Footwear Traction: Implications on Slips and Falls 9789819978229, 9789819978236

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Biomedical Materials for Multi-functional Applications

Arnab Chanda · Shubham Gupta · Subhodip Chatterjee

Footwear Traction Implications on Slips and Falls

Biomedical Materials for Multi-functional Applications Series Editors Arnab Chanda, Berlin, Germany Sarabjeet Sidhu, null, London, UK

The series ‘Biomedical Materials for Multi-functional Applications’ is intended for the audience with an interest in biomaterials for various applications, including bioimplants, bio-instrumentation, biomechanics, and tissue engineering. The book titles under this series will bring different areas of biomedical engineering, bioengineering, materials science, mechanical engineering, and biosciences under one umbrella of biomedical materials. The books in this series will provide deep insights into the stateof-art of biomedical materials and deal with diverse and interdisciplinary aspects of biomaterials, ranging from basic science to host responses, as well as real-time applications. Some of the topics include but are not limited to: functional materials, smart materials, biomimetic materials, polymers, composites, ceramics, metals, alloys, and nanomaterials. This series will provide the most recent advances in biomedical materials, including design, synthesis, processing, and characterization. This series will accept both authored and edited volumes, including monographs, textbooks, reference works, and handbooks. This series aims to publish new advances in biomedical materials that have been made in the fabrication and development of biomaterials for different applications and the advancement of principles, theories, and designs. Each book title in the series will give a comprehensive overview of pioneering topics, and the readers will gain new knowledge and insights about the topic. As a collection, the series on ‘Biomedical Materials for Multi-functional Applications’ is anticipated to benefit a wide audience in academia, the research community, and industry.

Arnab Chanda · Shubham Gupta · Subhodip Chatterjee

Footwear Traction Implications on Slips and Falls

Arnab Chanda Indian Institute of Technology (IIT) Delhi New Delhi, India

Shubham Gupta Indian Institute of Technology (IIT) Delhi New Delhi, India

Subhodip Chatterjee Indian Institute of Technology (IIT) Delhi New Delhi, India

ISSN 2731-9695 ISSN 2731-9709 (electronic) Biomedical Materials for Multi-functional Applications ISBN 978-981-99-7822-9 ISBN 978-981-99-7823-6 (eBook) https://doi.org/10.1007/978-981-99-7823-6 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore Paper in this product is recyclable.

Preface

The physics of footwear traction and its relationship with slips and falls are covered in this book titled Footwear Traction. It offers a thorough introduction to slips and falls and emphasizes the importance of footwear traction in preventing such accidents. These chapters emphasize the study of their impact on traction throughout commonly available footwear and systematically altered shoes are covered in the book, along with the ergonomics involved in the design of slip-resistant tread patterns. Chapter 1 introduces the problems of slips, trips and falls. It further introduces different types of slips, terminologies associated with these types of injuries and psychophysical approaches to ascertain slips and falls. This chapter sets the base to understand and ascertain the quantification approach towards slips and falls. Chapter 2 presents the fall-related accidents due to slips documented all over the world by the various government and private safety organizations. In addition to this, this chapter discusses the financial strain placed on various sectors by the need to compensate workers who sustain injuries as a result of falls. It also highlights how falls caused by slips and falls have contributed to occupational accidents throughout the years. Chapter 3 studies the basic principles of slip resistance. The study of the biomechanics of slips is attempted in this chapter, with a particular emphasis on how the ground reaction forces change when slipping occurs. This chapter also discusses how kinetic and kinematic variables change during slipping. Chapter 4 describes the idea of shoe slip testing and its effects. It also discusses various kinds of footwear that individuals wear at home and at work. Additionally, there is a strong association between the change in gait characteristics and the footwear used. This chapter also covers the issue of contact between shoes and floors when there are contaminants present. Chapter 5 describes the conventional human sliding tests used to evaluate the slip risk associated with wearing shoes and being barefoot. This chapter also discusses the significance of determining whether a slip is caused by shoes or bare feet and how it affects human safety. The several conventional techniques for determining how likely different types of footwear are to slip are discussed in Chap. 6. The chapter also presents an overview of all the research work conducted over time employing various slip testers.

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Chapters 7–10 represent the effect of systematically modified footwear tread orientations on slip performance. Chapter 7 studies the effect of horizontal tread patterns (or treads perpendicular to the slipping direction) on the slip resistance performance. The horizontal patterns were systematically modified along its width and gaps to ascertain an optimal design which provided better slip resistance. Similar strategies were used for Chaps. 8–10 which studied vertical, square and oblique tread orientations, respectively. Chapter 11 extensively discusses footwear wear and possible wear mechanisms. The mechanism of interaction between footwear imperfections and flooring surface abnormalities was also emphasized. The study examined the fundamental theory that underlies the interaction between the microscopic characteristics of the footwear surface and the flooring surface and how this interaction causes footwear wear. Chapter 12 presents the state-of-the-art research work on the study of the effect of footwear wear (or outsole wear) on the available traction due to dry, wet and oil-contaminated floor interactions. This chapter looked at the degree of slipperiness of different shoe types in both new and worn conditions. At last, Chap. 13 provides insight into the new developments in the realm of slip testers. The significance of traction testing in sports like running, football and soccer, basketball and tennis was discussed, with a focus on how it can be applied to the development of better athletic footwear and the prevention of foot injuries sustained by athletes while participating in a variety of sports. This chapter also provides a quick discussion on the various foot-loading conditions that may be encountered during different sporting activities. The key biomechanical factors observed during sporting activities and how they historically influenced the development of various traction testing equipment were also discussed. The application of slip testing devices on footwear worn in sports such as football, tennis and badminton was highlighted. This book is anticipated to serve as a key reference textbook for research in ergonomics, design engineering, material selection, biomechanics, footwear science and kinesiology. This book can be used as a comprehensive source for academics, ergonomists, designers, material scientists, civil engineers, biomedical engineers, footwear manufacturers and safety engineers in various areas and we are highly confident that this contribution will benefit all the readers in different ways. New Delhi, India October 2023

Arnab Chanda [email protected] Shubham Gupta [email protected] Subhodip Chatterjee [email protected]

Contents

1

Introduction to Slips and Falls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Introduction to Slips, Trips and Falls . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Types of Slips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Terminologies Associated with Slips and Fall Injuries . . . . . . . . . 1.4 Quantification Approach Towards Slips and Falls . . . . . . . . . . . . . 1.4.1 Extrinsic Factors in Slip-Related Falls . . . . . . . . . . . . . . . 1.4.2 Intrinsic Factors in Slip-Related Falls . . . . . . . . . . . . . . . . 1.5 Psychological and Physical Approaches to Ascertain Slips . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 2 3 5 6 7 8 9

2

Worldwide Statistics of Slips and Falls . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Slips-Related Accidents and Their Effect in the Twentieth Century . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Slips-Related Accidents and Their Effect in the Twenty-First Century . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Slips-Related Accidents in Bathrooms . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3

Basic Principles of Slip Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Biomechanics of Normal Gait . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Biomechanics of Slips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Human Gait Kinetics During Slips . . . . . . . . . . . . . . . . . . 3.3 Variation of Gait on Slippery Surfaces . . . . . . . . . . . . . . . . . . . . . . . 3.4 Recovery Responses After Slipping Incidents . . . . . . . . . . . . . . . . 3.5 Measurement of Slip Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 Available Co-Efficient of Friction . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

21 21 24 25 25 26 27 28 28

4

Slip Resistance of Footwear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Prevention of Falls by Footwear Implementation . . . . . . . . . . . . . . 4.2 Interrelationship Between Footwear Styles and Gait Parameters Related to Falls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

31 31

11 13 15 16

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4.3 Shoe-Floor Contaminant Friction . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Impact of Slip-Resistant Footwear . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Formal Footwear Slip Risk Scenario . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

32 34 35 36

5

Human Slipping Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Slipping Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Footwear Slipping Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

39 39 43 47

6

Mechanical Footwear Traction Testing Methods . . . . . . . . . . . . . . . . . . 6.1 Need for Mechanical Slip Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Introduction to Slip Testers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Horizontal Pull Slipmeter (HPS) . . . . . . . . . . . . . . . . . . . . 6.2.2 Portable Articulated Strut Tribometer (PAST) . . . . . . . . . 6.2.3 Portable Friction Tester (PFT) . . . . . . . . . . . . . . . . . . . . . . 6.2.4 British Portable Skid Tester (BPST) . . . . . . . . . . . . . . . . . 6.2.5 Step Simulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.6 Slip Simulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Comparison of Slip Testers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Slipping Experiments with Whole Footwear . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

49 49 50 51 51 51 52 52 52 53 58 62

7

Effect of Horizontal Outsole Tread Orientation on Slip Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Materials and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 Fabrication of Footwear Outsoles . . . . . . . . . . . . . . . . . . . 7.2.2 Slip Testing Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1 Frictional Assessment of Footwear Outsoles . . . . . . . . . . 7.3.2 Influence of Tread Width on Traction Performance of Footwear Outsoles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.3 Influence of Tread Gap on Traction Performance of Footwear Outsoles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8

Effect of Vertical Outsole Tread Orientation on Slip Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Materials and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Footwear Outsole Designs . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 Slip Testing Experiments . . . . . . . . . . . . . . . . . . . . . . . . . .

65 65 66 66 68 69 69 70 70 71 71 73 73 74 74 75

Contents

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8.3

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Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1 Traction Performance of Footwear Outsoles . . . . . . . . . . 8.3.2 Influence of Tread Width and Gap on Traction Performance of the Outsoles . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Effect of Square Outsole Tread Orientation on Slip Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Materials and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.1 Fabrication of Footwear Outsoles . . . . . . . . . . . . . . . . . . . 9.2.2 Slip Testing Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.1 Frictional Assessment of Footwear Outsoles . . . . . . . . . . 9.3.2 Influence of Tread Dimension on Traction Performance of Footwear Outsoles . . . . . . . . . . . . . . . . . . 9.3.3 Influence of Tread Gap on Traction Performance of Footwear Outsoles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

78 79 79 81 81 82 82 83 84 84 85 87 87 90

10 Effect of Oblique Outsole Tread Orientation on Slip Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 10.2 Materials and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 10.2.1 Fabrication of Footwear Outsoles . . . . . . . . . . . . . . . . . . . 94 10.2.2 Slip Testing Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . 95 10.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 10.3.1 Frictional Assessment of Footwear Outsoles . . . . . . . . . . 96 10.3.2 Influence of Tread Dimension on Traction Performance of Footwear Outsoles . . . . . . . . . . . . . . . . . . 99 10.3.3 Influence of Tread Gap on Traction Performance of Footwear Outsoles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 10.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 11 Footwear Wear and Wear Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Hypothesis of Footwear Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.1 Footwear Floor Interaction . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Theory Behind Footwear Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4 Adhesion and Hysteresis Friction During Footwear Wear . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

105 105 106 107 108 109 110

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12 Effect of Footwear Wear on the Available Traction . . . . . . . . . . . . . . . 12.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Materials and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.1 Development of Custom Outsoles . . . . . . . . . . . . . . . . . . . 12.2.2 Slip Testing Device and Protocol for Accelerated Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3 Results and Discussions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3.1 Friction Results of Footwear Outsoles in New Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3.2 Friction Results of Footwear Outsoles: First Wear . . . . . 12.3.3 Friction Results of Footwear Outsoles: Second Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3.4 Friction Results of Footwear Outsoles: Third Wear . . . . 12.3.5 Outsole Worn Region Area Versus ACOF Outcomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

113 113 114 114

13 New Developments and Challenges in the Area of Slip Testers . . . . . 13.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2 Foot Loading in Sports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.1 Running . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.2 Football and Soccer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.3 Basketball . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.4 Tennis and Badminton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.3 Traction Testing Methods Employed in Sports and Athletic Movements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.4 Challenges in Simulating Foot Loading in Traction Tests . . . . . . . 13.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

127 127 128 128 129 130 131

114 116 116 117 117 118 120 120 124

132 135 138 138

About the Authors

Dr. Arnab Chanda is an assistant professor at the Centre for Biomedical Engineering, Indian Institute of Technology (IIT) Delhi, India, and a joint faculty at the Department of Biomedical Engineering, All India Institute of Medical Sciences (AIIMS), Delhi, India. He is also the founder of a startup company BIOFIT Technologies LLC, USA. Dr. Chanda is an expert in the area of biomechanics, especially foot mechanics. Dr. Chanda has been studying slips and falls caused due to footwear for more than eight years. He has conducted experiments with more than 100 human subjects, with a range of new and worn footwear, in gait laboratories to study unexpected slips and measure the biomechanical parameters at different stages of slips and falls. Also, he has worked with several footwear traction measurement devices used across the industry, such as BPN and SATRA, for measurement of traction in 200+ footwear designs, and provided consultancy services to footwear companies, such as Nike. Recently, the most advanced footwear traction characterization device, which is also the first portable device, was developed in his with funding support from government and private agencies. This pioneering device is current being used to study traction across footwear in a range of cohorts. To date, Dr. Chanda has been involved in the research of footwear traction as a part of University of Pittsburgh, USA; IIT Delhi; AIIMS Delhi; and Indian Spinal Injury Centre (ISIC), Delhi. He has received young researcher awards from ASME and MHRD, holds 7 US patents and 2 Indian patents, and has authored more than 50 articles in reputed international journals, and his research has been featured in news in Times of India, NDTV, and IndiaWest, USA. Currently, Dr. Chanda heads the “Disease and Injury Mechanics Lab (DIML)”, where his team is working on developing cutting-edge technologies to study and improve traction in footwear. Shubham Gupta is a Ph.D. scholar in the Disease and Injury Mechanics Lab (DIML). He completed his B.Tech. in Mechanical Engineering from the University of Pune in 2019. His research is focused on the study of traction performance of worn footwear. He was instrumental in the development of the advanced portable footwear traction characterization device and has also experimented with a range of

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worn footwear to estimate their traction reduction and determine replacement thresholds. Shubham has also developed a pioneering computational fluid dynamics-based (CFD) model to study the underlying physics behind the traction properties presented by a new or worn footwear on slippery fluid contaminated floorings. He also aims to integrate different fields of engineering that would produce impressive technologies to benefit mankind. Subhodip Chatterjee is a Ph.D. Scholar in the Disease and Injury Mechanics Lab (DIML). He previously worked as an assistant professor at Regent Education and Research Foundation College of Engineering, West Bengal. He has completed his M.Tech. in Metallurgy and Materials Engineering from the Indian Institute of Engineering Science and Technology, Shibpur, West Bengal, and B.Tech. in Mechanical Engineering from the West Bengal University of Technology. His research focus is on the experimental study of barefoot and footwear traction. He has tested a range of footwear-floor-contaminant combinations and studied the science of footwear traction through systematically modifying the footwear tread patterns. His work has been valuable in establishing correlations among footwear tread design, traction, and the incidences of slips and falls.

Chapter 1

Introduction to Slips and Falls

1.1 Introduction to Slips, Trips and Falls Slip, trip and fall incidents place a significant financial and health burden on the workplace. Slips and falls, according to the National Floor Safety Institute, are the main reasons for workers’ compensation applications [1]. A fall, as defined by the World Health Organization, is an event that results in someone coming to rest unintentionally on the surface of the ground, a floor, or another lower level [2]. Falls are an important health issue, as the global population increasingly ages and people are living longer. With increasing age, the risk of a fall rises, along with the frequency and severity of fall-related injury [3]. Falls are frequently investigated due to their prevalence worldwide and their related costs, including the human misery and economic burden they generate. Falls can either be same-level falls or falls from a certain height. When a person encounters a fall from a certain height, the point of contact must be under the faller’s initial supporting surface. Same-level falls occur when the contact point is at or above the faller’s initial supporting surface. Although same-level falls are much more common, accidents from a certain height are thought to be more likely to result in serious injuries. Over 37.3% of the falls are caused due to slips. Hospitals are one of the most common areas where slip and fall accidents are known to occur [3]. Figure 1.1 represents the most frequent factors involved in slips, trips and falls. A big problem is workplace trips, slips and falls on such floors, which can result in serious harm [5–8] and expensive repairs. Slips, trips and falls on a comparable level of prevention have been the focus of safety researchers, and they have made progress in understanding the circumstances and causes of these accidents. However, because of the complex interrelationships among the triggering elements, both external and internal, that are present or influencing the genesis of the injury, developing and putting into practice effective prevention strategies is a significant challenge. However, some researchers argue that the term “slips, trips, and falls” and

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 A. Chanda et al., Footwear Traction, Biomedical Materials for Multi-functional Applications, https://doi.org/10.1007/978-981-99-7823-6_1

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Fig. 1.1 Most frequent factors in slips, trips and falls [4]

the propensity to emphasize slipping fail to adequately recognize other significant factors that lead to injuries on level floor surfaces, including loss of balance [9]. Slipping transpires when the friction that exists between the sole of the foot or shoe and the floor surface is insufficient to deal with the forces that are generated when moving forward or backward, i.e., the interaction between a person’s foot or shoe sole and the floor. Studies on slip biomechanics have shown that slides often happen when the proceeding limb makes contact with the flooring surface (heel contact) or when the following foot pushes off (toe-off) [3]. Despite the fact that the back foot is bearing the bulk of the weight, the forces generated there are used to propel the body forward during toe-off. The leading foot receives the vertical portion of weight transfer at heel strike. The body is naturally unsteady as weight is being transmitted, depending on the secure placement of the impacting foot for brief stabilization. Therefore, velocity and acceleration at the heel upon heel impact may result in loss of balance and possible falls. Therefore, toe-off slides are less dangerous than heel strike slips [10]. A trip happens when the foot’s swing phase is abruptly stopped by insufficient ground clearance. Just a 5 mm variation in the travelling area may be necessary for someone to trip [11].

1.2 Types of Slips Li et al. [12] described that based on the length of a slide, three types of slips occur when walking. A “microslip” is a slip that is no longer than 3 cm, a “slip” refers to one that is between 8 and 10 cm and a “slide” is an uncontrollable motion of the heel that often occurs when a slip distance is greater than 10 cm. Microslips typically go unnoticed, slips are followed by automatic attempts to regain postural control and slides are more likely to end in a loss of balance and a fall. A trip happens when the foot’s swing phase is abruptly stopped because the floor wasn’t properly cleared. It may simply require walking surface asperities having a height of above 5 mm to

1.3 Terminologies Associated with Slips and Fall Injuries

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initiate the process of tripping. Falls can also result from loss of balance brought on by sudden, abrupt collision with something or someone. Comparable to how standing in an automobile that moves might result in unexpected, forced floor movement that throws the person off balance to such an extent which is enough to induce fall on the same level.

1.3 Terminologies Associated with Slips and Fall Injuries It is still unclear how slips and falls occur and what causes them. Before effective prevention measures can be put into action, the injury and accident mechanism must first be properly understood. By utilizing the principles and techniques for tribophysical, biomechanical, psychological and psychiatric research, it is necessary to identify and comprehend the series of activities or events that make up risk exposure and the start of an event that ultimately result in injury and disability. The presence of a material (factor) in the worker’s surroundings, such as a change in surface topology (such as roughness) or friction [13–16], may be characterized as this (e.g., contaminant) [17, 18]. Hagberg [17] also took into account the injury process, also known as the incident-injury-impairment-disability chain. Figure 1.2 shows the conceptual framework for slips and falls. (1) Hazard—Rouhiain et al. [19] typically characterize this as a state that has the potential to have negative effects, such as harm to people, destruction of property or the environment or a system’s decreased capacity to carry out a specified function. (2) Risk—According to Rouhiainen et al. [19], risk is a measurement of the likelihood and seriousness of a bad outcome brought on by a hazard.

Fig. 1.2 Conceptual framework for slips and falls

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(3) Risk Factor—According to epidemiological research, this is widely understood to be a trait, exposure to the environment or aspect of one’s individual behaviour or lifestyle that is referred to be linked to physiologically related diseases that are crucial to prevent. (4) Accident—An instantaneous and accidental mishap to humans produced due to an incident or a series of incidents. (5) Injury—According to one definition, injury is “the outcome of occurrences and actions that have environmental, biological and behavioural factors that are frequently reducible or eliminateable”. (6) Surface Roughness—Roughness is measured on a microscopic scale and reported as the mean height of characteristics on a walkway’s surface. Despite the fact that traction is related to surface texture of the surface of the flooring and the footwear (or foot), the usefulness of roughness calculations is diminished by the need for averaging. Additionally, it has been noted that even tiny walkway “profiles” may have the same roughness value. (7) Asperities—Individual characteristics that stand out from the surface’s “average” foundation. For greater mechanical interaction with the shoe or foot, high, pointed asperities can extend over contaminants. Specifically, with broomfinished concrete, untreated slate and some textured ceramic tiles, the depth, precision and arrangement of asperities may differ significantly over a walkway surface. Walkways made of polished marble or terrazzo, which don’t have a lot of asperities or roughness, rely more on chemical bonding than on mechanically interlocking with the foot or footwear. (8) SCOF—The SCOF which is also known as the static coefficient of friction was determined at the initial stages of slippage while the object is stationary. (9) DCOF—DCOF is the coefficient of friction that is determined when an object slides along a surface. Typically, at a steady speed when the moving item is practically stopped, the highest value of DCOF occurs. (10) TCOF—The COF (coefficient of friction) resulting from simultaneous application of vertical and horizontal contact forces, evaluated at the point of transition from static friction to steady-state dynamic friction. (11) RCOF—The pathway forces used to calculate the COF, which is recorded when a pedestrian applies a certain force as determined by the force plate in a monitored environment is known as Utilized Coefficient of Friction (UCOF). (12) ACOF—The available coefficient of friction (ACOF) is the COF as calculated by the tribometer assessment of the frictional characteristics of the inherent walkway surface. (13) Slip Resistance—Slip resistance is the equivalent force preventing shoes or a foot from slipping across a walkway’s surface. Numerous factors, including the surface of the walkway, the bottom of the footwear and the presence of foreign items between them, might affect slip resistance (ASTM F1646).

1.4 Quantification Approach Towards Slips and Falls

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1.4 Quantification Approach Towards Slips and Falls A variety of human-centred methods for measuring slipperiness have been used to calculate the dangers and risks of slipping and falling. Both the initial events—from the start of the slip to the foot slide—and the following ones—from a lack of balance to falling—have been explored using these techniques. The findings include estimations of falling frequency, subjective assessments of sense of slip and slip distance, slipperiness ratings, measurements of heel velocity, heel and torso acceleration and postural instability [10, 20]. The following human-centred methods for measuring slipperiness are used: (1) Techniques that are “subjective” comprise ranking, grading, comparing the flooring and shoes in pairs and observing the defensive responses to slipping. (2) Techniques that are objectively focused on biomechanics include assessments of ground reaction forces, friction utilization, body section movements, joint angles and moments, slip distances and velocities, centre of mass and centre of pressure trajectories or electromyography. (3) Both subjective evaluations and objective measurements are included in “combined” approaches. There are known risk factors for slip-and-fall accidents, which can be external (environmental variables), internal (human factors) or combined (system factors), and they are not just random occurrences. By definition, inadequate traction or low friction between the footwear and the flooring surface (floor, pavement, etc.) constitutes the primary risk factor for slipping [10]. Dynamic friction would decide whether or not a foot slip could be recovered from and an injury avoided, or whether the slip could result in an injury-causing fall or any other sort of harm, for instance, due to vigorous body motions for regaining balance. Dynamic friction would determine if a foot slip would be recovered and an injury preventable, but static friction is regarded to be crucial in preventing the start of slipping. A few examples of secondary risk factors (or “predisposing factors”) for slipping accidents include inadequate lighting, uneven flooring, inadequate stairway design, failure to use railings and automobile exit aids, poor postural control, ageing, vertigo, vestibular disease, diabetes, alcohol consumption and the use of anti-anxiety medications [21–23]. Falls and other sliprelated accidents may not always be caused just by the slippery shoe/floor interaction. People are more likely to sustain accidental injuries under slippery circumstances and during quick, unexpected changes in slippery conditions when secondary risk factors are present. Measurements of slipperiness and the avoidance of slip-andfall accidents and injuries appear to be further complicated by the multiplicity of risk variables and their potential cumulative effects. Figure 1.3 represents objective, subjective and combined approached towards slips and falls.

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Fig. 1.3 Objective, Subjective and Combined approaches towards slips and falls

1.4.1 Extrinsic Factors in Slip-Related Falls When there is a chance of slipping, the gait characteristics are altered in an attempt to prevent slipping. However, the atmosphere, the shoes and internal factors like age, fatigue and obesity could affect how the slip turns out. The inherent postural control systems are externally perturbed during a slip in order to regain and maintain equilibrium. A fall could occur if the intrinsic system is unable to counteract the extrinsic system’s disruption. A poor support surface has been proven to be the cause of over 50% of these falls, which is an example of an extrinsic factor leading to slips and trips [24]. The existence of an impediment, the presence of a contaminant or extreme natural conditions like heavy rain, snow or ice as well as man-made conditions like poor lighting and insufficient warning signs are all on the list of extrinsic factors that cause falls. The human postural system may become unsteady as a result of these changes to the flooring or support surface, which could eventually lead to a failure of the postural control system and a fall. Slip, trip and fall accidents are the leading cause of work absence in occupational settings. In response, the Health and Safety Administration for Occupational Safety (OSHA) and the American National Standards Institute (ANSI) have developed recommendations to provide slip-resistant walking surfaces in the workplace.

1.4 Quantification Approach Towards Slips and Falls

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According to OHSA’s basic guidelines for walking and working on surfaces, a coefficient of friction of at least 0.5 is needed to offer an adequate level of slip resistance. A greater coefficient of friction may be necessary for some tasks, such as carrying, pushing or pulling objects or climbing up and down sloping surfaces. Regardless of the type of footwear used, OSHA advocates modifying the flooring’s composition rather than modifying the flooring itself to change the COF, particularly in moist, oily or dirty work environments. This is accomplished by creating slippery flooring from the same materials that are serrated, punched or textured to increase their roughness and possibly increase the COF that is reachable in slick, slippery and wet working situations. There are numerous techniques for determining the slipperiness of a floor, which is frequently done in work environments and for developing recommendations for constructing a workplace setting and working surfaces that are safe for workers. Torus digital tribometer, ASTM F609 horizontal pull slip meter, Sigler pendulum tester, ASTM F1678 portable inclinable articulated strut tribometer and slip meter—a roller coaster type tribometer—are a few tools that assist in the slip risk assessment.

1.4.2 Intrinsic Factors in Slip-Related Falls Falls can be caused by personal, internal causes such as postural control system dysfunctions or physiological abnormalities. Falls may be caused by problems with the visual, vestibular, somatosensory or musculoskeletal systems, as well as any excessive wear and tear on these systems brought on by an overly demanding work environment. The risk of trips, falls and slips increases due to age-related physiological decline in these postural control systems’ functionality and ability to move safely with the centre of mass (COM) inside the base of support (BOS) [25]. Slips and trips were responsible for 32% of falls among young adults, but 67% of falls among older people have been linked to slips and trips [26]. Due to a decline in muscle strength, vision, sensory functions, reduced somatosensory and lowered proprioceptive feedback that occurs with normal ageing, the elderly are more vulnerable to trips, falls and injuries from these falls than younger people are [27, 28]. Age-related sensory decline and muscular deterioration, which also prolong the time it takes for the body to adjust to slippery surfaces, may be the root of the inability to control sliding reactions [29]. The older population was disproportionately affected by this issue. a result of incorrectly estimating the slipperiness of the floor using uncompensated slip parameters [27]. Excess body weight has a negative impact on slip, gait and balance factors. This negative outcome is caused by a number of biomechanical and physiological factors, including increased forward pelvic tilt and lumbar lordosis during dynamic standing and walking, decreased muscle strength due to excess adipose tissue and the inability to produce enough muscular force to maintain stability under static and dynamic conditions. [30]. The higher risk of falling linked with obesity can be ascribed to the diminished capacity to preserve balance during dynamic conditions such as the onset, detection and recovery of slip-induced falls.

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Contrarily, a scant number of studies have found no differences between obese and normal-weight people’s propensities to slip, even though obese people tend to take wider steps when it’s slippery [30]. Prior research has been done on the relationship between walking speed and slipping [31]. For walking at speeds of 1.5 m/s (slow), 1.8 m/s (medium) and 2.1 m/s (fast), on marginally slippery floors with a COF ranging from 0.12 to 0.21, McGorry et al. noticed no significant differences in forward slip distance. However, according to the significant differences between fast and slow walking considering the instantaneous forward horizontal heel velocity, the heel velocity occurred 30 ms after heel strike [31]. Compared to the adolescents, older people reported walking at a slower pace with a longer stance phase and shorter steps than they did [29]. The elderly then exhibited decreased quadriceps activation in slippery circumstances. [29]. Younger participants were better able to adjust to slippery flooring because they were able to lower their quadriceps average muscle activity after one step, but older participants’ activity stayed constant throughout their whole gait cycle [29].

1.5 Psychological and Physical Approaches to Ascertain Slips Psychophysical methods may be used in human-centred approaches to the evaluation of slipperiness. To rate the level of felt “slipperiness” on a psychophysical scale, foot movement or postural instability might be utilized as the physical stimulus. The stimulus can be evaluated objectively as well as subjectively using opinions and preferences. In addition to ground response forces measured with force platforms, objective measures can also incorporate video recording or rapid imaging of the participant’s gait. Human-centred techniques might include the simultaneous gathering of objective physiological information and subjectively evaluated data [32]. A nominal scale can be used to analyse force and motion variables (such as slip distance, slip speed, friction usage, joint angles, etc.), whereas ordinal (category) scales are used to quantitatively treat subjective assessment data. Psychophysical scaling was considered as an alternative to the standard class internationalization of opinions and preferences. For assessing slipperiness, a variety of ‘purely’ subjective methods have been used (such as paired comparisons). Human test subjects appeared to be able to distinguish between whether floors are slippery when they are dry, wet or contaminated. Tactile slipping resistance cues are the most reliable measures of the coefficient of friction in a variety of experimental conditions, but particularly on wet surfaces, as stated by Cohen et al. [33]. In comparison to apparatus-based friction measurement techniques, Leamon et al. [10] claimed that monitoring microslip length or slip distance during slipping episodes would be a more precise way to assess slipperiness in contrast to apparatus-based friction measurement techniques. Research on employees’ perceptions of slipping when performing standing tasks, such as lateral reach tasks, was recently published by Chiou et al. [34]. They also

References

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linked their impression of the slipperiness scale to participants’ postural sway and instability. Researchers found that attentive workers had less postural instability when doing their duties. The angle of inclination at the point where utilizing the ramp became dangerous was translated geometrically into a traction coefficient to produce the estimate of slip resistance that was considered to be subjective.

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17. Mats H, David C (2023) Conceptual and definitional issues in occupational injury epidemiology. Hagberg [1997]. Am J Ind Med. Wiley Online Library, 1998, Accessed: Apr. 25, 2023. https:// doi.org/10.1002/(SICI)1097-0274(199708)32:2%3C106::AID-AJIM2%3E3.0.CO;2-X 18. Chang WR, Leclercq S, Lockhart TE, Haslam R (2016) State of science: occupational slips, trips and falls on the same level* 59(7):861–883. https://doi.org/10.1080/00140139.2016.115 7214. 19. Rouhiainen V (1992) QUASA: a method for assessing the quality of safety analysis. Saf Sci 15(3):155–172. https://doi.org/10.1016/0925-7535(92)90002-H 20. Myung R, Smith JL, Leamon TB (1993) Subjective assessment of floor slipperiness. Int J Ind Ergon 11(4):313–319. https://doi.org/10.1016/0169-8141(93)90081-N 21. Fathallah FA, Grönqvist R, Cotnam JP (2000) Estimated slip potential on icy surfaces during various methods of exiting commercial tractors, trailers, and trucks. Saf Sci 36(2):69–81. https:// doi.org/10.1016/S0925-7535(00)00016-3 22. Honkanen R, Ertama L, Kuosmanen P, Linnoila M, Alha A, Visuri T (2015) The role of alcohol in accidental falls 44(2):231–235. https://doi.org/10.15288/JSA.1983.44.231 23. Malmivaara A, Heliövaara M, Knekt P, Reunanen A, Aromaa A (1993) Risk Factors for Injurious Falls Leading to Hospitalization or Death in a Cohort of 19,500 Adults. Am J Epidemiol 138(6):384–394. https://doi.org/10.1093/OXFORDJOURNALS.AJE.A116871 24. Gauchard G et al. (2010) Falls and working individuals: role of extrinsic and intrinsic factors 44(14):1330–1339. https://doi.org/10.1080/00140130110084791 25. Kim S, Lockhart T, Yoon HY (2005) Relationship between age-related gait adaptations and required coefficient of friction. Saf Sci 43(7):425–436. https://doi.org/10.1016/J.SSCI.2005. 08.004 26. Lloyd DG, Stevenson MG (1992) An investigation of floor surface profile characteristics that will reduce the incidence of slips and falls. Trans Inst Eng Aust Mech Eng 17(2):99–105 27. Lockhart TE, Woldstad JC, Smith JL, Ramsey JD (2002) Effects of age related sensory degradation on perception of floor slipperiness and associated slip parameters. Saf Sci 40(7–8):689–703. https://doi.org/10.1016/S0925-7535(01)00067-4 28. Lockhart TE (2008) An integrated approach towards identifying age-related mechanisms of slip initiated falls. J Electromyogr Kinesiol 18(2):205–217. https://doi.org/10.1016/J.JELEKIN. 2007.06.006 29. Lockhart TE, Spaulding JM, Park SH (2007) Age-related slip avoidance strategy while walking over a known slippery floor surface. Gait Posture 26(1):142–149. https://doi.org/10.1016/J.GAI TPOST.2006.08.009 30. Capodaglio P, Cimolin V, Tacchini E, Parisio C, Galli M (2012) Balance control and balance recovery in obesity. Curr Obes Rep 1(3):166–173. https://doi.org/10.1007/S13679-012-00187/METRICS 31. DiDomenico A, McGorry RW, Chang CC (2007) Association of subjective ratings of slipperiness to heel displacement following contact with the floor. Appl Ergon 38(5):533–539. https:// doi.org/10.1016/J.APERGO.2006.09.001 32. Grönqvist R, Hirvonen M, Tuusa A (1993) Slipperiness of the shoe-floor interface: comparison of objective and subjective assessments. Appl Ergon 24(4):258–262. https://doi.org/10.1016/ 0003-6870(93)90460-Q 33. Cohen HH, Cohen DM (1994) Psychophysical assessment of the perceived slipperiness of floor tile surfaces in a laboratory setting. J Safety Res 25(1):19–26. https://doi.org/10.1016/00224375(94)90004-3 34. Chiou SY, Bhattacharya A, Succop PA (2010) Evaluation of workers’ perceived sense of slip and effect of prior knowledge of slipperiness during task performance on slippery surfaces. AIHAJ 61(4):492–500. https://doi.org/10.1080/15298660008984560

Chapter 2

Worldwide Statistics of Slips and Falls

2.1 Slips-Related Accidents and Their Effect in the Twentieth Century Workplace slipping has been a global issue for decades. Although the scope of the issue has been acknowledged for a number of decades, STFL continues to be a significant cause of workplace injuries [1–3]. For instance, according to the most recent US statistics, the cost of fall-based injuries leading to disabilities is approximately 9 billion dollars [4, 5]. Authorities in charge of occupational safety are extremely concerned about the frequency and seriousness of mishaps brought on by an employee’s foot slipping. When strolling on an even surface or walking up on elevated surfaces, slipping can lead to several different types of falls. Additionally, slips can lead to injuries due to excessive stretching and muscle efforts required to recover from a slip. As a result, foot slippage may not be listed as the primary cause of an injury in accident reports. Regardless of this, it is estimated that slip-and-fall accidents cause over 20% of all workers’ compensation costs in the United States each year [6]. There is a concern with the US standards organizations with the slipping potential of work surfaces which are mainly focused on aesthetics and cleanliness. It is therefore important to characterize the frictional properties of these work surfaces under realistic conditions. The National Bureau of Standards, which has a focus on flooring in public buildings, has already endorsed this overall goal [7, 8]. This interest necessitates extensive surface-friction quality measurements [9]. Figure 2.1 shows the historical slip and fall-based fatal work-related injuries reported in the USA. Slips and falls are a significant issue. Workplace-related injuries caused by slips and falls in the US are anticipated to reach $7 billion. [3]. Slips and falls that occur on the same level lead to over 55% of direct compensation claims at the workspace [10]. In the workplace, slips and falls are extremely dangerous events. According to the US National Safety Council, workplace falls result in between 250 000 and 300 000 injuries annually, including 1200–1600 fatalities. More than 20% of all injuries necessitating reimbursements in the US industry are caused by these falls © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 A. Chanda et al., Footwear Traction, Biomedical Materials for Multi-functional Applications, https://doi.org/10.1007/978-981-99-7823-6_2

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Fig. 2.1 Historical slip and fall-based fatal work-related injuries reported in the USA

[11]. In Finland, falls, typically from higher to lower levels, are said to be the cause of 12% of serious injuries [12, 13]. The shoe-floor friction is currently measured using numerous evaluation techniques and in the presence of different contaminated conditions, including dry, wet, oil, etc. [14]. Accidents involving slips and falls present specific ergonomic issues. Slip, trip and fall incidents made up approximately 21 of the total documented non-fatal work injuries in 1996 that required time off from work [15]. Over 20% of employees who suffer falls miss at least 31 days at work. Additionally, falling accidents contribute significantly to the overall medical care cost. Analysis of USA worker’s compensation benefits for the years 1989 and 1990 revealed a 24% direct cost contribution of all claims received during this time period from fall-related injuries [3]. There is little information on the specific role that slipping plays in falls-related injuries. However, information currently available indicates that slipping frequently precedes falling. For instance, according to a review of injury data from 1998 in Sweden, slips were responsible for up to 55% of falls [16]. Slips are the main contributors to falls on both level surfaces and stairs in the workplace, and it is estimated that this factor accounts for about 62% of underfoot incidents [17, 18]. Underfoot perturbations, which are characterized by unexpected interactions between a person’s foot and the ground, have also been identified as a significant source of injuries [19].

2.2 Slips-Related Accidents and Their Effect in the Twenty-First Century

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2.2 Slips-Related Accidents and Their Effect in the Twenty-First Century Millions of visits to the hospital emergency were reported due to slips, trips and falls (STF). According to an opinion survey of young individuals, slips account for roughly half of falling accidents [20]. Same-level falls are a leading cause of workplace accidents leading to disability and significant direct expenditures [21–23]. Over onethird of the instances resulted in employees missing work for a month or longer, and 28.3% of those incidents happened in the service sector [23]. Furthermore, it has been discovered that slipping is responsible for 40–50% of fall-related injuries [18]. In the private sector, falls have caused over 16% of injuries not leading to fatalities in 2017 [23]. There were 111 incidents of fatal injuries from accidents on a comparable level in 2011 and 134 cases in 2016 [24, 25]. Fall injuries at work are frequently caused by slippery flooring. In the developed world, it is believed that between 20 and 40% of debilitating injuries result from slip, trip and fall incidents [12]. Hospital visits are common in industries, particularly in the areas of services, production, sales and construction. [26]. In the workplace, floor contaminants were listed as the reason for same-level falls in 19% of cases for women and 32% of cases for men [26]. Slips, trips and falls made up about 27% of all occupational injuries in 2015 [22]. It has been determined that between 40 and 60% of workplace falling incidents begin with slipping. Figure 2.2 shows the slip and fall-based injuries leading to fatal injuries in the USA (2000–2018). According to the National Safety Council (NSC), there are over 25,000 falls in the US every day, costing over $70 billion in medical expenses and compensations each year. It is estimated that over six billion US dollars are spent directly on workplace

Fig. 2.2 Slip and fall-based injuries leading to fatal injuries in the USA (2000–2018)

14 Table 2.1 Fatal occupational injuries (2015–2016) [22]

2 Worldwide Statistics of Slips and Falls

Event

2015 2016

Transportation Incidents

2054 2083

Violence and other injuries by persons or animals

703

866

Falls, Slips and Trips

800

849

Contact with objects and equipment

722

761

Exposure to harmful substances or environments

424

518

Fires and Explosions

121

88

injuries each year [12]. As per reports on direct workers’ compensation, for slips and falls injuries at work, accidents on the identical level led to up to 65% of claim expenditures. In more than 30% of all such cases, slips caused while carrying a weight led to significant causes for lower back pain [27, 28]. Slippery flooring is a major contributor leading to falls observed in kitchens of restaurants. They are typically brought on by impurities like water and oil [29]. The third greatest workplace incidence in Taiwan in 2010 was 1,835 falling incidents at work, which made up 16%of all occupational incidents for the year. Assessing the shoe-floor friction is one of the most popular ways to determine how slippery a surface is [4]. The COF is frequently utilized as a predictor of the probability of accidents involving slips and falls. The COF is measured in order to determine the likelihood of slips. The COF reading between the shoe sole and the floor has been the subject of previous studies. The COF has been shown to be impacted by floor type, floor contaminants and footwear tread design [30]. However, it is uncommon to measure the COF while barefoot on the ground. In both public and private bathrooms, pool facilities and bathtubs, barefoot slippage is frequent. Therefore, barefoot friction characterization on different floorings is crucial [30]. Table 2.1 shows the fatal occupational injuries between 2015 and 2016. Slips and falls place a heavy weight on our society’s businesses and communities [31]. The direct costs of US workers’ compensation for the nation’s most severe workplace illnesses and accidents in 2009 came to USD 50.1 billion. Fall to lower level came in third with costs of USD 5.35 billion. A total of 1,828 workplace falling accidents occurred on the identical level in Taiwan in 2010, according to the Council of Labour Affair’s Labour Annual Inspection Report. For the course of the year, these incidents made up 17.26% of all workplace incidents [32]. In 2002, falls in Taiwan were responsible for 13.9% of all work-related injuries. 10.6% of all work-related injuries were caused by falls on the same level, which made up 76.6% of all reported falling instances [33]. Table 2.2 shows the number of days taken as leave due to workplace accidents in 2017. Older people’s fall prevention is a serious medical and public health concern. According to studies, wearing slippers, walking barefoot and wearing socks instead of shoes are all related to an increased risk of falling, especially indoor falling [34]. Expert panels, however, have deemed the proof insufficient to suggest changes to footwear. Older folks frequently go barefoot, wear socks instead of shoes and use

2.3 Slips-Related Accidents in Bathrooms

15

Table 2.2 Number of days taken as leave due to workplace accidents (2017) [24] Event or exposure

2017

Overexertion and bodily reaction

3,62,580

Falls, Slips and Trips

2,90,660

Contact with objects and equipment

2,61,900

Violence and other injuries by persons or animals

79,900

Transportation Incidents

61,530

Exposure to harmful substances or environments

45,050

Fires and Explosions

3, 120

Table 2.3 Fatal Occupational Injuries (2016–2018) [23] Event or exposure

2016

2017

2018

Transportation incidents

2083

2077

2080

Falls, Slips and Trips

849

887

791

Violence and other injuries by persons or animals

866

807

828

Contact with objects and equipment

761

695

786

Exposure to harmful substances or environments

518

531

621

slippers within the house [35]. Slippers were undoubtedly the most popular form of shoe among those who actually did wear shoes around the house. Very few older adults wear high heels, despite the fact that doing so may increase their risk of falling [36–39]. Bathtubs and showers range from the third to the tenth most frequent causes of these unintentional injuries, depending on age and gender [39]. About 10% of bathtub accidents, according to reports, involve slips and falls when entering or leaving the tub [40]. Table 2.3 shows the fatal occupational injuries between 2016 and 2018.

2.3 Slips-Related Accidents in Bathrooms One of the top causes of fatal injuries, falls are a worldwide problem. The agestandardized mortality rate and the number of deaths have increased in recent years, and older persons over 65 encounter the greatest number of fatal falls. Individuals above the age of 90 suffer the highest rate of fatal injuries as the proportion of deaths rises with age. Older adults frequently sustain injuries from falls, and the frequency of these accidents rises with age. In accordance with an investigation of the Canadian Community Health Survey, the rate of fall-related injuries among individuals over 65 jumped from 49.4 to 58.8 per 1000 people between 2005 and 2013. [41]. This may lead to impeded quality of life, independence loss and hospitalization rates that are high. As more people in Canada are predicted to age, the projected amount

16

2 Worldwide Statistics of Slips and Falls

of falls will rise to 3.3 million in 2036. Hip fractures are a common fall-related injury, and broken or fractured bones account for a great deal of fall-related injuries. Over 90% of hip fractures in Canada were due to falls, according to a prior survey of hospitalizations for falls, and about 20% of hip fractures resulted in fatalities. For older persons, falling can have a number of psychological and physical effects, including diminished autonomy, immobility and sadness. These modifications may also affect one’s health and quality of life. According to reports, fifteen percent of falls among people over 60 in Canada happen in the restroom [42]. Bathroom falls contribute to over one-third of fallrelated hospitalizations in older people [43]. More than half of bathroom falls in older persons happen while they are bathing, and 70% of these falls are brought about by unsuccessful transfers (such as stepping into or out of the bathtub) [43]. Injury risk is considerable if a person falls while getting in or out of the bathtub. Mobility, freedom and the standard of life may be restricted by fall-related injuries and hospitalization. Fall-related injuries place a heavy financial burden on the healthcare system as well; the projected 8.7 billion dollars in healthcare expenditures associated with falls each year are expected to climb as the percentage of elderly Canadians continues to rise. Up to 85% of older persons who live in communities experience anxiety about falling, another major side effect of falls [44]. Fear of falling can cause people to forgo social interactions and necessities of life, which can result in reductions in bodily function, independence, social engagement and quality of life [45, 46]. Deshpande and associates discovered that activity limitations brought on by a fear of falling are a distinct indicator of physical function decrease [47]. Fear of falling is linked to worse results for mental health and raises the chance of subsequent falls. Up to 60% of elderly people who live in the community have bathing disabilities [48–50]. It has been considered a pivotal event in the process of older persons becoming disabled; it is a powerful predictor of damage and disability, although it is not frequently preceded by incapacity in other daily life activities [51, 52]. Bathing impairment is linked to a number of negative outcomes, such as increased use of hospital and home care services, admission to a long-term nursing facility and a higher risk of bone injuries and mortality [48, 53–55]. The most significant daily activity to anticipate both compensated and unpaid hours of personal assistance is bathing, which is more significant than dressing, using the restroom and eating [56]. People who have trouble bathing, such as those who are afraid of falling, may limit their bathing activities, which may limit their involvement in social activities [30, 42]. Hence, friction has a larger effect on the overall problems of slips and falls [57, 58].

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25. Marucci-Wellman HR et al (2015) The direct cost burden of 13 years of disabling workplace injuries in the U.S. (1998–2010): findings from the Liberty Mutual Workplace Safety Index. J Safety Res 55:53–62. https://doi.org/10.1016/J.JSR.2015.07.002 26. Layne LA, Pollack KM (2004) Nonfatal occupational injuries from slips, trips, and falls among older workers treated in hospital emergency departments, United States 1998. Am J Ind Med 46(1):32–41. https://doi.org/10.1002/ajim.20038 27. “The reduction of slip and fall injuries: Part I — Guidelines for the practitioner. Int J Ind Ergon 10(1–2):23–27. https://doi.org/10.1016/0169-8141(92)90044-Z 28. Li KW, Wen HC (2014) Friction between foot and floor under barefoot conditions: a pilot study. IEEE Int Conf Ind Eng Eng Manag 1651–1655. https://doi.org/10.1109/IEEM.2013.6962690 29. Chang WR, Cotnam JP, Matz S (2003) Field evaluation of two commonly used slipmeters. Appl Ergon 34(1):51–60. https://doi.org/10.1016/S0003-6870(02)00074-1 30. Li KW, Chang WR, Leamon TB, Chen CJ (2004) Floor slipperiness measurement: friction coefficient, roughness of floors, and subjective perception under spillage conditions. Saf Sci 42(6):547–565. https://doi.org/10.1016/J.SSCI.2003.08.006 31. Leclercq S (1999) Prevention of same level falls: a more global appreciation of this type of accident. J Safety Res 30(2):103–112. https://doi.org/10.1016/S0022-4375(99)00004-3 32. Li KW, Huang SY, Wang CW (2014) Relationship between floor-type gait adaptations and required coefficient of friction. IEEE Int Conf Ind Eng Eng Manag 131–135. https://doi.org/ 10.1109/IEEM.2013.6962389 33. Li KW, Hsu YW, Chang WR, Lin CH (2007) Friction measurements on three commonly used floors on a college campus under dry, wet, and sand-covered conditions. Saf Sci 45(9):980–992. https://doi.org/10.1016/J.SSCI.2006.08.030 34. Menz HB, Morris ME, Lord SR (2006) Footwear characteristics and risk of indoor and outdoor falls in older people. Gerontology 52(3):174–180. https://doi.org/10.1159/000091827 35. Munro BJ, Steele JR (1999) Household-shoe wearing and purchasing habits. A survey of people aged 65 years and older. J Am Podiatr Med Assoc 89(10):506–514. https://doi.org/10.7547/ 87507315-89-10-506 36. Menant JC, Steele JR, Menz HB, Munro BJ, Lord SR (2008) Optimizing footwear for older people at risk of falls. J Rehabil Res Dev 45(8):1167–1182. https://doi.org/10.1682/JRRD. 2007.10.0168 37. Koepsell TD et al (2004) Footwear style and risk of falls in older adults. J Am Geriatr Soc 52(9):1495–1501. https://doi.org/10.1111/J.1532-5415.2004.52412.X 38. Kelsey JL, Procter-Gray E, Nguyen US, Li W, Kiel DP, Hannan MT (2010) Footwear and falls in the home among older individuals in the MOBILIZE Boston study 2(3):123–129. https:// doi.org/10.1080/19424280.2010.491074 39. Zaloshnja E, Miller TR, Lawrence BA, Romano E (2005) The costs of unintentional home injuries. Am J Prev Med 28(1):88–94. https://doi.org/10.1016/J.AMEPRE.2004.09.016 40. Siegmund GP, Flynn J, Mang DW, Chimich DD, Gardiner JC (2010) Utilized friction when entering and exiting a dry and wet bathtub. Gait Posture 31(4):473–478. https://doi.org/10. 1016/j.gaitpost.2010.02.003 41. Do MT, Chang VC, Kuran N, Thompson W (2015) Fall-related injuries among Canadian seniors, 2005–2013: an analysis of the Canadian community health survey. Heal Promot Chronic Dis Prev Canada Res Policy Pract 35(7):99. https://doi.org/10.24095/HPCDP.35.7.01 42. Aminzadeh F, Edwards N, Lockett D, Nair RC (2001) Utilization of bathroom safety devices, patterns of bathing and toileting, and bathroom falls in a sample of community living older adults. Technol Disabil 13(2):95–103. https://doi.org/10.3233/tad-2000-13202 43. Rosen T, Mack KA, Noonan RK (2013) Slipping and tripping: fall injuries in adults associated with rugs and carpets. J. Inj. Violence Res. 5(1):61. https://doi.org/10.5249/JIVR.V5I1.177 44. Jørstad EC, Hauer K, Becker C, Lamb SE (2005) Measuring the psychological outcomes of falling: a systematic review. J Am Geriatr Soc 53(3):501–510. https://doi.org/10.1111/J.15325415.2005.53172.X 45. Rand D, Miller WC, Yiu J, Eng JJ (2011) Interventions for addressing low balance confidence in older adults: a systematic review and meta-analysis. Age Ageing 40(3):297–306. https://doi. org/10.1093/AGEING/AFR037

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46. Brouwer B, Musselman K, Culham E (2004) Physical function and health status among seniors with and without a fear of falling. Gerontology 50(3):135–141. https://doi.org/10.1159/000 076771 47. Deshpande N, Metter EJ, Lauretani F, Bandinelli S, Guralnik J, Ferrucci L (2008) Activity restriction induced by fear of falling and objective and subjective measures of physical function: a prospective cohort study. J Am Geriatr Soc 56(4):615–620. https://doi.org/10.1111/j.15325415.2007.01639.x 48. Gill TM, Allore HG, Han L (2006) Bathing disability and the risk of long-term admission to a nursing home. J Gerontol Ser A 61(8):821–825. https://doi.org/10.1093/GERONA/61.8.821 49. Naik AD, Concato J, Gill TM (2004) Bathing disability in community-living older persons: common, consequential, and complex. J Am Geriatr Soc 52(11):1805–1810. https://doi.org/ 10.1111/J.1532-5415.2004.52513.X 50. Guay M, Dubois MF, Corrada M, Lapointe-Garant MP, Kawas C (2014) Exponential increases in the prevalence of disability in the oldest old: a canadian national survey. Gerontology 60(5):395–401. https://doi.org/10.1159/000358059 51. Golding-Day M (2017) Interventions to reduce dependency in bathing in community dwelling older adults: a systematic review. Syst Rev 6(1):1–6. https://doi.org/10.1186/S13643-0170586-4/TABLES/3 52. Jagger C, Arthur AJ, Spiers NA, Clarke M (2001) Patterns of onset of disability in activities of daily living with age. J Am Geriatr Soc 49(4):404–409. https://doi.org/10.1046/J.1532-5415. 2001.49083.X 53. David EK, Reuben B (2023) Development of a method to identify seniors at high risk for high hospital utilization on JSTOR. Med Care 782–793. Accessed: Apr 30, 2023. https://www.jstor. org/stable/3768144 54. Østbye T, Walton RE, Steenhuis R, Hodsman AB (2004) Predictors and sequelae of fractures in the elderly: the canadian study of health and aging (CSHA)*. Can J Aging / La Rev Can du Vieil 23(3):245–251. https://doi.org/10.1353/CJA.2004.0035 55. Carey EC, Walter LC, Lindquist K, Covinsky KE (2004) Development and validation of a functional morbidity index to predict mortality in community-dwelling elders. J Gen Intern Med 19(10):1027–1033. https://doi.org/10.1111/J.1525-1497.2004.40016.X 56. LaPlante MP, Harrington C, Kang T (2002) Estimating paid and unpaid hours of personal assistance services in activities of daily living provided to adults living at home. Health Serv Res 37(2):397–415. https://doi.org/10.1111/1475-6773.029 57. Gupta S, Chatterjee S, Malviya A, Kundu A, Chanda A (2023) Effect of shoe outsole wear on friction during dry and wet slips: a multiscale experimental and computational study. Multiscale Sci Eng 2023:1–15. https://doi.org/10.1007/S42493-023-00089-0 58. Gupta S, Chatterjee S, Malviya A, Chanda A (2023) Frictional assessment of low-cost shoes in worn conditions across workplaces. J Bio- Tribo-Corrosion 9(1):1–13. https://doi.org/10. 1007/S40735-023-00741-0

Chapter 3

Basic Principles of Slip Resistance

3.1 Biomechanics of Normal Gait For the purpose of identifying the reasons for slips from a human centric perspective, fundamental knowledge of gait patterns is required. Important biomechanical elements related to slips are the underlying ground reaction forces (GRF) and foot kinematics throughout the stance phase of the gait cycle. Gait techniques that reduce the danger of slipping may be revealed by biomechanical analyses, which may also provide supplementary data that directs traction measurement. Since it is the phase that maintains the body weight, the stance phase is essential for maintaining balance. Heel strike initiates the stance phase, which is followed by toe-off. Weight acceptance follows heel striking, and is followed by midstance and push-off. During heel strike and the support limb’s maximal knee flexion, the weight is accepted. Ankle plantar flexion happens during push-off, which is when the lower limb exerts pressure against the ground to propel the body forward. The period of time between accepting weight and pushing off is known as the midstance. Depending on the phase in which it occurs, slipping might have a negative impact on the body’s capacity to propel or maintain weight. Figure 3.1 represents different phases of normal gait. The frictional demand that a person must exert to avoid slipping is measured using the GRF throughout the stance stage of an ordinary gait on a linear pathway [2–4]. There are two different maxima in the vertical force. Peak one, which biomechanically reflects weight acceptance, appears at approximately 25% of the stance phase. At around 75% of the stance phase, the second peak, which corresponds to the push-off phase, appears. The pressure of the foot on the ground creates the initial peak of the longitudinal shear, or friction in the gait direction, which is in the anterior direction, at around 15% of the stance phase. The second peak emerges in the posterior direction just over 85% of the way through the stance phase. The first and second peaks, respectively, stand in for the braking and propelling phases. The ratio of shear to vertical forces is known as the necessary coefficient of friction (RCOF), and it is used to determine how much friction a person needs to maintain their balance

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 A. Chanda et al., Footwear Traction, Biomedical Materials for Multi-functional Applications, https://doi.org/10.1007/978-981-99-7823-6_3

21

22

3 Basic Principles of Slip Resistance

Fig. 3.1 Various stages of a normal gait [1]

when walking. A key predictor of slips is the highest RCOF throughout the weight absorption period (Table 1), which coincides with the maximum longitudinal shear force in the forward direction. Higher RCOF values are linked to a higher chance of slipping [5]. Figure 3.2 represents the GRF and RCOF during normal walking at different stances. Slip propensity is also influenced by the kinematics of walking, such as gait speed, step length and shoe-floor angle. Variations in slipping risk are correlated with changes in gait speed and step length [7, 8]. Increased gait speed is linked to higher peak RCOF, according to research by Powers et al. [7]. Nevertheless, speed of walking and step length are correlated, meaning that greater step lengths are typically seen when movement speed is increased [7]. The impact of step length and gait speed on RCOF was separated by Anderson et al. [9]. Increased step length was found to have a large beneficial impact on RCOF, but gait speed that was unrelated to step length had a weakly negative impact. This may be because increasing gait speed (where step length is controlled) results in only minor increases in vertical GRF, whereas longer step length has a favourable impact on shear forces and raises the individual frictional demand [9]. Figure 3.3 represents the typical variation of ground reaction forces during walking. Additionally, it was demonstrated by the dissociation of step length and gait speed that stability after the commencement of a slip was increased by shorter step length and quicker gait speed. [10]. It has been shown that other kinematic parameters, such as shoe-floor angle and frontal heel propulsion at heel strike, affect how serious slips are. Larger heel retardation (negative acceleration) lessens the frequency of serious slides, while larger shoefloor angles enhance slip severity [8, 11]. As a result, a number of gait metrics that consider human factors affect the likelihood of falls brought on by slips. Subject’s thinking, including anticipation, is another crucial aspect to take into account that could potentially impact the outcome of the slip. When subjects are aware of a slippery surface, their gait speed, step length and shoe-floor angle at heel strike tend to slow down [12]. Additionally, when they anticipate a slippery surface, individuals lower their peak RCOF [13]. These modifications are probably meant

3.1 Biomechanics of Normal Gait

23

Fig. 3.2 GRF and RCOF during normal walking [6]

to lower the possibility of slipping incidents [12]. Additionally, this expectation of slippery flooring could overestimate the actual danger of slipping[3]. So, when assessing the risk of slipping, the attitude or anticipation component must be taken into account. Changes in gait techniques may reduce slipping incidents since the gait biomechanics plays a significant role in determining the result of slips. Therefore, when developing shoe traction tests and evaluating an individual’s total slipping risk, biomechanical information about a person’s stride pattern should be taken into account.

24

3 Basic Principles of Slip Resistance

Fig. 3.3 Typical variation of ground reaction forces during walking [14]

Throughout the loading response phase, the gluteus medius also engages in an isometric muscular action that stabilises the pelvis. After that, the soleus muscle contracts eccentrically, allowing the front foot to be forced against the ground and the knee to be extended without using the knee extensors. Late stance causes the hip and knee to flex, and the ankle plantar flexors to start producing concentric muscle action that propels the human body forward during push off [15, 16].

3.2 Biomechanics of Slips A person’s movement is always influenced by the coefficient of friction (COF), which occurs when two surfaces, such as the shoe sole and the floor being walked on, come into contact. This is especially true with varied degrees of COFs (high, medium, and low). A medium coefficient of friction is required for a normal strolling, free from slips or loss of balance incidents. A very small coefficient of friction will offer the foot very little resistance as it moves, causing the foot to slide around excessively and maybe putting the foot at risk of falling. An extremely high coefficient of friction, on the other hand, may increase the resistance to the foot’s movement and may even impair regular stride, which could still make a fall more likely to be a trip than a slip. Therefore, an average or standard coefficient of friction is required, especially in work environments, to allow for fluid foot transitions from one to the other and to achieve gait with the least amount of energy expended. Under typical walking circumstances and at typical walking speeds, the applied coefficient of friction ranged from 0.17 to 0.20. [13, 17]. And the likelihood of slipping increases when the amount of space at the point where the footwear meets the floor exceeds the utilised coefficient of friction [13, 17]. Walking faster than usual results in longer steps or strides and a greater angle at which the lower part of the leg maintains contact with the ground, which raises the necessary coefficient of friction. The evaluation of the interaction between the footwear and floor interface and the explanation of motion of the body segments during a slip are both aided by the

3.3 Variation of Gait on Slippery Surfaces

25

biomechanical analysis of slips [18]. There are various terms for categorising slips according to the degree of seriousness of the outcome, using slip lengths and the impression of the slipping. Slip distance from the heel motion has been employed as a distinguishing metric to distinguish between microslips and macroslips in terms of slip severity [19]. Using slip perception and recovery as a criterion, slip-sticks were divided into three categories: mini-slips, during which subjects failed to notice the motion of slipping; mid-slips, throughout which slips are recovered with minimal disruption to gait; and maxi-slips, through which the slip recovery entails significant corrective actions and is dangerously close to falling [20].

3.2.1 Human Gait Kinetics During Slips For the purpose of predicting slips and falls, the ground response forces that occur shortly after a heel contact are crucial. The important moment when the majority of slips happen is the initial peak in the shear force, which happens between 90 to 150 ms after heel impact, roughly 19% of the gait cycle [13]. The gait cycle’s heel contact and push-off phases have the highest shear forces and are thought to be the times when slipping is most likely to happen [13]. The ratio of the shear force to the normal force experienced when moving across a slippery surface compared to a non-slippery surface is known as the available coefficient of friction (ACOF) [21–24]. The necessary coefficient of friction is the ratio of the shear force to the normal force during walking on dry surfaces (RCOF). Consequently, by comparing the RCOF and ACOF for a particular type of surface and a particular pair of shoes, the likelihood of slipping in a work setting may be determined. The individual shouldn’t fall if the ACOF is higher than the RCOF, but if the ACOF is lower than the RCOF, the danger of falling is increased [13]. Six peak forces that occur during a normal gait cycle in dry circumstances may be identified using the horizontal to vertical ground response forces ratio (FH/FV). The first peak, a forward force, is produced by the heel hit, and the second peak, a backward force, is delivered to the heel after contact throughout the early landing period. The forward pressures that slow the foot’s motion are represented by the third and fourth peaks. The fifth and sixth maxima of the gait cycle are finally in the opposite direction because of the push-off phase [13].

3.3 Variation of Gait on Slippery Surfaces Due to the increased frictional demand throughout heel strike and toe-off, slips are more likely to happen during these movements. Since they happen when the body’s COM is less stable, heel slips (slips that happen at heel strikes) are riskier. Through the use of a motion capture device, spots on the body are tracked in space to provide kinematic measurements. Previously, Lockhart [25] studied the kinematics and kinetics

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3 Basic Principles of Slip Resistance

during a typical heel slip-off. Since so little weight has been transferred to the stepping foot at the time of heel impact, the foot may not start to slip right away. From this point, the heel decelerates and its velocity drops. The amplitude of the shear force increases when the COM moves to the stepping foot, raising the frictional requirement, and causing the heel to start slipping forward. The heel will speed up to a maximum before decelerating if recuperation from the slip is successful. At this point, the heel reaches its maximum sliding speed before coming to a standstill at the completion of the slip. Humans modify their gait to improve stability when walking on slippery floors in order to prevent falling. Gait patterns when stepping on slippery surfaces and those on non-slippery floors were compared by Cappelini et al. [26]. Step length, step time and horizontal shear pressures were all dramatically reduced over a slick surface. While arm motion and trunk rotation were both enhanced, head orientation was more steadily oriented in space. There was less of a heel-to-toe rolling gait pattern. But with time and practice, the body’s COM and hips become more stable in the frontal plane. Chambers et al. [27] demonstrated that the foot kinematics during gait in known slippery situations differ from those in unanticipated slippery conditions. The vertical velocity of the heel before heel strike and the heel contact angle at heel strike both decreased when a participant was aware of the slippery conditions. Prior to this, recovery efforts at the heel were seen on known slippery surfaces. Additional effective techniques for keeping one’s balance while treading on slippery floors include lengthening one’s stance and stride and decreasing propagation speeds [28].

3.4 Recovery Responses After Slipping Incidents Depending on how the slip was viewed, an individual might or might not participate in a recovery reaction after experiencing a slip. Slip distances between 2 and 3 mm are thought to be a characteristic of normal gait [29]. Based on slip duration, perceptibility and if balance recovery is possible, longer slips have been classified. Microslips are thought to be undetectable. The microslip lengths reported by Perkins and Wilson ranged from 10 to 20 mm [19], Leamon and Li [30] reported microslips up to 30 mm long, while Strandberg and Lanshammar [31] found microslips up to 12 mm ± 4 mm. Additionally, Strandberg and Lanshammar [31] described detectable slips in gait sequences of up to 51 mm ± 47 mm without obvious problems. Macroslips are thought to be typically perceivable because they cause postural reactions such arm movement or jerking or lunging of the upper torso. Leamon [30] described macroslips with lengths ranging from 2 to 10 cm. According to several studies, slips that are greater than 10 cm cause a complete loss of equilibrium and frequently end in collisions with the ground or other objects [19, 20]. These classifications are by far not certain, however, research conclusions about the cutoff values for identifying microslips, macroslips and slips continue to differ [32, 33]. Brady et al.

3.5 Measurement of Slip Resistance

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[32] demonstrated that when moving along oily surfaces, participants could recover from unanticipated slides greater than 10 cm in length and 50 cm/s in speed. Cham and Redfern [34] investigated how people recover their balance after slipping during stride on greasy surfaces. A broad spectrum of remedial tactics was used by the subjects. Pure hip reflexes, pure ankle reactions and an intricate jiggling of lower body joint moments were among them. The slipping foot’s ankle operated more passively during the fall and recovery trials than the knees and hips, which increased their respective flexion and extension moments. The efficacy of the balance recovery responses was discovered to be influenced by the body’s postural conditions and the features of the ground reaction forces at the time of the disturbance. McGorry et al. [17] investigated 47 pairs of slip and non-slip trials on surfaces such as Delrin, Teflon with furniture polish and Teflon without polish, matched by participant and surface-contaminant combinations. About 25–30 ms after the heel contact, higher COF usage and horizontal heel velocity were noticed. These findings demonstrated the need for measures to stop slip propagation very quickly after heel striking. The walking task also affects slip reactions and the capacity to recover when walking. For instance, turning, carrying loads, and lowering slopes can all increase the likelihood of slipping and hamper recovery. In a variety of oily surface conditions, Chiou et al. [35] compared walking along a straight road versus walking along a turn and found that slip incidence on turns was significantly higher. In this investigation, Chiou et al. [35] also discovered that the RCOF measure of frictional demand was a subpar predictor of slip frequency. Cham and Redfern [29] used vinyl tiles with people walking down them wearing PVC hard-soled shoes at angles of 0°, 5° and 10° to investigate slips on inclined planes. Participants were not made aware of the circumstances being tested beforehand, and the two conditions under scrutiny were dry and oily. In the oily level planes, 25% of the trials involved falling and 50% involved slip-and-recovery episodes. On the greasy ramp at number 5, 44% slipped and got up, but 56% fell. On the 10-degree ramp, all individuals slipped and fell. Additionally, it was discovered that the peak UCOF needed to prevent slippage rose with ramp angle [29].

3.5 Measurement of Slip Resistance Walking tests and analysis of slip severity have both been used to determine the relative slip resistance between different footwear-contaminant-surface combinations. To produce comparative estimates of footwear slip resistance for the specified surface condition, many types of footwear can be evaluated on the same surface-contaminant condition. On the other hand, it is possible to compare the corresponding slip resistance of a variety of surfaces by varying the surfaces while maintaining constant the footwear and contaminants [36]. However, there is still debate concerning exact slip resistance measurements. Slip frequency or likelihood, slip measurements and maximum slipping velocities can all be utilized to assess the slipping severity

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employed in subject-based testing. These traditional measures of slip severity are used to test the reliability of readings obtained from mechanical instruments used to measure slip resistance when care is taken to design the conditions for testing to be corresponding to naturally occurring slip scenarios. However, biomechanical analyses for figuring out the level of slipping can be expensive, time-consuming and need specialized tools. Testing the slip resistance of various flooring materials and contaminants in real-world settings is necessary. Many researchers have sought to create mechanical tools for assessing slip resistance of floor surfaces as well as at footwear-surface interactions in order to meet this need. The maximum coefficient of friction (ACOF) at the surface contact is measured using a number of techniques by the slip resistance measurement instruments. Higher ACOF values should, in theory, lessen the likelihood of slipping because slips happen when the frictional demand (RCOF) surpasses the available amount, or when RCOF > ACOF. The complexity of the elements affecting ACOF makes measuring it difficult.

3.5.1 Available Co-Efficient of Friction Since footwear is elastomeric and doesn’t follow the traditional principles of friction, it is challenging to determine the available coefficient of friction at any given position on a surface during gait [37–39]. The first classic law of friction is as follows: Fµ = µ FN

(3.1)

According to this law, the normal force (FN ), the coefficient of friction, is proportional to the frictional force (Fµ ) across two surfaces that are moving relative to one another. When the relative velocity between the two surfaces is zero, the static coefficient of friction (µs ) and the kinetic coefficient of friction (µk ) relate the frictional force and normal force, respectively. According to Grönqvist et al. [40], the essential phase of gait for slipping occurs shortly after heel strike of the following foot, and the foot is typically moving at this moment. As a result, the kinetic frictional characteristics are more important than static friction while walking. But Grönqvist et al. [40] also acknowledged that static frictional characteristics might be significant if the foot is moving slowly, if there is a prolonged period of contact, or if the slip begins from a standing position.

References 1. Kim SC, Cho SH (2022) Effects of H-reflex onset latency on gait in elderly and hemiplegic individuals. Med 58(6):716. https://doi.org/10.3390/MEDICINA58060716

References

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2. Burnfield JM, Powers CM, Powers CM (2007) Prediction of slips: an evaluation of utilized coefficient of friction and available slip resistance. https://doi.org/10.1080/001401306006 65687 3. Siegmund GP, Heiden TL, Sanderson DJ, Inglis JT, Brault JR (2006) The effect of subject awareness and prior slip experience on tribometer-based predictions of slip probability. Gait Posture 24(1):110–119. https://doi.org/10.1016/J.GAITPOST.2005.08.005 4. Hanson JP et al. (2010) Predicting slips and falls considering required and available friction 42(12):1619–1633. https://doi.org/10.1080/001401399184712 5. Beschorner KE, Albert DL, Redfern MS (2016) Required coefficient of friction during level walking is predictive of slipping. Gait Posture 48:256–260. https://doi.org/10.1016/J.GAI TPOST.2016.06.003 6. Yamaguchi T, Masani K (2016) Contribution of center of mass–center of pressure angle tangent to the required coefficient of friction in the sagittal plane during straight walking. Biotribology 5:16–22. https://doi.org/10.1016/J.BIOTRI.2015.12.002 7. Powers CM, Burnfield JM, Lim P, Brault JM, Flynn JE (2002) Utilized coefficient of friction during walking: static estimates exceed measured values. J Forensic Sci 47(6):1303–1308. https://doi.org/10.1520/jfs15565j 8. Moyer BE, Chambers AJ, Redfern MS, Cham R (2007) Gait parameters as predictors of slip severity in younger and older adults 49(4):329–343. https://doi.org/10.1080/001401305004 78553 9. Anderson DE, Franck CT, Madigan ML (2014) Age differences in the required coefficient of friction during level walking do not exist when experimentally-controlling speed and step length. J Appl Biomech 30(4):542–546. https://doi.org/10.1123/JAB.2013-0275 10. Espy DD, Yang F, Bhatt T, Pai YC (2010) Independent influence of gait speed and step length on stability and fall risk. Gait Posture 32(3):378–382. https://doi.org/10.1016/J.GAITPOST. 2010.06.013 11. Beschorner K, Cham R (2009) Impact of joint torques on heel acceleration at heel contact, a contributor to slips and falls 51(12):1799–1813. https://doi.org/10.1080/00140130802136479 12. Menant JC, Steele JR, Menz HB, Munro BJ, Lord SR (2009) Effects of walking surfaces and footwear on temporo-spatial gait parameters in young and older people. Gait Posture 29(3):392–397. https://doi.org/10.1016/j.gaitpost.2008.10.057 13. Cham R, Redfern MS (2002) Changes in gait when anticipating slippery floors. Gait Posture 15(2):159–171. https://doi.org/10.1016/S0966-6362(01)00150-3 14. Pavei G, Cazzola D, La Torre A, Minetti AE (2019) Race walking ground reaction forces at increasing speeds: a comparison with walking and running. Symmetry 11(7):873. https://doi. org/10.3390/SYM11070873 15. Lee S-K, Lee S-Y (2016) The effects of changing angle and height of toilet seat on movements and ground reaction forces in the feet during sit-to-stand. J Exerc Rehabil 12(5):438. https:// doi.org/10.12965//JER.1632700.350 16. Cavagna GA, Margaria R (1966) Mechanics of walking 21(1):271–278. https://doi.org/10. 1152/JAPPL.1966.21.1.271 17. McGorry RW, DiDomenico A, Chang CC, The anatomy of a slip: kinetic and kinematic characteristics of slip and non-slip matched trials. Appl Ergon 41(1):41–46. https://doi.org/10.1016/ j.apergo.2009.04.002 18. Li KW, Chang WR, Leamon TB, Chen CJ (2004) Floor slipperiness measurement: friction coefficient, roughness of floors, and subjective perception under spillage conditions. Saf Sci 42(6):547–565. https://doi.org/10.1016/J.SSCI.2003.08.006 19. Perkins PJ, Wilson Shoe MP (2007) Slip resistance testing of shoes — new developments 26(1):73–82. https://doi.org/10.1080/00140138308963314 20. Strandberg L, Lanshammar H (1981) The dynamics of slipping accidents. J Occup Accid 3(3):153–162. https://doi.org/10.1016/0376-6349(81)90009-2 21. Gupta S, Chatterjee S, Malviya A, Chanda A (2022) Traction performance of common formal footwear on slippery surfaces. Surfaces 5(4):489–503. https://doi.org/10.3390/SURFACES5 040035

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22. Gupta S, Sidhu SS, Chatterjee S, Malviya A, Singh G, Chanda A (2022) Effect of floor coatings on slip-resistance of safety shoes. Coatings 12(10). https://doi.org/10.3390/COATINGS1210 1455 23. Gupta S, Chatterjee S, Malviya A, Chanda A (2023) Frictional assessment of low-cost shoes in worn conditions across workplaces. J Bio- Tribo-Corrosion 91 9(1):1–13. https://doi.org/10. 1007/S40735-023-00741-0 24. Chatterjee S, Gupta S, Chanda A (2022) Barefoot slip risk assessment of Indian manufactured ceramic flooring tiles. Mater Today Proc. https://doi.org/10.1016/J.MATPR.2022.04.428 25. Kim S, Lockhart T, Yoon HY (2005) Relationship between age-related gait adaptations and required coefficient of friction. Saf Sci 43(7):425–436. https://doi.org/10.1016/J.SSCI.2005. 08.004 26. Cappellini G, Ivanenko YP, Dominici N, Poppele RE, Lacquaniti F (2010) Motor patterns during walking on a slippery walkway. J Neurophysiol 103(2):746–760. https://doi.org/10. 1152/JN.00499.2009/ASSET/IMAGES/LARGE/Z9K0021099190011.JPEG 27. Chambers AJ, Margerum S, Redfern MS, Cham R (2003) Kinematics of the foot during slips. Occup Ergon 3(4):225–234. https://doi.org/10.3233/OER-2003-3404 28. Fong DTP, Mao DW, Li JX, Hong Y (2008) Greater toe grip and gentler heel strike are the strategies to adapt to slippery surface. J Biomech 41(4):838–844. https://doi.org/10.1016/J.JBI OMECH.2007.11.001 29. Grönqvist R et al (2010) Measurement of slipperiness: fundamental concepts and definitions 44(13):1102–1117. https://doi.org/10.1080/00140130110085529 30. Leamon TB (1992) The reduction of slip and fall injuries: Part II — The scientific basis (knowledge base) for the guide. Int J Ind Ergon 10(1–2):29–34. https://doi.org/10.1016/01698141(92)90045-2 31. Strandberg L (2007) On accident analysis and slip-resistance measurement 26(1):11–32. https:// doi.org/10.1080/00140138308963309 32. Brady RA, Pavol MJ, Owings TM, Grabiner MD (2000) Foot displacement but not velocity predicts the outcome of a slip induced in young subjects while walking. J Biomech 33(7):803– 808. https://doi.org/10.1016/S0021-9290(00)00037-3 33. McGorry RW, DiDomenico A, Chang CC (2007) The use of a heel-mounted accelerometer as an adjunct measure of slip distance. Appl Ergon 38(3):369–376. https://doi.org/10.1016/J. APERGO.2006.03.013 34. Redfern MS et al (2010) Biomechanics of slips 44(13):1138–1166. https://doi.org/10.1080/ 00140130110085547 35. Chiou SS, Bhattacharya A, Lai C-F, Succop PA (2003) Effects of environmental and jobtask factors on workers’ gait characteristics on slippery surfaces. Occup Ergon 3(4):209–223. https://doi.org/10.3233/OER-2003-3403 36. Gupta S, Chatterjee S, Malviya A, Singh G, Chanda A (2023) A novel computational model for traction performance characterization of footwear outsoles with horizontal tread channels. Computation 11(2):23. https://doi.org/10.3390/COMPUTATION11020023 37. Gupta S, Chatterjee S, Chanda A (2023) Influence of vertically treaded outsoles on interfacial fluid pressure, mass flow rate, and shoe–floor traction during slips. Fluids 8(3):82. https://doi.org/10.3390/FLUIDS8030082 38. Gupta S, Chanda A (2023) Biomechanical modeling of footwear-fluid-floor interaction during slips. J Biomech 156:111690. https://doi.org/10.1016/J.JBIOMECH.2023.111690 39. Gupta S, Chatterjee S, Chanda A (2022) Effect of footwear material wear on slips and falls. Mater Today Proc. https://doi.org/10.1016/J.MATPR.2022.04.313 40. Grönqvist R, Hirvonen M (1995) Slipperiness of footwear and mechanisms of walking friction on icy surfaces. Int J Ind Ergon 16(3):191–200. https://doi.org/10.1016/0169-8141(94)00095-K

Chapter 4

Slip Resistance of Footwear

4.1 Prevention of Falls by Footwear Implementation Footwear is described in falls guidelines as one of many important considerations when providing information to older adults on falls management. The Australian Commission on Safety and Quality in Healthcare Best Practice Guidelines for Preventing Harm from Falls in Older People in hospitals [1] and society states that “health care providers should provide education and information about footwear features that may reduce falls risk”. The guidelines report on the results of footwear research. Both guidelines also provide a qualifying statement that the studies informing this recommendation are of limited design and quality. Footwear styles that have been implicated as increasing falls risk include: slippers, high heels, bare feet and wearing socks. Footwear features that have been associated with increasing falls risk include: medium and high heel height, narrow heels, and inadequate fixation, such as no laces or buckle. The guidelines state that the footwear associated with the lowest risk of falls is athletic footwear, referencing only one study by Koepsell et al. in 2004 [2]. The falls prevention guidelines additionally provide information on footwear and falls with pictures of what is considered an optimal “safe” shoe, and a theoretical “unsafe” shoe. However, the guidelines state that the level of evidence is “low” for the recommended optimal footwear for falls management. The reason given is that there are no experimental studies with the described optimal footwear style that have examined falls as an outcome. It would be advantageous to evaluate the falls prevention guidelines with an appraisal tool to assess the guidelines’ quality dimensions [3]. Included in the quality criteria should be how the content of the guidelines is presented, their adaptability, information retrieval, scope and any potential conflicts of interest among the guidelines’ authors [4].

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 A. Chanda et al., Footwear Traction, Biomedical Materials for Multi-functional Applications, https://doi.org/10.1007/978-981-99-7823-6_4

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4.2 Interrelationship Between Footwear Styles and Gait Parameters Related to Falls The relationship between falls and footwear may be investigated by either the direct relationship between footwear and falls or the relationship between footwear and the gait parameters that then link to falls. It is important to consider both of these investigations separately, however, it is ultimately the relationship between footwear and falls that is the important issue, whether it is mediated through gait parameters or not. As guidelines appeared sparse with tenuous evidence proposing a relationship between footwear and falls, it is considered prudent to investigate gait parameters that are linked to falls and are affected by footwear. There are particular spatial and temporal gait parameters associated with falls. These include step length, cadence, step velocity, double support, step width, stride length and toe clearance. The gait parameters, such as velocity, step and stride length generally decreases as age increases. Whereas, other gait parameters, such as step width and double support usually increases with age [5]. Older adults also have greater variability in foot clearance therefore increasing the risk of a toe-trip-related fall [6]. A toe trip can result in an injurious fall as the body’s centre of gravity is moving forward and is less likely to rebalance its equilibrium [7]. Previous research has shown that bare feet, or the “unshod” foot may increase falls risk as compared to wearing footwear [8]. However, no particular footwear type has been adequately researched in comparison to bare feet, to determine a causal effect. A point for future research could be different footwear types versus no footwear. There is acknowledged infinite variations in the number of ways at the analysis level of dissecting the footwear and falls dilemma. It is impossible to separate out footwear features as part of the causative factors that lead to falls or as preventative strategies. In fact it is hard to separate out footwear as a single contributor to falls unless the research is carried out as an experimental laboratory-based study where many of the variables can be controlled.

4.3 Shoe-Floor Contaminant Friction Numerous research have sought to identify shoe and floor elements that affect and could possibly raise shoe-floor-contaminant friction in an effort to build safer shoefloor-contaminant pairings. The categories of footwear design (material, surface microstructure, and tread patterns), flooring design (floor material and microstructure), surroundings (contaminant) and physiological (loading pattern, heel velocity, and shoe-floor angle) variables that contribute to shoe-floor-contaminant friction are as follows [9–16]. To determine which of the aforementioned elements contributes to increased friction between shoes and floor contaminants, current research frequently uses empirical methods. The goal of earlier research into the elements that determine shoe-floor friction was probably to discover how to increase friction. The outcomes

4.3 Shoe-Floor Contaminant Friction

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of these research are nonetheless challenging to comprehend and apply due to the complicated interplay between these components. However, a fundamental comprehension of the contributing elements to shoe-floor-contaminant friction also offers guidance for which elements must be incorporated in computer modelling methods. The impact of flooring roughness has been thoroughly investigated to identify surface characteristics that produce higher friction. Research has demonstrated that both the roughness of the floor surface [17–19], waviness of the floor surface [20] and condition of contamination [20] contribute greatly to the coefficient of friction. Chang used various cut-off lengths to examine the relationship between common surface characteristics and COF that measure both roughness of the surface or surface waviness. This study demonstrated that in the occurrence of a liquid contaminant, roughness of the surface is the more significant factor when viscosity is low, whereas surface waviness predominates when viscosity is high. However, interaction of footwear imperfections with floor asperities also has an impact, especially in dry conditions, so the topography and texture of the footwear and the floor need to be taken into account when determining the impact of roughness on friction [21, 22]. Kim and Smith also showed that big asperities wear off over time and change the friction between shoes and floor contaminants [23]. It is also recognized that aspects of the shoe, such as the outer sole fabric and tread design, influence friction between the shoe and the floor’s contaminants. Although studies have shown that tougher shoe materials are linked to a reduced friction coefficient, it is still unclear what mechanisms are primarily to blame for this shift in friction. Footwear-floor-contaminant friction is clearly reliant on shoe material [17]. However, friction is more directly impacted by tread design. It was discovered that increasing tread width and depth both increased the amount of friction that was available [24, 25]. Even while it is obvious that a shoe’s design influences how slip-resistant it is, the traction mechanics that result in these alterations are not well understood. Footwear-floor friction may be influenced by biomechanical factors such as the usual loading of the footwear, the angle between the shoe and the floor and the speed at which the shoe slides. Lower COF values have repeatedly been demonstrated to be associated with larger speeds [26, 27]. Although research on the impacts of typical force and footwear angle on footwear-floor-contaminant friction have been less conclusive, footwear angle and normal force would probably modify the dimensions of the footwear-floor contact zone, which might potentially change the friction between the shoe and the floor [27, 28]. Proctor and Coleman used a simple modelling technique to illustrate the significance of the speed of sliding and perpendicular force on the lubricating action of the footwear-floor-contaminant interface by comparing the footwear-floor-contaminant friction to a slider bearing [29]. Figure 4.1 shows the biomechanical testing parameters that are involved during slip testing experiments.

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Fig. 4.1 Biomechanical testing parameters during slip testing

4.4 Impact of Slip-Resistant Footwear There is convincing evidence that wearing slip-resistant shoes decreases accidents. St. Lawrence University in Canton, New York, completed a pilot trial in 2001 by supplying its staff with slip-resistant shoes. This small school’s decrease in accident rates and workers’ compensation claims led to expected savings of $100,000. Additionally, a 250-person food service company at a significant airport decreased slip and fall incidents to zero for more than a year by supplying skid-resistant footwear. Jack in the Box has a policy requiring slip-resistant footwear. The business purchases slip-resistant overshoes and mandates that all employees wear them or slip-resistant footwear. They claim that the overshoe has been the means of getting the workers into the footwear, and that when it comes to voluntary programmes, just approximately 40% of people tend to sign up for them. 90% of employees at a bakery named “The Cheesecake Factory” considered slip-resistant footwear. The corporation claimed that since the program’s start, the incidence of slip-and-fall claims had decreased by 72% and the associated costs got decreased by 81%. A cost-sharing programme employed by Friendly’s Restaurants to assist staff in purchasing skid-resistant safety shoes led to a 30% decrease in slip and fall incidents in the first year. Slips and falls were surpassed by cuts as the most common cause of injury in that year, saving the business an estimated $750,000. One kind of “slip-resistant” footwear, though, can really be much more or significantly less slip-resistant than another. Additionally, shoes that are slip-resistant in one setting might not be in another. Slip-resistant footwear may be examined on many types of floors. Make sure the shoe you choose is appropriate for the location in which it will be used because environmental conditions can have a substantial impact on a shoe’s performance. The efficiency of slip-resistant footwear depends on a variety of elements, including the environment, the type of flooring surface and polish, the materials and chemicals used in the shoe soles and the efficacy of the design. To be successful, footwear programmes must be maintained and administered in addition to being chosen.

4.5 Formal Footwear Slip Risk Scenario

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Fig. 4.2 Formal footwear tread patterns commonly employed [11]

4.5 Formal Footwear Slip Risk Scenario Recent worker’s compensation claims for workplace or occupational injuries totalled more than $30 billion, of which 30% were attributable to slips and falls brought on by employees’ customary wearing of formal shoes to work. In North America, slipand-fall accidents are a major cause of industrial injuries, both deadly and not fatal. More than 50% of reported fall injuries are caused by slips [30]. It frequently results in typical lower-extremity injuries such as sprains, fractures and dislocations [31]. Reduced available coefficient of friction (ACOF) between formal footwear and the floor increases the probability of slip-related accidents since formal footwear serves as the body’s principal point of contact with the floor [32, 33]. Because there is little friction at the shoe-floor interface, a high slip risk is created [34, 35]. There must be enough ACOF between the formal footwear and the flooring for routine duties like walking and running to be performed in workplaces and warehouses [36]. ACOF is often measured at the point where the shoe meets the floor to precisely determine the slip risk of formal footwear. The ACOF is significantly impacted by formal footwear characteristics such as outsole design [24, 37], outsole material [38, 39], wear [40], contact area [32], type of flooring [41] and contaminants on various types of flooring [16]. Particularly, the ACOF is significantly impacted by the presence of external contaminants on various types of flooring surfaces. Figure 4.2 shows the formal tread patterns that are commonly employed in the marketplace.

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References 1. Baggoley C (2023) Preventing falls and harm from falls in older people, 2009. https://www. safetyandquality.gov.au/sites/default/files/migrated/Guidelines-RACF.pdf. Accessed 10 May 2023 2. Koepsell TD et al (2004) Footwear style and risk of falls in older adults. J Am Geriatr Soc 52(9):1495–1501. https://doi.org/10.1111/J.1532-5415.2004.52412.X 3. Morgott M, Heinmüller S, Hueber S, Schedlbauer A, Kühlein T (2020) Do guidelines help us to deviate from their recommendations when appropriate for the individual patient? A systematic survey of clinical practice guidelines. J Eval Clin Pract 26(3):709–717. https://doi.org/10.1111/ JEP.13187 4. Chastan N et al (2009) Gait and balance disorders in Parkinson’s disease: Impaired active braking of the fall of centre of gravity. Mov Disord 24(2):188–195. https://doi.org/10.1002/ MDS.22269 5. Gervásio FM, Santos GA, Ribeiro DM, de Menezes RL (2016) Falls risk detection based on spatiotemporal parameters of three-dimensional gait analysis in healthy adult women from 50 to 70 years old. Fisioter e Pesqui 23(4):358–364. https://doi.org/10.1590/1809-2950/156619 23042016 6. Barrett RS, Mills PM, Begg RK (2010) A systematic review of the effect of ageing and falls history on minimum foot clearance characteristics during level walking. Gait Posture 32(4):429–435. https://doi.org/10.1016/J.GAITPOST.2010.07.010 7. Pavol MJ, Owings TM, Foley KT, Grabiner MD (2001) Mechanisms leading to a fall from an induced trip in healthy older adults. J Gerontol Ser A 56(7):M428–M437. https://doi.org/10. 1093/GERONA/56.7.M428 8. Tencer AF et al (2004) Biomechanical properties of shoes and risk of falls in older adults. J Am Geriatr Soc 52(11):1840–1846. https://doi.org/10.1111/J.1532-5415.2004.52507.X 9. Gupta S, Malviya A, Chatterjee S, Chanda A (2022) Development of a portable device for surface traction characterization at the shoe-floor interface. Surfaces 2022, vol 5, no 4, pp 504–520, Dec 2022. https://doi.org/10.3390/SURFACES5040036 10. Gupta S, Sidhu SS, Chatterjee S, Malviya A, Singh G, Chanda A (2022) Effect of floor coatings on slip-resistance of safety shoes. Coatings 2022, vol 12, no 10, p 1455, Oct 2022. https://doi. org/10.3390/COATINGS12101455 11. Gupta S, Chatterjee S, Malviya A, Chanda A (2022) Traction performance of common formal footwear on slippery surfaces. Surfaces 2022, vol 5, no 4, pp 489–503, Nov 2022. https://doi. org/10.3390/SURFACES5040035 12. Gupta S, Chatterjee S, Malviya A, Kundu A, Chanda A (2023) Effect of shoe outsole wear on friction during dry and wet slips: a multiscale experimental and computational study. Multiscale Sci Eng 2023:1–15. https://doi.org/10.1007/S42493-023-00089-0 13. Gupta S, Chatterjee S, Malviya A, Chanda A (2023) Frictional assessment of low-cost shoes in worn conditions across workplaces. J. Bio-Tribo-Corrosion 9(1):1–13. https://doi.org/10. 1007/S40735-023-00741-0 14. Gupta S, Chatterjee S, Malviya A, Singh G, Chanda A (2023) A novel computational model for traction performance characterization of footwear outsoles with horizontal tread channels. Comput 2023, vol 11, no 2, p 23, Feb 2023. https://doi.org/10.3390/COMPUTATION1102 0023 15. Gupta S, Chatterjee S, Chanda A (2023) Influence of vertically treaded outsoles on interfacial fluid pressure, mass flow rate, and shoe–floor traction during slips. Fluids 2023, vol 8, no 3, p 82, Feb 2023. https://doi.org/10.3390/FLUIDS8030082 16. Chatterjee S, Gupta S, Chanda A (2022) Barefoot slip risk in Indian Bathrooms: a pilot study, vol 65, no 6, pp 977–990. https://doi.org/10.1080/10402004.2022.2103863 17. Chang WR, Matz S (2001) The slip resistance of common footwear materials measured with two slipmeters. Appl Ergon 32(6):549–558. https://doi.org/10.1016/S0003-6870(01)00031-X 18. Chang WR (2001) The effect of surface roughness and contaminant on the dynamic friction of porcelain tile. Appl Ergon 32(2):173–184. https://doi.org/10.1016/S0003-6870(00)00054-5

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19. Chang WR (2004) Preferred surface microscopic geometric features on floors as potential interventions for slip and fall accidents on liquid contaminated surfaces. J Safety Res 35(1):71– 79. https://doi.org/10.1016/J.JSR.2003.09.017 20. Chang W-R et al (2007) The effect of surface waviness on friction between Neolite and quarry tiles, vol 47, no 8, pp 890–906, Jun 2007. https://doi.org/10.1080/00140130410001670390 21. Kim IJ (2004) Development of a new analyzing model for quantifying pedestrian slip resistance characteristics: Part I-Basic concepts and theories. Int J Ind Ergon 33(5):395–401. https://doi. org/10.1016/J.ERGON.2003.10.010 22. Kim IJ (2004) Development of a new analyzing model for quantifying pedestrian slip resistance characteristics: Part II—Experiments and validations. Int J Ind Ergon 33(5):403–414. https:// doi.org/10.1016/J.ERGON.2003.10.011 23. Kim IJ, Smith R (2000) Observation of the floor surface topography changes in pedestrian slip resistance measurements. Int J Ind Ergon 26(6):581–601. https://doi.org/10.1016/S0169-814 1(00)00024-X 24. Li KW, Chen CJ (2004) The effect of shoe soling tread groove width on the coefficient of friction with different sole materials, floors, and contaminants. Appl Ergon 35(6):499–507. https://doi.org/10.1016/J.APERGO.2004.06.010 25. Li KW, Chen CJ, Lin CH, Hsu YW (2006) Relationship between measured friction coefficients and two tread groove design parameters for footwear pads. Tsinghua Sci Technol 11(6):712– 719. https://doi.org/10.1016/S1007-0214(06)70254-1 26. Leclercq S, Tisserand M, Saulnier H (1993) Quantification of the slip resistance of floor surfaces at industrial sites. Part II: choice of optimal measurement conditions. Saf Sci 17(1):41–55. https://doi.org/10.1016/0925-7535(93)90019-A 27. Redfern MS, Bidanda B (2007) Slip resistance of the shoe-floor interface under biomechanically-relevant conditions, vol 37, no 3, pp 511–524. https://doi.org/10.1080/001 40139408963667 28. Grönqvist R, Matz S, Hirvonen M (2003) Assessment of shoe-floor slipperiness with respect to contact-time-related variation in friction during heel strike. Occup Ergon 3(4):197–208. https:// doi.org/10.3233/OER-2003-3402 29. Proctor TD, Coleman V (1988) Slipping, tripping and falling accidents in Great Britain— Present and future. J Occup Accid 9(4):269–285. https://doi.org/10.1016/0376-6349(88)900 18-1 30. Courtney TK, Sorock GS, Manning DP, Collins JW, Holbein-Jenny MA (2010) Occupational slip, trip, and fall-related injuries can the contribution of slipperiness be isolated? vol 44, no 13, pp 1118–1137, Oct 2010. https://doi.org/10.1080/00140130110085538 31. Bell JL et al (2009) Evaluation of a comprehensive slip, trip and fall prevention programme for hospital employees∗∗, vol 51, no 12, pp 1906–1925. https://doi.org/10.1080/001401308 02248092 32. Iraqi A, Vidic NS, Redfern MS, Beschorner KE (2020) Prediction of coefficient of friction based on footwear outsole features. Appl Ergon 82:102963. https://doi.org/10.1016/J.APE RGO.2019.102963 33. Beschorner KE, Redfern MS, Porter WL, Debski RE (2007) Effects of slip testing parameters on measured coefficient of friction. Appl Ergon 38(6):773–780. https://doi.org/10.1016/J.APE RGO.2006.10.005 34. Jones T, Iraqi A, Beschorner K (2018) Performance testing of work shoes labeled as slip resistant. Appl Ergon 68:304–312. https://doi.org/10.1016/J.APERGO.2017.12.008 35. Hemler SL, Charbonneau DN, Beschorner KE (2017) Effects of shoe wear on slippingimplications for shoe replacement threshold. https://doi.org/10.1177/1541931213601839 36. Bini RR et al (2021) Comparison of ground reaction forces between combat boots and sports shoes. Biomech 2021, vol 1, no 3, pp 281–289, Oct 2021. https://doi.org/10.3390/BIOMEC HANICS1030023 37. Yamaguchi T, Katsurashima Y, Hokkirigawa K (2017) Effect of rubber block height and orientation on the coefficients of friction against smooth steel surface lubricated with glycerol solution. Tribol Int 110:96–102. https://doi.org/10.1016/J.TRIBOINT.2017.02.015

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38. Jakobsen L, Lysdal FG, Bagehorn T, Kersting UG, Sivebaek IM (2022) The effect of footwear outsole material on slip resistance on dry and contaminated surfaces with geometrically controlled outsoles, pp 1–8, Jun 2022. https://doi.org/10.1080/00140139.2022.2081364 39. Nishi T, Yamaguchi T, Hokkirigawa K (2022) Development of high slip-resistant footwear outsole using rubber surface filled with activated carbon/sodium chloride. Sci Rep 2022 121 12(1):1–12. https://doi.org/10.1038/s41598-021-04102-0 40. Gupta S, Chatterjee S, Chanda A (2022) Effect of footwear material wear on slips and falls. Mater Today Proc 62:3508–3515. https://doi.org/10.1016/J.MATPR.2022.04.313 41. Derler S, Huber R, Kausch F, Meyer VR (2015) Effectiveness, durability and wear of anti-slip treatments for resilient floor coverings. Saf Sci 76:12–20. https://doi.org/10.1016/J.SSCI.2015. 02.002

Chapter 5

Human Slipping Experiments

5.1 Slipping Experiments According to age and gender, Kleiner et al. [1] looked at the impact of flooring on barefoot gait. The old senior individuals and the middle-aged individuals were the two healthy subject teams that were examined. Every volunteer was instructed to cross two force plates on carpet, homogeneous vinyl (HOV), vinyl-heterogeneous (HTV) and surfaces. The first half of the walking pathway was applied by HOV while the HTV was used to cover the left-out part. Induced ground reaction forces and the resulting friction were measured by two force plates (Kistler 9286BA), which were infused with the floor of the data acquisition chamber. Analysis was done on the required/needed coefficient of friction (RCOF). A linear multivariate regression model for repeated measures was used for the statistical analysis. One of the well-known places where slip-related accidents occur is the restroom [2]. Water, soap and shampoo splashes on the flooring are to blame for this. The majority of accidents involving falls happen in India as a result of the bathroom floor tiles’ propensity to be slippery. According to a research by Stevens et al. [3], throughout a 12-month period, approximately 200,000 bathroom-related injuries have been documented in the US. In addition to this, leaving the bathtub accounted for 81.1% of the overall injuries sustained. Elderly persons who experience falls and associated injuries also develop a fear of falling, which can prevent them from engaging in physical activity and result in a loss in their physical well-being, social engagement, and quality of life [4, 5]. Over a third of senior emergency room visits related to falls are caused by falls in bathrooms [6]. Up to 85% of senior persons experience the dread of falling, a serious side effect of falls, which can make them avoid engaging in regular social and basic activities [7–9]. The inability to bathe is linked to a number of negative outcomes, such as an increase in hospital visits, the need for home care assistance, and admissions to long-term nursing facilities [10]. Elderly adults who have difficulty bathing may reduce their bathing activities out of fear of falling [11], which may affect their social interaction. Disability has

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 A. Chanda et al., Footwear Traction, Biomedical Materials for Multi-functional Applications, https://doi.org/10.1007/978-981-99-7823-6_5

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Fig. 5.1 Human slipping experiments using harness system [14]

also been linked to decreased participation in social and recreational activities away from residence because bathing takes more time and energy [12]. The risk of barefoot slipping can be significantly increased by ignorance about bathroom flooring options, flooring maintenance, and contaminants. The significance of additional study into barefoot slip evaluation in Indian bathrooms is highlighted by this. Derler et al. [13] looked into the relationship between the microscopic topographical characteristics of encountered hard substrates and the sliding friction coefficient of water applied plantar skin. Fourteen subjects—4 women and 10 men—performed six to seven sliding motions across distances of 40–50 cm while standing barefoot on each of the surfaces. The foot made flat contact with the substrates in each friction test. The participants solidly stood on the opposite foot and further regulated themselves by grabbing a handrail during the slide experiments to maintain balance. The subjects were instructed to apply equivalent normal forces to their foot over the course of six to seven frictional assessments on an identical surface. Using a triaxial force plate, the forces imparted to the surfaces were measured, and the coefficient of friction was calculated over intervals of sliding in stationary mode. The amount of adhesion and deformation was shown to rise as the substrate’s surface roughness rose. Figure 5.1 represents the camera, force plate and harness system for the experiments. Individual biomechanical investigations measured friction coefficients ranging from 0.072 to 0.0762, with average readings for various surfaces falling between 0.12 and 0.59. The experiment to determine the force plate-based coefficient of friction (COF) between both the foot and the floor was reported by Li et al. [15]. The study’s participant was a woman. She was 23 years old. Her height was 156 cm, and her weight was 43 kg. The individual moved her foot 20 cm in the posterior direction in less than one second, which is considered to be quick. The patient moved the same foot in its entirety four seconds after starting. The individual moved her foot 20 cm in the rearward direction in less than one second, which is considered to be quick. The patient moved the same foot in its entirety four seconds after starting. On the testing area, a 20 cm foot mobility space was designated. The force platform gathered the ground reaction force produced by the translation motion of the foot. Wood, vinyl, steel and ceramic were the four-floor materials that

5.1 Slipping Experiments

41

were evaluated. The force platform’s top was mounted with the tested floor. These floors were each put to the test in dry, water applied and oily circumstances. For these three trials, the force plate-based COF averages were 0.65, 0.30 and 0.446, respectively. The force plate-based COF on the dry surface was 0.82, which was significantly higher than the COF on the wet surfaces (p < 0.05), which came in second (p = 0.61). The oily surfaces (0.07) had the lowest value, which was considerably lower (p < 0.05). Nagata et al. [16] set out to measure and analyse the reaction forces that were applied to the flooring surface at the time of a slip. Each participant was instructed to take a single, heel-to-toe step in barefoot condition on the slick floor. During the testing, a motion tracking camera, an extended cable metre and a force plate were used to measure the movements of the foot, velocity of foot translation and reaction forces applied to the floor at the start of a slip event. 15 senior individuals are having 65 years of age and older participated in slip tests. Each subject wore a fall stopping harness device that prevented them from falling while walking on slippery flooring. Every subject was given instructions on how to advance before slip tests. Areas of the force plate were cleaned with water and soapy water for preliminary inspections. During slip tests, floor surfaces were dried. Friction between barefoot and flooring is also influenced by the film thickness of lubricants on floors. To actually create slips, dense soapsuds (27% by weight of alkyl ether sulphate) were used. Every participant was instructed to place their right foot flat on the ground from heel to toe. Three times the subjects were requested to walk. Before beginning the experiment, volunteers were shown how to walk barefoot and experience accelerative slips while wearing a fall stopping harness. It was discovered that the speed of the foot motions immediately increases linearly and quickly when the foot touches the ground. Accelerative slips began with initial velocities (V0) of about 0.43 m/s. For senior participants, the average slip duration was roughly 30 ms. The vertical force peaked about 22 ms after the foot landed on the ground on average (SD 10 ms). Additionally, it was discovered that the normal forces (Fv1 and Fv2) at the moment the slip occurred ranged from 47 to 489 N. In their investigation, Li et al. [17] measured the ACOF between the flooring and the foot on three different surfaces at two different slipping speeds. To gauge the flooring reaction impact of the foot on the flooring, a force platform was used. All measurements were taken without shoes. For this investigation, five men were chosen as participants. They were having age of 22.8 (±0.8) years old, height of 167.1 (±6.9) cm tall, body weight of 71.6 (±15.7) kg and had an area of 107.8 (±18.3) cm2 of their right foot contacting the ground, respectively. The ground response forces (Fx, Fy and Fz) applied to the foot were evaluated using a Bertec 4060 force platform. Wood, vinyl, steel and ceramic were the four-floor materials that were evaluated. The force platform’s top was where the evaluated floor was installed. These floorings have all undergone testing in dry, wet and oily situations. The participant was seated on a chair with his shank vertically on the floor and his thigh horizontally positioned. The test participant stepped barefoot onto the floor with his right foot. He was asked to walk barefoot on the floor being tested. He was asked to go quickly or slowly while still being barefoot on the floor being evaluated.

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The individual had to move his foot 20 cm in the posterior direction in less than 1 s, which was considered a fast speed. The person completed the identical foot slide in 4 s, which was considered slow. On the testing area, a 20 cm foot movement space was designated. Compared to wet and oily surfaces, the friction coefficient measurements for dry floorings were greater. The friction coefficient of the foot under surface and slipping velocity conditions ranged from 0.1 to 1.3 on average. The coefficient of friction was significantly influenced by the kind of flooring, the state of the surface and the pace at which the foot was moving (Mean COF = 0.80 to 1.33). All dry surfaces showed adequate slip resistance. Generally, the COF of wet surfaces (mean COF = 0.43 to 1.05) was lesser than that of dry floorings but significantly greater than that of oil-applied flooring surfaces (mean COF = 0.10 to 0.21). Comparing ceramic tile to other floor materials, ceramic flooring demonstrated lesser friction coefficient readings under all surface and slipping velocity-related circumstances. In both dry and wet conditions, Siegmund et al. [18] sought to analyse and measure the friction that barefoot participants used to enter and exit a standard shower enclosure. Ground reaction forces were measured using force plates inserted in the shower enclosure flooring and the surrounding bathroom flooring. From these measurements, the traction performance and double support timings were estimated. This study had 60 healthy participants. Considering three age clusters (20–30 years), (40–50 years), (beyond 50 years) and (60–70 years) about 10 males and 10 females were recruited. An informed consent form was read and signed by each participant. Force plates installed in the bathroom (Bertec 4060H, Columbus, OH) and in the shower enclosure and were used to measure ground response forces (Bertec 4060). The bathroom flooring was cut to accommodate a force plate, and the cutout section was fixed to the plate’s top and set flush with the surrounding tub floor. On the back wall, a grab bar was put in for safety. To increase subject safety, slip-resistant materials were added on the bathroom flooring tiles. First, subjects underwent a barefoot balance test that involved standing still on the bathroom flooring tile consisting of the force plate for 30 s while keeping their eyes closed. No respondents reported slipping, and they saw no slips. There were 30 trials with 17 participants that may have shown microslip. The shower enclosure force plate saw all of these micro-slips: 17 during wet experiments and 13 during dry testing. Age clusters (young 8, middle 12, old 10), genders (females 14, men 16) and movement orientations were represented by the microslips (entry 16, exit 14). Female participants had a longer centre of pressure distance than male subjects (F: 109 ± 15 cm, M: 97 ± 15 cm, P = 0.002). Older respondents displayed higher and more varied double support periods than both young and middle-aged subjects when entering the bathroom (P < 0.0006). When entering the bathroom while wet, double support times were likewise higher and more varied than when doing so while dry (P < 0.0001). When getting out of the water, the trend of age-related variations was identical (P < 0.0009).

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Thus, we can observe that the main motive behind the human slip testing experiments was to effectively replicate the actual slipping scenario across different floorings, in the presence of different contaminants. The experimental process is effective but there are some major shortcomings. The experimental setup for the human slip testing experiment is quite costly as force plates, motion sensors, videographic equipment and safety harness equipment are required for the slip testing trials. Furthermore, as human participants are involved, there is the requirement of the ethical clearance from the medical board, which also takes a substantial amount of time, leading to the delay in the slip testing experiments. There is also the psychological factor of the participants during the event of a slip fall, which can result in discrepancy of the recorded data also. Mainly due to these reasons, mechanical slip testing methods are now being increasingly adopted for doing slip testing experiments. Table 5.1 summarizes the works on barefoot slipping.

5.2 Footwear Slipping Experiments Jones et al. [19] sought to measure how shoe design elements affected the available coefficient of friction (ACOF) in footwear classified as SR (Slip Resistant). Friction coefficient and slippage rate variations amongst SR shoes were also measured. Utilizing a full footwear mechanical slip device, 12 pairs of shoes were evaluated on five different types of flooring and three different contaminant situations. Both a mechanical evaluation part and a human slip evaluation part were included in this investigation. Twelve shoe styles were tested in 15 contaminated surface scenarios as part of the mechanical testing component. By suddenly exposing test subjects to surfaces contaminated with canola oil, the slipping speed was determined for three different types of slip-resistant (SR) footwear on vinyl composite tile in the human participant’s evaluation phase. The contact area, heel breadth, and hardness of the shoe outsoles were evaluated as evaluated by other studies [20–22]. The shore A durometer was used to gauge the shoes’ short- and long-term hardness. Two vinyl flooring styles, two quarry flooring styles and one ceramic flooring style were all featured in the flooring. Water, water (99.5% by volume)/sodium laurel sulphate (0.5% by volume) mixture and canola oil were among the contaminants. A rheometer was used to measure the contaminants’ viscosity levels. Utilizing a force plate to quantify perpendicular and shear forces, ACOF data were collected. Apart from the mechanical devices and force plates, Jiang et al. presented individual sensors to estimate the reaction forces as shown in Fig. 5.2. Three vertical and one horizontal motors were utilized in a system that was based on the design of the transportable slip simulator to produce the slip dynamics. 36 participants, aged 18–35, were selected for the experiment’s human slippage testing component. They were divided into 16 females and 20 males, with an average age and sample variance of 21.9 ± 4.4 years, 70 ± 12.4 kg and 1.74 ± 0.8 m, respectively. The subjects were fitted with reflective markers all over their bodies, a safety belt, and were given a pair of shoes at random. At least three walking trials were

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Table 5.1 Summary of literature review related to barefoot slipping Author

Experimental method

Outcomes

Kleiner et al. [1]

They looked at the impact of flooring on barefoot gait of different genders. Ground reaction forces and friction were measured using two force plates, which were integrated in the floor of the data acquisition chamber

When gender effects were assessed, the male individuals displayed a lower relative friction coefficient than the female ones with a negligibly small effect size

Derler et al. The relationship between the microscopic [13] topographical characteristics of encountered hard substrates and the sliding friction coefficient of water applied plantar skin was looked upon

Individual biomechanical investigations measured friction coefficients ranging from 0.072 to 0.0762, with average readings for various surfaces falling between 0.117 and 0.590

Li et al. [15]

They determined the force plate-based coefficient of friction (COF) between both the foot and the floor based on a female subject. The force platform’s top was mounted with the tested floor. These floors were each put to the test in dry, water applied and oily circumstances

The force plate-based COF on the dry surface was 0.82, which was significantly higher than the COF on the wet surfaces (p < 0.05), which came in second (p = 0.61)

Nagata et al. [16]

They measured and analysed the reaction forces that were applied to the flooring surface at the time of a slip. Each participant was instructed to take a single, heel-to-toe step in barefoot condition on the slick floor and the motion was tracked by motion capture system

It was discovered that the speed of the foot motions immediately increases linearly and quickly when the foot touches the ground Accelerative slips began with initial velocities of about 0.43 m/s. Additionally, it was discovered that the normal forces at the moment the slip occurred ranged from 47 to 489 N

Li et al. [17]

The measurement of the ACOF between the flooring and the foot on three different surfaces at two different slipping speeds was performed The ground response forces applied to the foot were evaluated using a force platform

Compared to wet and oily surfaces, the friction coefficient measurements for dry floorings were greater The friction coefficient of the foot under surface and slipping velocity conditions ranged from 0.1 to 1.3 on average

Siegmund et al. [18]

The main focus was on analysis and measurement of the friction that barefoot participants exhibited while entering and exiting a standard shower enclosure

Older respondents displayed higher and more varied double support periods than both young and middle-aged subjects when entering the bathroom

5.2 Footwear Slipping Experiments

45

Fig. 5.2 Sensors mounted on the areas of concern [23]

completed by subjects in which their left foot totally touched the force plate. This investigation verified that there is significant difference among slip-resistant (SR) shoes. ACOF values were linked with area of contact, treaded heel dimension, shortterm indentation resistance and long-term indentation resistance when canola oil was present. This study lends support to earlier studies that looked at how ACOF is affected by shoe outsoles. The results of the trials on humans show that footwear with a greater frictional coefficient decrease slipping occurrences, although changes in the frictional coefficient throughout the low to medium frictional coefficient levels were not responsive enough to cause a noticeable shift in the rates of slipping. The findings of this investigation also suggest that not all shoes with the designation “SR” are equally efficient. For instance, ACOF values on the reference Vinyl tile with canola oil varied across SR shoes from 0.118 to 0.372. In order to direct available coefficient of friction (ACOF) evaluation procedures for shoes and tiles used for floorings, Iraqi et al. [24] attempted to measure the human slip heel biomechanics and energetics. For this secondary study, kinematic and kinetic data from 39 patients who had a slip occurrence were combined from four related human slip risk evaluation investigations. The following variables were measured: slipping velocity, interaction period, shoe-floor angle, lateral slip inclination, vertical ground reaction force (VGRF) and centre of pressure (COP) (PSS). To determine

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Table 5.2 Summary of literature review related to footwear slipping Author

Experimental method

Outcomes

Jones et al. The main focus was to measure The findings of this investigation suggest that [19] how shoe design elements not all shoes with the slip-resistant designation affected the ACOF in footwear are equally efficient classified and not classified as slip resistant Iraqi et al. [30]

The aim of this study was to create an analytical framework that forecasts the available coefficient of friction (ACOF) under circumstances for boundary lubrication

Increased tread surface area and a switch from a flat to a bevelled edge in the heel design improved traction. Additionally, it was discovered that when tread surface area grows, contact pressure decreases, which in turn raises hysteresis friction

Iraqi et al. [24]

In order to develop ACOF The average shoe floor angle at slip start was evaluation procedures for shoes greater than the 7° inclination required by the and tiles used for floorings, an ACOF testing procedures attempt was made to measure the human slip heel biomechanics and energetics

whether there are any disparities between the condition of the sliding limb and the existing frictional coefficient evaluation settings, statistical comparisons were employed. Four distinct human slipping tests carried out in the same laboratory yielded biomechanical and energetic readings for 39 people (including 18 females; average age: 22.3 ± 3.3 years; average height: 173.1 ± 8.3 cm; average body weight: 68.3 ± 10 kg; average BMI: 22.8 ± 3.2). The inclusion criteria of the experimental analysis were slips followed by minimum three gait trials and a slipping range of more than 3 cm. The subjects employed a safety harness device and implemented a full body marker set. In a laboratory setting outfitted with a motion camera system, subjects were given the task of walking over a level vinyl composite tile sidewalk. On the dry walkway, the subjects carried out three to five gait tests with their left appendage firmly planted on the force plate. Then a liquid contaminant was unexpectedly poured onto the force plate, exposing the participants to it. It was discovered that the VGRF, footwear flooring inclination, and period of contact at slip start (SS) testing parameters had considerably different central tendencies from each other. Additionally, the average shoe floor angle at slip start (SS) was greater than the 7° inclination required by the ACOF testing procedures. Table 5.2 summarizes the works on footwear slipping experiments. Other studies have also worked on the similar testing procedures using a different device [25–29].

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References 1. Rozin Kleiner AF, Galli M, Araujo do Carmo A, Barros RML (2015) Effects of flooring on required coefficient of friction: elderly adult versus middle-aged adult barefoot gait. Appl Ergon 50:147–152. https://doi.org/10.1016/j.apergo.2015.02.010 2. Chatterjee S, Gupta S, Chanda A (2022) Barefoot slip risk in Indian bathrooms: a pilot study. Tribol Trans 1–15. https://doi.org/10.1080/10402004.2022.2103863 3. Stevens JA, Haas EN, Haileyesus T (2011) Nonfatal bathroom injuries among persons aged ≥ 15 years-United States, 2008. J Safety Res 42(4):311–315. https://doi.org/10.1016/j.jsr.2011. 07.001 4. Scheffer AC, Schuurmans MJ, Van Dijk N, Van der Hooft T, De Rooij SE (2008) Fear of falling: measurement strategy, prevalence, risk factors and consequences among older persons. Age Ageing 37(1):19–24. https://doi.org/10.1093/ageing/afm169 5. Yiannakoulias N, Rowe BH, Svenson LW, Schopflocher DP, Kelly K, Voaklander DC (2003) Zones of prevention: the geography of fall injuries in the elderly. Soc Sci Med 57(11):2065– 2073. https://doi.org/10.1016/S0277-9536(03)00081-9 6. Vellas BJ, Wayne SJ, Romero LJ, Baumgartner RN, Garry PJ (1997) Fear of falling and restriction of mobility in elderly fallers. Age Ageing 26(3):189–193. https://doi.org/10.1093/ ageing/26.3.189 7. Howland J, Lachman ME, Peterson EW, Cote J, Kasten L, Jette A (1998) Covariates of fear of falling and associated activity curtailment. Gerontologist 38(5):549–555. https://doi.org/10. 1093/geront/38.5.549 8. Deshpande N, Metter EJ, Lauretani F, Bandinelli S, Guralnik J, Ferrucci L (2008) Activity restriction induced by fear of falling and objective and subjective measures of physical function: a prospective cohort study. J Am Geriatr Soc 56(4):615–620. https://doi.org/10.1111/j.15325415.2007.01639.x 9. Brouwer B, Musselman K, Culham E (2004) Physical function and health status among seniors with and without a fear of falling. Gerontology 50(3):135–141. https://doi.org/10.1159/000 076771 10. Brody KK, Johnson RE, Ried LD (1997) Evaluation of a self-report screening instrument to predict frailty outcomes in aging populations. Gerontologist 37(2):182–191. https://doi.org/10. 1093/GERONT/37.2.182 11. Aminzadeh F, Edwards N, Lockett D, Nair RC (2001) Utilization of bathroom safety devices, patterns of bathing and toileting, and bathroom falls in a sample of community living older adults. Technol Disabil 13(2):95–103. https://doi.org/10.3233/tad-2000-13202 12. Greiman L, Fleming SP, Ward B, Myers A, Ravesloot C (2018) Life starts at home: bathing, exertion and participation for people with mobility impairment. Arch Phys Med Rehabil 99(7):1289–1294. https://doi.org/10.1016/j.apmr.2017.11.015 13. Derler S, Huber R, Feuz HP, Hadad M (2009) Influence of surface microstructure on the sliding friction of plantar skin against hard substrates. Wear 267(5–8):1281–1288. https://doi.org/10. 1016/j.wear.2008.12.053 14. Ouattas A, Rasmussen CM, Hunt NH (2022) Severity of unconstrained simultaneous bilateral slips: the impact of frontal plane feet velocities relative to the center of mass to classify sliprelated falls and recoveries. Front Public Heal 10:898161. https://doi.org/10.3389/FPUBH. 2022.898161/BIBTEX 15. Li KW, Wen HC (2014) Friction between foot and floor under barefoot conditions: a pilot study. IEEE Int Conf Ind Eng Eng Manag 1651–1655. https://doi.org/10.1109/IEEM.2013.6962690 16. Nagata H, Kato M, Watanabe H, Inoue Y, Kim IJ (2008) A preliminary study on slip potentials of stepping barefoot on slippery floors. Contemp Ergon 2008:710–716 17. Li KW, Lin CC (2013) Effects of floor material, surface condition, and foot moving speed on the coefficient of friction on the floor. Appl Mech Mater 303–306:2704–2707. https://doi.org/ 10.4028/www.scientific.net/AMM.303-306.2704

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18. Siegmund GP, Flynn J, Mang DW, Chimich DD, Gardiner JC (2010) Utilized friction when entering and exiting a dry and wet bathtub. Gait Posture 31(4):473–478. https://doi.org/10. 1016/j.gaitpost.2010.02.003 19. Jones T, Iraqi A, Beschorner K (2018) Performance testing of work shoes labeled as slip resistant. Appl Ergon 68:304–312. https://doi.org/10.1016/J.APERGO.2017.12.008 20. Gupta S, Chatterjee S, Malviya A, Chanda A (2022) Traction performance of common formal footwear on slippery surfaces. Surfaces 2022, vol 5, no 4, pp 489–503, Nov 2022. https://doi. org/10.3390/SURFACES5040035 21. Gupta S, Sidhu SS, Chatterjee S, Malviya A, Singh G, Chanda A (2022) Effect of floor coatings on slip-resistance of safety shoes. Coatings 12(10). https://doi.org/10.3390/COATINGS1210 1455 22. Gupta S, Chatterjee S, Malviya A, Chanda A (2023) Frictional assessment of low-cost shoes in worn conditions across workplaces. J Bio- Tribo-Corrosion 2023 919(1):1–13. https://doi. org/10.1007/S40735-023-00741-0 23. Jiang X, Napier C, Hannigan B, Eng JJ, Menon C (2020) Estimating vertical ground reaction force during walking using a single inertial sensor. Sensors 2020, vol 20, no 15, p 4345, Aug. 2020. https://doi.org/10.3390/S20154345 24. Iraqi A, Cham R, Redfern MS, Vidic NS, Beschorner KE (2018) Kinematics and kinetics of the shoe during human slips. J Biomech 74:57–63. https://doi.org/10.1016/j.jbiomech.2018. 04.018 25. Gupta S, Malviya A, Chatterjee S, Chanda A (2022) Development of a portable device for surface traction characterization at the shoe & ndash; floor interface. Surfaces 2022, vol 5, no 4, pp 504–520, Dec 2022. https://doi.org/10.3390/SURFACES5040036 26. Gupta S, Chatterjee S, Chanda A (2022) Effect of footwear material wear on slips and falls. Mater Today Proc. https://doi.org/10.1016/J.MATPR.2022.04.313 27. Gupta S, Chanda A (2023) Biomechanical modeling of footwear-fluid-floor interaction during slips. J Biomech 156:111690. https://doi.org/10.1016/J.JBIOMECH.2023.111690 28. Gupta S, Chatterjee S, Chanda A (2023) Influence of vertically treaded outsoles on interfacial fluid pressure, mass flow rate, and shoe–floor traction during slips. Fluids 2023, vol 8, no 3, p 82, Feb 2023. https://doi.org/10.3390/FLUIDS8030082 29. Gupta S, Kumar M, Singh G, Chanda A (2023) Development of a novel footwear based power harvesting system”, e-Prime-Adv. Electr Eng Electron Energy 3:100115. https://doi.org/10. 1016/J.PRIME.2023.100115 30. Iraqi A, Vidic NS, Redfern MS, Beschorner KE (2019) Prediction of coefficient of friction based on footwear outsole features. Appl Ergon 82:102963, 2020. https://doi.org/10.1016/j. apergo.2019.102963

Chapter 6

Mechanical Footwear Traction Testing Methods

6.1 Need for Mechanical Slip Testing Performing slip testing studies with actual human subjects is a great way of replicating the actual slipping scenarios and a great deal of literature exists regarding such human slip testing experiments. But there are various shortcomings of human slip testing experiments, which are illustrated as follows: (1) Human slip testing experiments are cost and time-intensive due to the equipment needed ranging from videographic equipment to safety harness equipment. (2) Requirement of a gait analysis lab with sophisticated instruments. The cost is quite high (cost > 2.5 Crores). (3) Ethical clearance is a committee matter and it takes substantial amount of time, similar issues with the biosafety regulations. (4) Collecting adequate number of slip data from participants can become a challenging task. (5) Biomechanical variations in walking and slipping pattern among participants. (6) Psychological and physiological factors during slipping. To evaluate how well footwear and flooring resist slipping, mechanical slip testing instruments that evaluate available coefficient of friction (ACOF) are widely used. Since the kinematics and kinetics used to design footwear have an impact on the ACOF readings, these devices occasionally make an effort to recreate the dynamics of the foot slide in order to achieve biofidelity (i.e., similarity between test conditions and shoe dynamics when slipping) [1]. In order to evaluate friction at the shoe-floor interface, a variety of mechanical slip testing tools have been utilized up to this point [2, 3]. “Biofidelity” (the ability of replicating the footwear outsole mechanics of a person’s slip) and “environmental fidelity” (the process of replicating the flooring surfaces and spill conditions present during accidents resulting from slips and

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 A. Chanda et al., Footwear Traction, Biomedical Materials for Multi-functional Applications, https://doi.org/10.1007/978-981-99-7823-6_6

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falls) are two crucial parts of testing. Although footwear motions have been standardized from those seen in a real slip testing scenario with human volunteers due to design and repeatability difficulties [1, 4]. Owing to weight and portability restrictions, more extreme simplifications were performed with portable slip testers, which resulted in low accuracy with the actual shoe-floor contact [5]. There are significant discrepancies between the ACOF results that were assessed using different slipmeters. For a certain shoe, a variety of ACOF values have been discovered under various floor contamination test circumstances [6]. There isn’t a single test instrument or measurement technique that is recognized worldwide, despite the fact that many various types of traction measurement instruments have been created. Studies have revealed significant discrepancies in these instrument’s performance, even when the similar flooring scenarios and spillage agents are used. These discrepancies could be caused by the kinematic and dynamic properties of the recording equipment, the material attributes of the slide materials or the slip risk assessment environment (ambient atmospheric composition, atmospheric moisture content). Although the absolute values of the friction coefficient varied between the tested devices, other investigations revealed a substantial link between them.

6.2 Introduction to Slip Testers Slipperiness has frequently been measured using friction. There are still questions concerning how to link friction with slipperiness, though. These include the issues of different methodologies and different device variation, the concern of biofidelity between slipping as an event and friction as a parameter, and whether the type of friction is more important, a question (static, dynamic and transitional). Traction assessment is still one of the most widely used techniques for assessing the rate of slipping despite these drawbacks. Numerous tools based on friction assessments, commonly referred to as slipmeters, have been created to evaluate how slippery footwear and walking surfaces are. While using friction to address the subject of slipperiness, all of these devices have distinct measuring features. Numerous methods for measuring friction have been employed, including impact, constant velocity friction and static drag sled approaches. Applied pressure, area of contact and speed of slipping at the region of contact commonly vary dramatically amongst devices depending on a particular type of friction. Only under realistic settings, with proper footwear and daily life used contaminants, is it possible to evaluate floors as they are in actual use [7–12]. Field measurements are frequently taken to: (1) Analyse the impact of actual implication or fouling on the development of resistance against slips in similar locations throughout a construction site. (2) Evaluate slip resistance at several locations to determine the role that various elements play in slipperiness.

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(3) Analyse a new floor surface’s ability to prevent slipping. Even if the two have the same commercial reference, the condition of the flooring in the event of floorings set or placed on site may differ from that examined in the laboratory.

6.2.1 Horizontal Pull Slipmeter (HPS) Mainly static drag sled is the foundation of the HPS. The HPS applies the drag force with the aid of a motor. About 70.2 kPa of contact pressure exists between the floor surface and the shoe sample. Only dry surfaces are intended for usage with the HPS.

6.2.2 Portable Articulated Strut Tribometer (PAST) Ideally Brungraber Mark I and PAST are both articulated strut slipmeters. The inclination formed among the strut and the vertical component at which a significant slide is known to happen at the contact involving a footwear sample and a flooring tile is used to calculate the coefficient of friction (COF). About 9.2 kPa of contact pressure exists at the interface. Only readings on dry surfaces are recommended for use with the PAST.

6.2.3 Portable Friction Tester (PFT) The PFT has been talked about by Strandberg et al. (1985). The experimenter pushes this object across the floor surface at a steady speed. A front wheel that has been braked (the test wheel) makes contact with the flooring, creating the biomechanical slipping force required for the evaluation of kinetic traction. 112 N is the typical force exerted on the front wheel. An elastomer band that is smooth covers this wheel. Under static conditions, the observed tread contact area among the wheel and the flooring is 2.8 cm2 . The sliding ratio that a speed reducer imposes on the front wheel (0–100%) and the velocity at which the device is moved forward determines the friction and rotational velocities of the test wheel mechanism. The user can choose the timings of the recordings by a start and stop switch. A built-in computer offers estimations of the dynamics COF mean value and standard deviation, estimation distance, actual slipping ratio, and average forward moving velocity at the conclusion of each measurement.

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6.2.4 British Portable Skid Tester (BPST) According to the French standards NF-P-18–578 [13] and NF-P-90–106 [14], both define this device, which incorporates several enhancements over the Siegler device (1986). The horizontal plane and altitude adjustment capabilities are unbiased, which greatly reduces the amount of time needed for setup. The cylindrical shaped arm of the pendulum served as the basis for the spring mechanism that resulted in the application of the resultant force, making it more flexible. As a result, the frictional force between the slider and the walking surface is more nearly constant; nonetheless, the normal force’s strength is dependent on the COF value. Slow-motion images collected at the National Bureau of Standards (NBS) reveal that the rubber slider still rebounded across the flooring tile being tested despite these improvements.

6.2.5 Step Simulator The computer-controlled step simulator principle is used in the friction tester designated as the Toegepast Natuurwetenschappelijk Onderzoek (TNO) slip risk assessment device. The sole of the shoe moves, but the floor is still. The device comprises of prosthetic leg part accompanied by a rigid ankle connecting a robotic lower leg and foot. To get a perfect match between the foot and the shoe, the prosthetic foot can be adjusted in the required dimensions. The hydraulically generated applied load and shearing force required to shift the footwear heel or outsole in relation to the floor surface are used to calculate dynamic COF. The leg module’s load cells are where the shear and normal forces are measured. There are several possible footwear flooring tactile inclination (0–10°). The slipping speed can be changed (0.1–0.4 ms-1). After a brief standstill timeframe of typically 100 ms or less, shoe motion toward the forward direction starts as the normal force builds up. Typically, dynamic COF is calculated 100–450 ms after the slide starts. The range of the normal force is 350–750 N.

6.2.6 Slip Simulator A slip simulator was created by Gronqvist et al. [15] to mimic a slip that would occur after a heel strike during everyday walking. The shoe is attached to the system using an artificial foot. The footwear is translated against the floor surface by three hydraulic cylinders (vertical, horizontal and inclination). The cylinder applying vertical force descends the footwear onto the flooring surface at a velocity of 0.1 ms−1 while the cylinder applying the horizontal force begins translation in a situation just above the flooring surface and thereafter consistently holds a chosen equal velocity, commonly 0.4 ms−1 . The footwear heel then makes contact with the floor at the chosen shoe-floor contact angle that the inclination cylinder has applied.

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Fig. 6.1 Different Types of Slip Testers: a Old (British Pendulum Tester), b Custom-designed biofidelic slip tester by Gupta et al. [16]

The traction and vertical forces at the shoe-flooring tile contact region are recorded by a force platform. The moment when the downward force exceeds 100 N, which is typically described as the heel touchdown, the measurement begins. Ordinarily, 100 ms after heel touchdown is the start of the period of time of 50 ms for taking the average value the vertical force, traction force, COF, and slipping velocity data. During a typical test, the sliding velocity is 0.40 ± 0.02 m/s−1 , and the imposed normal force is 700 ± 20 N. The heel test commonly employs a 5° shoe-floor contact angle. Other tests include sole sliding to the side, sole flat, and sole sliding backward. The footwear can spin 360° in the ground plane and the angle of footwear-flooring contact can be modified to a range of ± 25° around an upright position. Figure 6.1 shows one of the old used slip tester and a latest slip tester.

6.3 Comparison of Slip Testers The effectiveness of three transportable floor traction devices specifically the towing sled (Gabbriell SM), a pendulum (portable skid resistance), articulated strut with combined action of perpendicular and shearing force (Brungraber Mark II) against a force platform and verified biomechanically with the help of a slip simulation device as reference apparatus, was examined by Grönqvist et al. [15]. According to the specification of a PSRT, it is a swing pendulum-style instrument, having high speed KCOF (frictional skid resistance) and can be evaluated across a slipping length of 125–127 mm. The power lost at the event of contact between the sliding component and the flooring is where the frictional skid resistance is derived from.

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According to the usage instructions, the frictional resistance measurements were converted to approximately frictional coefficient values by dividing by hundred. The BMII is an articulating truss device that applies vertical and shearing stresses concurrently by activating the sliding mechanism at a set oblique angle. It is identified at what angle the slip recently occurred. The GSM monitors minimum velocity KCOF with an uncertain region of contact duration between the sliding mechanism and the flooring. Similar to the Tortus Floor Friction Tester, it is a towed sled-type instrument. A main powered motor propels the instrument forward, and a transducer for linear measurement produces a signal that is equivalent to the friction coefficient. The three portable test devices were compared to a force platform (Bertec 4060H) and a slip device designed for assessing the frictional resistance of flooring, shoes and soling materials. The slip simulator equipment can determine an appropriate range for a variety of test variables, including the slipping velocity, time spent in contact with the flooring, and the development rate and amount of normal force and pressure. For the mean friction coefficient values, Pearson’s linear correlation analysis was used. A slider composed of nitrile rubber with an indentation resistance of 61 ± 1 and a surface roughness (Ra) of 0.6 ± 0.1 µm was employed. Glycerol was present in concentrations ranging from 85 to 91.5% weight and was employed as a detergent in water with sodium lauryl sulphate (0.5% wt), which had a viscosity of 0.2–0.1 PaS at 20 °C. The current study partially corroborates earlier studies’ findings on the PSRT pendulum’s reasonable repeatability, precision, and selectivity. On the other hand, there was a poor Spearman’s rank correlation (r = 0.63) between the results of the subjective ramp experiments and the data from the pendulum tester. Although the BMII worked satisfactorily, it lacked the PSRT’s accuracy and consistency. The current investigation demonstrated that the realistic vertical force generated by the BMII depended on the amount of time the sliding mechanism was in contact with the flooring as well as the friction coefficient level. It can be concluded that the BMII is a more reliable device for assessment, based on comparisons with the FIOH slip simulator, pedestrian slip resistance is greater than that of the PSRT pendulum. Chang et al. [17] attempted to assess the traction performance of 16 widely implemented shoe materials on three floorings using the Brungraber Mark II and English XL in dry, wet, oily and oily wet surface conditions. Every material mixture and set of surface conditions received three samples. The findings of this study may help end users make the best choice of footwear materials. For the same material combinations and surface conditions, three samples were used. The differences between the various samples were compared using an analysis of variance (ANOVA) to see if they were statistically significant. At the atmospheric condition of 21 ± 1.7° and an absolute moisture content of 50 ± 5%, all traction performance measurements were conducted. The specimen surfaces were washed with water and diluted mild detergent (1% by volume). After a thorough cleaning with tap water, the surfaces were dried with compressed air. The specimen surfaces were then cleaned with deionized water that has been combined with 50% ethanol.

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With the help of compressed air, the surface was dried. These cleaning techniques were used on all the samples, including the samples of shoes and floor tiles. Additionally, the cleaned shoe pads were manually sanded for a total of 10 full movements in the longitudinal axis of the diamond pattern using 400 grit silicon carbide abrasive paper (five strokes). After turning the specimen 90°, the abrasive paper sanding procedure was carried out once more. All shoes and flooring materials underwent dry and wet testing. The testing area on the flooring surface was flooded with deionized water in order to simulate wet circumstances. Only measurements using quarry tiles were allowed under oily and oily water applied scenarios. To simulate greasy conditions, two millimetres of vegetable oil were applied using a brush to the entire test surface. Measurements of friction were made parallel to the grooves on the back of the quarry tiles and in the longitudinal axis of the diamond pattern on the shoe item. Each sample pair, each surface condition and each slipmeter received three additional readings. There were three tile samples for dry and wet circumstances for each material combination, as well as three tile specimens for oily and oily wet conditions. The English XL’s range of friction coefficient measurements is 0–1.0, while the Brungraber Mark II’s range is 0–1.1. The findings of ANOVA analysis showed that the most statistically significant changes in frictional coefficients for various samples with the same material combination, surface condition and slipmeter were observed. The English XL in wet conditions had substantially higher standard deviations and a higher coefficient of variation of friction than the Brungraber Mark II over three distinct floorings. The Brungraber Mark II outperformed the English XL in terms of average standard deviation and coefficient of variation on oily surfaces. Under wet and oily wet conditions, the friction coefficients measured with the English XL were significantly greater than those determined with the Brungraber Mark II. Kim et al. [18] sought to determine how well three different types of slipmeters performed in actual situations and to compare the results of gait tests to these slipmeters’ motion and movement characteristics. 12 participants were also quantified using the same equipment using three distinct types of slipmeters to look at their kinematic and dynamic features. The BOT-3000 is an instrument that resembles a drag sled and estimates the static coefficient of friction (SCOF) of a surface area among its slider and the flooring. The dynamic coefficient can also be calculated or measured using the apparatus. The English XL is a gas pressure-driven inclined strut-slip metre that is also known as a variable Incidence tribometer. The sensor slider exerts force on the flooring at an angle of inclination from the perpendicular direction, giving its velocity a horizontal component. The BPT employs a swaying dummy heel that carefully sweeps across a predetermined area of flooring. This measurement tool uses British Pendulum Numbers (BPN). This investigation employed a ceramic, vinyl and asphalt tile for the floors, with and without a waxed finish. While ceramic is often utilized in restaurant kitchens, vinyl and asphalt tiles are frequently employed in eating space outlets. Particular categories of slider material were used to estimate the slip resistance capabilities because the three chosen slip testers were unable to use footwear (or sole) specimens. Each test device’s respective producers provided the slider materials.

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For all surface types, the BPT used a 4S rubber slider with a Shore A hardness of 96 ± 2. With the BOT, two separate sliders were included. Five different surface conditions were tested for slip resistance: dry, water applied, detergent solution, soyabean oil and engine oil. Each contaminant was stored in an airtight, container at a consistent temperature. For each trial, the contaminants were cleaned with a paper cloth, washed with a 50% solution of ethanol and distilled water and allowed to air dry. Twelve healthy male participants between the ages of 20 and 50 were included in this study. The patients were 171 ± 3.81 cm tall, 36.5 ± 8.54 years old and weighed 72.7 ± 5.9 kg. Each participant wore the identical pair of shoes and a safety harness. They moved across the force-plate-equipped, polluted flooring. The force plate and high-speed camera were used to record their trajectories as well as the response force of their foot when they slipped. The asphalt tile B, which had a lower roughness, had the highest SCOF according to the BPT (Ra, Rz). The BOT-3000’s SCOF measurement showed a roughly linear rise with surface roughness (Ra). With the exception of the Asphalt tile, the English XL had the lowest SCOF when contrasted to the other devices, and its SCOF was raised by Ra. The BOT-3000’s excellent repeatability was probably due to the fact that it moved forward automatically using a motorized motor and used a strain gauge to measure the SCOF. Overall, the findings demonstrated that the characteristics of the floor and the presence of contaminants on the floor had an impact on the traction performances of the three instruments used to measure flooring slip probability. Aschan et al. [19] studied how portable devices measuring friction were used assessing slipperiness of walking surfaces during the winter seasons of 2003 or 2004. Different walking surfaces’ slip resistance, or dynamic coefficient of friction (DCOF), was assessed on-the-spot using a portable slip simulator. The device’s measurement parameters were as follows: perpendicular forces with three values that could be selected—170, 250 or 500 N—depending on the coefficient of friction of the surfaces; horizontal sliding velocity; and shoe contact angle, or region of heel contact; of 5°. For each pair of shoes and surface condition, the mean and standard deviations of at least nine consecutive DCOF measurements at the same location were determined. A significant factor influencing test outcomes was the footwear sample utilized in DCOF measurements. The measurements were carried out in the later part of the winter of 2003 between 19 February and 7 April while wearing four different pairs of shoes: (i) Winter shoes that were discovered to have slip probability during usage, (ii) tractionenhancing safety shoes, and slippery footwear with an outsole made of thermoplastic polyurethane (TPU), a material that is frequently used in women’s shoes and is hard (Shore A hardness 92) and (iv) shoes with flexible polyurethane (PU) outsoles that have had their tread pattern completely erased using an orbital sander. Before doing the DCOF measurements, each operator subjectively rated how slippery the walking surface was. A graduated scale was used for the rating, with the options being: (1) highly prone to slipping, (2) prone to slipping, (3) unbiased, (4) preventing slipping and (5) having great resistance to slipping.

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The findings of personal DCOF measurements made in this study under various weather and paving conditions show that footwear choice is crucial in minimizing wintry sliding accidents. The in situ DCOF measurements highlighted the significance of regular winter maintenance. When the flooring tile was asphalted, friction was doubled even in icy circumstances. The transportable slip-simulating device showed to be accurate in wintry field measurements and capable of measuring the performance of winter footwear slip resistance. The transition of the type of slip testers used for slip risk assessment studies can be clearly understood in this section. The slipmeters evolved from a comparatively simple drag and slide type tester to the portable slip tester which mimics the slip biomechanics of an actual slip. The portability of the slip testers was also the main concern, as compared to the British Pendulum Skid tester which was comparatively heavy and posed a problem in transportation, the portable slip simulator can be taken to any location be it any hospital or outside environment with ease and the slip testing experiments could be performed more effectively. Table 6.1 summarizes the literature review associated with slip testing devices. Table 6.1 Summary of literature review related to slip testing devices Author

Experimental method

Outcomes

Grönqvist et al. [15]

The effectiveness of three transportable floor traction devices was examined

Significant different in the ACOF values was observed. The devices were not consistent in replicating the actual slip biomechanics

Chang et al. An attempt was made to assess the [17] traction performance of 16 widely implemented shoe materials on three floorings using the Brungraber Mark II and English XL in dry, wet, oily and oily wet surface conditions

The Brungraber Mark II outperformed the English XL in terms of average standard deviation and coefficient of variation on oily surfaces

Kim et al. [18]

An attempt was made to determine how well three different types of slipmeters performed in actual situations

The findings demonstrated that the characteristics of the floor and the presence of contaminants on the floor had an impact on the traction performances of the three instruments used to measure flooring slip probability

Aschan et al. [19]

The focus of this experimental study was to study how portable devices measuring friction were used assessing slipperiness of snow-affected walking surfaces

The transportable slip simulating device showed to be accurate in wintry field measurements and capable of measuring the performance of winter footwear slip resistance

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6.4 Slipping Experiments with Whole Footwear Jones et al. [20] sought to measure how shoe design elements affected the available coefficient of friction (ACOF) in footwear classified as SR (Slip Resistant). Friction coefficient and slippage rate variations amongst SR shoes were also measured. Utilizing a full footwear mechanical slip device, 12 pairs of shoes were evaluated on five different types of flooring and three different contaminant situations. Both a mechanical evaluation part and a human slip evaluation part were included in this investigation. Twelve shoe styles were tested in 15 contaminated surface scenarios as part of the mechanical testing component. By suddenly exposing test subjects to surfaces contaminated with canola oil, the slipping speed was determined for three different types of slip-resistant (SR) footwear on vinyl composite tile in the human participant’s evaluation phase. The contact area, heel breadth and hardness of the shoe outsoles were evaluated. The shore A durometer was used to gauge the shoes’ short- and long-term hardness. 2 vinyl flooring styles, 2 quarry flooring styles and 1 ceramic flooring style were all featured in the flooring. Water, water (99.5% by volume)/Sodium Laurel Sulphate (0.5% by volume) mixture and canola oil were among the contaminants. A rheometer was used to measure the contaminants’ viscosity levels. Utilizing a force plate to quantify perpendicular and shear forces, ACOF data were collected. Three vertical and one horizontal motors were utilized in a system that was based on the design of the transportable slip simulator to produce the slip dynamics. 36 participants, aged 18–35, were selected for the experiment’s human slippage testing component. They were divided into 16 females and 20 males, with an average age and sample variance of 21.9 ± 4.4 years, 70 ± 12.4 kg and 1.74 ± 0.8 m, respectively. The subjects were fitted with reflective markers all over their bodies, a safety belt, and were given a pair of shoes at random. At least three walking trials were completed by subjects in which their left foot totally touched the force plate. This investigation verified that there is significant difference among slip-resistant (SR) shoes. ACOF values were linked with area of contact, treaded heel dimension, shortterm indentation resistance and long-term indentation resistance when canola oil was present. This study lends support to earlier studies that looked at how ACOF is affected by shoe outsoles. The results of the trials on humans show that footwear with a greater frictional coefficient decrease slipping occurrences, although changes in the frictional coefficient throughout the low to medium frictional coefficient levels were not responsive enough to cause a noticeable shift in the rates of slipping. The findings of this investigation also suggest that not all shoes with the designation “SR” are equally efficient. For instance, ACOF values on the reference Vinyl tile with canola oil varied across SR shoes from 0.118 to 0.372. Based on low-cost observations of footwear outsole properties, Iraqi et al. [21] sought to create a analytical framework that forecasts the available coefficient of friction (ACOF) under circumstances for boundary lubrication. Fifty-eight shoe models that were designated as slip-resistant were used to measure geometric and material

6.4 Slipping Experiments with Whole Footwear

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hardness criteria. ACOF was measured using a robotic friction measurement system using canola oil as the contaminant. Models that were focussed on the footwear properties and flooring type were created using stepwise regression techniques to forecast the ACOF. Measurements of the shoe outsole tread, readings of the ACOF, and the construction of statistical models made up the study’s three primary sections. The outsole patterns of 58 different shoe models were measured geometrically and mechanically. For these footwear, two types of flooring were tested utilizing a robotic full shoe tester to assess ACOF. Slip-resistant shoes were labelled as gents and ladies unisex footwear and came in casual, work, athletic and dress styles. To create variability in footwear design and material indentation resistance between and within footwear, different shoe designs were chosen. While controlling for flooring type, it was discovered that tread design features (heel shape and tread surface area) and material hardness explained 87% of the variation in greasy ACOF. Increased tread surface area and a switch from a flat to a bevelled edge in the heel design improved traction. Additionally, it was discovered that when tread surface area grows, contact pressure decreases, which in turn raises hysteresis friction. It was discovered that the softer elastomer (low hardness level) experienced more deformation than the tougher material, which raises ACOF. To avoid repeating slip testing efforts, Chanda et al. [22] tried to quantify the relationships between shoe traction under various contaminated flooring situations. For 17 shoes, the available coefficient of friction (ACOF) was measured for five distinct flooring types and three different contaminant circumstances. When the friction coefficient values for a first-floor contamination condition were greatly linked with a second-floor contaminant condition, redundant testing conditions were found. Shoes from 10 different brands that were both slip-resistant (SR) and non-slip resistant (NSR) were selected. 17 shoes in all (6 oxford work shoes, 3 clogs, 7 easy wearable footwear, and 1 sports footwear) were evaluated. Five different floors with two polymer designs (both vinyl composite tiles) and three quarry designs, each had their ACOF values assessed (one designated as ceramic and the other labelled as quarry). Three contaminants that were examined were: water, Sodium Laurel Sulphate (SLS) mixed with 99.5% water and 0.5% SLS, and canola oil. Using a rheometer, the viscosities of these contaminants were evaluated. The study used a portable biofidelic slip simulator to measure ACOF levels. A force plate was used to comprehend the perpendicular and shearing forces. The floor was covered with enough contaminant to replicate flooded conditions. A perpendicular force of 250 ± 25 N and a sliding velocity of 0.5 m/s were applied while a shoe floor angle of 17 ± 1° was fixed. For 200 ms, the perpendicular force was maintained. During this time, there was a mean perpendicular force of 250 ± 10 N. Five repetitions were run for each slip testing scenario, and the average friction coefficient values were calculated across the repetitions. Strong correlations between reference vinyl tiles, ceramic tiles and quarry tiles, for all three contaminants, were found throughout testing across various flooring types. The link among vinyl tiles and ceramic or quarry tiles was less strong.

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Comparing the canola oil contaminate to the water and SLS contaminations, stronger connections were found across all flooring types. Comparatively larger frictional coefficient relationships were discovered between SLS and canola oil contaminants for vinyl flooring compared to water and SLS and water and canola oil contaminations. When both floorings and contaminants were modified, higher correlations between ceramic, quarry 1 and quarry 2 floorings were seen. In order to direct available coefficient of friction (ACOF) evaluation procedures for shoes and tiles used for floorings, Iraqi et al. [1] attempted to measure the human slip heel biomechanics and energetics. For this secondary study, kinematic and kinetic data from 39 patients who had a slip occurrence were combined from four related human slip risk evaluation investigations. The following variables were measured: slipping velocity, interaction period, shoe-floor angle, lateral slip inclination, vertical ground reaction force (VGRF) and centre of pressure (COP) (PSS). To determine whether there are any disparities between the condition of the sliding limb and the existing frictional coefficient evaluation settings, statistical comparisons were employed. Four distinct human slipping tests carried out in the same laboratory yielded biomechanical and energetic readings for 39 people (18 females; average age: 22.3 ± 3.3 years; average height: 173.1 ± 8.3 cm; average body weight: 68.3 ± 10 kg; average BMI: 22.8 ± 3.2). Young people (18–35 years old), slips followed by minimum three gait trials during which their left foot was clearly seen to contact the dry force plate before being exposed to the liquid contaminant, and a slipping range more than 3 cm were the inclusion criteria into the experimental analysis. The subjects employed a safety harness device and implemented a full body marker set. In a laboratory setting outfitted with a motion camera system, subjects were given the task of walking over a level vinyl composite tile sidewalk. On the dry walkway, the subjects carried out three to five gait tests with their left appendage firmly planted on the force plate. Then a liquid contaminant was unexpectedly poured onto the force plate, exposing the participants to it. It was discovered that the VGRF, footwear flooring inclination and period of contact at slip start (SS) testing parameters had considerably different central tendencies from each other. Additionally, the average shoe floor angle at slip start (SS) was greater than the 7° inclination required by the ACOF testing procedures. Amongst the present research work conducted and earlier biomechanics of slip risk evaluation studies, distinctions were identified in the biomechanical variables at SS. Since biomechanical factors have an impact on ACOF, it was also discovered that the dynamics of the heel had a considerable effect on slipping resistance assessments. The effects of continuous footwear abrasion on the available coefficient of friction (ACOF) and under footwear fluid dynamics were studied by Hemler et al. [23]. With the use of a hastened, abrasive wear regimen, five distinct pairs of slip-resistant shoes were gradually worn. As footwear were slid across such a flooring tile covered with a weak glycerol solution contaminant, the ACOF and fluid forces were measured. A preliminary rise in the traction performance was accompanied by a sustained decline as the shoes wore down. Prior to wear, modest fluid forces were seen, then as the worn region grew larger, fluid forces rose.

6.4 Slipping Experiments with Whole Footwear

61

To simulate wear of the right shoe heel, the equipment for enhanced wear, which consists of a constant speed abrasion instrument and an angle-adjustable platform, was employed. The gadget removed shoe tread by abrasively moving abrasive cloth (180 µm diameter particles) across the heel at 9.65 m/s and a normal force of 40 N, which is comparable to abrasion resistance criteria for footwear. Each shoe was abraded for 20 s each at three different inclinations: 17 ± 1, 7 ± 1 and 2 ± 1° during one wear cycle. The angles were selected to replicate the angles that one would perceive from a heel strike to a flat foot. Each wear cycle amounted to 580 m of total sliding distance. In the study, five pairs of shoes that are frequently used in the service sector were used, and the appropriate footwear for each pair was evaluated. The shore A indentation assessment device was used to measure the short-term indentation resistance. A robotic slip tester was used to measure ACOF and fluid pressure. The shear and normal forces were also measured using a force plate. On a vinyl composite tile that was contaminated with a weak glycerol mixture (90% glycerol, 10% water by volume), shoes were moved across the surface. The associations between each of the dependent factors, such as traction performance, fluid pressure and contact area without treads, and the independent factors, such as footwear type (classification), slipping distance while footwear were abraded and their interaction, were examined using three generalized linear regression models. The findings demonstrated that shoe traction varies with tread wear. The first-order degradation procedure (3 km worn distance) for four of the footwear is accompanied by an increment in ACOF and essentially constant fluid forces. ACOF decreased for all pairs of shoes with heavy wear. Mechanical slip testing has enabled us to widely understand the field of footwear slip testing. As the device replicates the biofidelic parameters of human slips, it is a very reliable way of testing different footwear. The main focus of the mechanical slip testing experiments was to understand the effect of the different footwear design factors such as tread contact area, heel dimension and shore A hardness of the shoes on the frictional effectiveness in various tiles and applied contaminant scenarios. Different types of slip impervious and non-slip impervious footwear were also evaluated to ascertain their probability of slipping. There is a lack on the study of basically athletic shoes, which involves slip testing in outdoor sports environments. The influence of different systematic tread patterns on the frictional performance is not that much explored, and the effect of tread patterns such as rectangular, square, pyramidal, pentagonal and hexagonal can be explored on a variety of floorings in the presence of different contaminants. Table 6.2 summarizes the literature review associated with footwear slip testing experiments.

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Table 6.2 Summary of literature review related to whole footwear slipping Author

Experimental method

Jones et al. [20]

The main focus was to measure The findings of this investigation suggest that how shoe design elements not all shoes with the slip-resistant designation affected the ACOF in footwear are equally efficient classified and not classified as slip resistant

Outcomes

Iraqi et al. The aim of this study was to [21] create an analytical framework that forecasts the available coefficient of friction (ACOF) under circumstances for boundary lubrication

Increased tread surface area and a switch from a flat to a bevelled edge in the heel design improved traction. Additionally, it was discovered that when tread surface area grows, contact pressure decreases, which in turn raises hysteresis friction

Chanda et al. [22]

Strong correlations between reference vinyl tiles, ceramic tiles and quarry tiles, for all three contaminants, were found throughout testing across various flooring types

In order to avoid repeating slip testing efforts, quantification of the relationships between shoe traction under various contaminated flooring situations was performed

Iraqi et al. In order to develop ACOF [1] evaluation procedures for shoes and tiles used for floorings, an attempt was made to measure the human slip heel biomechanics and energetics

The average shoe floor angle at slip start was greater than the 7° inclination required by the ACOF testing procedures

Hemler et al. [23]

The effects of continuous footwear abrasion on the ACOF and under footwear fluid dynamics were studied

The first-order degradation procedure (3 km worn distance) for four of the footwear was accompanied by an increment in ACOF and essentially constant fluid forces. ACOF decreased for all pairs of shoes with heavy wear

Gupta et al. [16]

Portable and biofidelic slip testing The tester was found to be repeatable, portable, device was used to compare a compliant with the standards, environmental wide range of shoes across fidelity, biofidelic and precise slippery floorings

References 1. Iraqi A, Cham R, Redfern MS, Vidic NS, Beschorner KE (2018) Kinematics and kinetics of the shoe during human slips. J Biomech 74:57–63. https://doi.org/10.1016/j.jbiomech.2018. 04.018 2. Chang WR et al (2001) The role of friction in the measurement of slipperiness, Part 2: survey of friction measurement devices. Ergonomics 44(13):1233–1261. https://doi.org/10.1080/001 40130110085583 3. Gupta S, Chanda A (2023) Biomechanical modeling of footwear-fluid-floor interaction during slips. J Biomech 156:111690. https://doi.org/10.1016/J.JBIOMECH.2023.111690 4. Chang WR, Leclercq S, Lockhart TE, Haslam R (2016) State of science: occupational slips, trips and falls on the same level*. Ergonomics 59(7):861–883. https://doi.org/10.1080/001 40139.2016.1157214

References

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5. Grönqvist R, Chang WR, Courtney TK, Leamon TB, Redfern MS, Strandberg L (2001) Measurement of slipperiness: fundamental concepts and definitions. Ergonomics 44(13):1102– 1117. https://doi.org/10.1080/00140130110085529 6. Grönqvist R, Roine J, Korhonen E, Rahikainen A (1990) Slip resistance versus surface roughness of deck and other underfoot surfaces in ships. J Occup Accid 13(4):291–302. https://doi. org/10.1016/0376-6349(90)90035-T 7. Gupta S, Sidhu SS, Chatterjee S, Malviya A, Singh G, Chanda A (2022) Effect of floor coatings on slip-resistance of safety shoes. Coatings, vol 12, no 10, p 1455, Oct 2022. https://doi.org/ 10.3390/COATINGS12101455 8. Gupta S, Chatterjee S, Malviya A, Chanda A (2022) Traction performance of common formal footwear on slippery surfaces. Surfaces 2022, vol 5, no 4, pp 489–503, Nov 2022. https://doi. org/10.3390/SURFACES5040035 9. Gupta S, Chatterjee S, Malviya A, Chanda A (2023) Frictional assessment of low-cost shoes in worn conditions across workplaces. J Bio-Tribo-Corrosion 9(1):1–13. https://doi.org/10.1007/ S40735-023-00741-0 10. Gupta S, Chatterjee S, Malviya A, Kundu A, Chanda A (2023) Effect of shoe outsole wear on friction during dry and wet slips: a multiscale experimental and computational study. Multiscale Sci Eng 2023:1–15. https://doi.org/10.1007/S42493-023-00089-0 11. Gupta S, Chatterjee S, Chanda A (2023) Influence of vertically treaded outsoles on interfacial fluid pressure, mass flow rate, and shoe–floor traction during slips. Fluids 2023, vol 8, pp 82, no 3, Feb 2023. https://doi.org/10.3390/FLUIDS8030082 12. Gupta S, Chatterjee S, Malviya A, Singh G, Chanda A (2023) A novel computational model for traction performance characterization of footwear outsoles with horizontal tread channels. Computer 2023, vol 11, no 2, p 23, Feb 2023. https://doi.org/10.3390/COMPUTATION1102 0023 13. Standard P18–578 14. Standard NF P90–106 15. Grönqvist R, Hirvonen M, Tohv A (2000) Evaluation of three portable floor slipperiness testers. Int J Ind Ergon 25(1):85–95. https://doi.org/10.1016/s0169-8141(98)00101-2 16. Gupta S, Malviya A, Chatterjee S, Chanda A (2022) Development of a portable device for surface traction characterization at the shoe-floor interface. Surfaces 2022, vol 5, no 4, pp 504–520, Dec 2022. https://doi.org/10.3390/SURFACES5040036 17. Chang WR, Matz S (2001) The slip resistance of common footwear materials measured with two slipmeters. Appl Ergon 32(6):549–558. https://doi.org/10.1016/S0003-6870(01)00031-X 18. Kim J (2012) Comparison of three different slip meters under various contaminated conditions. Saf Health Work 3(1):22–30. https://doi.org/10.5491/SHAW.2012.3.1.22 19. Aschan C, Hirvonen M, Rajamäki E, Mannelin T, Ruotsalainen J, Ruuhela R (2009) Performance of slippery and slip-resistant footwear in different wintry weather conditions measured in situ. Saf Sci 47(8):1195–1200. https://doi.org/10.1016/J.SSCI.2009.01.006 20. Jones T, Iraqi A, Beschorner K (2018) Performance testing of work shoes labeled as slip resistant. Appl Ergon 68:304–312. https://doi.org/10.1016/J.APERGO.2017.12.008 21. Iraqi A, Vidic NS, Redfern MS, Beschorner KE (2020) Prediction of coefficient of friction based on footwear outsole features. Appl Ergon 82:102963. https://doi.org/10.1016/j.apergo. 2019.102963 22. Chanda A, Jones TG, Beschorner KE (2018) Generalizability of footwear traction performance across flooring and contaminant conditions. IISE Trans Occup Ergon Hum Fact 6(2):98–108. https://doi.org/10.1080/24725838.2018.1517702 23. Hemler SL et al (2019) Changes in under-shoe traction and fluid drainage for progressively worn shoe tread. Appl Ergon 80:35–42. https://doi.org/10.1016/J.APERGO.2019.04.014

Chapter 7

Effect of Horizontal Outsole Tread Orientation on Slip Performance

7.1 Introduction The outsole tread pattern, tread contact region, material and shore hardness of the footwear outsole have all been determined to have an impact on ACOF; nonetheless, the outsole’s design such as outsole tread angles, breadth, gaps and depth predominate over the others [1–5]. During fluid-contaminated sliding, in accordance to a recent study by Yamaguchi et al. [6] demonstrated the impact of tread properties on the traction performance by dispersing the excess fluid through the tread channels. Another study by Li et al. [7] looked at modifications in the tread widths of outsoles with a commonly encountered tread pattern (i.e., treads that are horizontally oriented with respect to the slipping motion axis) in both dry and fluid-contaminated situations, and they discovered major variations in the friction coefficient results. Determining an optimum outsole design hence requires knowing the scientific features of the outsoles by modifying them parametrically during slipping [8–13]. Variables including increased abrasion and the presence of contaminants like water or other viscous contaminants significantly lower ACOF [14–16]. Furthermore, it has been claimed that a shoe’s ability to drain excess fluid during slipping is crucial to deciding how well it will maintain its grip in fluid-contaminated settings [17, 18]. Recent research by Beschorner et al. [17] assessed the impact of treaded and without tread outsoles on fluid pressures. High association between elevated fluid pressure and elevated slip risks was shown by the study. A tapered-wedge bearing approach was used in a different investigation by Hemler et al. [18] to measure the fluid forces generated during slippage in both new and used footwear. According to the investigation, worse traction results were obtained as fluid force increased across the worn zone. In this study, the traction performance of a commonly utilized tread orientation—horizontally oriented tread patterns found on the topography of the outsole of footwear used by hospital staff such as doctors, trauma centre staff and general hospital staff—was examined. The tread channels were tested using a biofidelic slip

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 A. Chanda et al., Footwear Traction, Biomedical Materials for Multi-functional Applications, https://doi.org/10.1007/978-981-99-7823-6_7

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7 Effect of Horizontal Outsole Tread Orientation on Slip Performance

testing apparatus while being parametrically changed across widths and gaps. The results of this research are expected to improve tread channel design for improved traction performance on dry and slick surfaces, potentially lowering the risk of slipping accidents in hospital floorings.

7.2 Materials and Methods 7.2.1 Fabrication of Footwear Outsoles The outsole geometry of the shoes chosen for this work featured treads derived from the imprints of the original footwear. In relation to the sliding motion, the treads on the chosen design were positioned horizontally. A surface profilometer (Precision Instruments, India) and a durometer (Precision Instruments, India) were used to assess the shore hardness and tread geometry of footwear outsoles, respectively. The investigated variables were simply assessed 50 mm from the rearward portion of the heel, and it has been claimed that this is enough to assess how well the shoes provide traction [19–21]. The outsole was made of polyurethane, which had a 50 shore A hardness. The durability, extended lifespan and efficacy of polyurethane to lessen plantar pain are well established [22, 23]. It was 50 in shore A hardness scale. The measured data were recorded and loaded into SolidWorks by Dassault Systèmes in France, a CAD modelling programme. The actual outsole had a 2 mm tread width and a 2 mm space between each tread (Fig. 7.1). Additionally, the depth was kept constant while the widths and gaps were adjusted at intervals of 2 mm and 1 mm, respectively (i.e., 2 mm). The parametrical changes to the tread designs are shown in Table 7.1. Nine tread dimensionally created models were 3D printed into moulds to create the outsoles (Voxelab Aquila, Flashforge, China). A pourable liquid silicone that was hydrophobic was used to fill the moulds (LSR 130, Chemzest Products, India). After being given 7 h to cure, the silicone was removed from each mould. The silicone moulds were filled with a two-part polyurethane and allowed to dry for 48 h. The polyurethane substance used has a shore A hardness of 50 to resemble the characteristics of the authentic footwear material. Extra edges were cut off and their

Fig. 7.1 Dimensions of the original tread design [11]

7.2 Materials and Methods Table 7.1 Variations in the outsoles tread parameters [11]

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Tread designation

Width (mm)

Gap (mm)

HP1 (Original design)

2

2

HP2

4

2

HP3

6

2

HP4

2

3

HP5

4

3

HP6

6

3

HP7

2

4

HP8

4

4

HP9

6

4

measurements were remeasured after the created outsoles were removed. The nine designed polyurethane outsoles are shown in Fig. 7.2.

Fig. 7.2 The developed footwear outsoles [11]

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7.2.2 Slip Testing Experiments An entire-footwear transportable biofidelic mechanical slip evaluation apparatus (Fig. 7.3) that was created based on the worldwide standard ASTM F2913-19 was used to measure the traction performance. On the basis of the aforementioned ASTM standard, the device has been created and put into use in earlier investigations [24, 25]. A pair of shoes had the outsole affixed to them, and the shoes were further fastened over the shoe last. The inclination between the footwear and the flooring was fixed at 17 ± 2 degrees during the testing, as seen in the most recent slip testing studies [24]. Additionally, during the slip testing procedures, a normal force of 250 ± 25 N and a slipping speed of 0.5 m/s were used. The nine designed outsoles underwent slip testing on an anti-skid surface in both dry and wet circumstances. Utilizing a digital surface profilometer, the flooring’s surface roughness was determined to be 32.5 µm. (Precision Instruments, India). Based on Chanda et al. observations [24] that indicated generalizable ACOF results in several dry and fluid contaminated floorings, a single flooring was taken into consideration, helping to reduce the total time and effort. Within 200 ms of the start of the slide simulation, the ACOF values were estimated. Each outsole underwent five trials, with the results being averaged.

Fig. 7.3 Entire-shoe portable slip evaluation apparatus [11]

7.3 Results

69

7.3 Results 7.3.1 Frictional Assessment of Footwear Outsoles When mechanically tested in dry and contaminated water applied environments, the outsoles’ ACOF ranged from 0.13 to 0.35. (Fig. 7.4). The friction coefficient values in the dry slip test ranged from 0.28 to 0.35. Among all the outsoles, HP1, HP4 and HP7 displayed the comparable and greater friction coefficient values. The maximum ACOF difference for these outsoles was 0.01; HP7 had the greatest ACOF (0.35), followed by HP4 (0.34), and HP1 (i.e., 0.33). When compared to HP7, HP5 saw an ACOF reduction of 12%, while HP2, HP6 and HP8 all demonstrated a comparable ACOF reduction of more than 14%. Comparing HP9 and HP3 to other outsoles, the lowest ACOF values were 0.28 and 0.29, respectively. The ACOF values in the wet slip test varied from 0.13 to 0.18. When slip testing was performed with water as a contaminant across the flooring, HP7 and HP9 showed the highest ACOF value of all the outsoles, 0.18. Additionally, HP8 performed comparably to these outsoles, with an ACOF result difference of no more than 0.01. Outsoles HP2, HP3 and HP5 displayed a comparable ACOF with a 22% decrease in comparison to HP7. HP1 had the lowest ACOF, 0.13, followed by HP4 and HP6, both of which showed an ACOF of 0.15.

Fig. 7.4 Traction performance of the outsoles evaluated employing the mechanical slip testing apparatus

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Fig. 7.5 Influence of tread width on ACOF: A in dry condition and B in wet condition [11]

7.3.2 Influence of Tread Width on Traction Performance of Footwear Outsoles By calculating the relationships of various tread widths with the traction performance, the effects of different tread widths (i.e., width = 2 mm, 4 mm, 6 mm) on the outsole traction were assessed (Fig. 7.5). Tread width was shown to be highly and negatively correlated with the friction coefficient in dry conditions (R2 = 0.84). (Fig. 7.5A). Particularly, the narrower outsoles HP1, HP4 and HP7 displayed greater ACOF than the other outsoles. The 4 mm-wide tread on the outsoles HP2, HP5 and HP8 displayed moderate ACOF values. In comparison to other outsoles, the remaining outsoles with broader treads displayed reduced ACOF. In slip testing with water as the contaminant, the variable parameter of tread width was shown to have a poor correlation (R2 = 0.01) with the ACOF (Fig. 7.5B). Regardless of tread width, outsoles displayed both higher and lower friction coefficient.

7.3.3 Influence of Tread Gap on Traction Performance of Footwear Outsoles Estimating the relationship between the tread gaps and the friction coefficient allowed researchers to quantify the effects of different tread gaps (gap = 2 mm, 3 mm, and 4 mm) on traction (Fig. 7.6). Tread gap and ACOF were only marginally associated in dry conditions (R2 = 0.11). (Fig. 7.6A). When tread gaps and ACOF were examined, the ACOF fluctuated significantly, and no trend was seen. Tread gap was discovered to have a substantial and favourable correlation (R2 = 0.86) with the ACOF during wet slip testing (Fig. 7.6B). In this instance, outsoles with wider tread gaps (HP7, HP8 and HP9) displayed higher ACOF in contaminated water than other outsoles (HP1, HP2 and HP3).

References

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Fig. 7.6 Influence of tread gap on ACOF: A in dry condition and B in wet condition [11]

7.4 Conclusion In conclusion, it was discovered that parametrical changes in the horizontal tread patterns had an impact on how well the outsoles provided grip. On hospital floors with fluid contamination, the total slipping risk may increase with outsoles that have larger treads and smaller gaps. While outsoles with bigger gaps displayed higher traction when slipping on wet floors, smaller tread widths created higher traction when slipping on dry surfaces. The findings of this study should assist companies that make footwear intended for healthcare professionals, in understanding how tread changes will affect the possibility of slipping and help them improve designs.

References 1. Strobel CM, Menezes PL, Lovell MR, Beschorner KE (2012) Analysis of the contribution of adhesion and hysteresis to shoe-floor lubricated friction in the boundary lubrication regime. Tribol Lett 47(3):341–347. https://doi.org/10.1007/S11249-012-9989-5/TABLES/2 2. Gupta S, Chatterjee S, Chanda A (2022) Effect of footwear material wear on slips and falls. Mater Today Proc 62:3508–3515. https://doi.org/10.1016/J.MATPR.2022.04.313 3. Blanchette MG, Powers CM (2015) The influence of footwear tread groove parameters on available friction. Appl Ergon 50:237–241. https://doi.org/10.1016/J.APERGO.2015.03.018 4. Jones T, Iraqi A, Beschorner K (2018) Performance testing of work shoes labeled as slip resistant. Appl Ergon 68:304–312. https://doi.org/10.1016/J.APERGO.2017.12.008 5. Tsai YJ, Powers CM (2008) The influence of footwear sole hardness on slip initiation in young adults*. J Forensic Sci 53(4):884–888. https://doi.org/10.1111/J.1556-4029.2008.00739.X 6. Yamaguchi T, Katsurashima Y, Hokkirigawa K (2017) Effect of rubber block height and orientation on the coefficients of friction against smooth steel surface lubricated with glycerol solution. Tribol Int 110:96–102. https://doi.org/10.1016/J.TRIBOINT.2017.02.015 7. Li KW, Chen CJ (2004) The effect of shoe soling tread groove width on the coefficient of friction with different sole materials, floors, and contaminants. Appl Ergon 35(6):499–507. https://doi.org/10.1016/J.APERGO.2004.06.010

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8. Gupta S, Sidhu SS, Chatterjee S, Malviya A, Singh G, Chanda A (2022) Effect of floor coatings on slip-resistance of safety shoes. Coatings 12(10):1455. https://doi.org/10.3390/COATINGS1 2101455 9. Gupta S, Chatterjee S, Malviya A, Chanda A (2022) Traction performance of common formal footwear on slippery surfaces. Surfaces 5(4):489–503. https://doi.org/10.3390/SURFACES5 040035 10. Gupta S, Chatterjee S, Chanda A (2023) Influence of vertically treaded outsoles on interfacial fluid pressure, mass flow rate, and shoe–floor traction during slips. Fluids 8(3):82. https://doi. org/10.3390/FLUIDS8030082 11. Gupta S, Chatterjee S, Malviya A, Singh G, Chanda A (2023) A novel computational model for traction performance characterization of footwear outsoles with horizontal tread channels. Computation 11(2):23. https://doi.org/10.3390/COMPUTATION11020023 12. Gupta S, Chatterjee S, Malviya A, Kundu A, Chanda A (2023) Effect of shoe outsole wear on friction during dry and wet slips: a multiscale experimental and computational study. Multiscale Sci Eng 2023:1–15. https://doi.org/10.1007/S42493-023-00089-0 13. Gupta S, Chatterjee S, Malviya A, Chanda A (2023) Frictional assessment of low-cost shoes in worn conditions across workplaces. J Bio- Tribo-Corros 9(1):1–13. https://doi.org/10.1007/ S40735-023-00741-0 14. Gupta S, Chanda A (2023) Biomechanical modeling of footwear-fluid-floor interaction during slips. J Biomech 156:111690. https://doi.org/10.1016/J.JBIOMECH.2023.111690 15. Hemler SL, Charbonneau DN, Beschorner KE (2017) Effects of shoe wear on slippingimplications for shoe replacement threshold. https://doi.org/10.1177/1541931213601839 16. Meehan EE, Vidic N, Beschorner KE (2022) In contrast to slip-resistant shoes, fluid drainage capacity explains friction performance across shoes that are not slip-resistant. Appl Ergon 100:103663. https://doi.org/10.1016/J.APERGO.2021.103663 17. Beschorner KE, Albert DL, Chambers AJ, Redfern MS (2014) Fluid pressures at the shoe– floor–contaminant interface during slips: effects of tread & implications on slip severity. J Biomech 47(2):458–463. https://doi.org/10.1016/J.JBIOMECH.2013.10.046 18. Hemler SL, Charbonneau DN, Beschorner KE (2020) Predicting hydrodynamic conditions under worn shoes using the tapered-wedge solution of Reynolds equation. Tribol Int 145:106161. https://doi.org/10.1016/J.TRIBOINT.2020.106161 19. Iraqi A, Vidic NS, Redfern MS, Beschorner KE (2020) Prediction of coefficient of friction based on footwear outsole features. Appl Ergon 82:102963. https://doi.org/10.1016/J.APE RGO.2019.102963 20. Beschorner KE, Redfern MS, Porter WL, Debski RE (2007) Effects of slip testing parameters on measured coefficient of friction. Appl Ergon 38(6):773–780. https://doi.org/10.1016/J.APE RGO.2006.10.005 21. Beschorner KE, Meehan EE, Iraqi A, Hemler SL (2021) Designing shoe tread for friction performance: a hierarchical approach, 13(sup 1):S97–S99. https://doi.org/10.1080/19424280. 2021.1917701 22. Chhikara K, Gupta S, Chanda A (2022) Development of a novel foot orthosis for plantar pain reduction. Mater Today Proc 62:3532–3537. https://doi.org/10.1016/J.MATPR.2022.04.361 23. Karkali´c RM, Radulovi´c JR, Jovanovi´c DB (2017) Characteristics of polyurethane and elastomer parts for shoe industry produced by liquid injection molding technology. Vojnoteh Glas 65(4):948–967. https://doi.org/10.5937/VOJTEHG65-10543 24. Chanda A, Jones TG, Beschorner KE (2018) Generalizability of footwear traction performance across flooring and contaminant conditions, 6(2):98–108. https://doi.org/10.1080/24725838. 2018.1517702 25. Aschan C, Hirvonen M, Mannelin T, Rajamäki E (2005) Development and validation of a novel portable slip simulator. Appl Ergon 36(5):585–593. https://doi.org/10.1016/J.APERGO.2005. 01.015

Chapter 8

Effect of Vertical Outsole Tread Orientation on Slip Performance

8.1 Introduction The performance of the footwear outsole’s treads (or topographical characteristics) in both dry and wet sliding situations is crucial [1–7]. Outsole characteristics including tread patterns, tread region area and tread alignment have been found to have a significant impact on the ACOF in a recent research conducted by Gupta et al. [8]. The impact of tread features on the traction performance was examined in a different study by Yamaguchi et al. [9], which looked at the excessive fluid dispersion along tread pathways during contaminant applied slipping scenario. Subsequently, prior research have found a correlation between increased slipping risks and greater fluid pressure and the production of hydrodynamic fluid film at the footwear-floor junction [10–13]. According to several investigations, decreasing traction occurred when fluid pressures were raised over the untreated or worn area [14]. The impact of different tread properties on the ACOF was examined in a number of slipping scenarios in a different study by Li et al. [15], and considerable differences in the ACOF results were noted. The slip testers need additional attachments for measuring fluid pressure and an outsole’s capacity to disperse extra fluid, and the results may be compromised by flow losses and sensor sensitivity. Additionally, conducting slip testing tests with a variety of footwear, floorings and contaminants can take a lot of time and effort. There is a dearth of study in the literature on the flow of fluids and pressures over the outsoles of shoes, as well as on the effect of outsole tread orientation on the generation of traction force. This study looked closely at a tread pattern that is frequently employed and has a vertical arrangement (i.e., parallel to the axis of the slipping movement) implemented in the footwear of hospital staff such as doctors, ward staff and hospital front desk personnel. The traction ability of the treads on dry and water-contaminated floorings was evaluated after parametric modification.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 A. Chanda et al., Footwear Traction, Biomedical Materials for Multi-functional Applications, https://doi.org/10.1007/978-981-99-7823-6_8

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8.2 Materials and Methods 8.2.1 Footwear Outsole Designs On the basis of the geometry of an original footwear, the shoe outsole design used in this work included treads that were vertically oriented, or parallel to the slipping motion. A durometer (Precision Instruments, India) and digital depth gauge were used to measure the shore hardness and tread dimensions of the outsoles (Precision Instruments, India). These factors were assessed over the heel area of the shoes, up to 50 mm from the back of the heel. Shore hardness, outsole tread shape and the 50 mm metric have all been found to be suitable for measuring how well shoes operate under any slippage conditions in the past [16–18]. Polyurethane with a shore A hardness of 60 formed the outsole. The actual shoe outsole design, depicted in (Fig. 8.1), had a tread width of 4 mm and a tread gap of 2 mm. To make the parametric changes across the tread widths and gaps, the shape for the outsole was acquired and recreated in a 3D modelling programme (SolidWorks, Dassault Systèmes, France). While tread gaps were changed with a 1 mm interval, tread width was changed with a 2 mm interval. The sizes of each of the nine parametrically adjusted outsoles are shown in Table 8.1. Nine tread dimensionally created models were 3D printed into moulds to create the outsoles (Voxelab Aquila, Flashforge, China). A pourable liquid silicone that was hydrophobic was used to fill the moulds (LSR 130, Chemzest Products, India). After being given 7 h to cure, the silicone was removed from each mould. The silicone moulds were filled with a two-part polyurethane and allowed to dry for 48 h. The polyurethane substance used has a shore A hardness of 50 to resemble the characteristics of the authentic footwear material. Extra edges were cut off and their measurements were remeasured after the created outsoles were removed. The nine designed polyurethane outsoles are shown in Fig. 8.2.

Fig. 8.1 Dimensions of the original footwear tread structure [7]

8.2 Materials and Methods Table 8.1 Dimensional changes in the outsole’s structure [7]

75

Outsole designation

Width (mm)

Gap (mm)

VP1

2

2

VP2

2

3

VP3

2

4

VP4 (original design)

4

2

VP5

4

3

VP6

4

4

VP7

6

2

VP8

6

3

VP9

6

4

Fig. 8.2 Manufactured outsoles with dimensionally modified tread widths and gaps [7]

8.2.2 Slip Testing Experiments An entire-footwear transportable biofidelic mechanical slip evaluation apparatus (Fig. 8.3) that was created based on the worldwide standard ASTM F2913-19 was

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used to measure the traction performance. On the basis of the aforementioned ASTM standard, the device has been created and put into use in earlier investigations [17, 19, 20]. A pair of shoes had the outsole affixed to them, and the shoes were further fastened over the shoe last. The inclination between the footwear and the flooring was fixed at 17 ± 2 degrees during the testing, as seen in the most recent slip testing studies [16, 17]. Additionally, during the slip testing procedures, a normal force of 250 ± 25 N and a slipping speed of 0.5 m/s [19] were used. The nine designed outsoles underwent slip testing on an anti-skid surface in both dry and wet circumstances. Utilizing a digital surface profilometer, the flooring’s surface roughness was determined to be 32.5 µm. (Precision Instruments, India). Based on Chanda et al observations [17] that indicated generalizable ACOF results in several dry and fluid contaminated floorings, a single flooring was taken into consideration, helping to reduce the total time and effort. Within 200 ms of the start of the slide simulation, the ACOF values were estimated. Each outsole underwent five trials, with the results being averaged.

Fig. 8.3 Entire-shoe portable slip evaluation apparatus [7]

8.3 Results

77

8.3 Results 8.3.1 Traction Performance of Footwear Outsoles When the outsoles were evaluated for slip resistance on dry and water applied floorings, the friction coefficient readings ranged from 0.13 to 0.35. (Fig. 8.4). The friction levels specifically in the dry sip tests varied from 0.28 to 0.35. When contrasted to the remaining footwear outsoles, Outsole VP7 had the greatest friction coefficient, 0.35. With the exception of VP7, outsoles VP8 and VP9 showed comparable friction coefficient results (i.e., 0.33). VP4 displayed the greatest ACOF of 0.34 after VP7. In the ACOF in dry conditions, VP1 decreased by around 20% compared to VP7, VP2 and VP3 by 25%, VP5 by 9%, VP6 by 13%, and VP8 and VP9 by 6% each. Of all the outsoles, VP2 and VP3 had the lowest ACOF (i.e., 0.28). The ACOF values in water contaminated condition ranged from 0.13 to 0.19. In wet slip risk assessment study, VP9, VP2 and VP3 exhibited identical results and the highest ACOF (i.e., 0.19) of all the outsoles. VP6’s outsole also displayed a comparable ACOF with a 0.01-point difference (i.e., 0.18). In the wet condition of the ACOF, VP1 experienced a drop of about 23% compared to VP9, VP2, and VP3, 2% compared to VP2, and 30% compared to VP3. The lowest ACOF (0.13) across all the outsoles was produced by outsoles VP4 and VP7, which demonstrated their poor performance.

Fig. 8.4 Traction performance of the outsoles evaluated employing the mechanical slip apparatus across dry and wet slipping scenarios

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Fig. 8.5 Influence of tread gap on traction performance: A in dry condition and B in wet condition [7]

8.3.2 Influence of Tread Width and Gap on Traction Performance of the Outsoles By measuring the correlation between gaps and ACOF, the impact of varying gaps (i.e., gap = 2 mm, 3 mm, 4 mm) between the treads of the outsoles on traction performance was examined (Fig. 8.5). When tested in dry slip conditions, distances between the treads had a modest association (R2 = 0.11) and the ACOF fluctuated considerably without any discernible trend (Fig. 8.5A). In contrast, Fig. 8.5 B shows a positive correlation between tread gaps and the ACOF under wet slip testing scenario (R2 = 0.72). The upward trend implied that the growing tread gaps were to blame for the higher ACOF under wet sliding conditions. The biggest tread gap and maximum friction coefficient readings were found in the outsoles VP3, VP6 and VP9 under wet sliding conditions. On the other hand, when tested on water-contaminated flooring, outsoles with gaps of 2 mm (i.e., VP1, VP4, VP7) displayed low friction coefficient. Figure 8.6 depicts the relationships between varying tread widths (width = 2 mm, 4 mm, and 6 mm) and shoe-floor traction in both dry and wet sliding circumstances. In dry conditions, wider tread produced better ACOF results. It was discovered that the width and ACOF were positively and substantially associated (R2 = 0.79). (Fig. 8.6A). Particularly, outsoles VP1, VP2 and VP3 have lower ACOF values than other outsoles due to their narrower tread widths. The 4 mm-wide tread on the outsoles VP4, VP5 and VP6 showed medium friction coefficient values. The remaining outsoles with the broadest treads (VP7, VP8, and VP9) displayed the greatest ACOFs when compared to other outsoles. Without consideration of the tread width, outsoles showed both low and high ACOFs during wet slip testing. The ACOF and tread width as a changing parameter did not significantly correlate (R2 = 0.12) (Fig. 8.6B).

References

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Fig. 8.6 Influence of tread width on traction performance in: A dry condition and B wet condition [7]

8.4 Conclusion Last but not least, the outsole design with the widest treads and biggest gaps showed improved grip in both dry and wet slip situations. Overall, it was demonstrated that proportional adjustments in vertical tread patterns had a considerable impact on how well shoes intended for hospital staff grip wet and dry surfaces when slipping. On dry floors, outsoles with wide treads and close spacing can reduce the overall danger of slipping, and vice versa. Outsoles with large tread gaps may further aid in improving overall grip performance on wet surfaces. In comparison to contact tread region area, tread gaps were determined to be the dominant factor in offering sufficient footwearfloor grip in slippery water applied circumstances. To the best of our knowledge, no other research on the connection between traction and vertical tread characteristics has been published. The techniques used in this study, together with the results, are expected to increase knowledge of the fundamentals underlying footwear friction and assist shoe companies in improving outsole designs for hospital staff to lower overall slip and fall hazard.

References 1. Li KW, Chen CJ (2004) The effect of shoe soling tread groove width on the coefficient of friction with different sole materials, floors, and contaminants. Appl Ergon 35(6):499–507. https://doi.org/10.1016/J.APERGO.2004.06.010 2. Gupta S, Sidhu SS, Chatterjee S, Malviya A, Singh G, Chanda A (2022) Effect of floor coatings on slip-resistance of safety shoes. Coatings 12(10):1455. https://doi.org/10.3390/COATINGS1 2101455

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3. Gupta S, Chatterjee S, Malviya A, Chanda A (2022) Traction performance of common formal footwear on slippery surfaces. Surfaces 5(4):489–503. https://doi.org/10.3390/SURFACES5 040035 4. Gupta S, Chatterjee S, Malviya A, Kundu A, Chanda A (2023) Effect of shoe outsole wear on friction during dry and wet slips: a multiscale experimental and computational study. Multiscale Sci Eng 2023:1–15. https://doi.org/10.1007/S42493-023-00089-0 5. Gupta S, Malviya A, Chatterjee S, Chanda A (2022) Development of a portable device for surface traction characterization at the shoe–floor interface. Surfaces 5(4):504–520. https:// doi.org/10.3390/SURFACES5040036 6. Gupta S, Chatterjee S, Malviya A, Singh G, Chanda A (2023) A novel computational model for traction performance characterization of footwear outsoles with horizontal tread channels. Computation 11(2):23. https://doi.org/10.3390/COMPUTATION11020023 7. Gupta S, Chatterjee S, Chanda A (2023) Influence of vertically treaded outsoles on interfacial fluid pressure, mass flow rate, and shoe–floor traction during slips. Fluids 8(3):82. https://doi. org/10.3390/FLUIDS8030082 8. Gupta S, Chatterjee S, Chanda A (2022) Effect of footwear material wear on slips and falls. Mater Today Proc 62:3508–3515. https://doi.org/10.1016/J.MATPR.2022.04.313 9. Yamaguchi T, Katsurashima Y, Hokkirigawa K (2017) Effect of rubber block height and orientation on the coefficients of friction against smooth steel surface lubricated with glycerol solution. Tribol Int 110:96–102. https://doi.org/10.1016/J.TRIBOINT.2017.02.015 10. Meehan EE, Vidic N, Beschorner KE (2022) In contrast to slip-resistant shoes, fluid drainage capacity explains friction performance across shoes that are not slip-resistant. Appl Ergon 100:103663. https://doi.org/10.1016/J.APERGO.2021.103663 11. Hemler SL et al (2019) Changes in under-shoe traction and fluid drainage for progressively worn shoe tread. Appl Ergon 80:35–42. https://doi.org/10.1016/J.APERGO.2019.04.014 12. Hemler SL, Charbonneau DN, Beschorner KE (2020) Predicting hydrodynamic conditions under worn shoes using the tapered-wedge solution of Reynolds equation. Tribol Int 145:106161. https://doi.org/10.1016/J.TRIBOINT.2020.106161 13. Beschorner KE, Albert DL, Chambers AJ, Redfern MS (2014) Fluid pressures at the shoe– floor–contaminant interface during slips: effects of tread & implications on slip severity. J Biomech 47(2):458–463. https://doi.org/10.1016/J.JBIOMECH.2013.10.046 14. Gupta S, Chatterjee S, Malviya A, Chanda A (2023) Frictional assessment of low-cost shoes in worn conditions across workplaces. J. Bio- Tribo-Corrosion 9(1):1–13. https://doi.org/10. 1007/S40735-023-00741-0 15. Li KW, Wu HH, Lin YC (2006) The effect of shoe sole tread groove depth on the friction coefficient with different tread groove widths, floors and contaminants. Appl Ergon 37(6):743– 748. https://doi.org/10.1016/J.APERGO.2005.11.007 16. Iraqi A, Vidic NS, Redfern MS, Beschorner KE (2020) Prediction of coefficient of friction based on footwear outsole features. Appl Ergon 82:102963. https://doi.org/10.1016/J.APE RGO.2019.102963 17. Chanda A, Jones TG, Beschorner KE (2018) Generalizability of footwear traction performance across flooring and contaminant conditions, 6(2):98–108. https://doi.org/10.1080/24725838. 2018.1517702 18. Beschorner KE, Redfern MS, Porter WL, Debski RE (2007) Effects of slip testing parameters on measured coefficient of friction. Appl Ergon 38(6):773–780. https://doi.org/10.1016/J.APE RGO.2006.10.005 19. Jones T, Iraqi A, Beschorner K (2018) Performance testing of work shoes labeled as slip resistant. Appl Ergon 68:304–312. https://doi.org/10.1016/J.APERGO.2017.12.008 20. Aschan C, Hirvonen M, Mannelin T, Rajamäki E (2005) Development and validation of a novel portable slip simulator. Appl Ergon 36(5):585–593. https://doi.org/10.1016/J.APERGO.2005. 01.015

Chapter 9

Effect of Square Outsole Tread Orientation on Slip Performance

9.1 Introduction The outsole tread pattern, tread contact region, material and shore hardness of the footwear outsole have all been determined to have an impact on ACOF; nonetheless, the outsole’s design has such as outsole tread angles, breadth, gaps and depth predominate over the others [1–5]. During fluid-contaminated slipping,Yamaguchi et al. [6] demonstrated the impact of tread properties on the traction performance by studying the dispersing ability of the tread channels. Another study by Li et al. [7] looked at modifications in the tread widths of outsoles with a commonly encountered tread pattern (i.e., treads that are horizontally oriented with respect to the slipping motion axis) in both dry and fluid-contaminated situations, and they discovered major variations in the friction coefficient results. Determining an optimum outsole design hence requires knowing the scientific features of the outsoles by modifying them parametrically during slipping. Variables including increased abrasion and the presence of contaminants like water or other viscous contaminants significantly lower ACOF [8–15]. Furthermore, it has been claimed that a shoe’s ability to drain excess fluid during slipping is crucial to deciding how well it will maintain its grip in fluidcontaminated settings [16, 17]. Recent research by Beschorner et al. [16] assessed the impact of treaded and without tread outsoles on fluid pressures. High association between elevated fluid pressure and elevated slip risks was shown by the study. A tapered-wedge bearing approach was used in a different investigation by Hemler et al. [17] to measure the fluid forces generated during slippage in both new and used footwear. According to the investigation, worse traction results were obtained as fluid force increased across the worn zone. In this study, the traction performance of a commonly utilized tread orientation— square-oriented tread patterns found on the topography of the outsole of footwear used by hospital staff such as doctors, trauma centre staff and general hospital staff— was examined. The tread channels were tested using a biofidelic slip testing apparatus while being parametrically changed across dimensions and gaps. The results of this

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 A. Chanda et al., Footwear Traction, Biomedical Materials for Multi-functional Applications, https://doi.org/10.1007/978-981-99-7823-6_9

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research are expected to improve tread channel design for improved traction performance on dry and slick surfaces, potentially lowering the risk of slipping accidents in hospital floorings.

9.2 Materials and Methods 9.2.1 Fabrication of Footwear Outsoles The outsole geometry of the shoes chosen for this work featured treads derived from the imprints of the original footwear. In relation to the sliding motion, the treads on the chosen design were positioned horizontally. A surface profilometer (Precision Instruments, India) and a durometer (Precision Instruments, India) were used to assess the shore hardness and tread geometry of footwear outsoles, respectively. The investigated variables were simply assessed 50 mm from the rearward portion of the heel, and it has been claimed that this is enough to assess how well the shoes provide traction [18–20]. The outsole was made of polyurethane, which had a 50 shore A hardness. The durability, extended lifespan and efficacy of polyurethane to lessen plantar pain are well established [21, 22]. It was 50 in shore A hardness scale. The measured data were recorded and loaded into SolidWorks by Dassault Systèmes in France, a CAD modelling programme. The actual outsole had a 5 mm tread dimension and a 3 mm tread gap between each tread (Fig. 9.1). Additionally, the depth was kept constant while the tread dimension and tread gaps were adjusted at intervals of 1 mm, respectively. The parametrical changes to the tread designs are shown in Table 9.1.

Fig. 9.1 Original tread design

9.2 Materials and Methods Table 9.1 Variations in the outsole’s tread parameters

83

Tread designation

Tread dimension (mm)

Tread gap (mm)

SP1

5

3

SP2

5

4

SP3

5

5

SP4

6

3

SP5

6

4

SP6

6

5

SP7

7

3

SP8

7

4

SP9

7

5

Nine tread dimensionally created models were 3D printed into moulds to create the outsoles (Voxelab Aquila, Flashforge, China). A pourable liquid silicone that was hydrophobic was used to fill the moulds (LSR 130, Chemzest Products, India). After being given 7 h to cure, the silicone was removed from each mould. The silicone moulds were filled with a two-part polyurethane and allowed to dry for 48 h. The polyurethane substance used has a shore A hardness of 50 to resemble the characteristics of the authentic footwear material. Extra edges were cut off and their measurements were remeasured after the created outsoles were removed. The nine designed polyurethane outsoles are shown in Fig. 9.2.

9.2.2 Slip Testing Experiments An entire-footwear transportable biofidelic mechanical slip evaluation apparatus (Fig. 9.3) that was created based on the worldwide standard ASTM F2913-19 was used to measure the traction performance. On the basis of the aforementioned ASTM standard, the device has been created and put into use in earlier investigations [23, 24]. A pair of shoes had the outsole affixed to them, and the shoes were further fastened over the shoe last. The inclination between the footwear and the flooring was fixed at 17 ± 2 degrees during the testing, as seen in the most recent slip testing studies [23]. Additionally, during the slip testing procedures, a normal force of 250 ± 25 N and a slipping speed of 0.5 m/s were used. The nine designed outsoles underwent slip testing on an anti-skid surface in both dry and wet circumstances. Utilizing a digital surface profilometer, the flooring’s surface roughness was determined to be 32.5 µm. (Precision Instruments, India). Based on Chanda et al. observations [23] that indicated generalizable ACOF results in several dry and fluid-contaminated floorings, a single flooring was taken into consideration, helping to reduce the total time and effort. Within 200 ms of the start of the slide simulation, the ACOF values were estimated. Each outsole underwent five trials, with the results being averaged.

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9 Effect of Square Outsole Tread Orientation on Slip Performance

Fig. 9.2 The developed footwear outsoles

9.3 Results 9.3.1 Frictional Assessment of Footwear Outsoles When mechanically tested in dry, water and floor cleaner (Lizol) applied environments, the outsoles’ ACOF ranged from 0.02 to 0.40. The friction coefficient values in the dry slip test (Fig. 9.4A) ranged from 0.29 to 0.40. Among all the square patterned outsoles, SP1, SP2, SP3, SP5 and SP6 displayed the comparable and greater friction coefficient values. The maximum ACOF difference for these outsoles was 0.01; SP3 had the greatest ACOF (0.40), followed by SP2 (0.39), SP1 (i.e., 0.38). Comparing SP3 and SP2 to other outsoles, the lowest ACOF values were 0.29 and 0.30, respectively. The ACOF values in the wet slip test (Fig. 9.4B) varied from 0.10 to 0.15. When slip testing was performed with water as a contaminant across the flooring,

9.3 Results

85

Fig. 9.3 Entire-shoe portable slip evaluation apparatus [13]

SP3, SP6 and SP9 showed the highest ACOF value of all the outsoles, 0.15 and 0.14. Additionally, SP2 performed comparably to these outsoles, with an ACOF result difference of no more than 0.01. SP7 had the lowest ACOF, 0.10, followed by SP4, SP8 and SP1, in which SP4 and SP8 both of which showed an ACOF of 0.11. When slip testing was performed with floor cleaner (Lizol) (Fig. 9.4C) as a contaminant across the flooring, SP7, SP1, SP4 and SP9 showed the highest ACOF value of all the outsoles, 0.06 and 0.05 respectively. Additionally, SP5 performed comparably to these outsoles, with an ACOF result difference of no more than 0.01. SP2, SP6 and SP8 had the lowest ACOF, 0.02, followed by SP3, which showed an ACOF of 0.03.

9.3.2 Influence of Tread Dimension on Traction Performance of Footwear Outsoles By calculating the relationships of various tread dimensions with the traction performance, the effects of different tread widths (i.e., tread dimension = 5 mm, 6 mm,

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9 Effect of Square Outsole Tread Orientation on Slip Performance

Fig. 9.4 Traction performance of the outsoles evaluated employing the mechanical slip testing apparatus: A Dry Condition, B Water Applied Condition and C Lizol Applied Condition

9.4 Conclusion

87

7 mm) on the outsole traction were assessed (Fig. 9.5). Tread dimension was shown to be highly and negatively correlated with the friction coefficient in dry conditions (R2 = 0.88) (Fig. 9.5A). Particularly, the outsoles having less tread dimensions such as SP1, SP2 and SP3 displayed greater ACOF than the other outsoles. The 6 mm-wide square tread dimension on the outsoles that is SP4, SP5 and SP8 displayed moderate ACOF values. In comparison to other outsoles, the remaining outsoles with broader treads displayed reduced ACOF. In slip testing with water as the contaminant, the variable parameter of tread dimension was shown to have a poor correlation (R2 = 0.19) with the ACOF (Fig. 9.5B). Similarly, in slip testing with floor cleaner (Lizol) as the contaminant, the variable parameter of tread dimension was shown to have a poor correlation (R2 = 0.08) with the ACOF (Fig. 9.5C). Regardless of tread dimension, the outsoles displayed both higher and lower friction coefficient.

9.3.3 Influence of Tread Gap on Traction Performance of Footwear Outsoles Estimating the relationship between the tread gaps and the friction coefficient allowed researchers to quantify the effects of different tread gaps (gap = 3 mm, 4 mm, and 5 mm) on traction (Fig. 9.6). Tread gap and ACOF were only marginally associated in dry conditions (R2 = 0.08) (Fig. 9.6A). When tread gaps and ACOF were examined, the ACOF fluctuated significantly, and no trend was seen. Tread gap was discovered to have a substantial and favourable correlation (R2 = 0.75) with the ACOF during wet slip testing (Fig. 9.6B). In this instance, outsoles with wider tread gaps (SP3, SP6 and SP9) displayed higher ACOF in contaminated water than other outsoles (SP1, SP4 and SP7). Tread gap was observed to not have a recognizable effect on the traction performance in the condition where floor cleaner (Lizol) was applied as the floor contaminant (R2 = 0.31) (Fig. 9.6C). In this instance, the formation of the viscous film caused by the floor cleaner present between the square tread patterned outsoles and the flooring hindered the effect of the tread geometry on the traction performance.

9.4 Conclusion In conclusion, it was discovered that parametrical changes in the square tread patterns had an impact on how well the outsoles provided grip. On hospital floors with fluid and floor cleaner contamination, the total slipping risk may increase with outsoles that have larger treads and smaller gaps. While outsoles with bigger tread gaps displayed higher traction when slipping on wet floors, smaller tread dimensions created higher traction when slipping on dry surfaces. The findings of this study should assist companies that make footwear based on square tread patterns intended

Fig. 9.5 Influence of tread width on ACOF: A in dry condition, B in wet condition and C in Lizol applied condition

88 9 Effect of Square Outsole Tread Orientation on Slip Performance

Fig. 9.6 Influence of tread gap on ACOF: A in dry condition, B in wet condition and C in Lizol applied condition

9.4 Conclusion 89

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9 Effect of Square Outsole Tread Orientation on Slip Performance

for healthcare professionals, in understanding how tread changes will affect the possibility of slipping and help them improve designs.

References 1. Strobel CM, Menezes PL, Lovell MR, Beschorner KE (2012) Analysis of the contribution of adhesion and hysteresis to shoe-floor lubricated friction in the boundary lubrication regime. Tribol Lett 47(3):341–347. https://doi.org/10.1007/S11249-012-9989-5/TABLES/2 2. Gupta S, Chatterjee S, Chanda A (2022) Effect of footwear material wear on slips and falls. Mater Today Proc 62:3508–3515. https://doi.org/10.1016/J.MATPR.2022.04.313 3. Blanchette MG, Powers CM (2015) The influence of footwear tread groove parameters on available friction. Appl Ergon 50:237–241. https://doi.org/10.1016/J.APERGO.2015.03.018 4. Jones T, Iraqi A, Beschorner K (2018) Performance testing of work shoes labeled as slip resistant. Appl Ergon 68:304–312. https://doi.org/10.1016/J.APERGO.2017.12.008 5. Tsai YJ, Powers CM (2008) The influence of footwear sole hardness on slip initiation in young adults*. J Forensic Sci 53(4):884–888. https://doi.org/10.1111/J.1556-4029.2008.00739.X 6. Yamaguchi T, Katsurashima Y, Hokkirigawa K (2017) Effect of rubber block height and orientation on the coefficients of friction against smooth steel surface lubricated with glycerol solution. Tribol Int 110:96–102. https://doi.org/10.1016/J.TRIBOINT.2017.02.015 7. Li KW, Chen CJ (2004) The effect of shoe soling tread groove width on the coefficient of friction with different sole materials, floors, and contaminants. Appl Ergon 35(6):499–507. https://doi.org/10.1016/J.APERGO.2004.06.010 8. Gupta S, Sidhu SS, Chatterjee S, Malviya A, Singh G, Chanda A (2022) Effect of floor coatings on slip-resistance of safety shoes. Coatings 12(10):1455. https://doi.org/10.3390/COATINGS1 2101455 9. Gupta S, Chatterjee S, Malviya A, Chanda A (2022) Traction performance of common formal footwear on slippery surfaces. Surfaces 5(4):489–503. https://doi.org/10.3390/SURFACES5 040035 10. Gupta S, Chatterjee S, Malviya A, Kundu A, Chanda A (2023) Effect of shoe outsole wear on friction during dry and wet slips: a multiscale experimental and computational study. Multiscale Sci Eng 2023:1–15. https://doi.org/10.1007/S42493-023-00089-0 11. Gupta S, Malviya A, Chatterjee S, Chanda A (2022) Development of a portable device for surface traction characterization at the shoe–floor interface. Surfaces 5(4):504–520. https:// doi.org/10.3390/SURFACES5040036 12. Gupta S, Chatterjee S, Malviya A, Singh G, Chanda A (2023) A novel computational model for traction performance characterization of footwear outsoles with horizontal tread channels. Computation 11(2):23. https://doi.org/10.3390/COMPUTATION11020023 13. Gupta S, Chatterjee S, Chanda A (2023) Influence of vertically treaded outsoles on interfacial fluid pressure, mass flow rate, and shoe–floor traction during slips. Fluids 8(3):82. https://doi. org/10.3390/FLUIDS8030082 14. Hemler SL, Charbonneau DN, Beschorner KE (2017) Effects of shoe wear on slippingimplications for shoe replacement threshold. https://doi.org/10.1177/1541931213601839 15. Meehan EE, Vidic N, Beschorner KE (2022) In contrast to slip-resistant shoes, fluid drainage capacity explains friction performance across shoes that are not slip-resistant. Appl Ergon 100:103663. https://doi.org/10.1016/J.APERGO.2021.103663 16. Beschorner KE, Albert DL, Chambers AJ, Redfern MS (2014) Fluid pressures at the shoe– floor–contaminant interface during slips: effects of tread & implications on slip severity. J Biomech 47(2):458–463. https://doi.org/10.1016/J.JBIOMECH.2013.10.046 17. Hemler SL, Charbonneau DN, Beschorner KE (2020) Predicting hydrodynamic conditions under worn shoes using the tapered-wedge solution of Reynolds equation. Tribol Int 145:106161. https://doi.org/10.1016/J.TRIBOINT.2020.106161

References

91

18. Iraqi A, Vidic NS, Redfern MS, Beschorner KE (2020) Prediction of coefficient of friction based on footwear outsole features. Appl Ergon 82:102963. https://doi.org/10.1016/J.APE RGO.2019.102963 19. Beschorner KE, Redfern MS, Porter WL, Debski RE (2007) Effects of slip testing parameters on measured coefficient of friction. Appl Ergon 38(6):773–780. https://doi.org/10.1016/J.APE RGO.2006.10.005 20. Beschorner KE, Meehan EE, Iraqi A, Hemler SL (2021) Designing shoe tread for friction performance: a hierarchical approach, 13(sup1):S97–S99. https://doi.org/10.1080/19424280. 2021.1917701 21. Chhikara K, Gupta S, Chanda A (2022) Development of a novel foot orthosis for plantar pain reduction. Mater Today Proc 62:3532–3537. https://doi.org/10.1016/J.MATPR.2022.04.361 22. Karkali´c RM, Radulovi´c JR, Jovanovi´c DB (2017) Characteristics of polyurethane and elastomer parts for shoe industry produced by liquid injection molding technology. Vojnoteh Glas 65(4):948–967. https://doi.org/10.5937/VOJTEHG65-10543 23. Chanda A, Jones TG, Beschorner KE (2018) Generalizability of footwear traction performance across flooring and contaminant conditions, 6(2):98–108. https://doi.org/10.1080/24725838. 2018.1517702 24. Aschan C, Hirvonen M, Mannelin T, Rajamäki E (2005) Development and validation of a novel portable slip simulator. Appl Ergon 36(5):585–593. https://doi.org/10.1016/J.APERGO.2005. 01.015

Chapter 10

Effect of Oblique Outsole Tread Orientation on Slip Performance

10.1 Introduction The outsole tread pattern, tread contact region, material and shore hardness of the footwear outsole have all been determined to have an impact on ACOF; nonetheless, the outsole’s design has such as outsole tread angles, breadth, gaps and depth predominate over the others [1–6]. During fluid-contaminated sliding, in accordance to a recent study by Yamaguchi et al. [7] demonstrated the impact of tread properties on the traction performance by dispersing the excess fluid through the tread channels. Another study by Li et al. [8] looked at modifications in the tread widths of outsoles with a commonly encountered tread pattern (i.e., treads that are horizontally oriented with respect to the slipping motion axis) in both dry and fluid-contaminated situations, and they discovered major variations in the friction coefficient results. Determining an optimum outsole design hence requires knowing the scientific features of the outsoles by modifying them parametrically during slipping. Variables including increased abrasion and the presence of contaminants like water or other viscous contaminants significantly lower ACOF [9, 10]. Furthermore, it has been claimed that a shoe’s ability to drain excess fluid during slipping is crucial to deciding how well it will maintain its grip in fluid-contaminated settings [3, 11–20]. Recent research by Beschorner et al. [20] assessed the impact of treaded and without tread outsoles on fluid pressures. High association between elevated fluid pressure and elevated slip risks was shown by the study. In this study, the traction performance of a commonly utilized tread orientation— oblique-oriented tread patterns (angle of inclination is 45°) found on the topography of the outsole of footwear used by hospital staff such as doctors, trauma centre staff and general hospital staff—was examined. The tread channels were tested using a biofidelic slip testing apparatus while being parametrically changed across dimensions and gaps. The results of this research are expected to improve tread channel design for improved traction performance on dry and slick surfaces, potentially lowering the risk of slipping accidents in hospital floorings.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 A. Chanda et al., Footwear Traction, Biomedical Materials for Multi-functional Applications, https://doi.org/10.1007/978-981-99-7823-6_10

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10.2 Materials and Methods 10.2.1 Fabrication of Footwear Outsoles The outsole geometry of the shoes chosen for this work featured treads derived from the imprints of the original footwear. In relation to the sliding motion, the treads on the chosen design were positioned in an oblique pattern having inclination angle of 45°. A surface profilometer (Precision Instruments, India) and a durometer (Precision Instruments, India) were used to assess the shore hardness and tread geometry of footwear outsoles, respectively. The investigated variables were simply assessed 50 mm from the rearward portion of the heel, and it has been claimed that this is enough to assess how well the shoes provide traction [21–23]. The outsole was made of polyurethane, which had a 50 shore A hardness. The durability, extended lifespan and efficacy of polyurethane to lessen plantar pain are well established [24, 25]. It was 50 in shore A hardness scale. The measured data were recorded and loaded into SolidWorks by Dassault Systèmes in France, a CAD modelling programme. The actual outsole had a 5 mm tread dimension and a 3 mm tread gap between each tread (Fig. 10.1). Additionally, the depth was kept constant while the tread dimension and tread gaps were adjusted at intervals of 1 mm, respectively. The parametrical changes to the tread designs are shown in Table 10.1. Nine tread dimensionally created models were 3D printed into moulds to create the outsoles (Voxelab Aquila, Flashforge, China). A pourable liquid silicone that was hydrophobic was used to fill the moulds (LSR 130, Chemzest Products, India). After being given 7 h to cure, the silicone was removed from each mould. The silicone moulds were filled with a two-part polyurethane and allowed to dry for 48 h. The polyurethane substance used has a shore A hardness of 50 to resemble the characteristics of the authentic footwear material. Extra edges were cut off and their

Fig. 10.1 Original tread design

10.2 Materials and Methods Table 10.1 Variations in the Outsole’s tread parameters

95

Tread designation

Tread width (mm)

Tread gap (mm)

OB1

2

3

OB2

2

4

OB3

2

5

OB4

4

3

OB5

4

4

OB6

4

5

OB7

6

3

OB8

6

4

OB9

6

5

measurements were remeasured after the created outsoles were removed. The nine designed polyurethane outsoles are shown in Fig. 10.2.

10.2.2 Slip Testing Experiments An entire-footwear transportable biofidelic mechanical slip evaluation apparatus (Fig. 10.3) that was created based on the worldwide standard ASTM F2913-19 was used to measure the traction performance. On the basis of the aforementioned ASTM standard, the device has been created and put into use in earlier investigations [26, 27]. A pair of shoes had the outsole affixed to them, and the shoes were further fastened over the shoe last. The inclination between the footwear and the flooring was fixed at 17 ± 2° during the testing, as seen in the most recent slip testing studies [26]. Additionally, during the slip testing procedures, a normal force of 250 ± 25 N and a slipping speed of 0.5 m/s were used. The nine designed outsoles underwent slip testing on an anti-skid surface in both dry and wet circumstances. Utilizing a digital surface profilometer, the flooring’s surface roughness was determined to be 32.5 µm. (Precision Instruments, India). Based on Chanda et al. observations [26], which indicated generalizable ACOF results in several dry and fluid-contaminated floorings, a single flooring was taken into consideration, helping to reduce the total time and effort. Within 200 ms of the start of the slide simulation, the ACOF values were estimated. Each outsole underwent five trials, with the results being averaged.

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Fig. 10.2 The developed footwear outsoles

10.3 Results 10.3.1 Frictional Assessment of Footwear Outsoles When mechanically tested in dry, water and floor cleaner (Lizol) applied environments, the outsoles’ ACOF ranged from 0.03 to 0.42. The friction coefficient values in the dry slip test (Fig. 4a) ranged from 0.34 to 0.42. Among all the oblique patterned outsoles, OB1, OB2, OB3, OB6 and OB7 displayed the comparable and greater friction coefficient values. The maximum ACOF difference for these outsoles was 0.01; OB3 had the greatest ACOF (0.42), followed by OB1 and OB2 (0.40), OB6 (0.38) and OB5 (0.38). Comparing OB3 and OB1 to other outsoles, the lowest ACOF values were 0.35 and 0.36, respectively. The ACOF values in the wet slip test (Fig. 4b)

10.3 Results

97

Fig. 10.3 Entire-shoe portable slip evaluation apparatus [16]

varied from 0.10 to 0.16. When slip testing was performed with water as a contaminant across the flooring, OB2, OB3, OB5, OB6 and OB9 showed the highest ACOF value of all the outsoles, 0.16 and 0.15. Additionally, OB3 and OB6 performed better comparably to these outsoles, with an ACOF result difference of no more than 0.01. OB1 had the lowest ACOF, 0.13, followed by OB8, OB2, OB5 and OB9, in which OB2, OB5 and OB9 showed an ACOF of 0.15. When slip testing was performed with floor cleaner (Lizol) (Fig. 4c) as a contaminant across the flooring, OB7, OB1, OB4 and OB9 showed the highest ACOF value of all the outsoles, 0.07 and 0.06 respectively. Additionally, OB5 performed comparably to these outsoles, with an ACOF result difference of no more than 0.01. OB2, OB6 and OB8 had the lowest ACOF, 0.03, followed by OP3, which showed an ACOF of 0.04.

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Fig. 10.4 Traction performance of the outsoles evaluated employing the mechanical slip testing apparatus: a dry condition, b water applied condition and c Lizol applied condition

10.3 Results

99

10.3.2 Influence of Tread Dimension on Traction Performance of Footwear Outsoles By calculating the relationships of various tread dimensions with the traction performance, the effects of different tread widths (i.e., tread width = 2, 4, 6 mm) on the outsole traction were assessed (Fig. 10.5). Tread dimension was shown to be highly and negatively correlated with the friction coefficient in dry conditions (R2 = 0.88) (Fig. 5a). Particularly, the outsoles having less tread dimensions such as OB1, OB2 and OB3 displayed greater ACOF than the other outsoles. The 4 mm-wide oblique tread dimension on the outsoles that is OB4, OB5 and OB6 displayed moderate ACOF values. In comparison to other outsoles, the remaining outsoles with broader treads displayed reduced ACOF. In slip testing with water as the contaminant, the variable parameter of tread dimension was shown to have a poor correlation (R2 = 0.13) with the ACOF (Fig. 5b). Similarly, in slip testing with floor cleaner (Lizol) as the contaminant, the variable parameter of tread dimension was shown to have a poor correlation (R2 = 0.08) with the ACOF (Fig. 5c). Regardless of tread width, the oblique patterned outsoles displayed both higher and lower friction coefficient.

10.3.3 Influence of Tread Gap on Traction Performance of Footwear Outsoles Estimating the relationship between the tread gaps and the friction coefficient allowed researchers to quantify the effects of different tread gaps (gap = 3, 4, and 5 mm) on traction (Fig. 10.6). Tread gap and ACOF were only marginally associated in dry conditions (R2 = 0.11) (Fig. 6a). When tread gaps and ACOF were examined, the ACOF fluctuated significantly, and no trend was seen. Tread gap was discovered to have a substantial and favourable correlation (R2 = 0.75) with the ACOF during wet slip testing (Fig. 10.6b). In this instance, outsoles with wider tread gaps (OB3, OB6 and OB9) displayed higher ACOF in contaminated water than other outsoles (OB1, OB4 and OB7). Tread gap was observed to not have a recognizable effect on the traction performance in the condition where floor cleaner (Lizol) was applied as the floor contaminant (R2 = 0.31) (Fig. 10.6c). In this instance, the formation of the viscous film caused by the floor cleaner present between the oblique tread patterned outsoles and the flooring hindered the effect of the tread geometry on the traction performance.

Fig. 10.5 Influence of tread width on ACOF: a in dry condition, b in wet condition and c in Lizol applied condition

100 10 Effect of Oblique Outsole Tread Orientation on Slip Performance

Fig. 10.6 Influence of tread gap on ACOF: a in dry condition, b in wet condition and c in Lizol applied condition

10.3 Results 101

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10 Effect of Oblique Outsole Tread Orientation on Slip Performance

10.4 Conclusion In conclusion, it was discovered that parametrical changes in the oblique tread patterns had an impact on how well the outsoles provided grip. On hospital floors with fluid and floor cleaner contamination, the total slipping risk may increase with outsoles that have larger treads and smaller gaps. While outsoles with bigger tread gaps displayed higher traction when slipping on wet floors, smaller tread dimensions created higher traction when slipping on dry surfaces. The findings of this study should assist companies that make footwear based on oblique tread patterns intended for healthcare professionals, in understanding how tread changes will affect the possibility of slipping and help them improve designs.

References 1. Gupta S, Chanda A (2023) Biomechanical modeling of footwear-fluid-floor interaction during slips. J Biomech 156:111690. https://doi.org/10.1016/J.JBIOMECH.2023.111690 2. Strobel CM, Menezes PL, Lovell MR, Beschorner KE (2012) Analysis of the contribution of adhesion and hysteresis to shoe-floor lubricated friction in the boundary lubrication regime. Tribol Lett 47(3):341–347. https://doi.org/10.1007/S11249-012-9989-5/TABLES/2 3. Gupta S, Chatterjee S, Chanda A (2022) Effect of footwear material wear on slips and falls. Mater Today Proc 62:3508–3515. https://doi.org/10.1016/J.MATPR.2022.04.313 4. Blanchette MG, Powers CM (2015) The influence of footwear tread groove parameters on available friction. Appl Ergon 50:237–241. https://doi.org/10.1016/J.APERGO.2015.03.018 5. Jones T, Iraqi A, Beschorner K (2018) Performance testing of work shoes labeled as slip resistant. Appl Ergon 68:304–312. https://doi.org/10.1016/J.APERGO.2017.12.008 6. Tsai YJ, Powers CM (2008) The influence of footwear sole hardness on slip initiation in Young Adults*. J Forensic Sci 53(4):884–888. https://doi.org/10.1111/J.1556-4029.2008.00739.X 7. Yamaguchi T, Katsurashima Y, Hokkirigawa K (2017) Effect of rubber block height and orientation on the coefficients of friction against smooth steel surface lubricated with glycerol solution. Tribol Int 110:96–102. https://doi.org/10.1016/J.TRIBOINT.2017.02.015 8. Li KW, Chen CJ (2004) The effect of shoe soling tread groove width on the coefficient of friction with different sole materials, floors, and contaminants. Appl Ergon 35(6):499–507. https://doi.org/10.1016/J.APERGO.2004.06.010 9. Hemler SL, Charbonneau DN, Beschorner KE (2017) Effects of shoe wear on slippingimplications for shoe replacement threshold. https://doi.org/10.1177/1541931213601839 10. Meehan EE, Vidic N, Beschorner KE (2022) In contrast to slip-resistant shoes, fluid drainage capacity explains friction performance across shoes that are not slip-resistant. Appl Ergon 100:103663. https://doi.org/10.1016/J.APERGO.2021.103663 11. Chatterjee S, Chanda A (2022) Development of a tribofidelic human heel surrogate for barefoot slip testing. J Bionic Eng 2022 192 19(2):429–439. https://doi.org/10.1007/S42235-021-001 38-0 12. Hemler SL, Charbonneau DN, Beschorner KE (2020) Predicting hydrodynamic conditions under worn shoes using the tapered-wedge solution of Reynolds equation. Tribol Int 145:106161. https://doi.org/10.1016/J.TRIBOINT.2020.106161 13. Gupta S, Chatterjee S, Malviya A, Chanda A (2022) Traction performance of common formal footwear on slippery surfaces. Surfaces 5(4):489–503. https://doi.org/10.3390/SURFACES5 040035

References

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14. Gupta S, Chatterjee S, Malviya A, Chanda A (2023) Frictional assessment of low-cost shoes in worn conditions across workplaces. J Bio- Tribo-Corrosion 9(1):1–13. https://doi.org/10. 1007/S40735-023-00741-0 15. Gupta S, Malviya A, Chatterjee S, Chanda A (2022) Development of a portable device for surface traction characterization at the shoe–floor interface. Surfaces 5(4):504–520. https:// doi.org/10.3390/SURFACES5040036 16. Gupta S, Chatterjee S, Chanda A (2023) Influence of vertically treaded outsoles on interfacial fluid pressure, mass flow rate, and shoe–floor traction during Slips. Fluids 8(3):82. https://doi. org/10.3390/FLUIDS8030082 17. Gupta S, Chatterjee S, Malviya A, Singh G, Chanda A (2023) A novel computational model for traction performance characterization of footwear outsoles with horizontal tread channels. Computation 11(2):23. https://doi.org/10.3390/COMPUTATION11020023 18. Gupta S, Sidhu SS, Chatterjee S, Malviya A, Singh G, Chanda A (2022) Effect of floor coatings on slip-resistance of safety shoes. Coatings 12(10):1455. https://doi.org/10.3390/COATINGS1 2101455 19. Gupta S, Chatterjee S, Malviya A, Kundu A, Chanda A (2023) Effect of shoe outsole wear on friction during dry and wet slips: a multiscale experimental and computational study. Multiscale Sci Eng 2023:1–15. https://doi.org/10.1007/S42493-023-00089-0 20. Beschorner KE, Albert DL, Chambers AJ, Redfern MS (2014) Fluid pressures at the shoe– floor–contaminant interface during slips: effects of tread & implications on slip severity. J Biomech 47(2):458–463. https://doi.org/10.1016/J.JBIOMECH.2013.10.046 21. Iraqi A, Vidic NS, Redfern MS, Beschorner KE (2020) Prediction of coefficient of friction based on footwear outsole features. Appl Ergon 82:102963. https://doi.org/10.1016/J.APE RGO.2019.102963 22. Beschorner KE, Redfern MS, Porter WL, Debski RE (2007) Effects of slip testing parameters on measured coefficient of friction. Appl Ergon 38(6):773–780. https://doi.org/10.1016/J.APE RGO.2006.10.005 23. Beschorner KE, Meehan EE, Iraqi A, Hemler SL (2021) Designing shoe tread for friction performance: a hierarchical approach 13(sup1):S97–S99. https://doi.org/10.1080/19424280. 2021.1917701 24. Chhikara K, Gupta S, Chanda A (2022) Development of a novel foot orthosis for plantar pain reduction. Mater Today Proc 62:3532–3537. https://doi.org/10.1016/J.MATPR.2022.04.361 25. Karkali´c RM, Radulovi´c JR, Jovanovi´c DB (2017) Characteristics of polyurethane and elastomer parts for shoe industry produced by liquid injection molding technology. Vojnoteh Glas 65(4):948–967. https://doi.org/10.5937/VOJTEHG65-10543 26. Chanda A, Jones TG, Beschorner KE (2018) Generalizability of footwear traction performance across flooring and contaminant conditions 6(2):98–108. https://doi.org/10.1080/24725838. 2018.1517702 27. Aschan C, Hirvonen M, Mannelin T, Rajamäki E (2005) Development and validation of a novel portable slip simulator. Appl Ergon 36(5):585–593. https://doi.org/10.1016/J.APERGO.2005. 01.015

Chapter 11

Footwear Wear and Wear Mechanisms

11.1 Introduction The prevalence of pedestrian falls is caused by a variety of factors, however, the existing research mostly emphasizes design flaws in workplace environments and sidewalk surfaces [1]. Since shoes have less efficient slip resistance capabilities, it appears that they are one of the final things to change or improve [2]. However, it appears that this practice undervalues or fails to recognize the complexity of mechanisms such as wear from abrasion and fatigue [2–6]. Because the definition of traction is the force that opposes the development of relative motion between two entities in contact, or more generally, as the underlying resistive force that lies parallel to the slip axis of two interacting surfaces [7, 8]. Because they play an equally essential part in preventing pedestrian slips and falls, the surface circumstances and tread design concerns of the footwear ought to be thoroughly investigated to determine their frictional qualities and their impact on slip risk reduction features [5, 6, 9, 10]. The primary purposes of footwear are to safeguard our feet, ensure our safety and comfort and improve performance in a variety of activities [9–13]. Shoes must provide effective traction or fall prevention against any slippery situation in order to provide safety and balanced performance. However, with continuous walking, footwear heels and soles appear to develop major abrasions and tears. As a result, the footwear’s original topographic structures (micro- and macro-tread patterns) are probably much different from what they were originally. This shows that the shoe’s surface wear seems to be an important concern and has a significant impact on its ability to resist slipping. Despite the significance of this topic, there haven’t been many studies on how shoes affect slipping effectiveness with wear developments and accessible frictional qualities [14]. Few research have examined how footwear heels and soles affect slip resistance findings to this point [8, 15]. These experiments showed that during friction measurements, shoe surfaces changed continually. For instance, repetitive rubbing with sand altered the morphological features of the shoe surfaces, and these differences affected the results of slip resistance.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 A. Chanda et al., Footwear Traction, Biomedical Materials for Multi-functional Applications, https://doi.org/10.1007/978-981-99-7823-6_11

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Fig. 11.1 Diagrammatic representation of a possible tribological phenomenon at the shoe-floor interface during sliding: a Macroscale, b Microscale

Additionally, specific studies were carried out by comparing the wear behaviours of footwear to dynamic friction coefficients [16–18]. According to these investigations, the heel and sole surface roughness of shoes has a considerable impact on their slip-resistance qualities. When worn down, surface roughness quickly loses its effectiveness in providing drainage holes to prevent squeezing film forms in lubricated settings. A recent study examined footwear wear behaviours during dynamic friction tests and developed a wear paradigm for shoe surfaces [15]. Figure 11.1 shows the possible tribological phenomenon at the shoe-floor interface during sliding.

11.2 Hypothesis of Footwear Wear Despite the fact that numerous factors lead to fall accidents, the wearer’s stability is greatly impacted by their shoes. The soles and heels of shoes should have strong traction and slip-resistance capabilities in addition to providing the foot with good support and protection. But with constant walking, it’s inevitable that the shoe heels and soles would gradually deteriorate. Gronqvist. [19] claimed that adhesion, which typically rises with the actual contact area, seems to be the primary factor in determining the dry friction of leather shoes. Kim and Smith [15] discovered that wearing mechanisms and the migration of polymer substance from the soles of shoes to the floor could account for the surface alterations. Kim and colleagues looked at the surface changes and microscopic wear trends for three sole materials [16]. Mechanical wear of the sole substances during measurements, according to Derler et al. [20], was the factor having the most systematic impact on the dynamic coefficients of friction. These results showed that the surface features of the footwear surface were constantly changing. As a result, wear developments dramatically and continually

11.2 Hypothesis of Footwear Wear

107

change the basic surface characteristics and topographical patterns of the shoe soles/ heels, which in turn greatly affects slip-resistance capabilities. The significance of surface texture in determining slip resistance has been documented in current research on slip and fall events [2, 21]. According to those research, under various surface circumstances, the topographical roughness of interacting entities had a significant impact on slip-resistance capabilities. Surface roughness creates the drainage gaps required to prevent the production of squeezing films under lubricated circumstances. In order to boost interaction with the floorings and provide empty areas for the elimination of contaminants, correct tread patterns on the footwear’s heels and soles might enhance traction properties. Therefore, the heel/ sole portions have frequently been created with macro texture or tread patterns to improve slip resistance, however, they quickly lose their effectiveness after use. Wear concerns of polymeric and rubbers have received substantial experimental and theoretical study due to their practical relevance [22]. However, only a small number of recent research ask questions about shoe-wearing habits [8]. These authors claimed that during slip-resistance testing, the surface contours of the shoes changed in precise ways continuously. For instance, prolonged friction on the shoe heels changed the initial surfaces’ structure and the resulting coefficient of friction measurements. In order to comprehend the impact of surface roughness and roughness characteristics on the creation of transfer layers and the coefficient of friction, Menezes et al. [23] conducted additional tests. Although those studies demonstrated a significant correlation between slip-resistance characteristics and surface roughness, little was known about the fundamental tribological characteristics of wear developments of shoe surfaces and how they affected slip-resistance performance. Few attempts have been made to tribologically model the shoe-floor-contaminant interface, as Beschorner et al. [24] indicated. Particularly, there are no conclusive theories and systematic studies on how shoe surfaces wear.

11.2.1 Footwear Floor Interaction One of the most significant tribological interactions between various, incompatible materials may cause material loss from the surface. A shoe-floor sliding friction device might use this idea. As shown in Fig. 1a, it is possible to assume that whenever a footwear heel is pressed against a floor, two surfaces initially only make tiny, discrete contact at their maximum asperities. Because the shoe heel’s elastic modulus would be much lower than that of the flooring surface, localized pressure at the contact areas would be sufficiently great to lead to a plastic stretching of the heel imperfections even at the weakest load. Because of this, the interaction mechanism would be in the interlocking position seen in Fig. 1b in an enlarged format. When the footwear heel slips on the floor, the wedge-shaped imperfections of the flooring surface will tear, distort and furrow the heel surface. The footwear surface appears to be continuously altered and harmed during this phase by wear development. Constant walking would require harsh imperfections of the ground’s surface to grind channels in the surface

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areas of the footwear’s heel in order to prevent repeated sliding events. Therefore, it is expected that these repetitive slipping or walking occurrences will lead to additional advances of shoe wear, making worn expansion of the heel area inevitable.

11.3 Theory Behind Footwear Wear Two surfaces appear to touch when two solid substances are in contact, but only in very small, defined places where their maximum asperities interact [25]. This idea might be used to create a model of the sliding friction mechanism between shoes and floors. In other words, a footwear heel’s soft asperities appear to interact and glide over a floor’s array of hard, wedge-shaped asperities. According to the aforementioned premise, Fig. 11.1 provides a geometric concept for a contact-sliding interaction between the footwear heel and the floor’s surface as a micro- and macroscopic form, respectively. The largest imperfections of the floor surface smash against the heel surface of the shoe, creating a contact region between the footwear heel and floor surface. There appears to be a mutually reinforcing mechanism between the two bodies because the elasticity modulus of the floor surface is significantly higher than that of the shoe. The hard imperfections of the flooring surface appear to penetrate, rupture, and distort the heel surface as the shoe glides across it. Additional rubbings would necessitate the floor’s asperities being particularly sharp in order to carve grooves into the shoe’s surface. Persistent walking or periodic rubbings would greatly contribute to the progression of shoe wear and result in significant cracks. Peak height density (the density of the peak asperity within the evaluation measurement of the flooring surface’s profile) is thought to have a significant role in this process in terms of the footwear’s sliding friction mechanism. It implies that the slipping interaction among the footwear heel and the floor’s surface can be viewed as a combination of the tangential force necessary to elevate the asperities over one another and the tangential force necessary to overcome adhesion at locations of intimate contact. An attack lope angle (θ) of each asperity wedge of the footwear surface appears to be crucial to the amount of peripheral forces exerted in the sliding direction and affects the arrangement of the heel surface deformation when asperity wedges of the floor’s surface are drawn across the shoe surface. In order to spot wear crack developments, it would be good to look into how the typical asperity angle of slope of the footwear surface alters over time. Previous studies provide specifics regarding the average asperity’s lope angle of a surface profile [26].

11.4 Adhesion and Hysteresis Friction During Footwear Wear

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11.4 Adhesion and Hysteresis Friction During Footwear Wear Whenever utilized in bearings, artificial joints or rubber seals, tribology research frequently focuses on minimizing friction between surfaces [27, 28]. However, in some tribological applications, friction must be raised in order to absorb a system’s energy and maintain stability. The tribological interaction that occurs while walking between a footwear material and the floor’s surface is one such use. Notwithstanding the safety risks associated with this kind of research, the tribology community has paid less attention to this connection. According to the National Floor Safety Institute, slips and falls are the main reason for filing a workers’ compensation claim. According to research, the majority of slips and falls occur when the amount of available friction is smaller than the amount of required friction [29]. So increasing the friction between shoes and lubricant on the floor is a good way to stop incidents involving slips and falls. Therefore, in order to create practical methods for raising the coefficient of friction between shoes and floor surfaces, tribological study is required. Walking is a dynamic and fleeting activity that is influenced by a number of factors, such as the characteristics of the shoe and floor materials, the roughness of the floor surface, the sliding speed and fluid contaminants. Shoe-floor-lubricant friction is complicated because of this [30–33]. Because of the wide range of circumstances between the shoe and the ground during gait, the interface operates in several lubrication regimes and is also affected by various tribological phenomena [34, 35]. These mechanisms include fluid dynamics between the shoe and the floor surface, adhesion and hysteresis during sliding and more. It is necessary to examine each of these variables separately in order to comprehend their effects completely. The goal of this investigation was to separate the contributions of hysteresis and adhesion to the overall coefficient of friction. Floor roughness has been repeatedly demonstrated to be positively correlated with lubricated coefficient of friction [36]. According to earlier studies, the asperities of various sizes that vary the adhesion and hysteresis of interacting surfaces cause increased void space or material deformation, which causes the roughness effect. But the experimental evidence hasn’t really backed up these claims. To identify the process by which floor roughness affects friction and to enable future research to take advantage of this mechanism to optimize friction, it is necessary to identify the kind of friction (adhesion or hysteresis) that is affected by roughness. As the sliding speed increases, the coefficient of friction at the shoe/floor/lubricant interaction drops [24]. The Stribeck effect, which states that greater speed increases layer thickness and decreases surface contact, may account for this phenomenon [24]. According to additional studies, increases in speed also have an impact on the deformation of the shoe’s material and the contact area, which are associated with adhesion and hysteresis friction, respectively. Knowing how sliding speed affects adhesion and hysteresis is therefore essential to comprehending the tribological interactions at the shoe/floor/fluid interface.

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The primary contributory frictional phenomena among interacting footwear and floor asperities are adhesion and hysteresis. Hysteresis friction happens because of an unpredictable energy loss during the deformation of the surface asperity, and adhesion friction happens at the molecular level [37]. In this type of system, adherence is more common on flat surfaces and in dry environments [38]. When surface areas are lubricated, there is less adhesion and more hysteresis [38]. Despite the fact that these elastomer friction mechanisms are well recognized, few research have shown how footwear, flooring and testing circumstances influence adhesion and hysteresis separately.

References 1. Maynard WS (2013) Prevention through design-slips, trips and falls 2. Kim IJ, Nagata H (2008) Research on slip resistance measurements —a new challenge. Ind Health 46(1):66–76. https://doi.org/10.2486/INDHEALTH.46.66 3. Gupta S, Malviya A, Chatterjee S, Chanda A (2022) Development of a portable device for surface traction characterization at the shoe-floor interface. Surfaces 5(4):504–520. https://doi. org/10.3390/SURFACES5040036 4. Gupta S, Chatterjee S, Malviya A, Chanda A (2022) Traction performance of common formal footwear on slippery surfaces. Surfaces 5(4):489–503. https://doi.org/10.3390/SURFACES5 040035 5. Gupta S, Chatterjee S, Malviya A, Kundu A, Chanda A (2023) Effect of shoe outsole wear on friction during dry and wet slips: a multiscale experimental and computational study. Multiscale Sci Eng 2023:1–15. https://doi.org/10.1007/S42493-023-00089-0 6. Gupta S, Chatterjee S, Chanda A (2022) Effect of footwear material wear on slips and falls. Mater Today Proc. https://doi.org/10.1016/J.MATPR.2022.04.313 7. Ludema KC (1987) Friction: a study in the prevention of seizure. ASTM Stand News 15(5):54– 58 8. Manning DP, Jones C, Rowland FJ, Roff M (1998) The surface roughness of a rubber soling material determines the coefficient of friction on water-lubricated surfaces. J Safety Res 29(4):275–283. https://doi.org/10.1016/S0022-4375(98)00053-X 9. Gupta S, Chatterjee S, Malviya A, Chanda A (2023) Frictional assessment of low-cost shoes in worn conditions across workplaces. J Bio- Tribo-Corrosion 9(1):1–13. https://doi.org/10. 1007/S40735-023-00741-0 10. Gupta S, Chatterjee S, Chanda A (2023) Influence of vertically treaded outsoles on interfacial fluid pressure, mass flow rate, and shoe–floor traction during slips. Fluids 8(3):82. https://doi. org/10.3390/FLUIDS8030082 11. Gupta S, Chanda A (2023) Biomechanical modeling of footwear-fluid-floor interaction during slips. J Biomech 156:111690. https://doi.org/10.1016/J.JBIOMECH.2023.111690 12. Gupta S et al (2023) Diabot: development of a diabetic foot pressure tracking device J 6(1):32– 47. https://doi.org/10.3390/J6010003 13. Gupta S, Chatterjee S, Malviya A, Singh G, Chanda A (2023) A novel computational model for traction performance characterization of footwear outsoles with horizontal tread channels. Computation 11(2):23. https://doi.org/10.3390/COMPUTATION11020023 14. Gupta S, Sidhu SS, Chatterjee S, Malviya A, Singh G, Chanda A (2022) Effect of floor coatings on slip-resistance of safety shoes. Coatings 12(10): 1455. https://doi.org/10.3390/COATINGS1 2101455 15. Kim IJ, Smith R (2000) Observation of the floor surface topography changes in pedestrian slip resistance measurements. Int J Ind Ergon 26(6):581–601. https://doi.org/10.1016/S0169-814 1(00)00024-X

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16. Kim IJ, Smith R, Nagata H (2001) Microscopic observations of the progressive wear on shoe surfaces that affect the slip resistance characteristics. Int J Ind Ergon 28(1):17–29. https://doi. org/10.1016/S0169-8141(01)00010-5 17. Kim IJ (2015) Wear observation of shoe surfaces: application for slip and fall safety assessments 58(3):407–417. https://doi.org/10.1080/10402004.2014.980593 18. Kim IJ (2004) Development of a new analyzing model for quantifying pedestrian slip resistance characteristics: part II—experiments and validations. Int J Ind Ergon 33(5):403–414. https:// doi.org/10.1016/J.ERGON.2003.10.011 19. Grönqvist R (2007) Mechanisms of friction and assessment of slip resistance of new and used footwear soles on contaminated floors 38(2):224–241. https://doi.org/10.1080/001401395089 25100 20. Derler S, Kausch F, Huber R (2008) Analysis of factors influencing the friction coefficients of shoe sole materials. Saf Sci 46(5):822–832. https://doi.org/10.1016/J.SSCI.2007.01.010 21. Chang WR (2002) The effects of surface roughness and contaminants on the dynamic friction between porcelain tile and vulcanized rubber. Saf Sci 40(7–8):577–591. https://doi.org/10. 1016/S0925-7535(01)00060-1 22. Chowdhury SKR, Chakraborti P (2008) Prediction of polymer wear—an analytical model and experimental validation 51(6):798–809. https://doi.org/10.1080/10402000802322753 23. Menezes PL, Kishore, Kailas SV (2009) Study of friction and transfer layer formation in copper-steel tribo-system: role of surface texture and roughness parameters 52(5):611–612. https://doi.org/10.1080/10402000902825754 24. Kurt B, Lovell M, Higgs CF, Redfern MS (2011) Modeling mixed-lubrication of a shoe-floor interface applied to a pin-on-disk apparatus 52(4):560–568. https://doi.org/10.1080/104020 00902825705 25. Tabor D (1974) Friction, adhesion and boundary lubrication of polymers, 5–30. https://doi.org/ 10.1007/978-1-4613-9942-1_2/COVER 26. Spragg RC, Whitehouse DJ (1970) A new unified approach to surface metrology 697–707. https://doi.org/10.1243/PIME_PROC_1970_185_081_02 27. Scholes SC, Unsworth A (2000) Comparison of friction and lubrication of different hip prostheses 28. Pei YT, Bui XL, Zhou XB, De Hosson JTM (2008) Tribological behavior of W-DLC coated rubber seals. Surf Coatings Technol 202(9):1869–1875. https://doi.org/10.1016/J.SURFCOAT. 2007.08.013 29. Hanson JP et al (2010) Predicting slips and falls considering required and available friction 42(12):1619–1633. https://doi.org/10.1080/001401399184712 30. Redfern MS et al (2010) Biomechanics of slips 44(13):1138–1166. https://doi.org/10.1080/ 00140130110085547 31. Redfern MS, Bidanda B (2007) Slip resistance of the shoe-floor interface under biomechanically-relevant conditions 37(3):511–524. https://doi.org/10.1080/001401394089 63667 32. Chang WR et al (2010) The role of friction in the measurement of slipperiness, part 1: friction mechanisms and definition of test conditions 44(13):1217–12321. https://doi.org/10.1080/001 40130110085574 33. Moore CT, Menezes PL, Lovell MR, Beschorner KE (2012) Analysis of shoe friction during sliding against floor material: role of fluid contaminant. J Tribol 134(4). https://doi.org/10. 1115/1.4007346/436402 34. Strobel CM, Menezes PL, Lovell MR, Beschorner KE (2012) Analysis of the contribution of adhesion and hysteresis to shoe-floor lubricated friction in the boundary lubrication regime. Tribol Lett 47(3):341–347. https://doi.org/10.1007/S11249-012-9989-5/TABLES/2 35. Beschorner KE, Redfern MS, Porter WL, Debski RE (2007) Effects of slip testing parameters on measured coefficient of friction. Appl Ergon 38(6):773–780. https://doi.org/10.1016/J.APE RGO.2006.10.005 36. Chang WR et al (2007) The effect of surface waviness on friction between Neolite and quarry tiles 47(8):890–906. https://doi.org/10.1080/00140130410001670390

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37. The relation between the friction and visco-elastic properties of rubber. Proc R Soc London Ser A Math Phys Sci 274(1356):21–39 (1963). https://doi.org/10.1098/RSPA.1963.0112 38. Heinrich G (1997) Hysteresis friction of sliding rubbers on rough and fractal surfaces. Rubber Chem Technol 70(1):1–14. https://doi.org/10.5254/1.3538415

Chapter 12

Effect of Footwear Wear on the Available Traction

12.1 Introduction Recent worker’s compensation claims for workplace or industrial injuries amounted more than $30 billion, of which around 30% were caused by slips and falls [1, 2]. More than 50% of reported fall injuries are caused by slips [3]. It frequently results in common lower extremity injuries such as fractures, sprains and dislocations [4]. Slip-related accidents are more likely when there is less traction between the shoes and the floor since footwear serves as the body’s primary point of contact with the ground [5, 6]. Because there is little friction at the shoe-floor interface, a high slip risk is created [7, 8]. To reduce the danger of slip-related accidents, it is therefore vital to comprehend the significance of footwear and its outsole properties [9, 10]. ACOF between the shoe and floor must be sufficient for regular activities like walking and running [11, 12]. In order to precisely determine the slip risk of footwear, the traction performance is often recorded at the shoe-floor contact. The ACOF is greatly influenced by the contacting surfaces between the shoe and the floor or the shoe treads. The characteristics of the shoe tread are thought to be crucial in assessing the risk of slipping [7]. The ACOF at the interface is impacted by aspects of the outsole design including tread direction, periodicity, depth, width and material hardness. Also, it has been shown that tread channels aid in the drainage of extra fluid contaminants, preserving an adequate ACOF between both the shoe and flooring and lowering overall slip risk. To measure the slip risk, it is necessary to examine the outsole pattern characteristics of the widely accessible shoes and comprehend its frictional performance [13]. The outsole characteristics of four popular formal shoes were examined in the current work. A unique technique was used to mimic the tread characteristics in order to assess the traction capability of each outsole. The outsoles were also worn three times regularly to simulate progressive wear. The created outsoles’ ACOF values were evaluated on three different types of flooring (glossy, matt and antiskid), under dry settings and with various contamination situations, such as with

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 A. Chanda et al., Footwear Traction, Biomedical Materials for Multi-functional Applications, https://doi.org/10.1007/978-981-99-7823-6_12

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water and canola oil. It is predicted that the current research’s findings will contribute to our understanding of how tread characteristics and wear affect slip hazards across pollutants and flooring types.

12.2 Materials and Methods 12.2.1 Development of Custom Outsoles In India, four popular formal shoes with high sales that are unaffected by brand heterogeneity were chosen. The footwear was from well-known manufacturers, including EGOSS and BATA. The chosen shoes’ outsoles were made of polyurethane (PU), which is frequently utilized in footwear because of its durability and flexibility [14, 15]. According to recent investigations by Iraqi et al. [5] and Jones et al. [7], shoe characteristics including shore hardness and outsole shape were sufficient in determining the ACOF. A Durometer was used to determine the shore hardness of the chosen shoes, and a depth gauge was used to measure the tread’s properties. Also, the tread shape was replicated using Fusion 360. A Voxelab Aquila 3D Printer was used to create negative moulds from the replicated projected geometry. Silicone was put into the moulds and allowed to cure for 5 h. Liquid PU (Aditya Polymers, India) was put into the silicone mould and afterwards allowed to harden for 24 h in order to replicate the precise material properties of the footwear. Extra edges and flaps were cut off after the PU was removed, and the geometry’s accuracy was checked by comparing it to the original geometry. The produced outsoles’ measured shore A hardness of 72 was commensurate with the original footwear material’s hardness. The consolidated process utilized to create the outsoles is shown in Fig. 12.1.

12.2.2 Slip Testing Device and Protocol for Accelerated Wear Using a British Pendulum Skid Tester (Fig. 12.2), the developed outsoles’ ACOF was evaluated. In the past, multiple studies have used this tool to assess slipping probability in various places, including kitchens and industrial workshops, due to its mobility and convenience of use [16–18]. In this research, the prepared outsoles were fastened to a footwear component that was then attached to the slip tester with a specially made connector. The heel strike angle with regard to the flooring during unintended slips is said to be 17 ± 1º, which was kept constant [5, 6, 19– 22]. Following each frictional coefficient recording across any specific flooring and contaminant condition, the slip tester was calibrated. A computerized surface profilometer was used to measure the flooring’s surface roughness. On floorings such laminate, ceramic and anti-skid flooring with peak-to-valley surface roughness of 1.2, 1.8 and 2.6 µm, respectively, the ACOF of the outsoles

12.2 Materials and Methods

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Fig. 12.1 The consolidated process utilized to create the outsoles

was measured. During the tests, contaminants were poured on the floors under three different conditions: dry, wet and with the application of canola oil. The current investigation used an accelerated wear approach to shorten the total amount of time needed to evaluate a pair of shoes across its lifetime. The produced outsoles were worn three times at regular intervals to better understand how the traction performance of the shoes changed as they were being used. According to a recent study by Verma et al., wearing shoes continuously for 6 months seem to reduce ACOF by 54%. As a result, for the purposes of our investigation, usage of the outsole was simulated at three, six and full levels of wear to the shoe outsole [23, 24]. Using a belt sander, the worn footwear situations were approximated based on a recent study by Hemler et al. [25].

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Fig. 12.2 Skid tester with modified shoe outsole attachment

12.3 Results and Discussions 12.3.1 Friction Results of Footwear Outsoles in New Condition The new outsoles’ traction performance values ranged from 0.144 to 0.164 when they were contaminated with Canola oil, from 0.251 to 0.270 when they were contaminated with water, and from 0.352 to 0.380 when they were dry (Fig. 12.3). In terms of friction, the S2 and S4 outsoles (which have horizontal or complicated tread patterns in the heel area) excelled in dry conditions. The high frictional behaviour of S1 and S3 in wet conditions revealed the capability of the tread designs to effectively channel the flow. High ACOF for S1 and S3 was made possible by the flow channelling, but overall, generalized values were reported because of the viscous oil contamination. Considering canola oil as the flooring contaminant, the outsole S2 and S4 exhibited the lowest ACOF. The outsole tread pattern S2 had horizontal arrangement of treads and S4 had oblique tread patterns, the high viscous contaminant was not able to drain effectively during slip testing. This can be the probable reason for which the traction performance was low for these two outsoles. S1 and S3 had the tread pattern geometry, which enabled the sufficient exit of the high viscous contaminant, for which their traction performance was comparatively high when viscous contaminant such as canola oil was implemented.

12.3 Results and Discussions

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Fig. 12.3 Variation of ACOF across outsoles in unworn condition

12.3.2 Friction Results of Footwear Outsoles: First Wear After the initial progressive wear simulation, the ACOF values for each outsole significantly decreased. The ACOF values of the worn outsoles ranged from 0.301 to 0.334 while dry, from 0.212 to 0.246 when contaminated with water, and from 0.139 to 0.147 when contaminated with Canola oil (Fig. 12.4). However, only two tread designs (S1 and S3) have been demonstrated to be effective in this scenario for retaining the general frictional performance in all conditions. The outsole tread designs of S1 and S3 enabled the water and subsequently canola oil to sufficiently drain out of the tread patterns and thus the traction performance was comparatively high. The outsole tread patterns S2 and S4 had started resembling similar traction performance in wet and canola oil condition. As the wear cycle caused some of the treads in the heel region to degrade, thus enabling the flow of the contaminant in a similar fashion throughout these outsole tread patterns.

12.3.3 Friction Results of Footwear Outsoles: Second Wear After the completion of the second progressive wear simulation, all of the outsoles’ ACOF values significantly dropped as compared to the original test settings. The traction performance values of the fully worn outsoles varied from 0.265 to 0.295 while dry, from 0.195 to 0.214 when contaminated with water, and from 0.121 to 0.138 when contaminated with Canola oil (Fig. 12.5). In this case, only two tread

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Fig. 12.4 Variation of ACOF after first wear cycle

designs (S2 and S4) were shown to be ineffective at maintaining the overall frictional performance. After the third progressive wear cycle, all the outsoles showed significantly lower ACOF values when compared to prior test settings. The tread pattern orientation of the outsoles, S2 and S4 were also the possible cause of reduction in the ACOF values as the contaminants were not able to drain out properly. The outsole tread patterns S1 and S3 were also exhibiting better traction performance in both dry and contaminated condition, owing to the oblique tread orientation. Another significant observation was also noted, that there was a similarity in the trend of the traction performance between S1 and S3, as with the outsoles S2 and S4. The wear progression on the outsoles was affecting the traction performance of the outsoles to become more generalizable.

12.3.4 Friction Results of Footwear Outsoles: Third Wear The traction performance values of the fully worn outsoles ranged from 0.231 to 0.263 while dry, from 0.174 to 0.207 when contaminated with water, and from 0.110 to 0.121 when contaminated with Canola oil (Fig. 12.6). In this case, fully worn-out outsoles performed similarly to the base material. It was found that the produced shoe outsoles S1, S2 and S3 showed a similar pattern in their traction performance, especially in conditions with canola oil. After undergoing a full wear cycle, the outsoles began to behave like a plain base material, and the implications of the tread pattern were completely lost. As a result, we were able to see how the ACOF dropped

12.3 Results and Discussions

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Fig. 12.5 Variation of ACOF after second wear cycle

with each wear cycle, and when high viscosity contaminants were used in the slip tests, we saw a similar and generalizable trend in the ACOF values.

Fig. 12.6 Variation of ACOF after third wear cycle

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12.3.5 Outsole Worn Region Area Versus ACOF Outcomes After the first wear phase, it was noticed that there was a stronger association between the ACOF values and the sole’s worn tread area. This was due to the growing worn area, which reduced the impact of the sole tread geometry and signalled the beginning of a general trend in the ACOF values. After the first wear cycle, there were significant correlation values for the dry (R2 = 0.508), wet (R2 = 0.530) and canola oil conditions (R2 = 0.621) between the outsole tread contact area and the ACOF (Fig. 12.7). After the second wear cycle, it was observed that the correlation between the outsole worn tread area and the ACOF values increased further because of the increasing worn area. The worn area increase has a positive effect on the trend of correlation between the outsole tread area and the traction performance recorded in wet and canola oil condition. In the previous wear cycle and in the unworn condition, negative trend in the correlation existed between the traction performance and outsole tread area for wet condition. After the second wear cycle, high correlation values were recorded between the outsole tread contact area and the ACOF for dry condition (R2 = 0.558), wet condition (R2 = 0.822) and canola oil condition (R2 = 0.899) (Fig. 12.8). After the completion of the third wear cycle, it was observed that the tread outsole features were almost worn out and the outsole behaved like a base polyurethane block, due to which the highest correlation between the ACOF values and the outsole tread areas were recorded. After the third wear cycle, very high correlation values were noted between the outsole tread contact area and the ACOF for dry condition (R2 = 0.824), wet condition (R2 = 0.857) and canola oil condition (R2 = 0.945) respectively (Fig. 12.9).

12.4 Conclusions Several of the study’s findings agree with earlier works. ACOF values for outsoles with vertical or oblique tread orientations were higher than those for other outsoles, for instance. This might be because such outsoles have easy fluid drainage, which improved traction and adhesion [8, 20, 26, 27]. Oil-contaminated glossy and anti-skid tiles were found to correlate well with the outsoles having horizontal tread patterns. The formation of fluid films at the shoe-floor interface and increased hysteresis could be the result of such outsoles’ poor drainage and the presence of a very viscous fluid contaminant. High levels of hysteresis would make the impact of flooring asperities less significant for traction performance and may have increased correlations between smooth and uneven flooring. Understanding the impact of wear on various patterned footwear outsoles is the main goal of the current work. As the outsoles gradually wore down, the ACOF values decreased by 40–90%. Compared to other tread geometries, oblique tread design outsoles were found to cause high friction. It was discovered that a decrease in friction had a substantial correlation with an increase in worn area. It

12.4 Conclusions

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Fig. 12.7 Correlation between outsole tread area and ACOF after first wear cycle: a dry condition, b wet condition and c canola oil condition

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Fig. 12.8 Correlation between outsole worn region and ACOF after second wear cycle: a dry condition, b wet condition and c canola oil condition

12.4 Conclusions

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Fig. 12.9 Correlation between outsole worn region and ACOF after third wear cycle: a dry condition, b wet condition and c canola oil condition

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is predicted that the study’s findings and economical modelling techniques will give footwear makers pointers on how to choose the best tread patterns and recommend replacement thresholds.

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17. Sudoł E, Szewczak E, Małek M (2021) Comparative analysis of slip resistance test methods for granite floors. Materials (Basel) 14(5):1–15. https://doi.org/10.3390/ma14051108 18. Terjék A, Dudás A (2018) Ceramic floor slipperiness classification—a new approach for assessing slip resistance of ceramic tiles. Constr Build Mater 164:809–819. https://doi.org/ 10.1016/j.conbuildmat.2017.12.242 19. Chatterjee S, Gupta S, Chanda A (2022) Barefoot slip risk in Indian bathrooms: a pilot study. https://doi.org/10.1080/10402004.2022.2103863 20. Beschorner KE et al (2020) An observational ergonomic tool for assessing the worn condition of slip-resistant shoes. Appl Ergon 88:103140. https://doi.org/10.1016/J.APERGO.2020.103140 21. Sundaram VH, Hemler SL, Chanda A, Haight JM, Redfern MS, Beschorner KE (2020) Worn region size of shoe outsole impacts human slips: testing a mechanistic model. J Biomech 105:109797. https://doi.org/10.1016/J.JBIOMECH.2020.109797 22. Chanda A, Jones TG, Beschorner KE (2018) Generalizability of footwear traction performance across flooring and contaminant conditions 6(2):98–108. https://doi.org/10.1080/24725838. 2018.1517702 23. Gupta S, Chatterjee S, Malviya A, Chanda A (2023) Frictional assessment of low-cost shoes in worn conditions across workplaces. J Bio- Tribo-Corrosion 9(1):1–13. https://doi.org/10. 1007/S40735-023-00741-0 24. Verma SK et al (2011) A prospective study of floor surface, shoes, floor cleaning and slipping in US limited-service restaurant workers. Occup Environ Med 68(4):279–285. https://doi.org/ 10.1136/OEM.2010.056218 25. Hemler SL, Pliner EM, Redfern MS, Haight JM, Beschorner KE (2021) Effects of natural shoe wear on traction performance: a longitudinal study. https://doi.org/10.1080/19424280.2021. 1994022 26. Beschorner KE, (Sophia) Li Y, Yamaguchi T, Ells W, Bowman R (2021) The future of footwear friction. Lecture notes in networks and systems, vol 223 LNNS, pp 841–855. https://doi.org/ 10.1007/978-3-030-74614-8_103 27. Yamaguchi T, Katsurashima Y, Hokkirigawa K (2017) Effect of rubber block height and orientation on the coefficients of friction against smooth steel surface lubricated with glycerol solution. Tribol Int 110:96–102. https://doi.org/10.1016/J.TRIBOINT.2017.02.015

Chapter 13

New Developments and Challenges in the Area of Slip Testers

13.1 Introduction Reduction of the COF between surfaces below a critical limit causes slipperiness [1– 5]. With respect to sports, over 10 million lower extremity injuries during running and different athletic moments are reported every year in the US only. Out of such injuries, approximately 30% are related to slipping and ankle twisting due to inappropriate shoe-floor traction. Additionally, in sports, footwear traction has been observed to significantly affect the performance of athletic movements such as sprinting, cutting, stopping and jumping [6, 7]. In literature, footwear traction testing methods [8] have been developed to characterize the traction (both translational and rotational) of footwear, and traction testing standards have been implemented to ensure footwear safety and performance [9– 11]. However, till date, there has been a major gap in the literature between the foot loading conditions observed in a gait activity, and the loading conditions employed in footwear traction testing [12, 13]. This inhibits the development of reliable footwear traction testing methods and standards. This work reviews the foot loading conditions (Fig. 13.1) during walking, running and various athletic movements in major sports including soccer, American football, basketball, tennis and badminton. Specifically, the ground reaction forces, loading rates and shoe-floor angles reported in various studies have been investigated during walking and slipping during natural gait [14–16]. Also, these loading characteristics have been reviewed across a range of running speeds, and during movements such as braking, cutting, jumping and landing specific to different sport conditions. In some cases, the studies investigating the variability in loading due to differences in surfaces and footwear have been added to include all possible loading condition information. The study of foot loading is followed by an up-to-date review of the whole-shoe traction testing devices employed in literature for the characterization of the shoe-floor traction and the loading conditions considered for each instrument. Finally, a discussion is presented on the comparison of the loading conditions observed in natural or

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024 A. Chanda et al., Footwear Traction, Biomedical Materials for Multi-functional Applications, https://doi.org/10.1007/978-981-99-7823-6_13

127

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13 New Developments and Challenges in the Area of Slip Testers

Fig. 13.1 Different forces at the point of footwear contact during athletic activities

athletic gait activities and those employed in the traction testing devices. The evaluation of current traction testing methods will inform the latest improvements in the traction-performance testing of both work and athletic footwear.

13.2 Foot Loading in Sports 13.2.1 Running Running shoe loading has been a topic of interest for researchers for decades. Several studies exist that have measured the ground reaction forces during running at different speeds. Hamill et al. [17] studied the changes in peak ground reaction force in 10 skilled distance runners during running at different running speeds (4–7 m/s). The peak vertical ground reaction force changed from approximately 2.1 BW to 3.3 BW, going from 4 to 7 m/s running speeds. Miller et al. [18] in 1978 studied shoe-loading conditions during slow jogging (1.5 m/s). 25 ms after ground impact, an estimated 2.1 BW of vertical force was recorded. About 80 ms after contact, the vertical force reached 2.5 BW and then decreased gradually for the rest of the support phase. The A-P force peak of about 0.4 BW occurred during braking. A-P force pattern with a peak of around 0.4 BW was observed during the propelling phase. Munro et al. [19] collected GRF data from 20 adult males running at 3–5 m/s. The vertical loading rate increased from 77.2 BW/s at 3 m/s running speed to 113.0 BW/s at 5 m/ s running speed. With increased speed from 3 to 5 m/s, the average vertical GRF changed from 1.4 to 1.7 BW. The anterior–posterior GRF went from 0.15 to 0.25 BW (during both braking and propulsion), to increase running speed from 3 to 5 m/ s, respectively. Breine et al. [20] later studied the foot strike angles for 52 runners running at approximately 3 m/s. A typical rear-foot strike exhibited a shoe-floor angle of 20.4o ± 4.8o, while a mid-foot strike had a significantly smaller shoe-floor

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angle of 1.6o ± 3.1o. Nilsson et al. [21] investigated the shoe loading during running (1.5–6.0 m/s) in 12 male subjects (6 rear-foot and 6 fore-foot strikers). The subjects were selected with respect to foot strike pattern during running. In running, the vertical impact peak for rear-foot strikers increased linearly from 1.2 BW at 1.5 m/ s to approximately 2.6 BW at 6.0 m/s. For fore-foot strikers, the peak vertical force slowly increased from 2.0 to 2.9 BW in a nonlinear pattern going from 1.5 to 6 m/s running speed. Keller et al. [22] in 1996 studied the relationship between running speed, style, shoe loading and gender. 13 male recreational athletes and 10 female athletes had their force–time histories captured while running at speeds between 1.5 and 6.0 m/s. Most of the individuals were rear-foot strikers at speeds less than 5 m/s, according to analysis of the foot striking. Forefoot and mid-foot impacts were more frequent at speeds greater than 3 m/s. Loading rate increased linearly with speed (1.5–5 m/s), going from 8.2 BW/s to 122.8 BW/s for males and 7.7 to 122.3 BW/s for females. For high-speed running (>5 m/s) recorded mainly in male subjects, the loading rates went as high as 158.5 BW/s. Tillman et al. [23] in 2002 conducted in-shoe plantar measurements in eleven male runners during running on four different surfaces (asphalt, concrete, grass and a synthetic track). Data collection was performed using a plantar pressure measurement system on actual running surfaces outside the lab. The average running velocity was 3.8 m/s, and the vertical reaction force was estimated in the range of 1.38–1.46 BW with a contact time in the range of 213–227 ms. It was concluded from the study that no significant differences existed on under-shoe loading conditions while running across the different surfaces tested. Keshvari et al. [24, 25] recently studied shoe-surface interaction in trail running with 14 trail runners. Trails with distinct surface conditions (gravel, mud, natural grass) and different path shapes were used in the experiment. The maximum vertical ground reaction force was measured using insole pressure sensors to be 1495 N (3.1 BW).

13.2.2 Football and Soccer One of the earliest studies, in 1988, measured the six components of ground reaction forces and moments for eight recreational and former college soccer players during straight-line running and two different lateral movements. The testing was conducted on artificial turf fitted with a force measuring platform. The two lateral movements investigated were a rapid stop with the right foot followed by a 90° cut to the left. A rapid stop with the right foot followed by a 180° pivot medially and a push-off in the opposite direction to the original run-up. The average running speed during the approach towards the force plate was 4.3 m/s. Mean ground reaction force during braking while running, cutting and pivoting was recorded to be 2.46 BW, 3.45 BW and 3.02 BW, respectively. The anterior–posterior (A-P) forces measured during these movements were 0.66 BW, 1.85 BW and 1.60 BW, respectively. Propulsion while running and cutting resulted in average ground reaction forces of 2.84 BW and 1.82 BW, respectively, in the vertical direction and 0.34 BW in the A-P direction.

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The peak moment resisting rotation was measured to be 48 Nm during pivoting movements. The ground reaction forces in 40 elite male and 15 elite female soccer players were previously studied. During a normal strike, the measurements were conducted while the soccer players were running at 2.7 m/s over a force platform covered with a natural grass. The results revealed that during the normal strike, the first reaction peak was high in males (1.48 BW) compared to in females (1.33 BW). The second peak was lower than the first peak. Ford et al. [26] examined the effect of differences in two surfaces (natural grass versus synthetic turf) on in-shoe foot loading patterns during cutting movement. Seventeen male football players were tested with an in-shoe pressure distribution measurement insole in the right shoe (14 stud, moulded cleat) of each athlete. The findings showed that the peak pressures inside the central forefoot were substantially greater in the turf condition (turf: 646.6 172.6 kPa, vs. grass: 533.3 143.4 kPa). The peak pressures on the toes were generally lower, and turf had greater peak pressures (429.3 200.9 kPa) than grass (348.1 119.0 kPa). Kent et al. [27] studied the loading conditions on American football cleats on natural grass and infill-type artificial playing surfaces with 19 elite athletes. Variations of 2.8–4.2 kN (4.0–6.0 BW) were observed in the peak forces and variations from 120 to 174 N.m were observed in torques across cleats when tested on natural grass in translation and in rotation, compared to the synthetic turf (greater than 4.8 kN (6.8 BW) in translation and 200 N.m in rotation).

13.2.3 Basketball Basketball is associated with numerous injuries, mainly at the lower extremity [28, 29]. For the first time, Valiant et al. [30], in 1985, investigated the force and CoP patterns underneath the foot during the landing phase of a vertical jump typical of a basketball landing. Eight of the 10 participants in this study’s CoP patterns showed that the toe and forefoot region made first contact, producing a peak ground response force of about 1.5 BW. The centre of pressure moved to a location right beneath the calcaneus during the second phase of these forefoot landers’ landing pattern, accompanied by concomitant ground reaction forces of over 4 BW. Two participants had ground response forces of over 6 BW and CoP patterns that were almost totally restricted to the mid-foot area. Nyland et al. [31] studied the effect of fatigue on ground reaction forces in basketball. Nineteen females (Division 1 collegiate basketball and volleyball players) were recruited for this investigation. Run and rapid stop movements were performed, and forces were recorded using force plates. The mean time of fatigue was estimated to be 14.59 ± 3.6 min with an upper and lower limit of 11.3 and 23 min, respectively. For a mean approach velocity of 4.15 m/s in an unfatigued running drill, the peak vertical ground reaction force was recorded as 2.35 BW, which changed to 2.97 BW during the fatigued running drill at 3.92 m/s. The peak braking force recorded for sudden stop during unfatigued and fatigued run were quantified to be 1.35 BW and

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1.57 BW, respectively. The average shoe-floor angles at strike were 51° and 52° for unfatigued and fatigued (both run and stop), respectively. The rate of shoe-floor angle change to 0° was estimated to be 160°/s and 74°/s, respectively, for stopping after unfatigued and fatigued run, respectively. Struzik et al. [32] studied the loading conditions in a jump shot, the most essential technique in basketball. The study was conducted on 20 s-league basketball players. The loading conditions were quantified using force plate and motion capture data. For jump shots without arm swing, the mean take-off time, peak ground reaction force in take-off phase and jump height were estimated to be 0.18 ± 0.03 s, 5.39 ± 1.3 BW and 0.365 ± 0.06 m, respectively. For jump shots, with arm swing, the mean take-off time, peak ground reaction force in the take-off phase and jump height were estimated to be 0.22 ± 0.04 s, 5.57 ± 1.22 BW and 0.368 ± 0.045 m, respectively. Tobalina et al. [33] studied the effect of basketball footwear on the vertical ground reaction force during the landing phase of drop jumps. Thirteen students performed three drop landings from 30 and 60 cm high with basketball footwear or with running footwear. Two forces, namely F1 and F2, representing the maximum magnitude of the vertical ground reaction force during the landing of the fore-foot and rear-foot, respectively. For jump from 30 cm height, the mean F1 and F2 were 2.27 BW and 6.20 BW, respectively, for basketball footwear. In the case of running footwear, the values were 2.49 and 5.72 BW. Significant differences in the loading rates were observed at the fore-foot for basketball footwear (166.3 BW/s) compared to running footwear (198.8 BW/s). The loading rates were comparatively closer for the rearfoot with basketball (93.2 BW/s) and running (105.5 BW/s) footwear. For jump from 60 cm height, F1 at fore-foot was almost double for basketball footwear (4.65 BW) that observed at 30 cm jump (2.27 BW), and also significantly higher for running footwear (4.18 BW) compared to 30 cm jump (2.49 BW). F2 at rear-foot was significantly higher for basketball footwear (9.34 BW) compared to running footwear (8.27 BW). The loading rates in the 60 cm jump were very similar to in the case of the 30 cm jump. Statistically, F2 was concluded to be the only distinctive parameter that was affected by height and shoe conditions, with lower values for non-basketball footwear.

13.2.4 Tennis and Badminton Gheluwe et al. [34] in 1986, for the first time, studied the overall force action of the human body during the tennis strokes, namely, the service, forehand and volley strokes. The ground reaction forces were recorded in all three directions as three players performed the tennis strokes while they were on a force plate. Overall, the ground reaction forces were low, with the upward thrust recorded as the strongest, exhibiting less than 0.3 BW. The patterns of the forces exerted during the execution of a service, forehand and volley were similar for each player and for each type of racket used. During service, the forward thrust (in the direction of the ball) was low initially, and the strongest action occurred in the vertical direction at the end of the

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service rising to 0.27 BW. In the forehand stroke, the highest forces were measured in the vertical direction at the instant of impact (like in the case of service stroke) with force values reaching to 0.3 BW. Compared to the service and forehand strokes, the force patterns in this study during the volley stroke exhibited wide variations with a peak of 0.3 BW. Stiles et al. [35] in 2006 studied the effect of artificial surfaces on under-shoe loading in tennis and their possible implications in overuse injury. Six subjects performed eight running forehand trials on four different surfaces, namely, sandfilled artificial turf, cushioned acrylic hardcourt, carpet and a concrete baseline representing high, moderate, low and zero cushioning, respectively. The running forehand movement for right-handed player involved an outstretched left leg followed by a foot plant made at the end of a dash to the ball, which occurs at the same time as ball contact was made with the racket held in the right hand. The typical ground reaction force exhibited three peaks prior to foot-off peak. The peak ground reaction force for baseline (2.59 BW) was significantly lower than the carpet (2.93 BW), acrylic (2.86 BW), and artificial turf (2.88 BW). The highest loading rate was observed for the artificial turf (507.05 BW/s), which was significantly higher than the baseline (360.89 BW/s). The peak anterior–posterior forces recorded while braking was significantly higher for the baseline (1.05 BW) compared to carpet (0.79 BW), artificial turf (0.78) and acrylic (0.70 BW). The initial shoe-floor angles were similar with the highest for acrylic (39.56o) and lowest for artificial turf (35.57o). Kuntze et al. [36] studied ground reaction forces in racquet sports for the lateral sidestepping (SS) and crossover stepping (XS) movements. Nine experienced male badminton players performing lateral SS, XS and forward running tasks at a controlled speed of 3 m/s. The highest vertical force recorded in SS was 2.19 BW, and in XS was 1.92 BW, which were less than in running (2.5 BW). The highest horizontal braking and push-off forces recorded were 0.44 and 0.56 BW, respectively. It was concluded from the study that the ground reaction forces do not lead to high forces that could cause injuries. Table 13.1 summarizes the foot loading conditions across each activity.

13.3 Traction Testing Methods Employed in Sports and Athletic Movements Traction testing methods in sports were developed historically for measurement of rotational traction (Fig. 13.2), to assess ankle and knee injury risks [37, 38]. Examining how American-style football cleats interact with the surface being played on has been one of the main areas of attention [39–41]. Translational tests have been mainly conducted within a laboratory setting using a small section of turf attached to the testing apparatus (due to the lack of machinery) which can generate forces in high magnitude for applying realistic normal loading similar to an athlete, and for displacing cleats on turf like surfaces. One of the earliest athletic traction testing

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Table 13.1 Under-shoe biomechanical information Type of gait

Biomechanical parameters affecting footwear traction Normal force (BW)

Sliding speed (m/s)

Shoe-floor angle (Degrees)

Walking

0.74

1.87 m/s

11.3o

Walking on ice

0.75

0.45 m/s

17.2o

Slow running (1.5 m/ 2.50 s)

–

–

Mid-speed running (3 m/s)

1.40

–

20.4o

Fast running (5 m/s)

1.70

–

21.3o

Very fast running (7 m/s)

3.30

14.4o

Normal force (BW)

Sliding speed (m/s)

Shoe-floor angle (Degrees)

Moment (Nm)

Football (Braking)

0.60–2.46

–

–

120–174

Football (Cutting)

3.45

–

–

48

Football (Pivoting)

3.02

–

–

200

–

–

– –

Football (Propulsion) 0.37–2.84 Football (Kicking)

1.08–1.29

–

45o

Basketball (Cutting)

3.00

–

20o

–

Basketball (Layup-take-off)

2.70

–

–

–

Basketball (Layup-land)

8.90

–

–

-

Basketball (Starting)

0.80

–

–

–

Basketball (Stopping)

1.35–2.70

–

52o

–

Basketball (Jump-take-off)

3.00

–

–

44–50

Basketball (Jump-land)

2.27–6.00

–

–

44–50

Basketball (Shuffling)

2.60

–

–

–

Tennis (Service)

0.27

–

–

–

Tennis (Strike)

0.30

–

–

–

Tennis (Volley)

0.30

–

–

–

Tennis (Serve-stance)

1.50–2.10

–

–

–

Tennis (Serve-propulsion)

0.16–0.20

–

35°–40o

– (continued)

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Table 13.1 (continued) Type of gait

Biomechanical parameters affecting footwear traction

Tennis (Serve-braking)

0.12–0.19

–

–

–

Tennis (Lateral sidestepping)

2.19

–

–

–

Tennis (Crossover Stepping)

1.92

–

–

–

Tennis (Lunge)

2.99–3.23

–

36°–38o

248–288

Fig. 13.2 Traction testing schematic for employed in sports and athletic movements

device, and the most widely adopted for measuring rotational traction was the Studded Boot Apparatus (SBA). The SBA consists of a base plate with six football studs in a circular configuration, a vertical shaft onto which weights are added, and a handle and torque wrench that is used to quantify the peak rotational traction up to 80 Nm. The TrakTester [42] was built to test soccer boots under high-risk loading conditions. The shoe was placed on an artificial leg with provision for adjusting shoe-floor angles. A pneumatic cylinder with 4700 N (6.7 BW) maximum force capacity applied loads along the axis of the shaft. Another pneumatic device was used to apply horizontal loads to initiate sliding of the boots on turf-based surfaces, and to measure translational traction. Up to a maximum torque of 219 Nm was applied on the shoe for rotational traction measurements. Zeller et al. [43] in 2008 developed an automated traction tester for high accuracy and repeatability in measurements, and operator safety. This device mechanized the dropping and twisting of the weighted studded disc (up to 40 kg). A peak rotational torque of 100 Nm can be applied for rotational traction measurements. The Boise State TurfBuster [44] was developed in 2009 for testing both translational and rotational traction for American-style football cleats and the playing surfaces. A dynamic motion assembly maintained all predefined shoe positions and guided the linear and rotational motion of the shoe throughout any series of tests.

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135

The shoe orientation could be adjusted in 15° increments from an external rotation of 90° to an internal rotation of 90°, to mimic motions such as a side cut. Horizontal and vertical motions of the shoe were invoked with pneumatic cylinders. Cleated athletic shoe translational traction testing was performed at normal loads ranging from 222 N (0.3 BW) to 1776 N (2.53 BW), with a horizontal velocity up to 10 cm/s, and 20 cm maximum displacement. Buckling and potential permanent damage to the turf surface were observed at loads greater than 1776 N (2.53 BW). The torque application capacity of this device was in the range of 0–350 Nm for rotational traction measurements. Clarke et al. [45] in 2013 constructed a laboratory-based testing device at the University of Sheffield to measure the translational traction of tennis shoes on dry acrylic hard court (AHC) and artificial clay court tennis surfaces in dry and wet conditions. Pneumatic rams were used to apply loads up to 1000 N (1.42 BW). Loading conditions during jumping have been simulated with an initial shoe-floor angle of 7°, and translational traction measurements between 0.05 and 0.2 m distance after the initiation of sliding. Another test rig was developed to study the translational and rotational traction of tennis shoes. A court surface was made with Decoturf (similar to US open). All translational tests were conducted with the shoe aligned parallel to the horizontal. The shoes were loaded vertically in the range of 0–2200N (0–3.14 BW) and horizontally to initiate sliding. For the rotational tests, toques were applied in the range of 0–180 Nm along the axis of rotation located directly under the fore-foot section of the shoe. Ura et al. [46] recently developed a portable device to measure translational traction in tennis shoes in realistic playing conditions. During operation, a test shoe slider was attached onto a sled with several weights mounted on top of the sled, to apply normal loads in the range 100–400 N (0.14–0.57 BW). The shoe was oriented a 50º angle from the horizontal. An increasing horizontal force was applied using pneumatic cylinders until sliding of the shoe test slider was initiated. Table 13.2 represents the athletic traction testers and corresponding loading conditions.

13.4 Challenges in Simulating Foot Loading in Traction Tests On comparing the shoe-loading conditions encountered in different gait activities and the ones simulated during footwear evaluation, several challenges were recognized. With respect to natural gait, the maximum normal forces, sliding speed at slip start, peak sliding speed and shoe-floor angles encountered are 0.74 BW, 0.1 m/ s, 1.87 m/s and 11.3o at peak sliding speed, respectively. Out of the whole shoe traction testers, only PSRT is not able to simulate the maximum normal forces. In terms of sliding speeds, all devices can generate sliding speeds at the slip start (0.1 m/s). However, none of the devices can simulate peak sliding speed of 1.87 m/ s. The shoe-floor angles adopted during testing are attainable in most devices except

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Table 13.2 List of athletic traction testers and corresponding loading conditions Device name

Under-shoe loading

Sport shoes studied

Studded Boot Apparatus (SBA)

Normal force: 0–360 N (0–0.5 BW) Rotational torque: 0–80 Nm

Football/Soccer (rotational traction)

McNitt’s “Pennfoot”

Normal force: 0–1136 N (0–1.6 BW) Rotational torque: 0–32.2 Nm

Football/Soccer (rotational/ translational traction)

TrakTester

Normal force: 0–4700 N (0–6.7 BW) Rotational torque: 0–219 Nm

Football/Soccer (rotational/ translational traction)

Automated Traction Tester

Normal force: 0–392 N (0–0.56 BW) Rotational torque: 0–100 Nm

Football/Soccer (rotational traction)

Boise State TurfBuster

Normal force: 222–1776 N (0.30–2.53 BW) Rotational torque: 0–350 Nm

Football/Soccer (rotational/ translational traction)

University of Sheffield Traction Testing Device

Normal force: 0–1000 N (0–1.42 Tennis (translational traction) BW)

Traction Tester-Tennis Warehouse

Normal force: 0–2200 N (0–3.14 Tennis (rotational/translational BW) traction) Rotational torque: 0–180 Nm

Portable Tennis Traction Tester

Normal force: 100–400 N (0.14–0.57 BW)

Tennis (translational traction)

Stevenson devices. However, the typical angles considered in four out of the seven traction testers reviewed is 5º, which is significantly lower than the shoe-floor angles observed during an actual slip (22.1º at slip start, and 11.3º at peak sliding speed). Additionally, among the traction testers, only the Slip Simulator and the Portable Slip Simulator measure COF within the first 250 ms after heel strike, while other testers record COF much later after heel strike. A few implications of these differences in loading conditions are described in [47], which includes the dependency of the COF estimation on the traction testing device, and limited understanding of critical parameters during slipping with different shoe-floor combinations. During running, a wide range of shoe-loading conditions have been observed, which are influenced by the running speed. Specifically, normal forces have been observed to vary from 1.2 to 3.3 BW going from a speed of 1.5 to 7 m/s. To date, among the athletic traction testers, only the TrakTester is able to simulate such low and high loads. The current ASTM standard for athletic shoe traction testing in running recommends 800 N (1.1 BW) normal loading, which is lower than the lowest normal forces studied in running. With respect to loading rates, the current standard does not have any protocol. A wide range of shoe-loading rates were reviewed in this work, going from 7.7 BW/s at 1.5 m/s running speed to 158 BW/s at 7 m/s running speed. Also, the current standard does not specify any shoe-floor loading angle during running. However, in this study, we observed the shoe-floor angles to vary widely

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137

(in the range of 1.6º–21.3º) with the foot strike pattern (which was observed to be influenced by the running speed). The implications of the differences in the normal forces, loading rates and shoe-floor angles observed in running, from the current traction testing standard, on running safety and performance are not very clearly known and needs further investigation. In football and soccer, the highest normal forces were measured during cutting movement (3.45 BW). None of the reviewed athletic traction testers except the TrakTester can simulate such high normal forces. Also, with change of surface from natural grass to artificial turf, normal loading over 6.8 BW was recorded. The current ASTM recommendation for the maximum loading of athletic shoe in football is 3000 N (4.3 BW). Thus, high loads observed in artificial turf are not possible to be simulated as per the current traction testing standards. Also, loading rates in the range of 20.0–27.2 BW/s were measured in soccer and football shoes, which are not specified in the current standard for athletic shoe testing, and may have implications in traction testing. With respect to rotational traction, moment loading over 200 Nm have been measured on artificial turf. Among the reviewed athletic traction testers, the TrakTester and Boise State TurfBuster are only able to measure such high peak moments. The ASTM traction testing standards recommend very low testing torques in the range of 20–60 Nm. Football and soccer being sports with high incidences of joint loading injuries, the high loads and torques observed while playing on artificial turf needs to be investigated further. In the game of basketball, normal forces as high as 9.34 BW have been recorded while jumping and landing (8.9 BW). Such high normal forces cannot be simulated by any of the existing athletic traction testers. Also, such normal forces are significantly high compared to the maximum athletic shoe-loading recommendation as per the current ASTM standards. Also, in basketball, fore-foot loading rates as high as 198 BW/s have been measured and shoe floor angles of 51°–52° have been observed during running and stopping. Such traction testing parameters are currently not specified in the ASTM athletic traction testing standards or adopted in testing on any device. This review also studied the ground reaction forces in unfatigued and fatigued runners, revealing higher under-shoe normal loading in fatigued runners. This is a new finding, which may have implication on the game performance. The shoe loading observed in tennis is low in most of the athletic movements, except in lateral side stepping (2.19 BW), crossover stepping (1.92 BW) and in lunges (2.99–3.23 BW). Also, across different surfaces, the normal forces were in the range of 2.59–2.93 BW. However, only the TrakTester is able to simulate such loads. The current standard for athletic shoe testing specifies a shoe loading of 2200 N (3.14 BW), which mostly covers the range of forces observed in an actual game. However, extremely high loading rates going up to 853.3 BW/s were recorded, which is not specified in the current shoe testing standard. Also, shoe-floor angles of 35.57°– 39.57° were recorded, which are not mentioned in the current testing standard. The highest rotational torque recorded (288 Nm) is significantly higher than the maximum of 60 Nm specified in athletic shoe testing standard.

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13.5 Conclusion This study reviews the up-to-date foot loading conditions measured in natural gait, running and in athletic movements in major sports including soccer, football, basketball, tennis and badminton. Understanding these loading conditions is critical for appropriately evaluating the safety and traction performance of footwear worn during these gait activities. A detailed review was conducted on the shoe-loading conditions adopted in various traction testing methods, which are used for evaluation of occupational and athletic footwear performance. In summary, this study indicated good agreement between the peak normal forces studied in natural gait and the ones simulated by the current traction testing methods. However, the peak sliding speeds and shoe-floor angles exhibit differences, which are attainable with the current traction testing devices, but need further investigation on their influence on traction measurements across shoes. With respect to the sports and athletic movements studied, only one out of the eight traction testers reviewed was able to simulate the peak normal forces. The rotational loads recommended in the current athletic shoe traction testing standards are significantly lower than that observed in sports. Also, the current test standards merely specify forefoot loading for testing of shoes in all sports, with no shoe-floor angle specification (which were observed to vary widely across sports). Additionally, no loading rates or the biomechanics observed in different shoe movements in sports were given any consideration in the current testing standards. All such areas need further investigation to better inform shoe safety and performance testing.

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