Measuring the Slip Resistance of Winter Footwear by Jennifer Anna Hsu A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Department of Mechanical and Industrial Engineering Institute of Biomaterials and Biomedical Engineering University of Toronto © Copyright by Jennifer Anna Hsu 2015 Measuring the Slip Resistance of Winter Footwear Jennifer Anna Hsu Doctor of Philosophy Department of Mechanical and Industrial Engineering Institute of Biomaterials and Biomedical Engineering University of Toronto 2015 Abstract Slip and fall accidents represent an enormous burden in terms of human suffering and economic costs. In winter conditions, outdoor falls increase as cold temperatures and precipitation create hazardous conditions underfoot. An effective method of reducing falls is the use of slip resistant footwear. However, the optimization of this footwear is hindered by the fact that slip resistance is not easily measured and the validity of existing measurement devices is questionable. The series of four studies presented here show the progression of work utilizing human-centred approaches to evaluating the performance of winter footwear on real winter surfaces and have led to the development of a new test method. To start, traditional measures of slip severity were used to evaluate the effectiveness of winter boots and cleated footwear devices while subjects walked in common winter conditions. Walking trials on level and uneven icy surfaces showed that while cleated footwear did reduce slip frequency, they did not prevent dangerous slips from occurring. Further testing showed that the current standard test conditions and parameters used for determining footwear slip resistance do not adequately simulate gait in winter conditions. As a result, a new user- centred protocol was developed for testing footwear. Subjects were asked to stand and walk ii across a wet, icy surface that was tilted progressively until they could no longer maintain balance, thereby establishing the maximum achievable incline angle for a variety of styles of footwear. This method proved to be reliable on wet ice so a refined protocol was applied to a larger sample of footwear which was then tested on dry ice as well as snow conditions. The results showed the importance of testing winter footwear in snow conditions, which had not been done before. These studies demonstrate the viability of testing footwear without removing the critically important human component. iii Table of Contents List of Tables .................................................................................................................... v List of Figures ................................................................................................................... vi List of Acronyms .............................................................................................................. viii 1 An Introduction to Slips and Falls ............................................................................. 1 1.1 Slips and Falls ...................................................................................................... 1 1.2 Slip Biomechanics ................................................................................................ 10 1.3 Measuring Slip Resistance ................................................................................... 20 1.4 Addressing the Gaps ............................................................................................. 44 2 Footwear and Anti-Slip Device Testing in Simulated Winter Conditions .................. 60 2.1 Introduction .......................................................................................................... 61 2.2 Methods ................................................................................................................ 64 2.3 Results .................................................................................................................. 71 2.4 Discussion ............................................................................................................ 81 2.5 Conclusion ........................................................................................................... 85 3 Comparison of Footwear Slip Resistance Measurement Parameters to Gait on Icy Surfaces ........................................................................................................................ 90 3.1 Introduction .......................................................................................................... 91 3.2 Subjects and Methods ........................................................................................... 93 3.3 Results .................................................................................................................. 99 3.4 Discussion ............................................................................................................ 105 3.5 Conclusion ............................................................................................................ 111 4 Assessing the Performance of Winter Footwear Using a New Maximum Achievable Incline Method ............................................................................................................. 114 4.1 Introduction .......................................................................................................... 115 4.2 Methods ................................................................................................................ 118 4.3 Results .................................................................................................................. 126 4.4 Discussion ............................................................................................................ 132 4.5 Conclusion ............................................................................................................ 139 5 Slip Resistance of Winter Footwear on Snow and Ice Measured Using Maximum Achievable Incline ....................................................................................................... 145 5.1 Introduction .......................................................................................................... 146 5.2 Methods ................................................................................................................ 148 5.3 Results .................................................................................................................. 155 5.4 Discussion ............................................................................................................ 161 5.5 Conclusion ............................................................................................................ 166 6 Conclusions .................................................................................................................. 169 6.1 Overall Discussion ............................................................................................... 169 6.2 Advantages and Limitations ................................................................................. 178 6.3 Future Works ........................................................................................................ 181 6.4 Significance .......................................................................................................... 185 Appendix ............................................................................................................................ 188 iv List of Tables 1.1. Summary of coefficient of friction terms and definitions. .......................................... 25 1.2. Reduction in slip probability with increased ACOF ................................................... 26 1.3. Classification of DCOF values obtained using a step simulator ................................. 37 2.1. Slip index readings from the English XL Sliptester ................................................... 68 2.2. Participant information ............................................................................................... 71 2.3. Responses to questionnaires ....................................................................................... 71 2.4. Pearson correlation coefficients for bivariate correlation analysis of total slip distance and foot angle ............................................................................................... 78 3.1. Frequency of forward accelerating slips out of total number of steps and associated slip distances by test condition ................................................................................... 100 4.1. SATRA COF values on wetted ice ............................................................................. 129 4.2. Mean and standard deviation of change in gait speed per degree of incline .............. 130 5.1. Estimated means of gait kinematic data at each level of the main effects of slope type, footwear, and surface ......................................................................................... 157 v List of Figures 1.1. Typical event sequence for slips and falls ................................................................... 2 1.2. Features of a gait cycle ................................................................................................ 11 1.3. Ground reaction forces during normal walking from heel strike to toe-off ................ 12 1.4. Kinematic and kinetic parameters of a heel slip .........................................................