Introduction. These last years, the research on protective function of sports footwear has been largely focus on cushioning and overuse injuries prevention (Lake ...
Morio et al. Footwear Science, 7 Supp1 63-64
Static vs dynamic friction coefficients, which one to use in sports footwear research? Cédric Morio a*, Lise Sissler a, and Nils Guéguen a a Department of Movement Sciences, Decathlon Research, Villeneuve d’Ascq, France Introduction These last years, the research on protective function of sports footwear has been largely focus on cushioning and overuse injuries prevention (Lake, 2000, Potthast, 2011). However, lot of acute injuries might occur because of an uneven sliding surface and a loss of grip, or on the contrary a blocking footground contact that induced an increased strain in the ankle or the knee. Although grip and slipperiness were well studied in the field of occupational biomechanics and ergonomics (Grönqvist et al. 2001), limited number of studies could be found on such question in sports footwear (Barry and Milburn, 2013). Dynamic friction coefficient was usually correlated to the perception of slipperiness (Hanson et al. 1999), but these studies were performed on contaminated floor at low walking speed. Purpose of the study Therefore, the aim of the study was to define whether the static (SFC) or the dynamic friction coefficients (DFC) was the best one to report the grip perception in sports-like movements. To answer that question and confirm the consistency of the results, two experiments were completed with different populations and shoe-floor conditions. Methods For the first experiment (E1), fifteen female participants tested three different shoes, a walking shoe, a running shoe and a sailing shoe; in four floor conditions, smooth linoleum, wood, rubber and the naked force
plate made of stainless steel. For the second experiment (E2), eighteen female participants tested six similar shoes that differed only in the outsole material, with different rubber or EVA compounds; in two floor conditions, wood and steel. The participants performed a forwardbackward cutting maneuveron a forceplate measuring ground reaction forces at 2000 Hz. A ten points Likert scale was used to measure the grip perception of each shoe-floor combination. Ground reaction forces were filtered with a low-pass Butterworth critically damped filter with a cut-off frequency of 30 Hz. The FC was then calculated as the ratio of the shear forces divided by the normal force as follow: FC = √(FML2 + FAP2) / FVert (Figure 1).
Figure 1. Example of the friction coefficient for a typical forward cutting step. The SFC corresponded to the maximum friction in the early 50 ms of the stance phase of the cutting step. The DFC corresponded to the mean of the friction during the 100 ms in the mid-stance phase of the cutting step. These variables were highlighted with grey zones in the Figure 1.
Morio et al. Footwear Science, 7 Supp1 63-64
The relationships between grip perception and SFC or DFC were evaluated through linear regressions and Pearson correlation coefficients. Results The two experiments presented similar and complementary results. In the first experiment, the grip perception was similarly correlated to both DFC (r=.88; p
