Estimation of slipping risk during level walking

Abstract

Slipping is one of the major causes of falls in the elderly. Walking is regarded safe when the coefficient of friction (COF) of the surface is greater than the ratio of the horizontal and vertical force components applied to the ground, the required coefficient of friction (RCOF). At present, very little is known on the effect of foot-fall variability on the likelihood of slipping. This study, therefore, aims to define slipping risk during long term level walking. Foot-ground reaction forces (GRF) were recorded at 500 Hz using two force plates while four subjects (3 young and 1 older adult) walked continuously for ~30 minutes. The RCOF values for various walking trials were calculated from the recorded GRF data, and a custom-made software program was used to model the RCOF distributions and to calculate the probability of different RCOF values appearing during gait and the probability of slipping (PS) on an unseen surface hazard. The RCOFMean for the older adult (Right 0.38, Left 0.48) was lower than that of the young adults (0.58). For all the subjects, the PS graph indicated a reduction in PS with an increase in COF between the shoe and the walking surface. The results also show a large variability among the subjects as well as between the two limbs. For a COF hazard between shoe and walking surface of 0.3, the probability of slipping for the older adult was 50% and 55% for the left and right limbs respectively, whereas for the young subjects PS varied between 86% to 99%. The PS data also suggests a safer walking style adopted by the older adult, however, the high RCOFmax (1.47) displayed by the older adult could initiate a sudden slip in certain surfaces. Overall, this study shows a new model for the estimation of slipping propensity on walking surfaces with varying frictional properties with the aim of minimizing slip-induced falling during walking.