained from the biceps involved in supination processes with clean and oil-contaminated hands are given in Fig. 5. As expected, no essential systematic differences occurred between the sEA values that can be ascribed to the different screwdrivers used during Maximum Voluntary Contractions (MVC), but highly significant differences in the sEA values with respect to supination and pronation were measured. Furthermore, for 4 of the 5 screwdrivers (the exception being SD B), the mean physiological costs with clean hands were higher than the values obtained with oily hands during both supination and pronation. However, for all screwdrivers used, the pronation values of physiological costs of the biceps were substantially less than the corresponding supination values. The biceps, as a flexor of the forearm and as a dominant supinator (outward rotator), naturally has to pay higher physiological costs during the driving in of screws with the right hand.

Figure 5:

Maximum sEA values obtained from the biceps during MVC while gripping the tool with oil-contaminated and clean hands in a power grip during both supination and pronation, using 5 various screwdrivers (means of 12 Ss)

Figure 6 allows the comparison of physiological costs that have to be paid by the flexor digitorum. During supination, the main strength is exerted by the muscle, which flexes the fingers (middle phalanges) and the forearm. As a result of lower activity during pronation, a better performance with regard to a corresponding higher torque could be determined. The sEA values, obtained for all screwdrivers under two different hand conditions in supination, exhibit higher physiological costs (up to 62.5 % of the maximum electromyographic activity) which were close to each other. Contrary to the reactions of the biceps and the flexor digitorum, which showed clearly higher strain during supination compared to pronation, the sEA values of the clavicular part of the deltoid are significantly higher during pronation (approx. 40 %) than during supination (approx. 30 %) (cp. Fig. 7). An exception can be seen by using SD E. The friction ratio of the surface is so unfavorable, that the clavicular part of the deltoid is stressed less than 30 %.

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Furthermore, it can be seen by all screwdrivers, that the variation of the friction due to clean or oilcontaminated surface provokes no significant differences in the sEA values. One reason may be that this muscle is the farthest to the impact point of the 4 measured muscles.

Figure 6:

Maximum sEA values obtained from the flexor digitorum during MVC while gripping the tool with oil-contaminated and clean hands in a power grip during both supination and pronation, using 5 various screwdrivers (means of 12 Ss)

Figure 7:

Maximum sEA values obtained from the clavicular part of the deltoid during MVC while gripping the tool with oil-contaminated and clean hands in a power grip during both supination and pronation, using 5 various screwdrivers (means of 12 Ss)

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As shown in Fig. 8, the results of the sEA values of the brachioradialis show almost no significant differences between direction (supination / pronation) and surface (clean / oil-contaminated), respectively. Only the use of SD B provokes weakly significant higher sEA values during MVC while gripping the handle with a clean surface during pronation. However, almost equal biological energy invested in order to exert a maximum torque led to considerable differences in the operational performance resulting from more or less favorable grasping and coupling conditions.

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Figure 8:

Maximum EA values obtained from the brachioradialis during MVC while gripping the tool with oil-contaminated and clean hands in a power grip during both supination and pronation, using 5 various screwdrivers (means of 12 Ss)

According to the results of this study, substantially different physiological costs for the analyzed muscles and various physiological costs due to the working conditions (supination and pronation) and coupling conditions (working with clean and oily hands), but no substantial differences due to screwdriver handles were obtained. These results are consistent with prior studies (cp. Chapter 10.1 of this book; STRASSER 1991; STRASSER 1996; STRASSER and WANG 1998; KLUTH et al. 2004; STRASSER et al. 2004). 4. Conclusions The effects of the screwdriver handle design on strength and upper extremity muscle activity during simulated manual screwdriving tasks were investigated. Significant and essential differences between maximum torque values produced by screwing in or unscrewing screws with 5 different screwdrivers by 12 subjects were found. Such differences were related to the handle dimensions and surface quality. It should be noted that the measurement of the electromyographic activity of muscles that are mainly involved in screwdriving tasks is a method for evaluating the required muscular strain. Despite of equal physiological costs during operating the various screwdrivers, considerable differences in the operational performance (maximum achievable torques) were found that resulted from “loose or tight” gripping as a result of the slipping of the handle in the palm and due to the characteristics of the handles. The test results show that differences in maximum torque values and physiological cost measurements of the working conditions obtained with “clean/oily hand” during “supination/pronation” are significant.

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5. References AGHAZADEH, F. and MITAL, A. (1987) Injuries due to hand tools. Applied Ergonomics 18 (4) 273-278 ARMSTRONG, T.J. and CHAFFIN, D.B. (1979) Carpal tunnel syndrome and selected personal attributes. Journal of Occupation Medicine 21, 481-486 HÄGG, G.M.; ÖSTER, J. and BYSTROM, S. (1997) Forearm muscular load and wrist angle among automobile assembly line workers in relation to symptoms. Applied Ergonomics 28 (1) 41-47 KLUTH, K.; CHUNG, H.-C. and STRASSER, H. (2004) Verfahren und Methoden zur Prüfung der ergonomischen Qualität von handgeführten Arbeitsmitteln – Professionelle Schraubendreher im Test. Dr. Curt HaefnerVerlag, Heidelberg LEWIS, W. and NARAYAN, C. (1993) Design and sizing of ergonomic handles for hand tools. Applied Ergonomics 24 (5) 351-356 MITAL, A. (1991) Hand tools: injuries, illness, design, and usage. In: MITAL, A. and KARWOWSKI, W. (Eds.) Workspace, Equipment and Tool Design. Elsevier, Amsterdam, pp. 219-256 REMPEL, M. (1994) Carpal tunnel pressure studies: implications for prevention and rehabilitation. In: MCFADDEN, S.; INNES, L. and HILL, M. (Eds.) Proceedings of IEA-94 Vol.3. Human Factors Association of Canada, Toronto, pp. 244-246 SILVERSTEIN, B.A.; FINE, L.J. and ARMSTRONG, T.J. (1986) Hand wrist cumulative trauma disorders in industry. British Journal of Industrial Medicine 43, 779-784 STRASSER, H. (1991) Different grips of screwdrivers evaluated by means of measuring maximum torque, subjective rating and by registering electromyographic data during static and dynamic test work. In: KARWOWSKI, W. and YATES, J.W. (Eds.) Advances in Industrial Ergonomics and Safety III. Taylor and Francis, London / New York / Philadelphia, pp. 413-420 STRASSER, H. (1996) Electromyography of upper extremity muscles and ergonomic applications. In: KUMAR, S. and MITAL, A. (Eds.) Electromyography in Ergonomics. Taylor and Francis, London / New York, pp. 183-226 STRASSER, H. and WANG, B. (1998) Screwdriver torque strength and physiological cost of muscles dependent on hand preference and direction of rotation. Occupational Ergonomics 1 (1) 13-22 STRASSER, H.; KLUTH, K. and KELLER, E. (2004) A computer based system for the use of electromyographic methods for the measurement of physiological costs associated with operating hand-held tools and computer input devices. Occupational Ergonomics 4 (2) 73-87

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ZIPP, P. (1982) Recommendations for the standardization of lead positions in surface electromyography. European Journal of Applied Physiology 50, 41-54

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Chapter 10.3

Torque Levels, Subjective Discomfort, and Muscle Activity Associated with Four Commercially Available Screwdrivers Under Static and Dynamic Work Conditions M.-J.J. Wang, C.-L. Lin, Y.-C. Shih, H.-C. Chung and H. Strasser 0. Summary This study evaluated screwdrivers with different handle designs and blade lengths. 10 men and 10 women voluntarily participated. A repeated-measures experiment design was employed. The three independent factors were gender of user, handle (four types), and blade length (130, 170, and 210 mm). The dependent measures were the maximum supination torque under a static task and the %MVC of EMG responses in biceps brachii and flexor digitorum, and a discomfort rating for the upper extremity under the dynamic task. Analysis showed that the in-line screwdriver with the combined characteristics of large handle diameter (3.8 - 4.1 cm), smooth rubber covering handle surface, triangular (or circular) shape, and adequate handle length (11 cm) had the greatest supination torque and a smaller discomfort rating than the screwdriver with the pistol-grip handle. Blade length was not significantly related to any dependent measure.

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1. Introduction Screwdrivers are frequently used in the workplace and daily life. The need for high force exertion using poor wrist posture to tighten screws may cause high loads in the forearm muscles and the increased risk of musculoskeletal disorders in the upper extremity. Thus, it is necessary to use the ergonomically designed hand tools to reduce the risk of wrist and upper limb musculoskeletal disorders in the workplace (cp. MIRKA et al. 2002). The screwdriver handle is an interface of transferring hand force into the screw and work piece. The handle design parameters such as handle length, size, shape, and surface friction tend to affect the hand force transfer and job performance. For ergonomic hand tool design, it is necessary to consider the hand’s anthropometric dimensions in handle design. To avoid high palmar forces, the tool handle length should be sufficient to distribute the forces on the lateral and medial sides of the palm and across the four non-thumb digits (cp. CHAFFIN et al. 1999). GARRET (1971) suggested the proper handle length of 115 mm and a clearance of 30 to 50 mm all around the handle. Further, handle diameter is another important parameter that affects torque exertion (cp. MITAL and SANGHAVI 1986). It has been reported that the greater the handle diameter, the greater the maximum torque produced, when the diameter was from 25.4 to 63.5 mm (cp. SHIH and WANG 1996). HABES and GRANT (1997) compared screwdrivers with handle diameters at 29 mm and 37 mm at different handle height, reach distance, and handle orientation. They reported that the torque performance at 37 mm handle diameter was significantly greater than that at 29 mm in all conditions. For the handle shape effect, a triangular handle can generate greater torque than square, circular or hexagonal shapes (cp. SHIH and WANG 1996). Handle shape also affects the wrist posture when operating the hand tool. The ergonomic design principle is to bend the hand tool's handle rather than

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force the user to bend the wrist to avoid cumulative trauma disorders. A screwdriver with a pistol-grip handle design can promote neutral wrist posture. This type of screwdriver is usually combined with a ratchet wheel device which rotates the blade efficiently. It was employed here for evaluation. Handle surface friction depends upon handle surface texture and handle material type. The handle surface friction tends to have a pronounced effect on shear force generation, which triggers the torque output. It has been reported that, even if the length, diameter, and shape of screwdrivers were the same, a handle with greater surface friction can generate greater torque (cp. SHIH and WANG 1997) and less muscle EMG activity (cp. STRASSER 1991). During sustained or repetitive muscle contractions, electromyography (EMG) has usually been used to evaluate the influence of screwdriver design on the upper limb muscles. STRASSER (1991) measured EMG signals of flexor digitorum and biceps brachii muscles to estimate physiological cost in using seven different screwdrivers for simulated screwing tasks. The different screwdrivers produced equal biological energy but created varied torque as a result of grasping or coupling conditions (cp. STRASSER 1991). Grasping force and supination torque exertion are mainly involved in the screw insertion task. The biceps brachii muscle was a dominant supinator and the flexor digitorum muscle was a grip musculature (cp. STRASSER and WANG 1998). Thus, it is necessary to measure these two muscles’ activities while assessing design of screwdrivers. In addition, workers sometimes have to use different lengths of screwdriver blade to perform screw insertion tasks, given workplace conditions and parts geometry. Very little information is available regarding the blade length effect on torque performance and the physiological cost in muscular effort. Many of the past studies have emphasized the effect of the design of hand tool handles on supination torque. Apart from static maximum force exertion, dynamic screw insertion tasks are commonly encountered in the workplace. It is important to evaluate the design by measuring physiological and psychophysical responses under dynamic screw insertion tasks. The pistol-grip handle screwdriver is a new type of ergonomic screwdriver design which can promote neutral wrist posture when performing such a task in the vertical plane. The main objective of this study was to compare the usability of pistol-grip handle design with different in-line screwdriver handles under static and dynamic screw insertion tasks. It was hypothesized that supination torque for men from using a pistol handle and shorter blade length would be greater than that for women using a circular handle and longer blade length. There should also be smaller EMG responses in biceps brachii and flexor digitorum and lower discomfort rating for the screwing task for men using a pistol handle and shorter blade length than that for women using a circular handle and longer blade length. 2. Methods 2.1. Participants Taiwanese university students (10 men and 10 women) voluntarily participated in this study. Their average age was 21.5 years. All subjects were right-handed, i.e., hand used to write, to use a screwdriver, and free from hand, elbow and shoulder musculoskeletal disabilities. The average body height for men and women was 172.5 cm and 161.7 cm, respectively. The average body weight for men and women was 62.7 kg and 50.7 kg, respectively. The anthropometric data for the 20 subjects are presented in Table 1. Table 1: Test subjects’ anthropometric data

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2.2. Experimental design A repeated-measures experiment design was employed. The three fixed factors were gender, handle shape (square, triangular, circular, and pistol), and blade length (130, 170, and 210 mm). Participants were an assumed random factor. A total of 12 (4 handles x 3 length) experimental combinations were involved. Four screwdrivers with ratchet wheel devices, as illustrated in Fig. 1, were evaluated. The square screwdriver was an in-line square shaped screwdriver with a 10.5-cm handle length and a 3.2-cm handle diameter. The triangular screwdriver was an in-line triangular shaped screwdriver with 11-cm handle length and 3.8-cm handle diameter. The circular screwdriver was an in-line round screwdriver with 11-cm handle length and 4.1-cm handle diameter. The pistol screwdriver had a pistol-grip handle with a 11-cm handle length and 3.4-cm handle diameter. Table 2 provides the descriptions of the handle characteristics of the four screwdrivers, including handle shape, diameter, and length.

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Figure 1:

Four types of screwdrivers used in experiment

Table 2: Screwdriver handle characteristics

The dependent measure was the maximum supination torque under static task. The dynamic aspect of the task was used to rate subjective comfort with the Borg CR-10 scale and physiological cost with EMG responses in biceps brachii and flexor digitorum. 2.3. Equipment and materials To measure the torque MVC, a SENSOTEC torque transducer (Model QWLC-8M, capacity: 300 in-lb.) was mounted onto an adjustable steel frame and connected to a personal computer through a 12 bit A/D card (ICP DAS ISO-LDH) at a sampling rate of 1 kHz.

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The Noraxon TeleMyo EMG System was used to measure muscle activities in the upper extremity. EMG signals were recorded at a sampling rate of 1 kHz, low-pass filtered at 6 Hz and processed with Root-Mean-Square (50 ms). The incoming EMG signals were monitored on-line to ensure sufficient quality of the data. EMG recordings were made using surface electrodes [ConMed Adult ECG Electrodes (Ag/AgCl)] placed on the longitudinal axis of biceps brachii and flexor digitorum muscle fibers. All EMG data were processed and analyzed by the MyoResearch (Noraxon) software. The maximal voluntary contraction of each muscle was measured independently before the experiment. The EMG amplitude during dynamic screw-insertion task was normalized to the maximal voluntary contraction and expressed as %MVC. The Borg CR-10 scale (cp. BORG 1982) was used for psychophysical rating of discomfort. This is a 10-point scale, with 0 denoted as “nothing at all” and 10 as “almost maximal.” At the end of each experimental condition, the subjects were asked to rate their subjective discomfort in the upper extremity. 2.4. Experimental procedure Before the experiment, the researcher explained the purpose and procedure to the subjects. Several trials were performed to familiarize the subjects with the devices and the experimental procedure. The experimental session was conducted in two phases, maximum supination torque measurement and dynamic screw-insertion task. Both tasks were on a vertical plane. The maximum supination torque measurement followed the procedure proposed by CALDWELL (1974). In the first experimental phase, the torque sensor was adjusted at the subject’s elbow height for maximal torque exertion. The subject was requested to exert the maximum supination torque over a 3-s period, and this was repeated three times. The maximum torque among the three trials was used in analysis. A 2-min. rest between successive trials was given. In the second experimental phase, the subject was instructed to screw five cross head (#2 head) screws in a row 1-cm deep into a wooden board. A beep sound feedback was given when a screw reached the 1 cm depth. The height of the wooden board was adjusted to each subject's elbow height. The EMG data for biceps brachii and flexor digitorum were collected. After completing the screw insertion task, the subject was asked to assess subjective upper extremity discomfort from using each screwdriver. A 10-min. rest break was given prior to the next session. The 12 treatment combinations were randomized for each subject. The entire experiment took about 4 h for each subject in one day.

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3. Results 3.1. Torque performance The analysis of variance (Table 3) indicated that gender ( F1,18 = 48.15, p < .01), handle (F3,54 = 43.17, p < .01), and gender by handle (F3,54 = 8.53, p < .01) were significant effects for maximum supination torque. The mean maximum supination torque and values of Duncan multiple-range tests under each level of the three independent variables are shown in Table 4. The Duncan grouping results showed the maximum supination torque for the four screwdriver handle types can be classified into three groups. The first group with the highest peak torque strength included circular and triangular screwdrivers, followed by pistol and square screwdrivers. Using a circular screwdriver produced the greatest supination torque (about 5.81 Nm). Using a square screwdriver produced the least supination torque (about 3.86 Nm), which was only 66 % of the torque for the circular screwdriver. The maximum supination torque for men (about 6.35 Nm) was significantly greater than that for women (about 3.98 Nm). The average maximum torque for women was 63 % of that of men. The mean supination torque for blade lengths of 130, 170, and 210 mm was 5.14, 5.15, and 5.20 Nm, respectively. No other effect was significant for maximum supination torque.

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Table 3: Summary of analysis of variance for maximum supination torque

Table 4: Mean maximum supination torque [Nm] by independent variable

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3.2. Subjective and physiological responses The mean discomfort rating and %MVC of EMG responses under each level of the three independent variables are shown in Table 5. Table 5: Mean discomfort rating and %MVC of EMG responses under three independent variables

Mean discomfort rating for women was greater than that for men. A summary of the analysis of variance is shown in Table 6. The effect of handle (F3,54 = 4.08, p < .01) and the interaction of gender by

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handle (F3,54 = 3.23, p < .05) were statistically significant. The Duncan grouping showed that use of the square screwdriver was associated with a significantly higher discomfort rating for the upper extremity than the use of circular, pistol, and triangular screwdrivers. But there is no statistically significant difference among the three. Table 6: Summary of F values for discomfort rating and %MVC of EMG response

3.3. EMG of biceps brachii and flexor digitorum Mean EMG responses in %MVC of biceps brachii and flexor digitorum for women were greater than those for men. But the analysis of variance (Table 6) showed that a significant main effect for gender was found only for EMG of biceps brachii (F1,18 = 20.14, p < .01). The EMG of biceps brachii and flexor digitorum were slightly smaller for a circular handle than others, but the analysis of variance showed no statistically significant effect for handle type. Blade length was not significantly associated with measured subjective and physiological responses.

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4. Discussion The hypothesis of this study was that the pistol handle should produce greater supination torque, smaller EMG responses in biceps brachii and flexor digitorum, and lower discomfort rating than in-line screwdrivers. Comparing the square and pistol screwdrivers in detail, both had plastic handles and the diameter difference between the two was rather small (0.2 cm). The main difference between square and pistol screwdrivers was handle shape. The maximum supination torque generated by a pistol screwdriver (5.30 Nm) was markedly greater than the supination torque of a square screwdriver (3.86 Nm). This is related to shape of the pistol-grip handle which can increase the moment arm for exertion of supination torque. Further, mean subjective discomfort rating for the pistol screwdriver was significantly lower than that for the square screwdriver, and the %MVC of EMG responses were similar during screw-insertion task. It showed that the pistol-grip handle shape can promote a neutral wrist posture, a higher torque exertion, and feeling of less discomfort while operating on the vertical plane. In addition to the handle size and shape factors, the triangular and circular screwdrivers had the rubber covered handle surface material. The past studies indicated that a triangular shaped handle tended to generate a greater torque than a square or circular handle, given the same handle diameter. If the factor of handle shape plays a dominant role in generation of peak torque strength, the ranking in maximum supination torque would be triangular, square, and circular screwdrivers. Here, the ranking of peak torque strength was circular, triangular, and square screwdrivers. It seems that the handle diameter and surface material play a more dominant role in generation of peak torque than the handle shape. The increase in handle diameter would increase the moment arm in torque exertion.

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Compared with the plastic handle, the soft rubber handle surface could increase the friction between the handle and palm, thereby generating a greater supination torque. Furthermore, although the surface materials for circular and triangular screwdrivers are both rubber, the smooth handle surface of the triangular screwdriver seems to generate greater feeling of comfort than the rubber textured grip surface. Overall, the handle diameter, handle shape, and surface material all contribute to the peak torque performance under a static torque exertion task as well as discomfort rating for the upper extremity for the dynamic screw-insertion task. The pistol-grip handle shape can promote a neutral wrist posture. The in-line screwdriver that combined the characteristics of large handle diameter (3.8 – 4.1 cm), smooth rubber covered handle surface, triangular (or circular) shape, and adequate handle length (11 cm) had the greatest supination torque, and the smaller discomfort rating of the upper extremity. The main contribution for the flexor digitorum was for grasping the handle in holding the screwdriver. The involvement of the biceps brachii activity was for turning the screwdriver handle. The average maximum torque for women was 63 % of that of men under the static torque exertion task and the average %MVC of EMG in biceps brachii for men was 62 % of that of women under the dynamic screw-insertion task. The maximum supination torque of women was smaller than that of men and required greater biceps brachii muscle activity to perform the screw-insertion task. Table 7 shows the interaction of gender and handle type on maximum supination torque and the discomfort rating for the upper extremity. For women, using a pistol-grip screwdriver produced smaller supination torque and greater discomfort rating than using a circular screwdriver. For men, the supination torque of using a pistol-grip screwdriver was similar to that of a triangular screwdriver but the discomfort rating of using a pistol-grip screwdriver was greater than that of using a triangular screwdriver. This is perhaps because women had smaller hands than men. While working on a vertical plane, it seems that the in-line screwdriver with circular shape and rubber-covered handle surface is better suited for women than the pistol-grip screwdriver with a plastic handle. On the other hand, the in-line screwdriver with a triangular shape and a rubber-covered handle surface is more suitable for men than the pistol-grip screwdriver.

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Table 7: Interaction for gender and handle type on maximum supination torque [Nm] and discomfort rating of upper extremity

Blade length was not significantly related to any of the four dependent measures. The lack of blade length effect on peak torque performance seemed related to there being no increase in the moment arm for torque exertion with an increase in blade length. It also explains the lack of blade length effect on biceps brachii and flexor digitorum muscular activity. The change in blade length was not related to the subjective discomfort rating in the upper extremity as well. 5. References BORG, G.A. (1982) Psychophysical bases of perceived exertion. Medicine and Science in Sports and Medicine 14, 377-381 CALDWELL, L.S. (1974) A proposed standard procedure for static muscle strength testing. American Industrial Hygiene Association Journal 35, 201-206 CHAFFIN, D.B.; ANDERSSON, G.B.J. and MARTIN, B.J. (1999) Occupational Biomechanics. 3rd edition, Wiley Interscience, New York

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GARRET, J.W. (1971) The adult human hand: some anthropometric and biomechanical considerations. Human Factors 13, 117-131 HABES, D.J. and GRANT, K.A. (1997) An electromyographic study of maximum torques and upper extremity muscle activity in simulated screwdriving tasks. International Journal of Industrial Ergonomics 20, 339-346 MIRKA, G.A.; SHIVERS, C.; SMITH, C. and TAYLOR, J. (2002) Ergonomic interventions for the furniture manufacturing industry: Part II. Hand tools. International Journal of Industrial Ergonomics 29, 275-287 MITAL, A. and SANGHAVI, N. (1986) Comparison of maximum volitional torque exertion capabilities of men and women using common hand tools. Human Factors 28, 283-294 SHIH, Y.C. and WANG, M.-J.J. (1996) Hand/tool interface effects on human torque capacity. International Journal of Industrial Ergonomics 18, 205-213 SHIH, Y.C. and WANG, M.-J.J. (1997) The influence of gloves during maximum volitional torque exertion of supination. Ergonomics 40, 465-475 STRASSER, H. (1991) Different grips of screwdrivers evaluated by means of measuring maximum torque, subjective rating and by registering electromyographic data during static and dynamic test work. In: KARWOWSKI, W. and YATES, J.W. (Eds.) Advances in Industrial Ergonomics and Safety III. Taylor and Francis, New York, pp. 413-420

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STRASSER, H. and WANG, B. (1998) Screwdriver torque strength and physiological cost of muscles dependent on hand preference and direction of rotation. Occupational Ergonomics 1, 13-22

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Chapter 10.4

The Effect of Screwdriver Handle Design and Blade Length on Muscle Activity and Torque MVC M.-J.J. Wang, C.-L. Lin, Y.-C. Shih and H. Strasser 0. Summary The objective of this study was to investigate the effect of screwdriver handle design and blade length on maximum torque and muscle activity. Five handle types and three blade length were evaluated. Twenty student subjects (10 males and 10 females) participated in this study. The subjects exerted maximum torque before and after screwing task. Physiological cost was simultaneously measured by electromyographic activities of 2 muscles (biceps brachii and flexor digitorum). The results indicate that handle effect was significant on anterior, posterior maximum torque (p < 0.001), and %MVC of biceps brachii muscle (p < 0.01). On the other hand, the effect of blade length was not significant on all measures. When the diameter of handle was greater, the MVC in anterior and posterior maximum torque exertion was increased, and the %MVC was decreased. The mean anterior and posterior maximum torque of females was 64 % and 62 % of that of males respectively.

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1. Introduction Hand tools are often used in industrial works to enhance the physical capabilities of workers. Develop a "single standard" for ergonomic hand tool to fit all purpose is virtually impossible because of the variations in hand anthropometry, human performance, work environments, and task. Therefore, the investigation of appropriate hand tool design and its applications continues to evolve. For hand tool handle design, four major parameters are considered. The first one is handle size (diameter); the greater the size, the greater the torque (cp. DEIVANAYAGAM 1994). The second one is handle shape; the triangular design could exert greater torque than circular shape, and the square design could exert greater torque than regular type (cp. MITAL and CHANNAVEERAIAH 1988). The third parameter is handle length; a length of 115 mm and a clearance of from 30 to 50 mm all around the handle was suggested (cp. GARRET 1971). The last aspect is handle surface material; the surface material generates the friction property between handle and hand, and the subjective hand discomfort feeling. Many basic hand tools such as wrenches, pliers, screwdrivers, or hammers are commonly used in the workplace. Some researchers have investigated screwdriver characteristics through experiment evaluations (cp. JOHNSON and CHILDRESS 1988; ORTENGREN et al. 1991; CEDERQVIST and LINDBERG 1993; STRASSER and WANG, 1998). Most of the early studies focused on the effect of handle size, shape, length and material on human performance. One important principle of the ergonomics hand tool design is to maintain a neutral wrist posture to avoid cumulative trauma disorders (CTD’s). Moreover, workers sometimes have to use long blade screwdriver to complete the task, and there is a need to know the performance and induced physiological cost. The objective of this study was to investigate the effect of screwdriver handle design and blade length on maximum torque and muscle activity.

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2. Methods 2.1. Subjects and apparatus

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Twenty student subjects, 10 males and 10 females, voluntarily participated in this study. The average age for males was 21.7 (±2.63) years, the average body weight and height was 62.7 (±4.92) kg and 172.5 (±4.5) cm. The hand length was 18.8 (±0.78) cm and hand width was 9.9 (±0.34) cm. The average elbow height was 105.5 (±4.4) cm. The average age for females was 21.3 (±2.31) years. The average body weight and height was 50.7 (±7.53) kg and 161.7 (±5.29) cm. The hand length was 17.2 (±0.85) cm and hand width was 8.8 (±0.42) cm. The average elbow height was 100.8 (±4.94) cm. All subjects were right-handed, and free from hand and musculoskeletal disabilities. Figure 1 shows the experimental hand tools. The five screwdrivers from left to right were coded as No. l to No. 5. The blade length was defined as the length from handle head to blade tip. Three types of blade length (130, 170, 210 mm) were evaluated in this study. The experiment task was to screw screws into wood.

Figure 1:

Screwdriver handles n through r examined

2.2. Experimental design and procedures A nested-factorial design was employed. The independent variables involved gender, subjects, handle type, and blade length. The subject factor was nested under gender factor. At first, the experimenter explained about the purpose and procedure to the subject. Then, the subject was requested to exert maximum torque over a 3-second period and then rest for 2 minutes. This test was repeated three times, and the largest torque value was considered as the anterior maximum torque force. Next, subjects screwed 5 screws into wood for 1 cm deep. Afterwards, the maximum torque was measured again. A 10-minutes rest was scheduled after each treatment. Physiological cost in EMG was measured from 2 muscles (biceps brachii and flexor digitorum). The EMG data were presented in %MVC. Each subject had to conduct a total of 15 treatments.

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3. Results and discussion Table 1 shows the ANOVA results in nested-factorial design. The gender effect was significant in anterior, posterior maximum torque and the %MVC of biceps brachii (p < 0.001). Also, handle effect was significant in anterior, posterior maximum torque, %MVC of flexor digitorum (p < 0.001) and the %MVC of biceps brachii (p < 0.05). The two-way interactions (Gender x Handle, Subject x Handle (Gender)) were found in anterior, posterior maximum torque and the %MVC of biceps brachii and flexor digitorum (p < 0.05). The blade length effect was not significant in all dependent variables. Table 1: The ANOVA results

Table 2 presents the results of Duncan's multiple range test and the corresponding mean values. According to Duncan's multiple test results, the anterior, posterior maximum torque of the five screwdrivers can be classified into four groups.

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Table 2: The results of Duncan's multiple test in handle

The first group had the highest torque performance, including No. 4 and No. 3 screwdriver, followed by No. 5, No. 2 and No. l screwdriver. For the %MVC of biceps brachii, it can be roughly classified into three groups. The first group (No. l and No. 2) had the highest muscle activity. The next group included No. 5 and No. 3 screwdrivers. The third group (No. 4) had the lowest muscle activity. For anterior maximum torque, the greatest torque appeared at No. 4 (about 51 inch-lb), the lowest torque appeared at No. l (25 inch-lb). The lowest was only 50 % of the greatest one. For posterior maximum torque, the greatest torque appeared at No. 3 (about 43 inch-lb), and the lowest appeared at No. l (about 20 inch-lb). Again, the lowest was about 50 % of the greatest one. It indicates that in anterior or posterior maximum torque, the lowest value appeared in No. 1 was only half of the greatest torque that appeared in No. 3 or No. 4 screwdrivers. Besides, the %MVC of biceps brachii in No. 3 or No. 4 screwdriver was substantially lower than that of No. 1 screwdriver. Figure 2 illustrates the effect of handle types on anterior, posterior maximum torque and %MVC of biceps brachii, flexor digitorum for males and females. For anterior maximum torque, the MVC ranking in male group was 62.0 inch-lb for No. 3, 61.7 inch-lb for No. 4, 61.1 inch-lb for No. 5, 40.0

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inch-lb for No. 2 and 28.5 inch-lb for No. 1 screwdriver. Similarly, the MVC ranking for female group was 41.1 inch-lb for No. 4, 38.8 inch-lb for No. 3, 32.7 inch-lb for No. 5, 28.5 inch-lb for No. 2, 20.7 inch-lb for No. 1 screwdriver. For posterior maximum torque, the ranking in male and female group was the same as anterior maximum torque exertion. The average anterior, posterior maximum torque was 50.7, 41.7 inch-lb for male and 32.4, 25.8 inch-lb for female. The mean anterior and posterior maximum torque of females was 64% and 62 % of that of males.

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Figure 2:

The effect of handle types on anterior, posterior maximum torque and %MVC of biceps brachii, flexor digitorum

For both males and females, the same handle that could exert greater force before the screwing task was also found to have greater force exertion after the task. The diameter of No. 3, No. 4 and No. 5 screwdriver was 38 mm, 41 mm and 34 mm respectively. They were greater than No. l and No. 2 screwdriver which was 30 and 32 mm. The handle length of the five screwdrivers was 100 mm, 105 mm, 110 mm, 110 mm and 110 mm respectively. When the handle was bigger and longer, subjects tended to exert greater torque force. The handle surface of No. l and No. 2 screwdriver was hard and smooth. On the other hand, the handle surface material of No. 3 and No. 4 was rubber, which induced greater friction for maximum torque exertion. Although the diameter of No. 5 screwdriver (pistol handle type) was smaller than the diameter of No. 3 and No. 4, the maximum torque performance was similar. The %MVC of biceps brachii for male was 52 % for No. l, 51 % for No. 2, 50 % for No. 3, 44 % for No. 4 and 47 % for No. 5 screwdriver. The %MVC of biceps brachii for female was 95 % for No. 1, 85 % for No. 2, 73 % for No. 3, 72 % for No. 4 and 84 % for No. 5 screwdriver. The %MVC of flexor digitorum for male was 60 % for No. l, 56 % for No. 2, 48 % for No. 3, 46% for No. 4 and 60 % for No. 5 screwdriver. The %MVC of flexor digitorum for female was 56 % for No. l, 62 % for No. 2, 56 % for No. 3, 58 % for No. 4 and 62 % for No. 5 screwdriver. The %MVC of flexor digitorum for males and females was similar, but the %MVC of biceps brachii for males was smaller than that of females. It was probably due to the fact that the force of females was smaller than that of males which resulted in higher muscle activity in biceps brachii. Figure 3 shows the effect of blade length on anterior, posterior maximum torque and the %MVC of biceps brachii, and flexor digitorum. The range of anterior maximum torque for male group was 51.3 to 50.5 inch-lb, and was 32.8 to 32.0 inch-lb for female group. In posterior maximum torque, the range was 43.4 ~ 40.9 inch-lb for males and 27.0 ~ 25.4 inch-lb for females.

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Figure 3:

195

The effect of blade length on anterior, posterior maximum torque and %MVC of biceps brachii, flexor digitorum

4. Conclusions

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Among the five screwdrivers, No. 3 and No. 4 seem to be the best screwdrivers, due to the handle diameter and surface material characteristics. The No. 5 screwdriver can not only induce neutral wrist posture but also exert high torque forces. The effect of blade length on maximum torque exertion and %MVC of the biceps brachii and flexor digitorum was not significant. The mean anterior and posterior maximum torque of females was 64 % and 62 % of that of males. 5. References CEDERQVIST, T. and LINDBERG, M. (1993) Screwdrivers and their use from a Swedish construction industry perspective. Applied Ergonomics 24 (3) 158-164 DEIVANAYAGAM, S. (1994) Hand torque strength for small fasteners. In: AGHAZADEH, F. (Ed.) Advances in Industrial Ergonomics and Safety VI. Taylor and Francis, pp. 579-586 GARRET, J.W. (1971) The adult human hand. Some anthropometric and biomechanical considerations. Human Factors 13 (2) 117-131 JOHNSON, S.L. and CHILDRESS, L.J. (1988) Powered screwdriver design and use: tool, task, and operator effects. International Journal of Industrial Ergonomics 2, 183-191 MITAL, A. and CHANNAVEERAIAH, C. (1988) Peak volitional torque for wrenches and screwdrivers. International Journal of lndustrial Ergonomics 3, 41-64 ORTENGREN, R.; CEDERQVIST, T.; LINDBERG, M. and MAGNUSSON, B. (1991) Workload in lower arm and shoulder when using manual and powered screwdrivers at different working heights. International Journal of Industrial Ergonomics 8, 225-235 STRASSER, H. and WANG, B. (1998) Screwdriver torque strength and physiological cost of muscles dependent on hand preference and direction of rotation. Occupational Ergonomics 1 (1) 13-22

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Chapter 11

Product-Ergonomic Evaluation of Diagonal Cutter Handles K. Kluth, D. Zühlke and H. Strasser 0. Summary For the development of ergonomically optimized types of diagonal cutter handles, 8 typical diagonal cutters were compared and evaluated with respect to the handle design. Eleven male, right-handed test subjects (Ss) between the age of 19 and 35 years repetitively had to cut medium-hard and soft wires according to the test methods of the German Institute for Standardization (published in DIN ISO 5744) at a special device and in a standardized execution. By means of surface electromyography the muscle strain (mean values from 20 cuts) of the m. flexor digitorum, m. extensor digitorum, m. flexor carpi ulnaris and m. biceps in the right hand-arm system were continuously registered. Following these tests, the data were standardized, analyzed and assessed in percentage of the electromyographic activity (EA) associated with maximum voluntary contractions. A complete mobile system for the recording of peripheral-physiological data was used. Furthermore, specific questionnaires with 35 items were developed with which the Ss subjectively evaluated cutting pliers and handles criteria such as design, material, dimensions, weight, and handling as well as work effects on the human body. The determined objective measured data in combination with the obtained subjective assessments allowed conclusions about design criteria and design approaches for the optimization of the diagonal cutter handles.

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1. Introduction Pliers are a type of hand-held, two-legged tools which are among the most versatile in the manageability (cp. left side of Fig. 1) of all the tool types. Therefore, it is no surprise that, over time, numerous handle and head variants have been developed for this fairly old tool and that nowadays a wide product variety is available. All in all DIN ISO 5742 differentiates between 24 types of pliers, but the construction of pliers has a lot in common. In general, the head with two jaws or cutters merges at first in a joint for opening and closing the jaws or cutters and afterwards in straight or cambered handles. High-quality pliers are made from oil-hardened and tempered vanadium electric steel. The metallic handles are often coated with 1- or 2-component plastic handles with a high-friction surface which covers the solid, metallic core. The plastic outer surface allows an individually handle design with respect to shape, material, dimensions, and surface which affects the quality of the product, too. Some of these products are considered as “generalists” and one such product are the diagonal cutters, which are used finger-dynamically. Among diagonal cutters, there are differences in the quality standards with respect to cutting behavior, the design of the nose end of the nippers, and, most importantly, the ergonomic design of the handles. The subsequent described investigation with objective and subjective methods should allow to draw conclusions about design criteria and design approaches for the ergonomic optimization of this type of tools.

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2. Methods In order to develop new, ergonomically optimized handles, a comparative investigation was carried out in which 6 common diagonal cutters (pliers 1-6, see the right side of Fig. 1) and 2 close-toproduction prototypes (pliers 7-8) were analyzed and evaluated. The dimensions and inspection values of all pliers were conforming to DIN ISO 5749 and comparable among each other. Merely, the shape of the upper part of the pliers handles 2 and 3 offered no support for the thumb (slip-guard), especially where forces took effect in longitudinal direction. Furthermore, pliers 2 were equipped with an opening spring and the metallic handles of pliers 3 were not coated with 1- or 2-component plastic handles.

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Figure 1:

Study of different hand postures while working with pliers (left) and photos of the 8 test objects (right)

The test subjects had to cut 20 pieces of wire in a minute in a standardized working position. In the test series according to DIN ISO 5744 a medium hard wire (steel, tensile strength 1600 N/mm2, diameter 1.6 mm) and a soft wire (CuSn6, tensile strength 780 N/mm2, diameter 1.5 mm) were used for the cutting tasks. The cutting frequency (1 cut per 3 s) was forced and controlled by a constant acoustical signal of a metronome. After a working phase of 1 minute with one of the in the sequence randomized diagonal cutters followed a rest of 10 minutes. The right part of Fig. 2 shows a subject during the test and a special cutting device. This special height adjustable device was developed for the standardized execution of the cutting tasks and the standardized manual supply of the wire. The height adjustability guaranteed for all participants to cut the wire in elbow height as the optimum working height with an angle between the upper and lower arm of 100°. The subjects stood in an upright but comfortable posture in front of the device. The angle between the fontal plane of the body and the lower arm was 75°. Eleven male, right-handed test subjects with an average age of 26.5 ±4.4 years, a body height of 184 ±3.4 cm, a hand width of 9.5 ±0.4 cm (without thumb) and a hand length of 19.7 ±1.3 cm participated in the tests. All persons had normal hand types and none of the test subjects stated physical complaints in the hand-arm-shoulder system at the beginning of the tests. With the help of surface electromyography (see, for example, STRASSER 1996; KLUTH et al. 1997; STRASSER et al. 1998), used as a standard method in the field of product ergonomics, the muscle strain of the right hand-arm system was objectified utilizing a completely mobile data recording system (cp. Chapter 3). Four muscles (m. flexor digitorum, m. extensor digitorum, m. flexor carpi ulnaris, and m. biceps) which act on the right hand and forearm were selected (cp. left part of Fig. 2). The EA time series as indicators of the “physiological costs” which are required from the muscular system in order to carry out different cutting tasks were continuously registered, evaluated, and assessed.

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Figure 2:

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Location and function of monitored muscles of the right hand-arm system and test subject during cutting

Furthermore, specific questionnaires with a bipolar 4-step scale were developed for investigating, e.g., the design, handle material, dimensions, weight, and overall assessment of the handling. With the help of these questionnaires, the test subjects subjectively evaluated pliers criteria as well as work effects on the hand-forearm system. 3. Results

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3.1. Influence of the diagonal cutters on the muscle strain Figure 3 shows the standardized electromyographic activities sEA of the 4 muscles measured (in percent of the EAmax values during maximum voluntary muscle contractions), as the means of 11 subjects and of standardized 20 cuts per pliers model. The results of the test series with medium-hard wire (cp. upper part of Fig. 3) show an average level of strain of approximately 40 % for the m. flexor digitorum and about 45 % for the m. extensor digitorum as well as for the m. flexor carpi ulnaris. Only the sEA values of pliers 1 and 3 were a bit higher. All pliers demand the use of the m. biceps – here only involved in the static holding of the arm – by mere 7 %. For the test series with medium-hard wire the means of the muscle strain of all subjects shows only small differences between the 8 diagonal cutters investigated. The two-sided t-test yielded that the differences between the pliers were at least significant at a level of 95 %, if the various pliers were compared with pliers 1 or 3 (cp. Table 1). Pliers 1 demanded most of all the monitored forearm muscles. The analysis of the sEA values after the cutting of soft wire in a standardized body posture reveals means of muscle strain which are substantially lower as in the test before (cp. lower part of Fig. 3). In contrast to the results while cutting medium-hard wire the gripping forces were reduced by approximately 50 %. Therefore, an average decrease in muscle strain of approximately 20 % could be measured for the muscles m. flexor digitorum, m. extensor digitorum and m. flexor carpi ulnaris. The highest muscle strain could be analyzed again for pliers 1 and 3. Significant differences in strain could be identified only between pliers 1 and 6 as well as 1 and 7. But the differences were too small for a differentiated discussion.

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K. Kluth et al. / Product-ergonomic evaluation of diagonal cutter handles

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Table 1: Comparison of muscular strain while cutting medium-hard wire in a standardized working posture and symbolized description of the significance of strain differences as well as direction of the strain alteration. Exemplary explanation for the comparison of pliers 1 and 2: Alteration “-“means that the sEA values of pliers 2 are lower than the sEA values of pliers 1 († = p > 5 %; ’ = p ” 5 %; ’’ = p ” 1 % )

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Figure 3:

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Standardized electromyographic activity sEA [%] for 4 muscles while cutting medium-hard wire (top) and soft wire (below) in a standardized posture (N = 11)

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3.2. Subjective assessment of the diagonal cutters The answers to 35 items gave a detailed inside in the relevant design criterions of diagonal cutters. This article can give only an overview. All diagonal cutters analyzed, possess handles with a longitudinal contour which largely meet the ergonomic demands. But the analyses showed partly differences in the objective assessment as well as decisive differences in the subjective assessment. The overall subjective assessment of the handle material (cp. Fig. 4) results in a recurring trend as to the assessment of the cutting pliers. Especially pliers 2, equipped with 2-component handles with a soft, high-friction outer surface and a solid, durable core made a clearly positive impression on the subjects. These 2-component handles are increasingly offered by the manufacturers with the objective of reaching a compromise between the necessary degree of friction and a sufficiently easy sliding of the hand on the pliers handle during the cutting process. But the rating under the impression of the cutting experience turned out less good. This result possibly reflected the assessment of these handles in case of hand perspiration. In contrast to pliers 2, the “unfavorable” rating of the hard and narrow metal handles of pliers 3 with regard to a risk of incurring pressure marks, but also of pliers 1 with a rough and hard surface of the handle material obviously determined the overall assessment too (cp. Fig. 6). The assessment of the handle shape (cp. Fig. 5) again shows the positive rating of pliers 2 and the very negative rating of pliers 3. Additionally, the analysis of the assessment shows an averaged neither positive nor negative rating of the two prototypes (pliers 7 and 8). At this time it seems that the design of the handle shape has not reached an optimum.

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Figure 4:

Subjective assessment of the handle material (means and standard deviations of 11 Ss)

Figure 5:

Subjective assessment of the handle shape (means and standard deviations of 11 Ss)

Even though the overall impression of the pliers (cp. Fig. 6) could not be the arithmetical conclusion of the preceding questions, a variety of the subjects’ prior assessments were confirmed: In the case of pliers 1 and 3 this means a negative evaluation following a consistently poor assessment of many details of the pliers. In the case of pliers 2 and 4 the final assessment of “quite favorable” was supported by the positive rating of many details. Also pliers 6 induced an overall positive impression even though they were rated rather inconspicuously during the tests. In addition to the design aspects, the test persons had to make comments with respect to possible physical complaints in different body regions. It is remarkable that the physical complaints in the back, lumbar vertebra section, neck, in the right front and rear shoulder region, and in the right upper arm, forearm and wrist were rather limited. With respect to the fingers and the palm of the hand, however, clear differences in complaints could be determined subjectively.

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Figure 6:

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Subjective overall assessment of the diagonal cutters (means and standard deviations of 11 Ss)

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As shown in Fig. 7, this was especially the case for the palm and fingers when using pliers 3 and exclusively for the fingers when using pliers 1. The lowest subjectively felt finger strain was caused by pliers 4. The smallest complaints in the palm were caused by pliers 2.

Figure 7:

Subjective assessment of complaints after cutting tests in the palm of the hand and the fingers (intensity of complaints; means and standard deviations of 11 Ss)

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The detailed analyses showed decisive differences in the assessment of the user-friendliness. For example, pliers 3 – without plastic handles – were clearly rated more poorly in the subjective as well as in the objective assessment than pliers 2, which were equipped with 2-component handles. The harder plastic component of pliers 2 guarantees the transmission of the necessary cutting forces needed in the cutting process whereas the softer, high-friction outer surface causes an even better adaptation of the handles to the user’s hand and thus helps to reduce physical complaints, e.g., pressure marks (cp. Fig. 8).

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Figure 8:

Subjective assessment of complaints in the hand-forearm system after the cutting of medium-hard wire with pliers 2 and 3 (intensity of complaints; means and standard deviations of 11 Ss)

4. Discussion The objective and subjective assessment of different aspects according to handle design, material, surface, and dimension of the diagonal cutter handles were just as substantial as the subjective ranking of the importance of these characteristics. The data helped to identify characteristics which were decisive for the test persons’ evaluation of the cutting pliers and characteristics which were less important – also in view of an imagined purchase decision. As shown in Fig. 9, the handle design as the formal element was given highest priority by the test subjects. A closer analysis of this feature leads to the conclusion that the suitability of the form for the cutting forces is the most important feature of diagonal cutters. Directly related to this aspect and of equal importance is the feature of the shape of the arched handle contour adapted to the human hand. This last feature is the most significant among those features whose characteristics can be directly influenced. Another subjectively dominant feature of diagonal cutter handles is the gripping diameter which is also influenced by the form of the arch. On the whole, it can be seen that the test persons considered the overall effect of the handle design on the work process with the diagonal cutters as the most relevant criterion or rather the most important deciding factor. The handle surface was considered less important than the handle design, but this feature still remains significant because hand perspiration might cause slippery handles, which again would make it a dominant characteristic of cutting pliers handles.

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Figure 9:

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Rank category of the design elements from a subjective point of view

Of minor importance were the width, the length, and the diameter of the different handles. Neither the material of the handles – polyurethane or PVC of varying degrees of hardness – nor aesthetic factors like the appearance of the cutting edges or the slip-guard as well as the color of the handles were of major importance for the evaluation of this kind of pliers. This means that diagonal cutters were primarily evaluated according to their working quality and not according to the personal taste of the subjects.

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5. Conclusions The most relevant design elements for the different diagonal cutter handles were the formal elements and the analysis of the objective and subjective assessments led to several design recommendations. A rounded trapeziform cross-section of the handles is an ergonomically favorable shape for good coupling requirements and low frictional influences during relative movements between the cutting pliers and the finger phalanges. The longitudinal contour of the handles (arched handle contour) should be in accordance with the position of the middle finger phalanges and the middle finger joints in the palm of the hand, respectively. The cross contour should be in accordance with the length of the middle finger phalanges with an increasing width due to the middle finger dimension and a decreasing width due to the little finger dimension. A reduction of the demanded cutting forces at the diagonal cutter handles is possible by means of a long lever arm. The appropriate coupling with the greatest possible lever arm needs a depression on the handle surface in the coupling area of the little finger and an optional depression for the forefinger. A short length of the cutting edges reduces the negative effect of a change of the lever arm caused by a potential incorrect grasping of the cutting material at the cutting edge point. The anthropometrical properties of the hand should be the requirements for the dimensions of all design elements, such as gripping diameter, course of the arched handle contour, cross-section and width of the handles, handle length and the slip-guard. An optimum coupling condition can be realized through handles in three sizes and the frictional behavior of the handles could be optimized by 2-component handles with different degree of hardness (several materials). A solid, durable core is responsible for a better force transmission on the metal grips, as well as the sliding of the fingers during closing of the pliers and direct force transmission from the hand to the handle by a lower frictional resistance of the harder material. A soft, high-friction outer skin supports a necessary degree of friction and a sufficiently easy sliding during lower cutting forces and nestling of the handles in the hand during higher cutting forces. An aid for the tactile identification of the cutting edge side is the coding of the handle surface – possibly fitted into the logo of the manufacturer or the cutting pliers’ model or at a significant place as a design element. A high quality of the cutting edges helps to reduce the cutting forces.

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6. References KLUTH, K.; KELLERMANN, H. and STRASSER, H. (1997) Electromyographic and subjective methods for the assessment of the ergonomic quality of file handles. In: SEPPÄLÄ, P.; LUOPAJÄRVI, T.; NYGARD, C.-H. and MATTILA, M. (Eds.) From Experience to Innovation Vol. 4. Proceedings of the 13th Congress of the International Ergonomics Association in Tampere. Finnish Institute of Occupational Health, Helsinki/Finland, pp. 515517 STRASSER, H. (Ed.) (1996) Beanspruchungsgerechte Planung und Gestaltung manueller Tätigkeiten – Elektromyographie im Dienst der menschengerechten Arbeitsgestaltung. Ecomed-Verlag, Landsberg/Lech STRASSER, H.; KLUTH, K. and KELLER, E. (1998) A knowledge-based system for utilizing advanced electromyographic and subjective methods for the evaluation of the ergonomic quality of hand-held tools and computer input devices. In: SCOTT, P.A.; BRIDGER, R.S. and CHARTERIS, J. (Eds.) Global Ergonomics. Proceedings of the Cape Town Ergonomics Conference, Elsevier Science, Amsterdam, pp. 557-560 Standards, guidelines, regulations DIN ISO 5742 (2006-09) Pliers and nippers - Nomenclature. German Institute for Standardization, Beuth Verlag, Berlin DIN ISO 5744 (2006-09) Pliers and nippers - Methods of test. German Institute for Standardization, Beuth Verlag, Berlin

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DIN ISO 5749 (2006-09) Pliers and nippers – Diagonal cutting nippers. Dimensions and test values. German Institute for Standardization, Beuth Verlag, Berlin

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Chapter 12

Ergonomic Snap-On-Handles for a Hand-Powered Hacksaw B. Das 0. Summary

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Hand-powered hand tools in the past have mainly focused on designing hand tool handles based on ergonomics principles and anthropometric data. No consideration was given to accommodate the hand size of the individual user or the entire male and female populations. The present investigation was undertaken to redesign ergonomic handles for a hand-powered hacksaw to accommodate the entire male and female populations by considering their hand sizes. Based on the ergonomics evaluation of existing hand-powered hacksaws with original/horizontal and conventional/market handles, ergonomically designed hacksaw handles are proposed. To accommodate the entire male and female populations, the hand dimensions are categorized into three groups: small, medium and large. The proposed handles give special emphasis to hand size, length, cross-section dimension and curvature. The three-sized handles for both the preferred (rear) and non-preferred (front) hands are interchangeable to suit the individual hand size. Thus, the concept of “snap-on-handles” with a fixed hacksaw (blade) can be promoted. The ergonomically designed hacksaw handles were tested/compared with original/horizontal and conventional/market hacksaw handles, in terms of performance or productivity (depth of cut), muscular effort or strain (EMG) and subjective scores (acceptance/comfort). The experimental results conclusively proved that the ergonomically designed hacksaw handles were significantly better than the other handles in terms of the stated criteria. The performance or productivity improvements of the ergonomically designed handles were about 25 and 148 %, when compared with the conventional/market and original/horizontal handles, respectively. Furthermore, when the ergonomically designed handle was not matched with the proper or appropriate hand size, there was a significant reduction in performance or productivity, increase in muscular effort and decrease in subjective scores of acceptance/comfort.

1. Introduction A major concern in the past has been the proper selection, evaluation and use of hand tools from an ergonomics viewpoint. The hand tool must be able to perform the intended function effectively. To maximize operator efficiency, it must be compatible or proportionate to the body dimensions and in particular, the hand size of the operator. In the hand tool design due consideration must be given to match the strength and work capacity of the operator. Stated otherwise, it should avoid undue fatigue and would not require unusual postures. The main purpose of hand tools is to extend and reinforce range, strength and effectiveness of hand movements. The use of the proper hand tool will help to: (1) expand reach capabilities; (2) increase force output capabilities; and (3) perform motions with a high degree of precision and efficiency. Additionally, a good tool, when properly used, can enhance both operator’s safety and well being (cp. TICHAUER and GAGE 1977). In the design of hand tools, all aspects of the tool characteristics should be considered to prevent cumulative trauma disorders (CTDs) of the hand and forearm, as well as to improve work efficiency or productivity. The most significant factor associated with CTDs was found to be the amount of muscle effort or force exerted by the muscles in the forearm and hand. Basically a hand tool is designed to minimize the muscle forces in the forearm and hand during its use (cp. AGHAZADEH and MITAL 1987).

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Based on the data collected between 1979 and 1985, AGHAZADEH and MITAL (1987) found that hand tool injuries in the US comprised about 9 % of all work related injuries and 9 % of these were disabling injuries. About 80 % of hand tool injuries were caused by hand-powered hand tools. The medical cost alone associated with these injuries was about $400 million per year. These estimates are believed to be low because injury produced by cumulative trauma has been recognized only recently for compensation and thus for reporting purposes (cp. CHAFFIN et al. 1999). To reduce the risk of injury associated with its use, more attention should be given to the design of the hand tool. This can be achieved by incorporating ergonomic principles and data in the design of hand tools and in particular by incorporating anthropometric measurements of hands of the user population. This approach is especially important for the safety and health of the workers engaged in the extensive use of hand tools (cp. KADEFORS et al. 1993). This investigation focused on the design of hand-powered hand tools in general and in particular of handles for a hand-powered hacksaw by incorporating ergonomic principles of hand tool design (cp. DAS et al. 2005). In the past, hand-powered hacksaw handles used to be cylindrical in shape and horizontal or in line with the hacksaw blade (termed as original/horizontal handle for the purpose of this research). At present, the hand-powered hacksaw handle found generally in the market is held in a clamp formed partly by the flexed fingers and the palm (termed as conventional/market handle). The ergonomically designed handles developed in this study for a hand-powered hacksaw gave special emphasis to the integration of knowledge of the hand physiology and the anthropometric aspects of the hand. The main objectives of this investigation were to: (1) state the ergonomic principles for the design of hand-powered hand tools; (2) perform ergonomic evaluation of two existing hand-powered hacksaw handles: original/horizontal and conventional/market; (3) redesign hand-powered hacksaw handles based on ergonomics principles; and (4) test/compare ergonomically designed hand-powered hacksaw handles with the existing hand-powered hacksaw handles, in terms of task performance (depth of cut), muscle activity/strain (through electromyography) and subjective scores (acceptance/comfort).

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2. Ergonomic principles for the design of hand-powered hand tools In the design of hand-powered hand tools due emphasis must be given to the physiology and anthropometric characteristics of the hand. Functional capabilities of the hands are of great importance for work efficiency of humans. Hands, as part of the human body, require a certain level of strength and precision, depending on the type of task being performed. The right combination of strength and precision in handling the task involves a delicate management of the sensory system of the hand. The consideration of anatomical and physiological characteristics of the hand-arm system based on the equation “in conformity with human = in conformity with hand” is an absolutely necessary principle when designing working hand tools (cp. STRASSER 1991; STRASSER and BULLINGER 2007). The knowledge of the hand is of great essence to facilitate the process of designing the hand tool. 2.1. Hand anthropometry In the design of hand tools, the hand anthropometry should include the various parts of the hand for both the male and female populations. Table 1 presents the relevant hand dimensions for the 5th, 50th and 95th percentiles male and female US populations (cp. WOODSON et al. 1992; KONZ 1995).

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Table 1: Relevant anthropometric measurements (cm) of hand for the design of hand tool handles (from WOODSON et al. 1992; KONZ 1995)

2.2. Hand-powered hand tool design considerations In the design of the hand-powered hand tool and in particular the design of the tool handle, consideration must be given to the following factors: length, size, shape, material and angulations. 2.2.1. Handle length Length of handle depends on the type of grip used and hand size of the user population. KONZ (1995) provides guidelines for handle lengths for a power grip, with all four fingers making the contact. A minimum handle length of 10 cm is recommended; however, 12.5 cm would be more comfortable. Use of a 10 cm length is considered the minimum for an external precision grip (where the handle passes over the thumb and thus is “external” to the hand). For an internal precision grip (the handle passes under the thumb and thus is “internal” to the hand), the tool handle must extend beyond the tender palm but not so far as to contact the wrist. SELAN (1994) has suggested that handles should be lengthened, so that they do not end in the palm. The handle should be 12.5 cm long and, if the worker is wearing gloves, an additional length of 1.25 cm should be added.

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2.2.2. Handle size (diameter) Size of the tool handle is of major importance in the design of hand tools. If the handle diameter is too large, so that force must be applied with the tip of fingers, internal tendon forces can be 2 - 3 times larger than the applied forces. For large handles, lever arms of hand muscles work at a disadvantage (cp. KROEMER et al. 1994). As a consequence, a considerable force has to be produced by the muscle, which will lead to muscle fatigue in a short period of time. Overexertion of muscle has been linked to many CTDs (cp. PUTZ-ANDERSON 1988). If the handle size is too small, finger muscles are shortened to the point that they cannot produce enough tension. Thus, much force cannot be exerted and large local tissue pressures might be generated (cp. KROEMER et al. 1994). COCHRAN and RILEY (1986) found the greatest thrust forces in handles of about 4.1 cm equivalent circular diameter (based on their 13.0 cm circumference) for both males and females. Eastman Kodak Company (cp. N.N. 1983), based on company experience, recommends 3.0 - 4.0 cm with an optimum of 4.0 cm for pistol or power grips (tool axis or wrist alignment parallel to forearm) and 0.8 - 1.6 cm with an optimum of 1.2 cm for precision grips. 2.2.3. Handle shape The shape of the handle cross-section should vary over the length of the handle. A change in crosssection would (1) reduce the movement of the tool forward and backward, (2) permit greater force to be exerted along the tool axis due to the better bearing surface and (3) act as a shield if placed at the front (cp. KONZ 1995). A rectangular cross-section also allows tactile orientation of the tool. Another strategy is to improve the coefficient of friction of the handle. If the rotation of the tool is neither good

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nor bad, then a circular cross-section is more forgiving on the hand since there are no sharp edges (cp. KONZ 1995). A flange at the end of the handle prevents the hand from sliding off the handle (cp. KROEMER et al. 1994). For tasks that have a predominance of both orthogonal push and pull activities, rectangular handles with a cross-sectional width-to-height ratio of about 1 to 1.25 appears to be the best compromise. If the task involves much more orthogonal push than pull, then circular handles or circular handles with two flat sides are preferred (cp. COCHRAN and RILEY 1986). SELAN (1994) recommends that the tool handles should be curved such that the concave surface formed by the fingers and the convex surface formed by the heel of the palm and thumb are accommodated. 2.2.4. Handle material The type of material used for the handle is important, since it will determine the surface friction property and subsequently the ability to grasp and manipulate the hand tool. The frictional characteristics of the tool surface vary with the pressure exerted by the hand, the smoothness and porosity of the surface, and the type of contamination. Sweat increases the coefficient of friction, whereas oil reduces it (cp. BUCHOLZ et al. 1988; BOBJER et al. 1993). Compressive grip materials such as rubber, compressible plastic or wood are better for the hand than hard plastic or metal. Such materials tend to dampen vibration and allow better distribution of pressure, reducing the feeling of fatigue and hand tenderness (cp. FELLOWS and FREIVALDS 1991). KONZ (1995) has recommended that an improvement in the moment arm can be accomplished by providing a longer moment arm or by having good bearing surface to minimize slippage. 2.2.5. Handle angulation Angulation of handles may be necessary for tools, to maintain a straight wrist. The handle should reflect the axis of the grasp, which is about 70° to 80° from the horizontal, and should be oriented in order that the eventual tool axis is in line with the index finger (cp. KONZ 1995). Optimum tool angle depends on the posture. The amount of torque and force that can be exerted also depends on the working posture. Repetitive screw driving should not be done on a horizontal surface above the elbow (cp. KONZ 1995).

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3. Ergonomics evaluation of existing hand-powered hacksaw handles For the purpose of this investigation two existing (non-ergonomically designed) hand-powered hacksaws, and in particular the hacksaw handles, were considered: (1) original/horizontal and (2) conventional/market. These are shown in Figs. 1 and 2. The characteristics of the two handles were assessed from an ergonomics viewpoint. 3.1. Original/horizontal hand-powered hacksaw handle The original/horizontal hacksaw handle has one size of cylindrical (horizontal) handle for the preferred hand and one size of rectangular (vertical) handle for the non-preferred hand (as shown in Fig. 1). The shortcomings of the original/horizontal hacksaw handle from an ergonomics viewpoint are: (1) The preferred hand cylindrical handle length of 10.6 cm meets the minimum length requirement of 10.0 cm but does not meet the desirable length of 12.5 cm to avoid the handle ending in the palm for the entire population. (2) The preferred hand handle size (or diameter) of 3.3 cm meets the minimum diameter requirement of 3.0 – 4.0 cm but does not meet the desirable diameter of 4.0 cm, especially for a pistol or power grip. (3) The hacksaw handle causes the wrist to bend from a neutral position, causing a repetitive awkward posture, resulting in decreased strength and possibly increased risk of injury to the operator due to extreme ulnar deviation of the wrist.

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(4) Only one size of handle (both for preferred hand and for non-preferred hand) is provided for the entire population including males and females. (5) The (non-preferred hand) handle with a rectangular cross-section is uncomfortable to hold and inadequate sizewise (with dimensions 1.8 x 0.5 cm).

Figure 1:

Hand-powered hacksaw with original/horizontal handle (all dimensions are in cm)

3.2. Conventional/market hand-powered hacksaw handle

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The conventional/market hacksaw handle shown in Fig. 2 has one size pistol-grip (rectangular) handle for the preferred hand and one size rectangular (vertical) handle for the non-preferred hand.

Figure 2:

Hand-powered hacksaw with conventional/market handle (all dimensions are in cm)

The shortcomings of this handle from an ergonomics viewpoint are: (1) The (preferred hand) pistol grip handle is desirable ergonomically, but the size and shape are not based on ergonomic principles and data. The rectangular cross-section is uniform throughout the handle length. It does not provide a curved surface, such that the concave surface formed by the fingers and the convex surface formed by the heel of the palm and thumb are accommodated.

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(2) Only one size of handle (both for preferred hand and for non-preferred hand) is provided for the entire population. (3) The (non-preferred hand) handle with a rectangular cross-section is uncomfortable to hold and the size of 1.7 x 2.0 cm is inadequate. 4. Redesigned hand-powered hacksaw handles based on ergonomics principles Based on ergonomics principles, redesigned hand-powered hacksaw handles were developed especially to introduce or promote the concept of “ergonomically designed snap-on-handles.” For a fixed-sized hacksaw blade, three sizes (small, medium and large) of handles can be snapped-on (or interchanged) to suit the specific size of the (individual) hand. The necessary groove and bolting arrangement can be provided to snap-on the handle to the (hand-powered) hacksaw. 4.1. Handle size From an ergonomics viewpoint, it is apparent that one size of handle will not accommodate or satisfy the entire population (male and female) in industry. For this purpose, three different sized handles have been determined to accommodate the 5th to 95th percentile of both male and female population. The small size handle will accommodate the 5th to 50th percentile of the female population. The medium size handle will accommodate the 5th to 50th percentile of the male population and the 50th to 95th percentile of the female population. The large size handle will accommodate the 50th to 95th percentile of the male population. The specific design dimensions for handle length, handle crosssection and handle curvature were used to fulfil the design criteria. The manner in which the specific design dimensions were determined is stated below. 4.1.1. Handle length

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The handle length is defined as the length of the contact surface between hand grip and handle. Hand breadth at metacarpal is used to arrive at this dimension. For the rear (preferred hand) handle of the hacksaw, a flange under the little finger is used to guard the hand from slipping down the handle. A thickness of 1 cm along the handle is assigned. For the handle head a thickness of 2 cm is allowed (Fig. 3). Additionally, clearances of 0.5 cm are given at the top and bottom of the handle length to arrive at the total length (TL, Fig. 3). For better visualization pictures of the three snap-on-handles are provided in Fig. 4.

Figure 3:

Hand-powered hacksaw with ergonomically designed handle. The dimensions for small, medium and large hacksaw handles are provided in Table 2 (TL = Handle length, (A & B) Rear handle cross-section at C and (D) Front handle cross-section) (all dimensions are in cm)

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Figure 4:

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Ergonomically designed snap-on-handles for a hand-powered hacksaw (front (non-preferred hand) handle on left and rear (preferred hand) handle on right): n small size handles, o medium size handles, and p large size handles. For dimensions refer to Table 2 and Fig. 3

Table 2 shows the total length (TL) of the handle for the three different sizes of handle. In arriving at these dimensions, the upper limits of the percentile value stated under three sizes of handles is used. For example, 95th percentile value of the male population determines the large size handle length. Table 2: Ergonomically designed hacksaw handle dimensions (cm)

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4.1.2. Handle cross-section The hand, when it is in a “hand shake” position, has its unique curvature such that it minimizes the strain imposed in the underlying tissues. For this reason, it is desirable to duplicate this curvature, especially in the palmar region, in the design of a hand tool. This will require a change in the crosssection along the handle (cp. KROEMER et al. 1994; KONZ 1995). Thus, the cross-section specified here is the maximum cross-section dimension, which is located approximately at the middle of this handle length. The grip breadth-inside diameter for the circular cross-section is used as the basis of this dimension (Table 1). Accordingly, this handle diameter should be approximately the inside diameter minus 1.0 cm (cp. GRANT et al. 1992). In arriving at these dimensions, the lower limits of the percentile value under three sizes of handle is used. This is necessary to assure that the handles allow the smallest hand grip of each group (small, medium and large) to wrap around them. As a consequence, the 5th percentile of the female population can be accommodated. The shape of the cross-section is not a circle, but more of a rectangle with round edges. The recommended width to height ratio of 1 to 1.25 is used (cp. COCHRAN and RILEY 1986) to provide better orthogonal push and pull which is involved in the use of a hacksaw. The grip breadth-inside diameter will be used, in this case, as the length, to ensure that the 5th percentile of the female population is included. The dimensions shown in Table 2 are in agreement with the suggested diameter of between 2.5 and 5.0 cm. 4.1.3. Handle curvature The surfaces which need to be grasped but which are not handles should have edges rounded to at least 0.3 cm, with a desired radius of 0.6 to 1.0 cm. The curvature of the handle area in contact with the fingers (concave surfaces) occupies approximately a 60° arc of a circle. For the curvature of the handle area in contact with the base of the palm (convex surface), a 60° arc of a circle has been used, for the top portion, which is the base of the thumb and also for the bottom portion, which is the heel of the

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palm (length of 1/2 metacarpal breadth). SELAN (1994) recommends a bend angle of the handle of about 100 - 110° with the axis of the forearm, and for this design 105° has been used. To accommodate the different positions of grasping of the front-handle, a uniform cross-section along the handle is used. The cross-section is a circle with a diameter approximately the grip breadthinside diameter minus 1.0 cm as in the guidelines. The dimensions are shown in Table 2. 5. Testing of the (ergonomically) redesigned hand-powered hacksaw handles An experimental investigation was conducted to evaluate the three types of hand-powered hacksaws and in particular their hacksaw handles: (1) original/horizontal; (2) conventional/market; and (3) ergonomically redesigned which consisted of three sizes: improper size 1, improper size 2 and proper size, in terms of performance or productivity, muscle effort or strain, and subjective score (acceptance/comfort). Consequently, five types or sizes of hand-powered hacksaws were tested for this investigation. Each participant performed a sawing task employing five types or sizes of hand-powered hacksaw handles. 5.1. Participants Eight male participants (students of Dalhousie University) with no prior experience in sawing steel, volunteered in this study and they were paid $10.00 / h to participate in the experiment. Their ages ranged between 19 and 30 years. All the participants were right-handed and they had no previous experience of hand or arm injuries. For each participant, the relevant information and anthropometric measurements were recorded and are given in Table 3. Based on the grip breadth-inside diameter, two participants had hand size to fit the small handle size, four to fit the medium size handle and two to fit the large handle size.

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Table 3: Relevant anthropometric characteristics of the participants (males)

5.2. Tools Two hacksaws were actually used in this investigation to conduct the testing of five types or sizes of hand-powered hacksaws. The original/horizontal handle hacksaw (Fig. 1) was custom made. The back and front of this handle could be easily converted to accommodate small, medium and large ergonomic rear and front handles (as shown in Fig. 3). For the three ergonomically designed handles, necessary grooves with tape (rear) and slots with nuts and bolts (front) were provided to attach the handles to the hacksaw frame (as shown in Fig. 4). The other hacksaw was the typical conventional/market hacksaw (Fig. 2). No change was required for this hacksaw.

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5.3. Apparatus and measurements The sawing task was simulated for electromyography (EMG) measurement by employing the FlexCom system. The EMG signals were detected by self-adhesive gel electrodes with a 25 mm diameter surface area. The FlexCom system was used to collect, monitor and save the EMG data. The system provided small amplifiers that were attached to the surface electrodes. The amplified signals were sent to an encoder box and then transmitted through the fiber-optic cable to a personal computer (PC). For the purpose of the normalization of the raw EMG data, maximum voluntary contractions (MVC) for both the preferred and non-preferred hand grips were obtained using a Lafayette/TEC hand (grip strength) dynamometer. The unit features an adjustable stirrup to fit the hand. Double pointer has been employed so that the participant does not have to continue his/her effort until the scale is read. Once the grip is pressed one hand remains in position until manually reset. The unit is calibrated from 0 to 100 kg (220 lb) force. For obtaining MVC of ulnar deviation of the preferred hand and MVC of elbow flexion for the purpose stated above, a BTE (Baltimore Therapeutic Equipment) work simulator was used. The BTE work simulator measures or records the force applied during an isometric contraction in which there is no movement of the joint being tested. Recording this force allows for the evaluator to document peak or maximum (isometric) strength and consistency of effort. 5.4. Experimental procedure

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5.4.1. Determination of appropriate handle size for the (individual) participant The relevant anthropometric data were recorded and used to fit hand size to specific handle size during the conduct of experimental study. The grip breadth-inside diameter was used for determining the appropriate handle size for the participant: small 3.80 - 4.29 cm, medium 4.30 - 4.89 cm and large 4.90 - 5.90 cm. Three handle types were investigated: (1) original/horizontal; (2) conventional/market; and (3) ergonomically designed. For the ergonomically designed handle, three handle sizes were investigated: (1) improper size 1; (2) improper size 2; and (3) proper size. The improper sizes 1 and 2 were determined in the following manner. If the proper size handle selected for the hand was small, then the improper size 1 would be the large size handle (largest difference in hand size) and the improper size 2 would be the medium size handle. If the proper size handle selected was medium, then the improper size 1 was either the small or large size handle, depending on the difference between the participant’s hand breadth-inside diameter with the upper-end range value for the small handle size (4.29 cm) or the lower end range value of large handle size (4.89 cm). Whichever difference was larger was designated as the improper size 1 and the other handle (with smaller difference) was designated the improper size 2. 5.4.2. Collection of EMG data Using the FlexCom system, the raw EMG signals were collected for each selected muscle during the 15 min sawing task in each trial. The raw EMG data were subsequently normalized as stated above and integrated over the 15 min period. Four EMG sensors were used on the preferred (rear) hand and another one was placed on the non-preferred (front) hand. For the preferred hand the muscles of interest were: (1) flexor digitorum as the muscle for closing the hand; (2) extensor carpi as the muscle involved in ulnar deviation of the hand; (3) flexor carpi ulnaris as the muscle for palmar flexion; and (4) triceps as the muscle extensor of forearm. For the non-preferred hand, the pertinent muscle was flexor digitorum as the muscle for closing the hand. In the sawing operation, the selected muscles are extensively used. Bipolar surface electrodes were used to measure electromyographic potentials and the locations of the electrodes were selected in accordance with the recommendations provided by SODERBERG (1992). The surface electrodes were attached over the appropriate muscles. The EMG data of maximum

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voluntary contractions (MVCs) were recorded for the normalization of the raw EMG data, before the start of the experimental task. The following strength data were collected: (1) MVC of both the preferred and non-preferred hand grips using the grip strength dynamometer; (2) MVC of ulnar deviation of the preferred hand using the BTE work simulator; and (3) MVC of elbow flexion using the BTE work simulator.

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5.4.3. Sawing task The workpiece was a mild steel bar with dimensions of 2.6 x 7 x 2.4 cm. Three steel bars (2.4 cm thickness each) were taped together before the start of the sawing task. Some participants did cut more than one piece. The steel bars were held by a vise grip mounted on an adjustable table. The table was adjusted, so that the height of the workpiece was at the participant’s elbow height. The work table height must be compatible with the worker height, whether sitting or standing. KONZ (1967) found that the best working height is about 2.5 cm below the elbow. However, he found that working height can vary several centimeters up or down without any significant effect on performance. The height of the workpiece with respect to the surface of the table was 14 cm. To pace the speed of the sawing task, a metronome was activated at 90 cycles per min. This meant that in 1 min the participant would perform 90 cutting cycles (motions forward and backward), one cycle consisting of the movement of one push and pull of the sawing operation. At this speed, a moderate effort was required for the participant to perform the task. However, this speed generated a significant EMG signal in the performance of the task. The metronome produced both sound and light signals for pacing. The saw blade was marked in front and rear before the start of the sawing task to assure that the travel distance of the blade for each stroke was approximately equal. The work piece was marked at mid-length (approximately) to identify the location of the sawing cut. To control the posture, the participant was asked to move the left foot forward to about the participant’s shoulderwidth distance. At the same time, the participant was asked to hold the front handle of the hacksaw with the non-preferred hand to assume the sawing posture. The execution of the experimental task was controlled by standardizing the method of operation, so that main effect of the experiment could be ascertained properly. Work method standardization is often practiced in industry. This was monitored during the performance of the task by the experimenter. The participant was allowed a trial of 10 cycles before the start of the experimental sawing task. The EMG signal was recorded during the sawing task. Each participant performed the same sawing task with each of the five different handles. The sequence of experiments with the different handles was administered randomly. The duration of each of the five sawing tasks was 15 min with a 15 min rest period in between. 5.4.4. Collection of subjective scores The participants assessed the various hacksaw handle design characteristics and the effort required and extent of tiredness involved in performing the sawing task through a five-point rating scale questionnaire. The questionnaire was administered at the end of the 15 min experimental sawing task for a particular handle type/size. During each resting interval (between the experimental sawing tasks), the participant was asked to answer the subjective questionnaire for the hacksaw handle just used. 5.5. Statistical analysis The nine questions on the five-point rating scale questionnaire dealt with hacksaw handle grip diameter (front and rear), handle length (front and rear), handle shape (front and rear), rear handle position (vertical or horizontal with respect to hacksaw blade), effort required in task performance and the extent of tiredness in task performance. The first seven questions were basically concerned with the acceptability of various hacksaw handle design features and were rated on a scale of “1 = unacceptable, 2 = poor, 3 = fair, 4 = good, 5 = excellent.” Effort was rated on a scale of “1 = unacceptable, 2 = poor, 3 = fair, 4 = good, 5 = completely acceptable” and extent of tiredness was rated on a scale of “1 = extreme, 2 = large, 3 = fair, 4 = little, 5 = none.”

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The statistical methods analysis of variance (ANOVA) and Student-Newman-Kuel’s range test were used in this investigation. The statistical analysis was performed to determine the effects of (1) handle type and (2) handle size on the EMG activity and subjective score. Experimental results are presented in terms of three dependent variables: (1) performance/productivity; (2) EMG/muscle efficiency parameter; and (3) subjective scores. (1) Performance/productivity analysis (i) In this study, the performance or productivity was determined in terms of depth of cut on the workpiece achieved by the participant in 15-min work time.

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(2) EMG activity analysis (i) The first step in the EMG data analysis was to compute the maximum voluntary contraction (MVC) signals for each muscle group being tested for each participant. The EMG data during steady state for 3 s were extracted and integrated. Two trials were performed and, for the normalization purposes, the average EMG value during MVC of the two trials was used. (ii) The EMG data collected for 15 min for each trial were normalized by their corresponding EMG values during MVC. The normalized EMG values were then integrated over the 15-min period. Statistical analysis using the Statistical Analysis System (SAS) software package was used for calculating the integrated EMG values. (iii) The muscle activity determined by EMG is a function of the muscular effort used in performing the sawing task. The same muscular effort will produce different performance or depth of cuts in the sawing (steel) task due to the use of different or more efficient (ergonomically designed) hacksaw handles. Consequently, it is necessary to relate muscular effort, as determined by EMG, to performance or depth of cut (mm). To develop an EMG muscle efficiency parameter, the input and output from the system need to be considered. In operating the hand tool, the input would be the neuroelectric signal generated by the muscle, which is reflected by the EMG signal collected. The output of the system, in the context of this research, would be the depth of cut (mm) on the workpiece. The ratio of these two values represents the EMG muscle efficiency parameter. (3) Subjective scores analysis (i) Since the relative importance of the nine questions on the five-point rating scale questionnaire is not known, it was decided to give equal weight to all the nine questions and a combined or composite subjective score was obtained for each participant. 6. Results 6.1. Performance/productivity A comparison of performance/productivity between hacksaw handles, based on depth of cut, is presented in Table 4. The ergonomically designed hacksaw handle (proper size) was superior to the original/horizontal and conventional/market handles by about 148 and 25% respectively (Table 4). A randomized complete block experimental design was used to determine the effect of the handle type. The type of handle had three levels: original/horizontal handle, conventional/market handle, and ergonomically designed handle. Participants were used as the blocking variable to eliminate the variability among the participants. Here, the participant effect was also a random effect. A randomized complete block experimental design was used for the experimental design to determine the effect of the handle size. The hand size had three levels: proper size, improper size 1 and

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improper size 2. Analysis of variance (ANOVA) and the Student-Newman-Kuel’s (SNK) range tests were used to analyse the data. Only SNK range test results are presented here, in Table 5. The ergonomically designed handle (proper size) was significantly better than the original/horizontal and conventional/market handles. The ergonomically designed handle (proper size) was also significantly superior to the improper handle sizes 1 and 2. Table 4: Comparison of performance/productivity, based on depth of cut (mm), between hacksaw handles

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Table 5: Statistical analysis of performance/productivity based on depth of cut (mm), between hacksaw handles and sizes: Student-Newman-Kuel’s range test results

6.2. EMG/muscle efficiency parameter The normalized integrated EMG values (in % MVC) of selected muscle groups obtained during the sawing task are presented in Table 6. The statistical analysis of the EMG muscle efficiency parameter between hacksaw handle types (differences in mean values) is presented in Table 7. The EMG muscle efficiency parameters of the selected muscles of the ergonomically designed handle (proper size) were higher than the conventional/market and original/horizontal handles and the differences were significant (p < 0.05) or highly significant (p < 0.01). No significant difference was found between the original/horizontal and conventional/market handles in terms of EMG muscle efficiency parameter. The ergonomically designed handle (proper size) was also significantly better than the improper size 1 and improper size 2 handles, in terms of EMG muscle efficiency parameter (Table 8).

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Table 6: Normalized integrated (over 15 min) mean EMG values (% MVC) for hacksaw handle types/sizes

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Table 7: Statistical analysis of EMG muscle efficiency parameter between hacksaw handle types (differences in mean values): Student-Newman-Kuel’s range test results

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Table 8: Statistical analysis of EMG muscle efficiency parameter between hacksaw handle sizes (differences in mean values): Student-Newman-Kuel’s range test results

6.3. Subjective scores The subjective scores obtained at the end of the experimental sawing task for the different types/sizes of handles are presented in Table 9. The statistical analysis of the composite subjective scores between hacksaw handle types and sizes are shown in Table 10.

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Table 9: Subjective scores on a 5-point rating scale questionnaire for hacksaw handle types/sizes

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Table 10: Statistical analysis of total combined subjective scores between hacksaw handle types and sizes: StudentNewman-Kuel’s range test results

The composite subjective score of the ergonomically designed handle (proper size) was higher than those of the original/horizontal and conventional/market handles and the differences were significant or highly significant. A highly significant difference was also found between the conventional/market and original/horizontal (worst) handles. The ergonomically designed hacksaw (proper size) and improper 2 size handles proved to be significantly superior to the improper 1 size handle, in terms of composite subjective score. No significant difference was found between the proper size and improper size 2 handles. In terms of anthropometric dimensions, the differences between the proper size and improper size 2 handles were less than the differences between the proper size and improper size 1 handles. 7. Discussion

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7.1. Significance of the research Past research dealing with hand-powered hand tools has mainly focused on designing hand tool handles, based on ergonomics principles and anthropometric data. This investigation has made it possible to accommodate the entire (5th to 95th percentile) male and female populations through the use of three ergonomically designed handles for a hand-powered hacksaw. To accomplish this, the hand dimensions were categorized by overlapping percentile values of male and female populations into three groups: small, medium and large. However, it would be possible to categorize hand dimensions into six groups based on 5th, 50th and 95th percentile of male and female populations (without overlapping the percentile values of male and female populations). This would permit more accurate fit between hand size and handle size but would obviously increase the cost of production and be more difficult to administer in an industrial situation. The three sizes of handles for both the preferred and non-preferred hands could be interchanged to suit the specific size of the hand. This would promote the new concept of “snap-on-handles” with a fixed-size hacksaw blade. 7.2. Ergonomic design of a hand-powered hacksaw For designing handles of a hand-powered hacksaw, ergonomic principles dealing with hand physiology, anthropometry, grip, length, size (diameter), shape, material and angulations were given due consideration. The shortcomings of the currently available hand-powered hacksaw handles, as seen in the original/horizontal and conventional/market hacksaws in Figs. 1 and 2 were identified from an ergonomics viewpoint. Based on ergonomic guidelines for designing hand-powered hand tools, an ergonomically designed hacksaw handle was proposed and developed in three sizes. The ergonomically designed hacksaw handles (small, medium and large) gave special emphasis to appropriate handle size, length, cross-section and curvature (Figs. 3 and 4). Handle length was based

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on hand breadth at metacarpal. The grip breadth-inside diameter was used to determine the crosssection dimension. A 60° segment of a circle was used to obtain the curvature of the handle, for both the concave and convex surfaces. A rectangular handle cross-section with a width to height ratio of 1 to 1.25 appeared to be the best compromise for orthogonal push and pull activities involved in the sawing task. The three ergonomically designed handles gave due consideration to the anthropometry of the hand, in terms of the grip breadth-inside diameter and the hand breadth at the metacarpal joints. For an ergonomically designed handle, these dimensions were tailored or matched to the user’s hand (proper size) and thus provided maximum cutting force output or productivity and least muscular strain, as well as providing concave-convex surfaces for the handle shape and curvature. 7.3. Experimental results

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7.3.1. Performance/productivity The experimental results based on the 15-min sawing steel task revealed that the ergonomically designed hand-powered hacksaw handle was significantly superior to the original/horizontal and conventional hacksaw handles in terms of task performance or productivity (Table 5). The performance or productivity (depth of cut) improved through the use of the ergonomically designed hacksaw handle by about 25 and 148 %, when compared with the conventional/market and original/horizontal handles respectively (Table 4). The performance improvement of the conventional/market handle compared to the original/horizontal handle was about 98 % (Table 4). Both the conventional/market and ergonomically designed handles were superior to the original/horizontal handle because of the forward orientation of the hand grip, which kept the wrist in a neutral position. The original/horizontal handle has proved to be the worst since it demands extreme wrist bending and finger flexion. When the ergonomically designed handle was matched with the appropriate hand size (small, medium and large), the proper handle was significantly superior to the improper size 1 and 2 handles, in terms of performance/productivity. Also, the improper size 1 handle (with the larger discrepancy between the hand and handle sizes) was significantly inferior to improper size 2 handle (Table 5). This indicated that the greater the discrepancy between handle size and hand size, the lower would be the performance/productivity, in spite of the fact that the handles are ergonomically designed. Consequently, the (selected) ergonomically designed handle must be matched with the individual hand size. The experimental results confirmed that an ergonomically designed single size handle is not adequate for all hand sizes, male and female, in terms of performance/productivity and confirmed the fallacy of design based on average or one size for a population. 7.3.2. EMG muscle efficiency parameter In the evaluation of muscular effort or strain through EMG, a method was developed to study sawing (steel) task performance (measure) through depth of cut (in mm) when using different hacksaw handles. It is recognized that the same muscular effort with different hacksaw handles will produce different depths of cut. Therefore, an EMG muscle efficiency parameter was defined by calculating a ratio of depth of cut (mm) by the normalized integrated EMG signal (Table 6) for each muscle group. Significant or highly significant improvement in EMG muscle efficiency parameter was found for the selected muscles when the ergonomically designed proper size handle was compared with the original/horizontal and conventional/market handles (Table 7). Similarly when the ergonomically designed proper size handle was compared with the ergonomically designed improper sizes 1 and 2 handles, significant or highly significant improvements in EMG muscle efficiency parameter were found.

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7.3.3. Subjective scores For the purpose of this investigation, subjective scores from nine questions on a five-point rating scale questionnaire were combined to obtain a composite subjective score for each participant for subsequent statistical analysis, covering the perception or acceptability of various hacksaw handle design features and the extent of effort and tiredness involved in performing the sawing steel task. For the different types/sizes of handles the subjective scores were obtained at the end of the sawing task (Table 9). The ergonomically designed handles/sizes recorded higher (more positive) subjective scores, compared to the original/horizontal and conventional/market handles (Table 10). The subjective score for the conventional/market handle was significantly higher than that for the original/horizontal handle. The ergonomically designed proper size handle obtained a significantly higher subjective score than the ergonomically designed improper size 1 handle. However, statistically there was no difference between the subjective scores for the proper size and improper size 2 handles, which was probably due to the fact there was little difference between the two ergonomically designed handles in relation to hand size. The ergonomically designed improper size 1 handle obtained a significantly lower subjective score that the ergonomically designed improper size 2 handle. 7.4. Limitations of the study

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The limitations in the conduct of this investigation should be recognized. Only eight right-handed male participants were used in the experimental research. The reason for selecting male participants and sample size were to obtain a homogenous group of participants for subsequent statistical analysis, time involvement in conducting the experiment and cost. Since the sawing steel task involves human strength and female strength is on the average twothirds that of male (ASTRAND and RODAHL 1986), the group would not have been homogenous, if male and female participants had been combined in a statistical analysis. However, in this investigation, the hand sizes of the male participants adequately represented the target design population range (including the female population). On the basis of grip breadth-inside diameter, the numbers of participants’ hand sizes fitting the specific (ergonomically designed) handles were: two for the small size handle, four for the middle size handle and two for the large size handle. The main purpose of the experimental investigation was to evaluate the effectiveness (production efficiency) of the ergonomically designed handles. It is believed that the results obtained from this investigation employing right-handed male participants will also be true for the female population. 8. Conclusions In summary, the conclusions reached from this investigation are: 1. The shortcomings of the currently available hand-powered hacksaws: original-horizontal (handle) and conventional/market (handle) are evaluated from an ergonomics viewpoint. 2. Based on ergonomics guidelines for designing hand-powered hand tools, an ergonomically designed hand-powered hacksaw with particular reference to hacksaw handle is proposed. 3. For designing suitable sized hacksaw handles, hand dimensions are categorized into three groups: small, medium and large. The small handle can accommodate the 5th and 50th percentile female population. The medium handle can accommodate the 50th to 95th percentile female population and the 5th to 50th percentile male population. The large handle can accommodate the 50th and 95th male population. 4. The ergonomically designed hacksaw handles give special emphasis to handle size, length, cross-section dimension and curvature. Handle length is based on hand breadth at metacarpal. The hand grip-inside diameter is used to determine cross-section dimension. A 60° of a circle is used to obtain the curvature of the handle, for both concave and convex surfaces. Rectangular handle cross-section with a width to height ratio of 1 to 1.25 appears to be the best compromise for orthogonal push and pull activities involved in sawing.

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5.

The neutral (handshake) wrist position can be maintained by providing the bending angle of hand to 105° with the axis of the arm. 6. The three sizes of handles for both the right and left hands can be interchanged to suit the specific size of the hand. This will promote the concept of “snap-on-handles” with a fixed size hacksaw (blade). 7. The ergonomically designed hand-powered hacksaw handle (proper size) was significantly superior than original-horizontal and conventional/market hacksaw handles, in terms of task performance or productivity, muscle activity/strain and subjective perception. 8. The performance or productivity (depth of cut) improved through the use of the ergonomically designed hacksaw handle (proper size) by about 25 and 148 %, when compared with the conventional and original-horizontal handles, respectively. The performance improvement of the conventional handle, compared to the original-horizontal handle was about 98 %. Both the conventional and ergonomically designed (proper size) handles were superior to horizontal handle because of the forward orientation of the hand grip, which kept the wrist in a neutral position. The horizontal handle proved to be the worst, since it demands an extreme wrist bending and finger flexion. 9. The ergonomically designed (proper size) handle takes into consideration the anthropometry of the hand, in terms of the grip breadth-inside diameter and the hand breadth at the metacarpal. In the proper size handle, these dimensions are tailored or matched to the user’s hand and thus providing maximum cutting force output or productivity and least muscular strain. 10. In the ergonomically designed (proper size) handle due consideration was given to hand physiology and in particular to providing concave-convex surface while designing the handle shape and curvature. This would further reduce muscular strain of the hand and produce a comfortable handle for gripping, which would result in increased productivity.

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9. Concluding remarks The experimental results proved that an ergonomically designed single size handle is not adequate for all hand sizes, male and female, in terms of performance/productivity, muscular effort and acceptance/comfort and thus confirmed the fallacy of design based on average or one size for an entire population. This investigation introduced the concept of “snap-on-handles” with a fixed size blade for a hand-powered hacksaw to accommodate the entire male and female populations by considering their (individual) hand sizes. The concept can be utilized advantageously for the design of handles for other hand tools to improve performance/productivity, reduce muscular effort and enhance (operator) acceptance/comfort. 10. References AGHAZADEH, F. and MITAL, A. (1987) Injuries due to handtools. Applied Ergonomics 18, 273-278 ASTRAND, P. and RODAHL, K. (1986) Textbook on Work Physiology. McGraw-Hill, New York BOBJER, O.; JOHANSSON, S.E. and PIGUET, S. (1993) Friction between hand and handle: effect of oil and lard on textured and non-textured surfaces; percept of discomfort. Applied Ergonomics 24, 190-202 BUCHHOLZ, B.; FREDERICK, L.J. and ARMSTRONG, T.J. (1988) An investigation of human palmar skin friction and the effects of materials, pinch force and moisture. Ergonomics 31, 317-325 CHAFFIN, D.B.; ANDERSSON, G.B.J. and MARTIN, B.J. (1999) Occupational Biomechanics. 3rd edition, Wiley, New York COCHRAN, D.J. and RILEY, M.W. (1986) The effects of handle shape and size on exerted forces. Human Factors 8, 253-265 DAS, B.; JONGKOL, P. and NGUI, S. (2005) Snap-on-handles for a non-powered hacksaw: An ergonomics evaluation, redesign and testing. Ergonomics 48, 78-97

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FELLOWS, G.L. and FREIVALDS, A. (1991) Ergonomics evaluation of a foam rubber grip for tool handles. Applied Ergonomics 22, 225-230 GRANT, K.; HABES, D. and STEWARD, L. (1992) An analysis of handle design for reducing manual effort: The influence of grip diameter. International Journal of Industrial Ergonomics 10, 199-206 KADEFORS, R.; ALEXANDER, A.; DAHLMAN, S.; KILBOM, A.; SPERLING, L.; WIKSTROM, L. and OSTER, J. (1993) An approach to ergonomics evaluation of hand tools. Applied Ergonomics 24, 203-211 KONZ, S. (1967) Design of workstations. Journal of Industrial Engineering 18, 413-423 KONZ, S. (1995) Work Design: Industrial Ergonomics. 4th edition, Publishing Horizons, Arizona, USA KROEMER, K.H.; KOEMER, H.B. and KROEMER-ELBERT, K.E. (1994) Ergonomics: How to Design for Ease and Efficiency. Prentice Hall, New Jersey N.N. (1983) Ergonomic Design for People at Work Vol. 1. Lifetime Learning, Eastman Kodak Company, Belmont, CA, 140-159 PUTZ-ANDERSON, V. (1988) Cumulative Trauma Disorders: A Manual for Musculoskeletal Diseases of Upper Limbs. Taylor and Francis, London SELAN, J.L. (Ed.) (1994) The Advanced Ergonomics Manual. Advanced Ergonomics Inc., Dallas SODERBERG, G.L. (1992) Recording Techniques, in Selected Topics in Surface Electromyography for Use in Industrial Settings. Expert Perspectives; Department of Health and Human Services, DHHS (NIOSH) Publication No. 91-10, 23-41 STRASSER, H. (1991) Different grips of screwdrivers evaluated by means of measuring maximum torque, subjective rating and by registering electromyographic data during static and dynamic test work. In: KARWOWSKI, W. and YATES, J.W. (Eds.) Advances in Industrial Ergonomics and Safety III. Taylor and Francis, London, England, 413-420 STRASSER, H. and BULLINGER, H.-J. (2007) In: STRASSER, H. (Ed.) A systematic approach for the analysis and ergonomic design of hand-held tools and control actuators – Visualized by some real-life examples. Assessment of the Ergonomic Quality of Hand-Held Tools and Computer Input Devices. IOS Press, Amsterdam, The Netherlands, Chapter 1, pp. 1-22 TICHAUER, E.R. and GAGE, H. (1977) Ergonomic principles basic to hand tool design. American Industrial Hygiene Association Journal 38 (11) 622-634

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WOODSON, W.E.; TILLMAN, B. and TILLMAN, P. (1992) Human Factors Design Handbook. 2nd edition., McGraw Hill, New York

11. Acknowledgement In the preparation of the book chapter, the contributions made by Pornsiri Jongkol and Su Min Ngui are duly acknowledged.

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Chapter 13

Handle Design of Hedge-Clippers Assessed by Means of Electromyography and Subjective Rating J. Böhlemann, K. Kluth, K. Kotzbauer and H. Strasser 0. Summary Stress and strain during manual tool handling not only depend on factors such as weight to be handled, but are also determined by the design of the man-machine interface. In this study, three different handles of electric hedge-clippers were analyzed. Muscular strain was measured via surface electromyography in laboratory experiments with nine male subjects. The results showed significant differences in physiological cost depending on both work height and the handles’ shape, too, despite the fact that all clippers were compensated in respect to weight and location of centre of gravity. One of the handle designs enabled working under varying conditions (work height and direction) at a reduced level of muscular strain of the right arm. Results from the physiological evaluation were partly supported by the working persons’ own subjective experience. The results of this investigation show that further ergonomic tool and handle design is necessary.

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1. Introduction Electric hedge-clippers can be found in a variety of different shapes. Those used in the do-ityourself area differ more in the design of their handles and control actuators than in their technical data. A survey of the market of electric tools shows that the manufacturers of electric hedge-clippers follow different concepts in design and arrangement of the handles. From an ergonomic point of view it seemed necessary – in accordance with working groups in Scandinavia or the USA, e.g., KILBOM et al. (1991), KADEFORS et al. (1991), MARRAS (1990), and MARRAS and LAVENDER (1991) – not only to analyze relevant functions technically, but also to asses global and muscular strain caused by the use of the different types of tools. This study describes a comparative investigation of three different handles. 2. Methods 2.1. Materials Figure 1 shows three hedge-clippers, which were analyzed in a series of laboratory experiments in which the influence of the handles on human strain was measured. The characteristics of the different handles can be explained as follows. On Models A and M, the handle for the right hand is horizontally directed, whereas Model G – as an innovation – is equipped with forward-sloping pistol butt. Among the grips for the left hand, greater differences can be noticed. Model A has a horizontally placed cylindrical handle; Model G – which gained several awards for good design – offers a forwardinclined handle divided into three parts with security buttons placed inside and three different handling positions; Model M has also a forward-inclined handle, yet it is semicircular, with a one-part security button that allows any handling position.

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Figure 1:

Models of electric hedge-clippers (A, G, and M)

One of the hedge-clippers had a weight of approximately 3.200 g whereas the other two weighted approximately 3.900 g. In order to exclude any influence of the different weights of the hedge-clippers, the less heavy Model A was weight-compensated in alternating trials. Additionally, to avoid influences of the location of the centre of gravity, all clippers were equipped with swords of the same length. 2.2. Apparatus

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Cutting work was simulated at a test structure (see Fig. 2), which was designed and built to allow working with the hedge-clippers under standardized conditions at differing work heights and positions.

Figure 2:

Test equipment for simulating work with hedge-clippers (front, side and top view)

To simulate a hedge, the test structure consisted of a 3 m long frame with 40 wooden sticks to be shortened (7 x 7 mm cross section); the sticks were placed 75 mm apart from one another. All subjects had to work at the same predetermined speed and with the same predetermined swivel movements. Working speed was set at cutting the 40 sticks within 45 s. For each trial, the speed was paced by an arrow that moved along the sticks on the upper side of the structure and which had to be followed with the clippers. To ensure that all subjects did the same swivel movements with a tool manipulated in an unrestricted working area, a sample pattern was placed on the mounting of the sticks. This pattern showed the swivel movements that had to be reproduced during the cutting process. Hand posture and angle of abduction become comparable through the use of this pattern.

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Work heights of 1100 mm and 1400 mm were chosen for horizontal cutting (1100 H, 1400 H), as being representative of the heights of garden hedges. 1100 mm was selected for a vertical cutting task (1100 V). In this context, vertical cutting meant that the working persons had to cut the front instead of the top side of the hedge. This was simulated by turning the test structure into the horizontal plane, as may be seen from the lower right side of Fig. 2. Here, the clippers had to be controlled with a different kind of grip. As the height of the elbow for the 50th percentile of male persons is about 1100 mm (acc. to DIN 33402), the chosen heights seemed to be anthropometrically representative. A height of 1100 mm requires only a small angle of abduction for the left arm. The height of 1400 mm requires a more extensive abduction of both upper arms. The 1100 mm vertical cutting position was chosen to demand a further type of handling of the hedge-clippers (especially on the left hand) but would also allow comparison with the horizontal cutting work. 2.3. Experimental design Nine right-handed male subjects participated in the study (cp. Table 1). Each had to work with all three hedge-clippers in the three different working positions in a randomized order. The resulting nine tasks of the different working-position combinations were tested twice with all subjects.

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Table 1: Characteristics data of the volunteer subjects participating in the investigations

Each session was preceded by some training sequences, so that the subjects became familiarized with the tools and the test structure before the data collection began. In between each trial, the test structure was changed in height and direction of cutting (horizontal or vertical), according to the task condition, and the shortened sticks were extended to the 100 mm cut height. These technically necessary breaks lasted approximately 3 min. The subjects had this time to rest between trials. 2.4. Procedure Figure 3 shows the selection of the examined muscles, as well as the lead positions for the electromyographic recordings. With respect to the left arm, electromyographic activity (EA) of the middle part of the deltoid, the finger extensor, and the ulnar-sided hand flexor were recorded. The deltoid, especially the monitored middle part, is primarily responsible for the abduction of the upper arm, and thus should be active during the lifting of loads. The finger extensor is both responsible for the extension of the fingers, and is involved in the ulnar abduction of the wrist. The ulnar-sided hand flexor, from which myopotentials were recorded from both arms, is involved in palmar flexion, i.e., flexion towards the inner side of the hand. The ulnar-sided hand flexor is also involved in the ulnar abduction of the wrist. Recordings were also made from the radius-sided flexor of the right hand. Heart rate was recorded by means of an electrocardiogram (ECG).

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Figure: 3

Location and lead positions of the recorded peripheral-physiological parameters

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2.5. Data acquisition and evaluation After preparing the skin of the subjects with a conductive cream, bipolar surface electrodes were placed on the line of action of the muscles listed above. To ensure stability of the EMG signal, ground electrodes were placed at the shoulder and the elbow, where no muscle activity could be expected. Twenty minutes were allotted for the skin preparation to ensure that optimal skin-to-electrode conductivity was reached before the data collection began. Using miniaturized preamplifiers in SMD technology (small enough to be worn on the body), electromyographic raw signals were picked up, amplified, filtered, rectified, and integrated through a fixed time-constant, as can be seen in Fig. 4. The EMG data were transformed to EA data, which is a suitable indicator for the physiological cost of local muscle work. The output signal of the EA amplifiers consisted of a voltage proportional to the activity of the monitored muscle. These EA time series and the heart rate were then converted from analogue to digital data at sampling rate of 20 Hz. The data were then synchronized and stored in portable data reorders, which were fixed to a belt worn by the subjects. After the termination of a series of experiments, the contents of the portable registration system were transferred to a personal computer and the EA data were processed through self-developed software programs for the calculation of ergonomically relevant parameters. This process included the splitting up of the EA time series into static and dynamic components according to a method (cp. Fig. 5) which has been presented and published by MÜLLER et al. (1989), among others. Electromyographic data can only be interpreted as an expression of interindividual strain if they are standardized. The maximum EA, which was necessary as basis for the standardization procedure, was recorded separately for each muscle. Maximum voluntary contractions (MVC) were required. The subjects had to respond to special prefixed movements and directions similar to those required by the cutting tasks, both before and after the test sessions. During these maximum static exertions, peak values of the EA signal within a 3 s period of time were collected. The recorded electromyographic values EAmax were the basis for standardization.

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Figure 4:

Scheme of the data acquisition system with portable data recorder and data evaluation by means of a personal computer

Figure 5:

Standardization procedure and calculation of static and dynamic components of a time series of the electromyographic activity (EA)

Each EA data set was normalized using the formula given in Fig. 5, where sEA stands for the standardized electromyographic activity, EA0 for the resting activity, and EAmax represents maximum activity as described above. After the normalization procedure, mean values of the EA, the average peak of all respective swivel movements, and the static component of the EA for each subject and each trial were computed for a 42 s period in the middle of each trial, so that disturbances, possibly having occurred at the beginning and end of the cutting work, could not influence the data evaluation. Heart rate was recorded by means of a special ECG amplifier similar to the EA devices. The ECG amplifier produced a voltage between 0.6 V and 2.0 V proportional to heart rates between 60 and 200 beats per minute. In the data evaluation process, heart rate was computed as mean value over the whole 42 s of data analysis.

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3. Results The following results are means of nine subjects averaged over both trials (see Table 2); the results were not significantly affected by the clippers’ differing weights. A comparison of the differences in the subjects’ anthropometric data with the differences in their interindividual physiological responses did not show any significant correlation between body or elbow height and physiological strain. It could therefore be estimated that the results in systemic reaction and muscle response depend on test parameters such as the type of tool and work orientation, and not on interindividual reaction differences caused, for example, by variations in anthropometry. Table 2: Standardized electromyographic activities EA [%] of selected muscles (means of nine subjects)

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3.1. Heart rate The average heart rate, when working with the different machines under all three test conditions, is presented in Fig. 6. Heart rate does not show any differences that could be associated with specific characteristics of the machines or the test conditions. Heart rate was always at a normal level of about 80 to 85 bpm during the test series. This was to be expected, as the weights of the three hedge-clippers varied little (referring to Model A, which was weight-compensated in alternating trials). The higher working position did, however, cause slightly higher heart rates because of greater lifting distance.

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Figure 6:

233

Mean heart rate when working with hedge-clippers A, G, and M at varying work heights and directions

3.2. Muscular responses

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The normalized EMG activity is shown in Fig. 7 in bar charts assigned to the respective muscles. For the EA values, the grey part of each column represents the static component, whereas the yellow part represents the dynamic component. Each value is given as a percentage of the maximum activity at MVC conditions. Mean values are marked by a solid black line. In all diagrams, the letters A, G, and M stand for the different tool models (see Fig. 1).

Figure 7:

Physiological responses of selected muscles (means of nine subjects) when working with hedge-clippers A, G, and M in dependence on work position

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Results were tested for statistical significance, using the Wilcoxon test for matched pair differences. Univariate test results for the work-orientated formulation of questions in order of muscle, type of tool, and strain parameter are shown in Table 3. Table 4 indicates tool-orientated levels of significance; multivariate analysis for the m. extensor digitorum, univariate for the other four muscles. Table 3: EA significance for work-oriented formulation of questions

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Table 4: EA significance for tool-oriented formulation of questions

The middle part of the right shoulder muscle, the m. deltoideus pars acromialis, responds strongly, thus reflecting the influence of work height and handle design. Muscular strain – in terms of the use of one’s maximum power potential – is quite low when working at elbow height; it is lower than 10 % at the 1100 mm horizontal position when working with any of the clippers. However, an increase in height of only 300 mm causes a significant increase of strain, with values twice as high as at 1100 mm. Furthermore, the hedge-clipper of type A yields the highest EA values for the deltoid muscle under the horizontal test conditions. This is statistically significant at the higher position of 1400 H (cp. Table 4).

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The results for the fingers’ extensor, the m. extensor digitorum, show that the use of the power potential, with values of about 40 % of the maximum activity, is about 50 % higher at the vertical position than at the horizontal one. In spite of the working position’s significant influence, no toolbased difference in the physiological response for the flexor digitorum when working with the different tools can be observed. The left forearm muscle flexor carpi ulnaris shows no marked tool-based differences between the positions 1100 H and 1100 V. At the position of 1400 H, a correlation between the type of tool and muscular strain seems to exist. A decrease in all EA components from Model A to Model G and M seems to prevail, but this is of low significance and applies to only some aspects of comparison (cp. Table 4). Clearly, tool-specific differences in strain can be seen in the results from the other forearm muscle of the right arm. The results from the use of Model G are associated with the significant lowest strain values under all test conditions and in almost all components of EA data sets. The dominance of Model G in strain reduction in the right flexor carpi ulnaris is due to the fact that this model offers a special design for the right hand. The sloping arrangement of the handle places a lower demand on the flexing muscle of the hand; the other two models with comparable designs of horizontally arranged handles for the right hand are both associated with high strain levels. The right forearm muscle flexor carpi radialis shows the same tendencies at position 1400 H as at 1100 H (see Fig. 7). The EA average value of Model A is significantly higher than the values from Models G and M at the 1400 H and 1100 V positions. Working with vertical machine handling results in slightly higher EA levels, although this trend was not statistically significant. 3.3. Subjective rating

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Subjects assessed the various handling characteristics on a bipolar four-step scale both before and after the working sessions (see Fig. 8).

Figure 8:

Subjective rating of the handling qualities of three electric hedge-clippers after the working sessions (mean values of nine subjects)

When analyzing the results of the questionnaire, the physiological results are partly supported by the subjects’ own experience. As can be seen in Fig. 8, Model A was always associated with the worst rating, with values between -1 and -2, whereas Model M was estimated as the best in handling, with values of +1 to +2 under all conditions. Both physiological data assessment and subjective rating show that Model M was significantly better with respect to handling qualities and strain reduction (see Table 5). Model A was rated the poorest with respect to ergonomic qualities in both objective measurements and subjective ratings. Model G, although associated with positive results in the physiological responses, was not rated as well as Model M, a fact which can be explained by some problems in the design of the security buttons of these hedge-clippers (see Fig. 9).

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Table 5: Significance of subjective rating after the working sessions

Figure 9:

Unfavorable arrangement of the safety switches of Model G, which results in increased muscular strain

4. Discussion When considering the EMG results of this study, it becomes evident that there are significant differences in muscular effort resulting from working with the different tools. Higher working positions are associated with a higher demand on the muscles involved in lifting the upper arm and there are clear differences caused by the handle design. For the left hand the horizontally placed handle of Model A is the poorest. The horizontally handle arrangement demands an extreme ulnar abduction in the wrist, which can easily be seen at the work height of 1400 mm. The best handle for the left hand appears to be the circular handle of Model M. This always allows for comfortable strain-lowering grasping and controlling of the machine under all conditions. Nevertheless, the right hand grip has a great influence on the muscular strain of the right hand. For the right arm, the new handle design and arrangement of Model G shows distinct advantages, especially seen in the results of the right flexor carpi ulnaris. Model G offers the possibility of working at a lower level of strain, which could allow working for a longer period of time without fatigue.

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When looking at the absolute values of muscle activity of the extensor digitorum and the right flexor carpi ulnaris, it must be mentioned that there is a strain demand of nearly 50 % in the dynamic components and a demand of more than 20 % in the static ones. Since a demand of more than 15 % of the maximum strain causes muscle fatigue, the quality of the strain-reducing handle design of Model G is most important. An example of a good ergonomic solution, which only displays its advantages if all system parameters are working together, can be seen in the arrangement of the security buttons of Model G, as illustrated in Fig. 9. As can be seen from Fig. 9, the arrangement of the security buttons set deep inside the handle, prevents pressing with the tips of the fingers. At the position of 1100 H, the security button lies opposite the finger joints and not beneath the finger tips. To press the button, the whole hand must be extended in a dorsal direction. This effect becomes visible in the increased static components of the left finger extensor. At the working position of 1400 H, the forearm is turned further into pronation and the handle can be clutched more favorably. The security button can be pressed easily; this is reflected by the lower strain-values. The three-part handle allows for the same conditions for vertical cutting at the position of 1100 V as for the horizontal position 1100 H.

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5. Conclusions In conclusion, ergonomically designed tools and handles can reduce muscular strain. However, this does not mean improvements in detail alone, which often result only in cosmetic corrections, but a systematic approach to all shaping aims. In this context, it is interesting that the design potential for the right hand offers many more possibilities in strain reduction than the handle for the left hand. The myoelectric activities of the right forearm muscle are more than twice as high as those of the corresponding left muscle, and these differences are highly statistically significant. This leads to the conclusion that even the design of the right handle – which is traditionally arranged in a horizontal direction – should be ergonomically optimized. Aside from the problems with the safety switches, the best handle for the right hand is the one represented by Model G. For the left hand, a handle like Model M is clearly the best. An optimized model of electric hedge-clippers could be designed after this study, but this was not the aim. The objective of the study was primarily to show how advantages and disadvantages in certain tool and handle designs can be quantified by surface electromyography (see also STRASSER 1991; WANG and STRASSER 1993). Even with a portable registration system and a PC, strain parameters can be evaluated to verify the degree of more or less biological effects in designing tools. 6. References KADEFORS, R.; ARESKOUG, A.; ÖSTER, J.; DAHLMAN, S.; WIKSTRÖM, L. and SPERLING, L. (1991) Tools and hand function: Methods for evaluation of hand tools. In: QUINNEC, Y. and DANNIELLOU, F. (Eds.) Designing for Everyone Vol. 1. Proceedings of the 11th Congress of the International Ergonomics Association, Taylor and Francis, London / New York / Philadelphia, pp. 185-187 KILBOM, A.; FRANSSON, C.; BYSTROM, S.; SPERLING, L.; KADEFORS, R.; ARESKOUG, A.; ÖSTER, J.; LANDERVIK, E.; DAHLMAN, S.; WIKSTRÖM, L. and LIEDBERG, L. (1991) Tools and hand function: Design, selection and use of hand tools with regard to optimal hand-arm function and performance. In: QUINNEC, Y. and DANNIELLOU, F. (Eds.) Designing for Everyone Vol. 1. Proceedings of the 11th Congress of the International Ergonomics Association, Taylor and Francis, London / New York / Philadelphia, pp. 173-175 MARRAS, W.S. (1990) Industrial Electromyography (EMG). International Journal of Industrial Ergonomics 6, 89-93 MARRAS, W.S. and LAVENDER, S.A. (1991) The effects of use, tool design and roof height on trunk muscle activities during underground scaling bar use. Ergonomics 34 (2) 221-232

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MÜLLER, K.-W.; ERNST, J. and STRASSER, H. (1989) Ein Normierungsverfahren der elektromyographischen Aktivität zur Beurteilung einseitig dynamischer Muskelarbeit. Zeitschrift für Arbeitswissenschaft 43 (15 NF) 3, 129-135 STRASSER, H. (1991) Different grips of screwdrivers evaluated by means of measuring maximum torque, subjective rating and by registering electromyographic data during static and dynamic test work. In: KARWOWSKI, W. and YATES, J.W. (Eds.) Advances in Industrial Ergonomics and Safety Vol. III. Taylor and Francis, London / New York / Philadelphia, pp. 413-420

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WANG, B. and STRASSER, H. (1993) Left-and-right-handed screwdriver torque strength and physiological cost of muscles involved in arm pronation and supination. In: MARRAS, W.S.; KARWOWSKI, W.; SMITH, J.L. and PACHOLSKI, L. (Eds.) The Ergonomics of Manual Work. Taylor and Francis, London / Washington DC, pp. 223-226

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Chapter 14

Assessment of the Ergonomic Quality of Fire Nozzles K. Kluth, O. Pauly, E. Keller and H. Strasser

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0. Summary Firefighters are subject to high physical and psychological stress and fire fighting often requires mastering complicated tasks under adverse conditions. In this job, the handling of the hose/nozzle combination is a central and often performed task. The objective of this field study was to ergonomically evaluate different designs of 3 fire nozzles – a multi-purpose nozzle according to a German standard, a pistol nozzle (AWG), and a supposedly ergonomic nozzle (Quadrafog) – with respect to the muscle strain associated with performing standardized working tests. Eleven firefighters (10 males and 1 female, aged 27-54) used the 3 nozzles in the practice area of a fire station. For 3 different working tasks (straight stream, wide fog, and alternating operation), electromyographic activity was monitored continuously from 7 muscles of the right and left hand-arm-shoulder system using a PCbased mobile data registration system. Specially developed questionnaires provided subjective assessments of the ergonomic quality of the fire nozzles. The standard nozzle – which is still very frequently used – is only suited for “water go” for extended periods of time, but not for dynamic work. Especially the nozzle operator’s arm musculature is subjected to high strain by the hose forces, which depend on water pressure and flow. AWG and Quadrafog led to substantially lower overall strain and smaller static components. Only in pure straight stream fire fighting was there no difference between the standard nozzle and the other models. High static portions characterize straight stream as well as wide fog operation. Both operations required small movements of the body so that the static portions became more noticeable. Overall, the AWG fire nozzle exhibited the most balanced strain profile with non-critical static values and a tolerable overall strain for alternating operations. These results are in accordance with the fire fighters’ subjective preference for this model. It is unfortunate that currently the sole focus remains still on the price rather than the usability, which determines the physiological costs that must be paid by the operator. In the future, more attention should be paid to the compatibility between the characteristics of the human organism and the technical components of the tool.

1. Introduction Firefighters are often subject to high physical and psychological stress and, for example, when fighting fires, must be ready for duty even in the most adverse conditions (cp. REUM et al. 1974; ROSENKRANZ 1997). On occasion, they have to go close to their stress limit. Regarding risks taken and energy expenditure demanded, e.g., in wildland fire fighting see HEIL (2002). Success in attempting to save material goods and human lives without risking their own safety requires optimal equipment (cp. DIN EN 469). This is not always the case. The fire nozzles used to fight fires can provide water principally in both a straight stream – which allows a wide range and high pressure – and a wide fog. With the latter, the stream is divided into tiny droplets of water during short-range fighting, e.g., when, according to RIECK (1991), poisonous gas or steam needs to be “held down.” Fire nozzles are nowadays available in a large number of different models, which differ in terms of water outlet, water pressure, and stream angle (cp. GÜTTLER 1980; DIN 14200; DIN 14302; DIN 14307-1/-2; DIN 14800-1). Because the hose-nozzle combination often must be handled for extended

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periods of time, the design of the operating elements is of high importance. Therefore, it was the objective of this study to examine and compare three common nozzles. In this context, the subjective ease of handling and the electromyographically determined muscle strain during the handling of the test objects in various work situations were of interest. 2. Methods 2.1. Nozzles examined

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The upper part of Fig. 1 shows the often-used multi-purpose nozzle according to a German standard (DIN 14365-1/-2) which has a handle (for the right hand) and a valve handle which supposedly makes it easy to control the flow, and the stream shaper. Switching from straight stream to wide fog (by moving the valve handle from front to back) and vice versa requires that the water is turned off temporarily, when the valve handle is at a 90-degree angle to the nozzle (cp. HERTERICH 1960) The pistol nozzle AWG CS-A, which is shown in the middle part of Fig. 1, has a valve handle for the flow control with automatic shutoff, and an angled handle in elbow configuration. According to EBERT (1988) and REH (1996) it is supposed to provide more comfortable operation (with the left hand), improved safety, and optimal use of available water resources. A rotating stream shaper at the nozzle tip is supposed to allow quick switching from straight stream to wide fog (including the continuum of stream angles in between) without changing the grip of the right hand and without interruption of the water flow. The nozzle “Quadrafog 150” – shown in the lower part of Fig. 1 – has a rectangular-shaped valve handle for the flow control, a pistol handle which allows a power grip of the hand in normal position, and, just like the model shown in the middle of Fig. 1, a rotating stream shaper at the nozzle tip (cp. SCHALOMON 1995). It claims to be largely optimized with respect to ergonomic aspects. Its price, however (~ 250 US $), is a multiple of the prices for the other two models (Standard Nozzle: ~ 40 US $; Pistol Nozzle: ~ 180 US $).

Figure 1:

Design and functions of the tested fire nozzles

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2.2. Test tasks Figure 2 shows a fire fighter in protective gear (according to GUV 7.13) operating the three models. The three types of nozzles were used in a field study with a pressure of 5 bar at the nozzle according to instructions, which simulate reality well. C52 pressure hoses (cp. DIN 14811-1) supplied the water via the pumping apparatus of a water tank truck of a professional fire-fighting department.

Figure 2:

Fire fighter operating the 3 models of fire nozzles

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In a first task (TS 1), the nozzles were used at a vertical angle of 32°, as required by official instruction FwDV2/2, in order to achieve the maximum range (cp. SCHOTT and RITTER 1988). In this TS 1, the test subject had to maintain the straight stream on target for 60 s (left part of Fig. 3). In TS 2, the operator held the nozzle horizontally in wide fog mode (middle part of Fig. 3).

Figure 3:

Test series provided for each nozzle model with various flow procedures “straight stream” (TS 1), “wide fog” (TS 2) and “alternating operations” (TS 3)

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During a 60-s period, he or she had to move the nozzle to the left and to the right. The movements from one side to the other lasted 4 s, and the angle between the two endpoints was 150°. The rhythm for the direction changes was dictated by the test conductor. Since the controlled use of water and the alternating nozzle handling during close range fire fighting is of increasing importance according to recent developments in fire protection, various consecutive tasks had to be carried out in TS 3. As presented in the right part of Fig. 3, the alternating operation required the nozzle operator to switch back and forth within 5 s between horizontal straight stream and wide fog, as well as “water go” and “water stop” during a 60-s period. Before, during, and after these tests, myoelectric activity from several muscles was measured in order to obtain information about muscle strain associated with operating the test models during the various tasks. 2.3. Selected myoelectric recordings and test design Video studies on real-life use and pre-tests on the handling of fire nozzles had shown that the following muscles of the left and right hand-arm-shoulder system are involved in the tasks of the nozzle operator (Fig. 4):

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• the m. flexor digitorum, • the m. flexor carpi ulnaris, and • the m. biceps brachii (both body halves).

Figure 4:

Muscles of the left and right hand-arm-shoulder system which have been monitored continuously

These are muscles which are involved in the holding of the nozzle as well as the right hand’s operating of control elements with an ulnar deviation of the hand to a certain degree. Since holding and stabilizing of the nozzle-hose combination can be done with various shoulder positions, the strain on • the m. trapezius, pars descendens (dexter) and • the m. trapezius, pars descendens (sinister) was of interest as well.

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This muscle of the left and right part of the body was examined because operating a nozzle requires both hands in a kinematic chain which is closed via the hand tool. Since the left arm had to exert a torque-compensating counter pressure on the nozzle from above in a more or less pronounced abducted position, the extensing musculature rather than the flexing musculature, i.e., • the m. triceps brachii was of interest in addition to • the m. deltoideus, pars acromialis which determines the abduction of the upper arm. The electromyograms from the above-mentioned muscles could be recorded continuously with portable data recording systems (Fig. 5).

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Figure 5:

Test subject with a portable data recording system (left) and utilized baby-electrodes and SMD-preamplifiers for the EMG-recordings (right)

The schematic representation in Fig. 6 shows how the electromyographic activity, EA, was first calculated from the raw EMG and then – after the tests – transferred as a time series to a laptop computer before it was edited in a PC for the presentation of the results. Software, as described in Chapter 3 of this book (see also STRASSER et al. 2004), was used to smooth, standardize, and analyze the EA signals. In addition to the calculation of conventional averages, an ergonomically relevant separation of the standardized electromyographic activity, sEA, into a static and a dynamic part was carried out. It could be expected that the ergonomic quality of a hand-held device cannot be evaluated by the physiological cost alone, which can be quantified via electromyographic methods. Several characteristics which may neutralize each other with advantages and disadvantages can only be assessed via subjective ratings. Thus, specially structured questionnaires were developed, which were supposed to capture the dominance of various characteristics and their effects on the subjectively felt performance. Figure 7 gives an overview of the test program, which lasted 2½ hours. After some preparatory work, subjective assessment of the 3 nozzle models (without immediate work experience), and preassessment of possible complaints, i.e., the subjective state of health, were completed; the test subjects (Ss) had to operate the 3 nozzles in 3 test series with “straight stream,” “wide fog,” and “alternating operation” in randomized order. EA values from 7 muscles were registered continually during the handling of the fire nozzles. After the immediate work experience, the Ss again subjectively assessed the nozzles. Thereafter the maximum electromyographic activity, EAmax, of the 7 muscles in typical working posture during maximum voluntary muscle contractions was measured. Finally, the nozzles were subjectively assessed once more and possible physical complaints were noted.

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Figure 6:

Schematic representation of the recording and processing of electromyographic data

Figure 7:

Schedule of the test program

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2.4. Test subjects Eleven experienced members of a professional fire department (10 males and 1 female) with a mean age of 36 ±10 years participated in the field study. Further characteristics of the test subjects (who were all right-handed) can be seen in Table 1. Table 1: Characteristics of the test subjects

3. Results 3.1. Muscle strain dependent on work tasks and test objects

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The graphic presentation of the electromyographic results (averaged over the 11 Ss) is explained in Fig. 8, which shows the results of TS 1 with the model which is allegedly largely optimized with respect to ergonomic aspects.

Figure 8:

Static and dynamic parts as well as mean values of the standardized electromyographic activity sEA [%] for 7 muscles of the left and right hand-arm-shoulder system when working with the “Quadrafog 150“ during the work process “straight stream“ (TS 1) (means from 11 Ss)

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From left to right, the static and the dynamic components of strain on the monitored muscle groups of the left and right hand-arm-shoulder system are represented by the black and yellow column portions respectively. Without going into too much detail about the interpretation of the sEA values, it becomes clear that the trapezius musculature is most likely not a bottleneck with respect to strain in this task. There is, however, a relatively large amount of strain on the triceps of the left arm, as an antagonist of the biceps, and the muscles which affect the right forearm. Figure 9 shows the sEA values for all muscle groups during the task “straight stream” (TS 1) with the 3 models. Since this task requires mostly static work, the static part is relatively high. A comparison of the 3 rows of columns shows that the model AWG CS-A (middle part of Fig. 9), and not the Quadrafog 150 leads to the lowest levels of strain during this task.

Figure 9:

Static and dynamic parts as well as mean values of muscle strain associated with operating the three nozzle models during the work process “straight stream” (TS 1) (sEA [%] of 7 muscles, averaged over 11 Ss)

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During wide fog operation in TS 2 (Fig. 10), both the static and dynamic components of strain are generally substantially higher for all muscles. Again, the model which was rated best in TS 1 fared well, in that it almost always led to the lowest sEA values.

Figure 10: Static and dynamic parts as well as mean values of muscle strain associated with operating the three nozzle models during the work process “wide fog” (TS 2) (N = 11)

Figure 11 shows how the required manual manoeuvres in TS 3 resulted in extremely high muscle strain, especially with the standard nozzle (upper part of Fig. 11). This reflects the difficult handling when somewhat more complex manual switching movements have to be carried out simultaneously to static work. Due to the alternating operation, the static components of strain are less dominant than the dynamic ones. Again, the AWS CS-A model is more favorable than the allegedly ergonomically

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optimized Quadrafog 150 nozzle. The former exhibits by far the lowest sEA values of all muscles. The static components in the muscle groups which influence the position of the right forearm and hand are especially low (right part of Fig. 11).

Figure 11: Static and dynamic parts as well as mean values of muscle strain associated with operating the three nozzle models during the work process “alternating operations” (TS 3) (N = 11)

The reason for the clearly lower strain of the flexor carpi ulnaris with the AWG CS-A (relative to the Quadrafog 150) is presumably the elbow configuration, which redirects the water flow in such a way that the force vector and the torque which needs to be compensated are reduced. A two-sided t-test showed that most of the differences in sEA values between the models in Table 2 are statistically significant at least at the 95 % level.

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Table 2: Results of the significance tests (two-tailed t-test) for the comparison of the sEA mean values of the 7 muscles involved in operating the 3 nozzle models (D, A, and Q)

3.2. Subjective assessment of the nozzles with and without working experience (prior to, during, and after the tests)

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In order to gain an insight into the very differentiated subjective assessment of various criteria of the nozzles (such as shape and dimensions of handles and controls for various conditions, ease of applying forces, etc.), the orientation of body parts in the kinematic chain (left and right hand and forearm, upper arm, and shoulder), and the effects of work on possible physical complaints, a selection of figures from the large set of evaluated data will be presented here. Figure 12, for example, presents the test subjects’ overall impression of the 3 test models prior to, during, and after the tests. A priori existing reservations about the standard model (left part of Fig. 12) were reinforced by working with the nozzle and remained after the end of the test series, when immediate comparisons were possible.

Figure 12: Overall assessment about the handling of the nozzles (means and standard deviations from 11 Ss)

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The pistol nozzle in elbow configuration (AWG CS-A in the middle part of Fig. 12) generally received the best evaluations, which improved during the course of the tests. The presumed ergonomically optimized model (right part of Fig. 12), which also was evaluated positively prior to the tests (around “+1” on a bipolar scale ranging from “-4” to “+4”), was evaluated substantially less favorably after the work experiences. Figure 13 shows that the subjective assessment of the handle design is distinctly different for the left and the right hand. The standard model had received only slightly positive or negative evaluations for the left hand which changed over time, that is, they depended on experience with this model and the possibility of comparing it to the other models. The assessment for the right hand was clearly negative. The test model AWG CS-A (shown in the middle) again received very clear positive evaluations which were quite robust in repeated tests. The positive evaluation for the Quadrafog 150 nozzle prior to the tests, however, was reduced substantially.

Figure 13: Subjective assessment of the handle design for the left hand (left) and for the right hand (right) (means and standard deviations from 11 Ss)

Figure 14, finally, shows that the valve handle for the “water go” and “water stop” positions (left part of Fig. 14) and the stream shaper (right part of Fig. 14) were assessed very negatively for the standard model. The assessments for the other two models were approximately equally good for the left hand, but were different, with less appreciation of using the stream shaper of the Quadrafog 150 with the right hand (during the test and after gaining working experience with all three models).

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Figure 14: Subjective assessment of the valve handle (left) and of the stream shaper (right) (means and standard deviations from 11 Ss)

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4. Discussion The main goal of this study was to objectify the ergonomic quality of 3 models of fire fighting nozzles. The scheduled test sequence included various ways of handling, and was intended to measure the various levels of strain associated with the test objects. Substantial disadvantages of the standard nozzle – which is most commonly used – became apparent in the strain analyses. With standardized electromyographic activity (sEA) values in the arm musculature of up to 60 % for the overall strain and with static portions of more than 15 %, the endurance level is already exceeded in the short test portions with alternating operation. AWG and Quadrafog lead to substantially lower overall strain and smaller static portions. Only in pure straight stream fire fighting is there no substantial difference between the standard nozzle and the other models. High static portions characterize straight stream operation (up to more than 15 % for the standard model, 10 % for AWG, and up to almost 20 % for Quadrafog) as well as wide fog operation (up to 20 % for the standard model, 15 % for AWG, and up to 20 % for Quadrafog). Both operations require little movement of the body, so that the static portions become more noticeable. However, the endurance level in practice should not be exceeded, as it was in this study. Overall, the AWG fire nozzle exhibited the most balanced strain profile with non-critical static values and an overall strain of up to no more than 30 % for alternating operation, the most stressful working task. These results are in accordance with the fire fighters’ subjective preference for this model. Via electromyographic techniques, muscle strain could be determined exactly and subjective assessments of 11 professional fire fighters led to consistent, clear results which, however, were not in agreement with the producers’ opinions. The allegedly ergonomically designed nozzle did not live up to its promise. In most test criteria, it did not perform as well as a second test model with elbow configuration, a special way of directing the pressurized water stream through the nozzle. The effects of the elbow configuration have been known for some time now (cp. DIN 14368), but, unfortunately, the producer of the allegedly ergonomic model did not take them into account. The special handshaped pistol grip which is used led instead to high strain of the hand’s abduction musculature, which must exert counter forces when the nozzle is not just held, but operated under high pressure.

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5. Conclusions

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This study is an example of how misleading the term “ergonomic” for a tool can be when the producer and manufacturer do not have a comprehensive understanding of ergonomics. It is not enough to call a handheld product “ergonomic” when ergonomic considerations are limited to the coupling of hand and tool – such as the pistol grip in this example – as it is all too common with today’s inflationary ergomania. Ergonomics as a science far exceeds traditional measurements of body parts and involves at the very least an understanding of dynamic situations with respect to the interaction of the user with the product in a comprehensive work process. To perceive ergonomics as nothing more than an anthropometric consideration (as too many engineers do) is to miss the major part of the point. Instead of this, the term has to be defined as the “study of the efficiency of persons in their working environment.” This amounts to considerably more than percentiles of limb segments, so that the transatlantic term “human factors engineering” is more appropriate (cp. HANIFAN 1990). According to the International Ergonomics Association (IEA), ergonomics is the scientific discipline concerned with the understanding of interactions among humans and other elements of a system, and the profession that applies theoretical principles, data, and methods to design in order to optimize human well-being and overall system performance. Professional ergonomists need a profound education and training, e.g., in planning, design, and the evaluation of products and complex systems, and not just simple recipes or checklists in order to make just such a simple tool as a nozzle compatible with the needs, abilities and limitations of the operator (cp. STRASSER 2002). Furthermore, when a suspect ergonomic product is more than three times as expensive, its success will certainly be quite limited. The tests, of course, also showed that the standard nozzle (which costs roughly a quarter of the ergonomically best model) was by far the worst model tested. There is still room for quite obvious improvements, however, for the model with elbow configuration, which fared best in this study (Fig. 15).

Figure 15: Recommendations for the ergonomic design of fire nozzles

Instead of the valve handle for the flow control, which must be grasped and operated by the right hand in conjunction with the pistol grip, a rectangular-shaped valve handle (as featured on the allegedly ergonomically optimized model) would be such an improvement. It is regrettable that purchasing decisions are almost exclusively based on monetary considerations (which favor the very inexpensive standard nozzle) and that the “physiological costs” which must be “paid” by the operator are not considered. In the future, more attention should be paid to the compatibility (cp. STRASSER 1995) between the human organism’s characteristics and the controllable technical components of a tool.

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6. References EBERT, K. (1988) Handbuch Feuerwehrarmaturen. 2. Auflage, Druckerei Schmid/GmbH & Co., herausgegeben von WIDENMANN, M., Giengen/Brenz GÜTTLER, E. (1980) Vergleich zwischen Hochdruck und Normaldruck. Fire International 37, 94-101 HANIFAN, P.V.A. (1990) Front end. What’s in a Word. Control Systems, May 1990 HEIL, D.P. (2002) Estimating energy expenditure in wildland fire fighters using a physical activity monitor. Applied Ergonomics 33, 404-413 HERTERICH, O. (Ed.) (1960) Wasser als Löschmittel. Dr. A. Hüthig Verlag, Heidelberg REH, W. (1996) Konzept über Maßnahmen zum Schutz von Einsatzkräften vor den Gefahren durch flashover hinsichtlich der Vorhersehbarkeit eines flashover, der Einsatztaktik und der Ausstattung des Löschzuges bei der Berufsfeuerwehr Münster. Abschnittsarbeit, Feuerwehr Siegen REUM, P.-J.; KÄSEBERG, H. and KLINGBEIL, M. (1974) Arbeitsmedizinische Kriterien für die Beurteilung der Tauglichkeit bei Feuerwehrangehörigen im operativen Dienst. Unser Brandschutz 23 (6) 49-54 RIECK, L. (1991) Feuerlöscharmaturen. 10. überarbeitete Auflage, Verlag W. Kohlhammer, Stuttgart / Berlin / Köln ROSENKRANZ, U. (1997) Untersuchungen zur aktuellen physischen Leistungsfähigkeit Leipziger Berufsfeuerwehrleute. Dissertation an der Universität Leipzig SCHALOMON, T. (1995) Untersuchung der bei der Berufsfeuerwehr Gelsenkirchen vorhandenen Strahlrohrtypen und Strahlrohrgrößen hinsichtlich ihrer Verwendbarkeit und Effizienz. Abschnittsarbeit, Feuerwehr Datteln SCHOTT, L. and RITTER, M. (1988) Feuerwehrgrundlehrgang. 5. Auflage, Wenzel-Verlag, Marburg STRASSER, H. (1995) Ergonomics efforts aiming at compatibility in work design for realizing preventive occupational health and safety. International Journal of Industrial Ergonomics 16 (3) 211-235 STRASSER, H. (2002) Ergonomics education and experience required for the certification as a professional European ergonomist – Quality and quality of academic provision and ergonomics training from a German point of view. Journal of Ergonomic Study 4 (2) 67-94 STRASSER; H.; KLUTH K. and KELLER, E. (2004) A computer-based system for the use of electromyographic methods for the measurement of physiological costs associated with operating hand-held tools and computer-input devices. Occupational Ergonomics 4 (2) 73-87 Standards, guidelines, regulations Copyright © 2007. IOS Press, Incorporated. All rights reserved.

DIN 14200 (1979-06) Nozzle discharge. German Institute for Standardization, Beuth Verlag, Berlin DIN 14302 (1985-04) Aluminium alloy delivery coupling type C; nominal pressure 16. German Institute for Standardization, Beuth Verlag, Berlin DIN 14307-1 (1985-04) Aluminium alloy solid coupling type C with sealing ring for suction purposes; nominal pressure 16. German Institute for Standardization, Beuth Verlag, Berlin DIN 14307-2 (1986-11) Aluminium alloy solid coupling type C with sealing ring for suction purposes; nominal pressure 16. German Institute for Standardization, Beuth Verlag, Berlin DIN 14365-1 (1991-02) Multi purpose branch pipes for nominal pressure 16; dimensions, materials, construction, marking. German Institute for Standardization, Beuth Verlag, Berlin DIN 14365-2 (1986-09) Multi purpose branch pipes for nominal pressure 16; requirements, testing. German Institute for Standardization, Beuth Verlag, Berlin DIN 14368 (1985-09) Branchpipe-holder for nominal pressure 16. German Institute for Standardization, Beuth Verlag, Berlin DIN 14800-1 (1984-07) Appendix 3, Fire-fighting equipment for fire-fighting vehicles; group 3: hoses, armatures and fittings. German Institute for Standardization, Beuth Verlag, Berlin DIN 14811-1 (1990-01) Fire hoses; requirements, testing, treatment. German Institute for Standardization, Beuth Verlag, Berlin

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DIN EN 469 (2007-02) Protective clothing for firefighters - Performance requirements for protective clothing for firefighting. German Institute for Standardization, Beuth Verlag, Berlin FwDV (1980) Feuerwehrdienstvorschriften, insbesondere 2/1 Ausbildung der freiwilligen Feuerwehren, Rahmenvorschriften 2/2 Ausbildung der freiwilligen Feuerwehren, Musterausbildungspläne vom 01.01.1980

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GUV 7.13 (1991) UVV, Unfallverhütungsvorschriften Feuerwehren mit Durchführungsanweisungen. Feuerwehr Unfallkasse Westfalen-Lippe, gültig ab dem 01.04.1991

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Chapter 15.1

Ergonomics in the Rescue Service – Part 1: Strain-Oriented Evaluation of Ambulance Cots K. Kluth, E. Keller and H. Strasser

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0. Summary In addition to work analyses during the use of ambulance cots, the strain on the circulatory system of 12 professional carriers was measured in 4 standardized carrying tests: carrying of the stretcher in a staircase at normal speed and at increased speed, lifting of the stretcher onto the gurney, and loading the ambulance cot into as well as unloading it from an ambulance. Additionally, static and dynamic components of the muscle strain of 6 muscle groups were determined electromyographically. The tests consisted of “normal” carrying as well as explicitly rapid carrying of a dummy (78 kg) up and down a flight of stairs using 3 different commercially available ambulance cots (roll-in systems – stretchers with incorporated transporter which weighed between 48.5 and 50 kg including a pad for the patient). The paramedics had to carry both the front and the rear of all the stretchers. The “normal” carrying times to cover one floor (the typical real-life situation) were approximately 35 to 40 s. Rapid carrying reduced the carrying times by approximately 10 s. Model-specific influences of the roll-in systems aside, the rapid carrying led to substantially increased strain on the circulatory system (work-related increases of approximately 10 beats per minute (bpm)). The “lifting of the stretcher onto the gurney” and the “loading/unloading of the roll-in system” cause significantly less strain, but still lead to substantial “extra physiological costs” of approximately 50 bpm. The strain on the flexor digitorum, the upper part of the trapezius muscle, and the erector spinae substantially exceeds the strain on the 3 parts of the delta muscle in all tests. Increased speed significantly increases muscle strain. The static components of the standardized electromyographic activity sEA [%] with values of 50 % and more (especially for the flexor digitorum) show that even carrying times of only approximately 30 s cause fatigue. It is well known that just 50 % of maximum output over 30 s already require recovery times of approximately 400 %, i.e., approximately 2 min. Carrying the rear of the stretcher upstairs leads to significantly increased strain on the erector spinae related to the carrying position at the front. Similarly, the carrier at the front of the stretcher experiences substantially higher strain while bearing the stretcher downstairs. Even though model-specific influences cause some differences, they are not displayed consistently across all work elements. The data can be used by job analysts to grade the level of muscle activity required by different carrying tasks during the transport of a patient, and by product designers to justify changes in the design of ambulance cots to reduce muscular strain on the upper extremities and the back.

1. Introduction Problems with the intervertebral discs in the lumbar area due to lifting and carrying of heavy loads over the course of years are considered rare long-term results of unfavorable working conditions. In the early stages of such serious long-term effects, analyses of real work situations and physiological strain on the muscles of the upper extremities as well as the entire body (when measured under controlled conditions) can be expected to yield valuable information. In this context, manual patient transport is of high interest since it can gradually lead to long-term damage of the body’s support structure, especially of the intervertebral discs in the lumbar area. There is not much information available on the ability of paramedics to carry patients. As far as known to the authors, only FURBER et al. (1997), KNAPIK et al. (1998, 2000), V. RESTORFF (2000) and RICE et al. (1996a, 1996b) published comprehensive data on stretcher carriage by ambulance officers as well as male and female soldiers.

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In order to increase risk prevention, ergonomically optimized stretchers and gurneys should be utilized. Design features of common commercial products (roll-in systems – as combination of stretcher and gurney) as well as their resulting stress and strain were determined in a comparative ergonomic study in order to document the existing state and derive suggestions for improvements. The assessed roll-in systems – with respect to the ergonomic state – are two frequently used products by the companies Ferno and Stollenwerk; well known in Europe and the USA (cp. NIESSER 1998), as well as a newly developed product by the American company Stryker (cp. Fig. 1).

Figure 1:

Ferno gurney 50-6 with stretcher 409-14A (top), Stollenwerk gurney 4002 with stretcher 3002 (middle), and Stryker roll-in system M1-6100 (bottom)

The Stryker gurney weighs 26 kg, and the stretcher including a pad for the patient weighs 24 kg. Thus, the total weight of the Stryker roll-in system is 50 kg, making it the heaviest of the tested models. With a total weight of 48.5 kg, resulting from the weight of the stretcher plus pad (26 kg) and 22.5 kg for the gurney, the Ferno system is the lightest ambulance cot examined in this test. The Stollenwerk stretcher’s weight including pad is 23 kg, and the gurney’s weight is 26 kg for a combined weight of 49 kg.

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2. Methods The test series attempted to simulate real work situations. Therefore, data on patient transportation were collected by the German Red Cross through questionnaires for an extended period of time before the tests were carried out. An analysis of 263 questionnaires was used to determine the weight of the patient dummy that was to be used in the tests as well as the floor of a building to which the patient dummy had to be carried. A patient’s weight was recorded in one of five classes: less than 50 kg / 50-69 kg / 70-79 kg / 80-89 kg and more than 90 kg. The statistical analysis has resulted in an average patient weight of 78 kg (cp. left part of Fig. 2). Therefore, the tests were carried out with a patient dummy weighing 78 kg. Its shape and weight distribution replicate the human body. The middle part of Fig. 1 shows the patient dummy buckled onto the Stollenwerk roll-in system. The data of the pretest further permitted the calculation of the mean number of floors to which the patients had to be carried. The resulting value was slightly less than the 2nd floor (cp. right part of Fig. 2). Therefore, the dummy had to be carried over a distance of one floor.

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Figure 2:

Weight distribution of 263 transported patients (left) and frequency distribution of floors to which patients are transported (right) (Source: German Red Cross)

Paramedics attempt to make as much use of the gurney as possible, e.g., when a patient needs to be picked up, since doing so allows the pushing rather than the carrying of the patient on the stretcher. This is only possible, however, if sufficient space is available in apartments or elevators. Since this is often not the case, the stretcher is often detached from the gurney at the entrance door to the building. Then, the stretcher is carried to the patient, and the patient is placed on the stretcher. After that, the stretcher with the patient is carried back and reattached to the gurney. Afterwards, the entire system gets rolled away and placed into the ambulance. The following four segments of this overall process “patient transportation” were included in the approximately 4-hour testing procedure: • Carrying of the stretcher in a staircase at normal speed (both upstairs and downstairs; TS01), • Lifting of the stretcher onto and off the gurney including lifting of the roll-in system (TS02), • Loading the roll-in system into and unloading it from an ambulance (TS03), and • Stretcher carrying in a staircase at increased speed (both upstairs and downstairs; TS04). The tests were carried out with professional carriers. Professional firefighters as well as paramedics participated. Most of the test subjects have many years of experience with the various roll-in systems. Their characteristics are represented in Table 1. In order to balance the conflicting goals of representative results on the one hand and justifiable time commitment for the on-call test subjects on the other hand, the number of persons was limited to 12, and the entire testing procedure was distributed over two days.

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In the evaluation of products and working tools which are operated by hand, the physiological costs (the muscle strain) which can be electromyographically determined in standardized work tests during the handling of working tools is of utmost importance (cp. BÖHLEMANN et al. 1994; STRASSER et al. 1999). Quite recently STRASSER et al. (2004) have pointed out again that advanced methods of electromyography enable the measurement of the intensity of muscle exertions which are demanded when working with hand-held tools (see amongst others LEWIS and NARAYAN 1993; BÖHLEMANN et al. 1994; KUMAR 1995; KUMAR and MITAL 1996; STRASSER, 2006). The same is true when repetitive manual movements during material-handling tasks have to be performed (cp. STRASSER and MÜLLER 1999). Utilizing multi-channel recording devices, physiological responses of those muscles involved in work can be quantified in figures and numbers, whereby more or less ergonomically designed tools lead to different physiological costs associated with work (cp. KLUTH et al. 1997; STRASSER et al. 1999).

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Table 1: Characteristics of the test subjects

Therefore, the objective strain data during the various tasks were determined via surface electrodes, miniature preamplifiers, and mobile data recording devices and afterwards evaluated on stationary PC systems (cp. KLUTH et al. 2004). In addition to the muscle strain, the work-related increases in heart rate, i.e., the respective work pulses, which were associated with the lifting, carrying, and transferring were measured during the test work. These peripheral-physiological data were collected in order to allow conclusions about model-specific strain differences and to allow the overall rating of strain on the muscles and the whole body. The analysis of sequences of movements during the lifting, carrying, and handling of the test objects allowed the determination of especially active muscles or functional parts of muscles. They are: • Musculus flexor digitorum superficialis dexter, as main flexor of the right wrist and fingers, • Musculus trapezius pars descendens dexter, which allows the stabilization and pulling up of the right shoulder, • Musculus erector spinea sinister, which is necessary for the upright posture of the torso (the upper part of the body), and the three components of the musculus deltoideus, the • pars clavicularis dexter, which moves the right arm forward, • pars acromialis dexter, which is responsible for the sideways abduction of the right arm, and • pars spinalis dexter, which is involved in the backward motion of the right arm.

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The raw EMG, which was derived from these muscles or muscle components with the help of surface electromyography went through the following course of signal processing (cp. STRASSER 2006), which is also shown in Fig. 3. After it was rectified and integrated, it was smoothed (with the help of software), standardized (cp. MARRAS 1987; MIRKA 1991; MÜLLER et al. 1989), and then – after the cycle identification (with maxima and minima in the time series) – evaluated. In addition to the determination of conventional means, an ergonomically relevant splitting up of the standardized Electromyographic Activity (sEA) into a static and dynamic component took place (cp. STRASSER 1996).

Figure 3:

Calculation of EA from time series of the “raw” EMG and smoothing of the “rough” electromyographic activity EA, standardization, identification of cycles, and splitting up of sEA in dynamic and static parts in addition to the mean value

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3. Results

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In the first test series (TS01), the stretcher with the patient dummy was transported at “normal” speed to the second floor and back to the first floor. The total weight of more than 100 kg causes high static (the grey parts of the columns in Fig. 4) and only minor dynamic strain (the blue parts of the columns in Fig. 4), shown in the example of the Ferno stretcher. The stress from the test causes static strain which, for most of the muscles, largely exceeds the permanent output limit of 15 %, i.e., the endurance level for an 8-hour workday. This fact alone, however, would not be particularly relevant, since continuous patient transports for 8 hours are not a realistic situation. Nonetheless, static strain of close to 45 % of the shoulder-stabilizing descendent part of the trapezius and total strain of approximately 50 % combined with prolonged carrying duration – e.g., in a narrow staircase or over several floors – will likely lead to distinct myogelosis in the neck-shoulder area. It should be mentioned in this context that 50 % of maximum force can only be kept up for 1 minute. If a muscle contraction of 50 % of maximum force is required for only 30 s, the subsequently required restitution time according to ROHMERT and LAURIG (1993) is already approximately 400 % of the holding time, i.e., approximately 2 minutes. A similar issue exists for the erector spinae, which is responsible for the straight posture of the upper part of the body. In this case, the static strain of approximately 30 % will also lead to fatigue after only a few minutes.

Figure 4:

Static and dynamic parts as well as means of the standardized electromyographic activity sEA [%] over 12 test subjects for 6 muscles and muscles parts during carrying the Ferno stretcher in the staircase at “Individually Chosen Carrying Speed (TS01)”

The different test objects of the manufacturers Stryker, Ferno, and Stollenwerk cause comparable strain. As can be seen in Fig. 5, it was not possible to determine a significant strain difference in either of the muscles or functional muscle parts, which was due to the use of the different products. The undoubtedly existing differences between models are too minor to be noticeable given the extremely high basic strain due to the heavy load. Figure 6 illustrates the results of Test Series TS04. They can be directly compared to those of the first Test Series (TS01) since the only difference between the two was the increased speed at which the subjects had to carry the test objects in the staircase. The faster trials in the fourth test series were, on average, approximately 10 s or 30 % shorter in terms of the total carrying time. The results for the

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Stryker product show that carrying the stretcher at increased speed always leads to higher strain relative to the previous individually chosen carrying speed. Model-specific differences, however, can once more not be found due to the high basic strain, which results from the heavy weight of the stretcher and the dummy.

Figure 5:

Comparison of static and dynamic parts as well as means of the standardized electromyographic activity sEA [%] over 12 test subjects for 6 muscles and muscles parts during carrying three different stretchers in the staircase at individually chosen carrying speed

Figure 6:

Static and dynamic parts as well as means of the standardized electromyographic activity sEA [%] over 12 test subjects for 6 muscles and muscles parts during carrying the Stryker stretcher in the staircase at “Individually Chosen Carrying Speed (TS01)” and at “Fast Carrying Speed (TS04)”

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During TS02 – transferring the stretcher to the gurney, lifting and lowering of the roll-in system, as well as detaching the stretcher from the gurney – the functionality of the attachment mechanism of stretcher and gurney was examined as well. To that end, the stretcher was attached to and released from the gurney several times. As expected and shown in Fig. 7, the overall strain is clearly lower than during the tests that involved actual carrying. The dynamic parts are greater than during the “normal” and fast carrying in the staircase. For all parts of the delta muscle and the trapezius, the mean strain is below the permanent output limit for an 8-hour workday. The sEA-values of the erector spinae and the flexor digitorum are similarly high.

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Figure 7:

Static and dynamic parts as well as means of the standardized electromyographic activity sEA [%] over 12 test subjects for 6 muscles and muscles parts during “Transferring of the Stretcher onto the Gurney (TS02)”

The loading of the roll-in system into an ambulance and unloading it from the ambulance (TS03) examined the muscle strain during the loading process of the gurney with stretcher and patient dummy. The results for the Ferno roll-in system – as illustrated in Fig. 8 – exhibit very high strain for the flexor muscles acting on the wrist and fingers. On the one hand, such a high level correlates with the high force required to release the height locking mechanism of the Ferno gurney. On the other hand, the increased effort during the loading of the Ferno roll-in system into the ambulance can be explained by the fact that the roll-in system must be lifted during the pushing since, by design, the front leg and the foot end leg are released simultaneously (cp. KLUTH and STRASSER 2007). This can also be seen in the results for the erector spinae. Contrary to Ferno’s design, the other models are supported by the foot end legs during the pushing-in process for as long as possible. Thus, the roll-in system only needs to be lifted up for a brief period of time. That is, the lifting phase is substantially shorter for the Stollenwerk and Stryker models. The EMG analyses allowed the rating of additional work situations common during the transportation of patients. For example, it was possible to compare the carrying positions “Head” and “Foot” during the carrying in the staircase. As can be seen from the in Fig. 9 represented electromyographic activities during the simulated patient transport from the second floor to the first floor with individually chosen carrying speed, the person at the foot (i.e., the front) of the stretcher must endure higher physical strain during the descent. The strain was higher in 5 of the muscles. The position at the front of the stretcher during the descent is beneficial for the erector spinae, however, which is reduced by approximately 7 % despite a higher overall strain level.

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Figure 8:

Static and dynamic parts as well as means of the standardized electromyographic activity sEA [%] over 12 test subjects for 6 muscles and muscles parts during “Loading of the Roll-In System into and Unloading It from an Ambulance (TS03)”

Figure 9:

Static and dynamic parts as well as means of the standardized electromyographic activity sEA [%] over 12 test subjects for 6 muscles and muscles parts during the comparison of the carrying positions “Head” and “Foot,” down the stairs, at individually chosen carrying speed

The activity of the spinal part of the deltoid (which is responsible for the arm’s backward motion) is markedly increased. This is the result of additional thrust forces, which act on the person in front of the stretcher, which is carried down a staircase at an oblique angle. Not shown in a figure are the results concerning the reversal in the direction in which the stretcher must be carried (from the first floor to the second floor). With the exception of the trapezius, a reversal in the direction leads to exactly opposite strain on the carriers. Carrying at the foot end of the stretcher

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(i.e., bottom or back) now leads to a clear reduction of strain on the spinal (back) part of the deltoid. The same change in the pattern of strain was measured for the erector spinae. The strain on the back musculature and the trapezius increases substantially for the person in the back (i.e., at the bottom). That is, carrying up the stairs causes substantially more strain for the back and shoulder muscles of the person in the back. A final comparison between “carrying upstairs” and “carrying downstairs” (using the carrier at the head as an example) yielded higher strain of the spinal part of the deltoid and lower strain for the trapezius and the erector spinae for the case of “carrying upstairs.” Noticeable differences could not be measured for any of the other muscles. In addition to the electromyographic data concerning the determination of local strain, the test subjects’ heart rate (HR) was measured as an indicator of the global strain (cp. STRASSER 1986). The results depicted in Fig. 10 show that the “handling” with all models in all test series temporarily causes high global strain which exceeds of the permanent output limit (endurance level) of approximately 30 work pulses. It is noteworthy that carrying a patient in a staircase (TS01) with approximately 60 work pulses already represents very high physical strain. A slight increase in speed (in TS04) leads to a substantial strain increase of 10 beats per minute. While clear differences between the test series do exist, the differences between the models are not significant. Considering all 4 tests, however, the Stollenwerk product appears most favorable with respect to whole-body strain.

Figure 10: Means and standard deviations of the work pulses during the different tests (N = 12)

4. Conclusions This study found that the total weight differences of 1.5 kg – total weight ranging from 22.5 kg (Ferno) to 24 kg (Stryker) – are not associated with substantial effects on the overall strain. Weight differences would have to be much larger in order to be objectively established. This does not mean, however, that a further reduction of the stretchers’ weight is not desirable, especially since modelspecific differences were quite noticeable subjectively (cp. the following Chapter 15.2). The test subjects’ very detailed subjective ratings suggest a number of concrete improvements, especially in the design of the various stretchers with the goal of reducing the extraordinarily high strain on paramedics which was determined via peripheral-physiological methods.

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5. References BÖHLEMANN, J.; KLUTH, K.; KOTZBAUER, K. and STRASSER, H. (1994) Ergonomic assessment of handle design by means of electromyographic and subjective ratings. Applied Ergonomics 25 (6) 346-354 FURBER, S.; MOORE, H.; WILLIAMSON, M. and BARRY, J. (1997) Injuries to ambulance officers caused by patient handling tasks. Journal of Occupational Health and Safety 13 (3) 259-265 KLUTH, K.; KELLERMANN, H. and STRASSER, H. (1997) Electromyographic and subjective methods for the assessment of the ergonomic quality of file handles. In: SEPPÄLÄ, P.; LUOPAJÄRVI, T.; NYGARD, C.H. and MATTILA, M. (Eds.) From Experience to Innovation. Proceedings of the XIIIth Congress of the International Ergonomics Association. Vol. 4, Finnish Institute of Occupational Health, Helsinki/Finland, pp. 515-517 KLUTH, K. and STRASSER, H. (2004) Subjective assessment of the ergonomic quality of ambulance cots (roll-in cots). In: SCHULZE, L.J.H. (Ed.) Building Bridges to Healthy Workplaces. Proceedings of the XVIIIth Annual Conference of the International Society for Occupational Ergonomics & Safety (ISOES), Houston, pp. 48-51 KLUTH, K.; PAULY, O.; KELLER, E. and STRASSER, H. (2004) Muscle strain associated with operating three models of fire nozzles and subjective assessment of their ergonomic quality. Occupational Ergonomics 4 (2) 89-104 KLUTH, K. and STRASSER, H. (2007) Ergonomics in the rescue service – Part 2: Subjective evaluation of ambulance cots. In: STRASSER, H. (Ed.) Assessment of the Ergonomic Quality of Hand-Held Tools and Computer Input Devices. IOS Press, Amsterdam, Chapter 15.2, pp. 267-279 KNAPIK, J.J.; HARPER, W. and CROWELL, H.P. (1998) Physiological factors in stretcher carriage performance. European Journal of Applied Physiology 79, 409-413 KNAPIK, J.J.; HARPER, W.; CROWELL, H.P.; LEITER, K. and MULL, B. (2000) Standard and alternative methods of stretcher carriage: performance, human factors, and cardiorespiratory responses. Ergonomics 43, 639-652 KUMAR, S. (1995) Electromyography of spinal and abdominal muscles during garden raking with two rakes and rake handles. Ergonomics 38 (9) 1793-1804 KUMAR, S. and MITAL A. (Eds.) (1996) Electromyography in Ergonomics. Taylor and Francis, London LEWIS, W.G. and NARAYAN, C.V. (1993) Design and sizing of ergonomic handles for hand tools. Applied Ergonomics 24 (5) 351-356 MARRAS, W.S. (1987) The preparation, recording, and analysis of the EMG signal. In: ASFOUR, S. (Ed.) Trends in Ergonomics. Human Factors IV. Elsevier, Amsterdam, pp. 701-707

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MIRKA, G.A. (1991) The quantification of EMG normalization error. Ergonomics 34 (3) 343-352 MÜLLER, K.-W.; ERNST, J. and STRASSER, H. (1989) A standardization procedure for electromyographic activity for assessing unilateral dynamic muscle work. Z.Arb.wiss. 43 (15 NF) 4, 129-135 (in German) NIESSER, C. (1998) Rollkommando: Fahrtragen der Firma Stollenwerk und Ferno. In: GAESE, J. (Ed.) Rettungsmagazin 6. Ebner-Verlag, Ulm, pp. 30-38 RESTORFF, W. VON (2000) Physical fitness of young women: Carrying simulated patients. Ergonomics 43 (6) 728-743 RICE, V.J.B.; SHARP, M.A.; THARION, W.J. and WILLIAMSON, T.L. (1996a) The effect of gender, team size, and a shoulder harness on a stretcher-carry task and post carry performance. Part I: A simulated carry from a remote site. International Journal of Industrial Ergonomics 18, 27-40 RICE, V.J.B.; SHARP, M.A.; THARION, W.J. and WILLIAMSON, T.L. (1996b) The effect of gender, team size, and a shoulder harness on a stretcher-carry task and post carry performance. Part II: A mass-casualty simulation. International Journal of Industrial Ergonomics 18, 41-49 ROHMERT, W. and LAURIG, W. (1993) Physical strain by muscular stress. In: SCHMIDTKE, H. (Ed.) Ergonomie. 3. Auflage. Hanser Verlag, München / Wien, pp. 121-143 STRASSER, H. (1986) Physiological principals for the assessment of human work. Work load/strain/endurance level/fatigue/stress. REFA-Nachrichten 39 (5) 18-29 (in German) STRASSER, H. (1996) Electromyography of upper extremity muscles and ergonomic applications. In: KUMAR, S. and MITAL, A. (Eds.) Electromyography in Ergonomics. Taylor and Francis, London, pp. 183-226

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STRASSER, H. and MÜLLER, K.-W. (1999) Favorable movements of the hand-arm system in the horizontal plane assessed by electromyographic investigations and subjective rating. International Journal of Industrial Ergonomics 23, 339-347 STRASSER, H.; KLUTH, K. and KELLER, E. (1999) Multi-channel electromyography and subjective methods for the evaluation of the ergonomic quality of hand-held tools and computer-input-devices. In: LEE, G.H. (Ed.) Advances in Occupational Ergonomics and Safety III. IOS Press, Amsterdam, pp. 347-352 STRASSER, H.; KLUTH, K. and KELLER, E. (2004) A computer-based system for the use of electromyographic methods for the measurement of physiological costs associated with operating hand-held tools and computer input devices. Occupational Ergonomics 4 (2) 73-87

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STRASSER, H. (2006) Electromyography: Methods and Techniques. In: KARWOWSKI, W. (Ed.) International Encyclopedia of Ergonomics and Human Factors Vol. III. Methods and Techniques. 2nd Edition, Taylor and Francis, London / New York, pp. 3115-3118

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Chapter 15.2

Ergonomics in the Rescue Service – Part 2: Subjective Evaluation of Ambulance Cots K. Kluth and H. Strasser

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0. Summary In a comparative ergonomic study, three combinations of stretchers with incorporated transporters of brand-name manufacturers – so-called “ambulance cots” or “roll-in systems” – were tested with respect to their ergonomic quality. Twelve male test subjects (Ss) from the invalid transportation sector subjectively assessed the design elements of the stretchers as well as the gurneys via a questionnaire which had been developed specifically for that purpose. Additionally, the Ss were asked to express potential physical complaints resulting from the carrying activity. To that end, the Ss – under controlled conditions – had to carry out 4 different test tasks: carrying in a staircase at two different speeds, attaching of the stretcher onto the transporter as well as loading and unloading of the ambulance cot into and out of the ambulance. For each task, a 78-kg patient dummy was on the stretcher. Thus, differences in strain across the different test objects became visible or could be subjectively experienced. The presentation of results reflects the advantages and disadvantages of the different models’ specified design characteristics and possibly permits suggestions for design improvements. The very detailed subjective assessment of “roll-in systems” and the Ss’ subjective evaluation of them via approximately 50 items offer a differentiated view of the work situation. They suggest several concrete changes in order to improve the design. In particular, changes in the design of the different stretchers were recommended in order to reduce the extraordinarily high strain on the paramedics which was also measured via peripheral-physiological methods. It became clear that one system which is widely used in several countries has marked weaknesses. The biggest disadvantage is the unfavorable grip during height adjustments. Furthermore, it is not possible to utilize the so-called “switching technique” in order to relieve the back with the complete ambulance cot. However, even with the other two models, which are similar to each other in terms of the operating elements and their handling, promising approaches to improve these products do exist with respect to the stretchers’ weight, their shape and positioning of handles, and the positioning of the release mechanism to adjust the length of the handle. The results of this study reveal the necessity for industry to manufacture user-friendly and safe ambulance cots for the market. Paramedics cannot risk to use an equipment which is inadequate or works deficiently. Furthermore, an ergonomic design of the product additionally increases the safety and user-friendliness of the system during a rescue operation. Thanks to the ergonomic design, less effort is needed during the transport of the patient, which at the same time means lower physical strain for the paramedics’ back and their hand-arm-shoulder system.

1. Introduction Even though physical stress in the workplace is of somewhat lesser importance today than it used to be, prevention of work-related back and joint problems remains an important task which can only be solved by occupational medicine and ergonomics together (cp. HARTMANN 2000). Although the lifting of heavy loads at work is limited and regulated by the Lastenhandhabungsverordnung (cp. N.N. 1996) – a special “Ordinance on Safety and Occupational Health during Manual Material Handling,” it is not always possible to impose weight restrictions in the transportation of sick and injured people and for emergency medical services. Such heavy lifting can lead to long-term damage (see, e.g., AYOUB and

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MITAL 1989), which by now is recognized as an occupational disease. In the course of transporting sick and injured people, paramedics experience the “heavy loads” which are mentioned in “Occupational Disease No. 2108.” This applies to both male and female paramedics in the civilian as well as the military sector (cp. FURBER et al. 1997; KNAPIK et al. 1998, 2000; V. RESTORFF 2000; RICE et al. 1996a, 1996b). The latter often operate under extreme time pressure combined with alarmingly bad overall conditions. The weight to be carried (the sum of the weights of stretcher and patient) frequently exceeds 100 kg. Thus, it appears necessary to make use of all possibilities to reduce strain. 2. Methods

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An important starting point to improve the paramedics‘ stress and strain situation is the selection of an ergonomically optimized combination of stretcher and gurney, which is often referred to as “roll-in system.“ In addition to the objective evaluation of the work situation via local and global strain analyses (cp. KLUTH et al. 2007a, i.e., the preceding Chapter 15.1 in this book), the three mentioned “roll-in systems“ by the manufacturers Stryker, Ferno, and Stollenwerk were rated subjectively with respect to their ergonomic quality in this second part of the comparative ergonomic study. The subjective rating of design elements of both stretchers and gurneys was carried out with the help of 12 male test subjects (Ss) whose occupation is patient transportation. They all had sufficient – and in some cases very extensive – experience in their jobs. All of them were well versed in the use of the different ambulance cots. A specially developed questionnaire – which included bipolar and unipolar 4-step scales (cp., e.g., KELLER and STRASSER 1998; STRASSER 2000), and more than 50 items of different topics (cp. Fig. 1) – was used in order to highlight advantages and drawbacks of specified design elements of the different models after carrying out the 4 different tasks under controlled conditions (i.e., carrying in a staircase at two different speeds, attaching the stretcher to the gurney, and loading the roll-in system into an ambulance and unloading it with a patient dummy weighing 78 kg on the stretcher in each case). Especially the control elements and the handles were in the focus of interest.

Figure 1:

Excerpt from the questionnaire about the subjective rating of the stretchers manufactured by Stryker (Sy), Ferno (F), and Stollenwerk (S)

As shown in Fig. 2, the control elements of the Stryker product are at the head and foot of the gurney. Release levers for the gurney´s height adjustment are positioned underneath the handlebars and the foldout handles. There are two foldout handles at the head to help with lifting and lowering of the

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system with the patient on it. The handlebars at the head and the foot are intended for pushing, pulling, and carrying of the system. To unlock the gurney from the locking mechanism in the ambulance, there is a release button which – by design – is only at the foot of the system. The handles – which are identical on the gurney and the stretcher – are made from soft rubber with pronounced naps on the textured surface to improve the grip. The handles on the Stryker stretcher can by extended after a locking button has been pressed. This increases the distance between the stretcher and the carrier, which allows lifting with a straight upper part of the body. The gurney weighs 26 kg, and the stretcher including a patient pad 24 kg. Thus, the total weight of the Stryker ambulance cot is 50 kg, making it the heaviest one of the tested models.

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Figure 2:

Control elements of the Stryker gurney 6100 at the head (left) as well as carrying handles, locking buttons, and release button of the Stryker stretcher 6100 (right)

At the head as well as the foot of the Ferno gurney, there is a pull lever to release the height locking mechanism (cp. Fig. 3). If both levers are used simultaneously, the gurney can be adjusted to different heights during the transport or transferring of patients. A ball-shaped handle is used to lock the stretcher on the gurney. In order to release the stretcher from the gurney, the release lever to the left of the ball-shaped handle must be used and the stretcher must be moved slightly towards the patient’s feet. During simple pushing, pulling, and carrying, the paramedics hold on to the gurney’s handlebars at the gurney’s head and foot. In order to increase the comfort for the paramedics’ during the carrying of the stretcher, the carrying handles – which have a rectangular cross section with straight plastic covers – can be adjusted to five different lengths by depressing a locking button. The total weight of 48.5 kg results from the weight of 26 kg of the stretcher plus pad and 22.5 kg for the gurney. The Ferno system is the lightest roll-in system examined in this test.

Figure 3:

Control elements of the Ferno gurney 50-6 at the foot (left) as well as carrying handle and locking button of the Ferno stretcher 409-14A (right)

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The in Fig. 4 illustrated controls and the operation of the Stollenwerk model are almost identical to those of the Stryker model. Therefore, only relevant differences will be mentioned here. The also present foldout handles on the gurney – which are covered in soft rubber material – feature less pronounced naps on the textured surface. The handlebars have a round cross-sectional shape, are not contoured, and are not covered. The Stollenwerk stretcher’s carrying handles can be adjusted to two different lengths by depressing a locking button, which is located at the end of the handle and cannot be operated during carrying. The stretcher’s weight including pad is 23 kg, and the gurney’s weight is 26 kg for a combined weight of 49 kg.

Figure 4:

Control elements of the Stollenwerk gurney 4002 at the head (left) as well as carrying handle, locking button, and release button of the Stollenwerk stretcher 3002 (right)

Additionally, the Ss were asked to express potential effects of the carrying activity resulting in possible physical complaints. In addition to objective strain differences between the test objects, a subjective evaluation is useful to ultimately derive design suggestions. 3. Results

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3.1 Subjective evaluation of physical wellbeing or complaints prior to and after the tests The documentation of the physical wellbeing or complaints prior to and after the tests was part of the questionnaire. Questions regarding the occurrence, intensity, and duration of possible complaints in various body regions were recorded before and after the test series. Surprisingly, only one test subject expressed any physical complaints at all. They already existed before the tests in his left upper arm. In the course of the tests, none of the Ss indicated a worsening of their physical wellbeing. Apparently, the test subjects’ muscles are so well trained through the daily patient transport that the test series’ requirements did not cause any complaints. One should not draw the conclusion, however, that working as a paramedic for many years does not cause any complaints. 3.2 Subjective evaluation of the stretchers’ handles and control elements The following results are an excerpt from the whole catalog of results regarding design aspects of the roll-in systems. Figure 5 shows the results regarding the rating of the stretchers’ handles on a bipolar scale. The results for the Stryker model are shown as red columns, those for the Ferno product are shown as blue columns, and the Stollenwerk results are shown as yellow columns. The rating of the handles clearly shows that the round cross section of the Stryker and Stollenwerk models are preferred to the rectangular shape of the Ferno model. The rating of the cross-sectional shape is rather positive for the Stryker (overall mean of 2.3 ±1.8 standard deviation) and the

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Stollenwerk models (2.0 ±1.7). The Ferno model seems to cause high area pressure in the palm, which results in the negative rating of the cross-sectional shape (-3.2 ±1.2) as well as the risk of getting tender spots and blisters.

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Figure 5:

Subjective assessment of stretchers´ handles (means and standard deviations of 12 Ss)

However, the Stryker model also reveals a tendency to cause tender spots (-2.6 ±1.8 on a unipolar scale). This rating probably results from the height of the naps on the textured surface of the handles. While they ensure “fairly good” grip (1.8 ±1.6), they also increase the tendency to cause tender spots and blisters. While the handles on the Ferno stretcher provide sufficient grip in the dry state, this is no longer the case when substantial perspiration occurs in the palm. Palm perspiration mainly occurs while carrying the stretcher in the staircase and it is due to the high associated strain, resulting in substantially reduced grip of the Ferno model’s handles, which is clearly shown in the “bad” rating (-2.7 ±2.0). The Stollenwerk stretcher does not particularly lead to tender spots (-0.6 ±0.7), and the grip provided by its handles is considered neither good nor bad with a mean value of 0.8 and a standard deviation of ±2.2. As can be deduced from Fig. 6, the rating of the Stryker model’s hardness of the handle surface shows a high degree of variation. The handles are considered to be too hard by half the persons asked. This may be due to the pronounced naps on the textured surface. However, 17 % of the Ss felt that the surface was too soft – rated with -3.5 –, and only one third of those asked was satisfied with the handles’ hardness. The Stollenwerk stretcher seems to have the right choice of handle material. 75 % are satisfied with the hardness, and the remaining 25 % only considered the material “somewhat” too hard (-0.5 ±1.0). The result for the Ferno stretcher is completely different. The handle surface is considered to be too hard by 83 % of the Ss (-3.0 ±1.5). Only 17 % were satisfied with the chosen material. In order to improve the paramedics’ freedom of motion while handling the stretcher, the carrying handles of all three products are length-adjustable. The increased distance between the handles and the stretcher makes lifting easier. If the handles are extended too far, however, the handling becomes difficult in narrow rooms, hallways, or staircases due to the stretcher’s increased overall length. An optimum must be found in which the handles are extended sufficiently, but there are no adverse effects on the handling yet. This goal can only be achieved, however, if a large number of adjustment possibilities exist. The Stryker and Ferno models’ locking button is positioned on the handle (cp. Fig. 2

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and Fig. 3) and can be operated relatively well with the thumb. The Stollenwerk model‘s button is at the end of the handle (cp. Fig. 4), which results in unsatisfactory positioning and operability. The Ss prefer to have the locking button positioned on the handle to the positioning at the end of the handle. The Ferno stretcher’s carrying handles can be set to 5 different lengths while the Stryker model has three settings and the Stollenwerk model has two. A larger number of settings is preferred.

Figure 6:

Subjective assessment of hardness of stretchers´ handle surface (evaluation in % of all Ss (N=12) as well as means and standard deviations of the scaled assessments)

Figure 7 illustrates that the handling of the stretcher during the attaching to the gurney is apparently “fairly” easy (1.7 ±2.0) in the Stryker system. The same is true for the Stollenwerk system (with values of 1.3 ±1.9). This process is markedly more difficult for the paramedics with the Ferno system (-2.1 ±1.9). It is also relatively easy to release the stretchers’ side bow on the Stryker model (1.3 ±1.6) and the Stollenwerk model (0.4 ±1.9), whereas the operation is somewhat difficult in the Ferno model (–0.8 ±2.3). There is room for improvement in all systems. Three different mechanisms for the release of the backrest are realized at the different stretchers: a large handlebar, which can be reached with either hand on the Stryker backrest, a handle, which must be pulled with the fingers of the left hand on the Ferno backrest, and a button, which must be depressed with the thumb of the right hand on the Stollenwerk backrest. The rating of the release mechanism for the backrest shows that the new developments on the Stryker model have resulted in “good” operability (3.0 ±1.0), reach (3.0 ±1.0), and design (2.8 ±1.3). The rating for the very similar roll-in system manufactured by Stollenwerk (operability: 2.1 ±1.4; reach: 1.4 ±1.4; design: 0.9 ±1.4) is also positive for all three design parameters, but the Stryker model’s advantage for this operational element is apparently subjectively noticeable. The Ferno system is in need of improvement. The release mechanism is generally rated negatively (operability: -1.3 ±1.7; reach: -0.3 ±2.1; design: -0.7 ±1.8).

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Subjective assessment of the stretchers´ handling during attaching to the gurney as well as different release mechanisms on the stretchers (means and standard deviations of 12 Ss)

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3.3 Subjective evaluation of the gurneys’ handles and control elements Not only the stretcher is handled during patient transport, but also the gurney. The round crosssectional shapes of the handlebars of the Stollenwerk and Stryker models are clearly preferred to those of the Ferno model, which, in this case, also has a round cross section (cp. Figs. 2-4). Analogous to the rating of the stretchers’ handles, an increased risk of tender spots exists (-2.4 ±1.8). The Stryker model exhibits an increased risk as well (-1.9 ±1.8). This drawback apparently results from the height of the naps on the textured surface of the handlebars. The surface is designed in the same way as on the handles, which are on the stretcher. They again ensure “good” grip (2.7 ±1.7), but tender spots are more likely to occur. The grip in the touch area of the Ferno gurney is insufficient (ratings of -3.1 ±1.4). While the “manipulating” of the Stollenwerk gurney is not associated with a substantial risk of tender spots (-0.4 ±0.7), it is rated markedly worse with respect to the grip of the handlebars than the contoured area on the Stryker gurney. The foldout handles on the Stryker and Stollenwerk gurneys (see Figs. 2 and 4) are a useful aid. For one, the arms do not have to be positioned in front of the body during lifting, but can be on the sides of the body. Furthermore, folding out these handles increases the distance between carrier and the system, which allows a straight posture of the torso during lifting. Figure 8 shows that the Ss did indeed prefer the foldout handles to the handlebars (Stryker: 2.9 ±1.5; Stollenwerk: 2.8 ±1.4). On the Stryker model, the handles at the head of the gurney are identical to the ones on the handlebar and the stretcher. Thus, the ratings of the cross-sectional shape (2.6 ±1.7), the risk of developing tender spots (-2.2 ±1.6), and the grip (1.4 ±2.6) are similar. Among the results for the Stollenwerk model, the better rating of the risk of developing tender spots and the grip of the handles at the head of the gurney (1.9 ±2.4) versus the grip on the stretcher (0.8 ±2.2) is noticeable. A comparison of the handles shows that the handles on the stretcher have a smooth surface, whereas the handles on the gurney are slightly gummy and have minor naps on the textured surface.

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Figure 8:

K. Kluth and H. Strasser / Subjective evaluation of ambulance cots

Subjective assessment of the foldout handles at the head of the Stryker and Stollenwerk gurney (means and standard deviations of 12 Ss)

The gurneys’ height adjustment through the release of the legs’ locking mechanism exhibits further differences in the systems’ handling. The Stollenwerk and Stryker models use handlebars and release levers underneath to lift and lower the gurney (see Figs. 2 and 4). The palms face the user, and the release levers are operated with the index fingers. The Ferno model requires maximum supination (outward rotation of the arms) to grip the handlebars, i.e., the palms face away from the user. In order to release the height adjustment, the right hand has to reach for and pull the “pull lever” (see Figs. 3 and 11). Once the front leg at the head of the Ferno gurney has been released, the loading into the ambulance requires the paramedics to fully lift the gurney until it has been completely placed in the ambulance since the gurney’s foot end leg gets released at the same time as the front leg. The other models allow the separate release of the front leg. The roll-in system – still supported by the foot end leg – is then placed in the ambulance. Only once the foot end leg reaches the ambulance does this leg get released, and the gurney needs to be lifted. Thus, the phase during which the system must be lifted is substantially shorter in the Stollenwerk and Stryker models, which corresponds with substantially lower strain. Figure 9 illustrates the negative ratings of the Ferno model with respect to the uncomfortable shape (-3.3 ±0.9), the bad reach (-3.2 ±1.3), the increased danger of slippage at the lever for the height adjustment (-2.9 ±1.3) as well as the operability of the height adjustment (-2.4 ±1.6). The Ss clearly preferred the two competitors’ products, with a slight edge for the Stryker gurney.

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Figure 9:

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Subjective assessment of the gurney´s height adjustment lever (means and standard deviations of 12 Ss)

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3.4 Subjective evaluation of strain and comfort while using stretcher and gurney The weight of none of the stretchers (without a patient dummy) is generally rated as “acceptable,” i.e., the values – as shown in Fig. 10 – are in close proximity to zero. The weight of the lightest stretcher (22.5 kg for the Ferno stretcher) is rated as “fairly” high just like the Stollenwerk stretcher, which is 0.5 kg heavier. The weight of the only marginally heavier Stryker stretcher (approximately 24 kg), however, is already rated as “rather” high (with –1.6 ±1.5). The repeatedly worse ratings of the Ferno stretcher find their continuation in the ratings of the strain from carrying a patient dummy horizontally (-2.0 ±1.5) and in a staircase (-2.6 ±1.2) as well as the transferring of the loaded stretcher onto the gurney (-1.3 ±1.4). The Stryker stretcher is easier to carry horizontally (-1.2 ±1.5), in a staircase (-1.3 ±1.5), and during transferring (-0.8 ±0.9). The lightest stretcher (Stollenwerk) receives the best subjective ratings. During carrying horizontally (-1.0 ±1.2), in a staircase (-0.9 ±1.1), as well as during the transferring onto the gurney (-0.8 ±1.0), it receives the best marks. Since the Ferno gurney requires the pulling of the height adjustment lever during lifting (see Fig. 11), it is not possible for the right hand to grip the handlebar in the same fashion that the left hand can. Conversations with the Ss confirmed that this causes a large portion of the weight to rest on the left arm, which explains the negative rating for the items “comfort during lifting the system into a moving position” (-1.5 ±2.0), “comfort during loading the stretcher into the ambulance” (-1.8 ±2.5), and the high “strain during loading the stretcher into the ambulance” (-1.9 ±2.2) for the Ferno gurney. The Stollenwerk gurney is rated slightly better than the Stryker model on these three items.

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Figure 10: Subjective assessment of the stretcher´s weight without a patient dummy and of the strain while carrying and transferring the stretcher with a patient dummy (means and standard deviations of 12 Ss)

Figure 11: Subjective assessment of the comfort during lifting the system into the moving position and of the comfort and strain during loading the system into the ambulance (means and standard deviations of 12 Ss)

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3.5 Subjective overall assessment of the handling of stretcher and gurney The summary of the ratings of the various details leads to an overall evaluation of the three tested models based on the work tests (cp. Fig. 12). The handling of both the stretcher and the gurney of the Ferno model is clearly rated negatively by the 12 Ss. With average ratings of -1.7 ±1.9 and even -2.8 ±1.1, respectively, virtually none of the 12 persons who participated in the work tests submitted a positive rating. In contrast, the other two models received rather positive ratings, with little differences between the 2 models. In light of the relatively small standard deviations of no more than ±2.1 and means of approximately +3.0, it can be assumed that all Ss submitted a positive assessment of the handling of both the stretcher and the gurney.

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Figure 12: Subjective overall assessment of the handling of stretcher and gurney (means and standard deviations of 12 Ss)

4. Conclusions It was shown that weight differences of 1.5 kg – with weights of 22.5 kg to 24 kg – do not have any relevant effects on the overall strain objectified via physiological measurements (cp. preceding Chapter 15.1 of this book). The weight differences would have to be substantially larger for objective measuring results to be obtained. This does not mean, however, that a further reduction of the stretchers’ weight is not desirable, especially since model-specific differences were quite noticeable subjectively. The handle design should be based on the anatomy of the hand. If handles have a circular cross section instead of a rounded rectangle or hexagon, they should at least possess a bulbous longitudinal section for better distribution of the pressure. The tests showed that the handles’ surface material and surface structure on the Stollenwerk gurney best meet these requirements. Foldout handles on the gurney are a useful aid. For one, the arms do not have to be positioned in front of the body during lifting, but can be on the sides of the body. Furthermore, folding out these handles increases the distance between carrier and the system, which allows a straight posture of the torso during lifting. This is the main reason why the paramedics use these handles. Thus, future designs should incorporate additional handles at the foot of the gurney despite the lower loads that can be expected at that end. These and other details (not mentioned here) of the extensive subjective ratings offer a sufficient amount of suggestions for several concrete improvements, especially in the design of the different systems.

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The goal must be to reduce the paramedics’ extraordinarily high physical strain, which was measured through peripheral-physiological methods (KLUTH et al. 2007). More attention should be paid to the compatibility (cp. STRASSER 1995) between the human organism’s characteristics and the controllable technical components of the systems. Anatomical and physiological characteristics – e.g. of the hand – are a guiding principle that must not be neglected in the design of working tools according to the principle “fitting the hand equals fitting the person”. The quality of a handle depends on whether ergonomic demands dominate over the ideas of designers or perhaps also over historical forms. Ergonomic variations of the design should be based not only on theoretical considerations, but they must also be quantified through their actual effects at work. It must be mentioned that the results from the subjective ratings, which are often not free of bias and uncontrollable transfer effects (see, e.g., KENDALL et al. 1994), must be evaluated in relation to objective measurements, e.g., electromyographic registrations. This fact, however, is not new but has already been observed and demonstrated in different studies (cp. BÖHLEMANN et al. 1994; STRASSER 2000). Nevertheless, the objective measurements should also never be considered independently. Only the multidimensional approach (cp. STRASSER 2002/2003) can ensure more consistent results in the evaluation of a working tool. 5. References AYOUB, M.M. and MITAL, A. (1989) Manual Handling. Taylor and Francis, London BÖHLEMANN, J.; KLUTH, K.; KOTZBAUER, K. and STRASSER, H. (1994) Ergonomic assessment of handle design by means of electromyographic and subjective ratings. Applied Ergonomics 25 (6) 346-354 FURBER, S.; MOORE, H.; WILLIAMSON, M. and BARRY, J. (1997) Injuries to ambulance officers caused by patient handling tasks. Journal of Occupational Health and Safety 13 (3) 259-265 HARTMANN, B. (2000) Prävention arbeitsbedingter Rücken- und Gelenkerkrankungen – Ergonomie und arbeitsmedizinische Praxis. Ecomed-Verlag, Landsberg/Lech KELLER, E. and STRASSER, H. (1998) Electromyographic and subjective evaluation of a wrist rest for VDT operators. Occupational Ergonomics 1 (4) 239-257

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KENDALL, C.B.; SCHOENMARKLIN, R.W. and HARRIS, G.F. (1994) A method for the evaluation of an ergonomic hand tool. In: AGHAZADEH, F. (Ed.) (1994) Advances in Industrial Ergonomics and Safety VI. Taylor and Francis; London, pp. 539-545 KLUTH K.; KELLER, E. and STRASSER, H. (2007a) Ergonomics in the rescue service – Part 1: Strain-oriented evaluation of ambulance cots. In: STRASSER, H. (Ed.) Assessment of the Ergonomic Quality of Hand-Held Tools and Computer Input Devices. IOS Press, Amsterdam, Chapter 15.1, pp. 255-266 KLUTH, K.; PAULY, O.; KELLER, E. and STRASSER, H. (2007b) Assessment of the ergonomic quality of fire nozzles. In: STRASSER, H. (Ed.) Assessment of the Ergonomic Quality of Hand-Held Tools and Computer Input Devices. IOS Press, Amsterdam, Chapter 14, pp. 239-254 KNAPIK, J.J.; HARPER, W. and CROWELL, H.P. (1998) Physiological factors in stretcher carriage performance. European Journal of Applied Physiology 79, 409-413 KNAPIK, J.J.; HARPER, W.; CROWELL, H.P.; LEITER, K. and MULL, B. (2000) Standard and alternative methods of stretcher carriage: performance, human factors, and cardiorespiratory responses. Ergonomics 43, 639-652 RESTORFF, W. VON (2000) Physical fitness of young women: Carrying simulated patients. Ergonomics 43 (6) 728-743 RICE, V.J.B.; SHARP, M.A.; THARION, W.J. and WILLIAMSON, T.L. (1996a) The effect of gender, team size, and a shoulder harness on a stretcher-carry task and post carry performance. Part I: A simulated carry from a remote site. International Journal of Industrial Ergonomics 18, 27-40 RICE, V.J.B.; SHARP, M.A.; THARION, W.J. and WILLIAMSON, T.L. (1996b) The effect of gender, team size, and a shoulder harness on a stretcher-carry task and post carry performance. Part II: A mass-casualty simulation. International Journal of Industrial Ergonomics 18, 41-49 STRASSER, H. (1995) Ergonomics efforts aiming at compatibility in work design for realizing preventive occupational health and safety. International Journal of Industrial Ergonomics16,: 211-235

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STRASSER, H. (Ed.) (2000) Ergonomic Quality of Hand-Held Tools – Electromyographic and Subjective Analysis of Strain. Ergon-Verlag; Stuttgart (in German) STRASSER, H. (2002/2003) Work physiology and ergonomics in Germany: From the past to future challenges. Occupational Ergonomics 3 (1) 19-44 Standards, guidelines, regulations

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N.N. (1996-12) German manual handling of load decree (LasthandhabV). Verordnung über Sicherheit und Gesundheitsschutz bei der manuellen Handhabung von Lasten bei der Arbeit vom 4. Dezember 1996, BGBl. I; p. 1841

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Authors • Dr. Jürgen Böhlemann Sales Manager Fa. Leonhard Breitenbach GmbH P.O. Box 11 11 52 57081 Siegen GERMANY E-mail: [email protected] • Prof. Dr.-Ing. habil. Prof. e. h. mult. Dr. h. c. mult. Hans-Jörg Bullinger Präsident der Fraunhofer-Gesellschaft Hansastr. 27c 80686 München GERMANY E-mail: [email protected] • Professor Hsiu-Chen Chung Department of Industrial Management Huafan University No. 1, Huafan Road Shihtin Hsiang Taipei Hsien 223 TAIWAN, R.O.C. E-mail: [email protected] • Ph. D. P. Eng. Prof. Biman Das Editor-in-Chief Occupational Ergonomics Department of Industrial Engineering Dalhousie University P.O. Box 1000 Halifax, Nova Scotia B3J 2X4 CANADA E-mail: [email protected] • Dipl.-Wirt. Ing. René Fleischer University of Siegen Ergonomics Division Paul-Bonatz-Str. 9-11 57068 Siegen GERMANY • Dipl.-Ing. Adolf Hoffmann University of Siegen Ergonomics Division Paul-Bonatz-Str. 9-11 57068 Siegen GERMANY • Dr. Erwin Keller University of Siegen Ergonomics Division Paul-Bonatz-Str. 9-11 57068 Siegen GERMANY E-Mail: [email protected] • Dr. Horst G. Kellermann University of Siegen Ergonomics Division Paul-Bonatz-Str. 9-11 57068 Siegen GERMANY • PD Dr. Karsten Kluth Darmstadt University of Technology Institute of Ergonomics Petersenstr. 30 64287 Darmstadt GERMANY E-mail: [email protected]

• Dipl.-Ing. Knut Kotzbauer University of Siegen Ergonomics Division Paul-Bonatz-Str. 9-11 57068 Siegen GERMANY • M. Sc. Chih-Long Lin National Tsing Hua University Department of Industrial Engineering and Engineering Management 101 Kuang Fu Road Sec. 2, Hsinchu 300 TAIWAN, R.O.C. • Dr.-Ing. Karl-Werner Müller Institut für Ergonomie Technische Universität München Boltzmannstr. 15 85748 Garching GERMANY E-mail: [email protected] • Dr. Türker Özalp Uludag Universitesi Muhendislik Mimarlik Fakultesi Endustri Muhendisligi Bölumu 16059 Görukle Bursa TURKEY E-mail: [email protected] • Dipl.-Wirt. Ing. Olaf Pauly University of Siegen Ergonomics Division Paul-Bonatz-Str. 9-11 57068 Siegen GERMANY • Prof. Ph. D. Yuh-Chuan Shih National Defense University Institute for Logistics Management P.O. Box 90046-15 Chung-Ho, Taipei TAIWAN, R.O.C. E-mail: [email protected] • Prof. Dr. Helmut Strasser University of Siegen Ergonomics Division Paul-Bonatz-Str. 9-11 57068 Siegen GERMANY E-mail: [email protected] • M. Sc. Baoqiu Wang Korschenbroicherstr. 140 41065 Mönchengladbach GERMANY • Prof. Ph. D. Mao Jiun J. Wang National Tsing Hua University Department of Industrial Engineering and Engineering Management 101 Kuang Fu Road, Sec. 2, Hsinchu 300, TAIWAN, R.O.C. E-mail: [email protected] • Dipl.-Wirt. Ing. Dirk Zühlke University of Siegen Ergonomics Division Paul-Bonatz-Str. 9-11 57068 Siegen GERMANY

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Author Index Böhlemann, J. ..................................................................................................................................... 227 Bullinger, H.-J. ....................................................................................................................................... 1 Chung, H.-C. .............................................................................................................................. 153, 183 Das, B. .......................................................................................................................................... 23, 207 Fleischer, R. .................................................................................................................................... 75, 89 Hoffmann, A. ...................................................................................................................................... 127 Keller, E. .......................................................................................... 41, 75, 89, 101, 111, 173, 239, 255 Kellermann, H.G. ............................................................................................................................... 143 Kluth, K. ....................................................................................... 41, 143, 153, 197, 227, 239, 255, 267 Kotzbauer, K. ..................................................................................................................................... 227 Lin, C.-L. .................................................................................................................................... 183, 191 Müller, K.-W. ....................................................................................................................................... 57 Özalp, T. ............................................................................................................................................. 173 Pauly, O. ............................................................................................................................................. 239

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Shih, Y.-C. .................................................................................................................................. 183, 191 Strasser, H. ............................................................... 1, 41, 57, 67, 75, 89, 101, 111, 127, 143, 153, 173 ............................................................................................................ , 183, 191, 197, 227, 239, 255, 267 Wang, B. ........................................................................................................................................ 67, 127 Wang, M.-J.J. ............................................................................................................................. 183, 191 Zühlke, D. ........................................................................................................................................... 197

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