Static friction of outdoor shoe soles on common alpine terrain surfaces and its association with microroughness, hardness and contact pressure

Abstract To avoid potentially dangerous slips during alpine activities like hiking, soles of outdoor shoes are supposed to provide grip on natural outdoor surfaces with varying characteristics. The aim of the study was to investigate how six commercially available soles perform on three different alpine surfaces and how parameters that are known to be linked to friction outcomes are associated with the static coefficient of friction ( µ s ). In addition, the effect of different gait phases from heel-strike to toe-off phase were considered. The µ s of outdoor shoe soles were determined at constant temperature in a linear tribometer on ice, snow and granite. The parameters hardness, contact pressure and roughness of the soles were determined to explore their potential relationships with µ s outcomes. To simulate the different phases of human gait, six angles from 10° heel-strike, stance-phase and 5°, 11°, 22°, 28° toe-off phase were analyzed. The tested soles performed significantly different on ice and granite, but not on snow, which was expected due to known hysteresis and adhesion mechanisms. All tested parameters appeared to be associated with friction, showing relationships. However, the associations shown in the correlation coefficient varied between the tested surfaces. Moreover, interaction effects of soles and angles could be observed. The measured angles indicated an alteration of friction performance, with respect to the phase of gait. In dependence of the intended purpose of a shoe sole, material characteristics and geometry should be considered when manufacturing outdoor soles to increase the friction on different surfaces.