Frictional requirements of skateboarding shoes during a push-off

Fifteen male amateur skateboarders were recruited to participate in the study. Nine of the subjects were regular footed and the other six were "goofy" footed (right foot forward). Subjects were required to wear the same model laboratory skate shoe (éS EK-01) to ensure the geometrical construction of the shoe outsole did not provide a bias to the ground reaction force data. A large AMTI force platform (Model: BP12001200) was used to collect 3-D GRF and moment data. The platform was placed in the middle of a long runway to ensure subjects had plenty of room to both start and stop their skateboards. In addition, RSscan in-shoe pressure insoles were inserted into the subject's shoes for the experiment. Testing began by having the subjects roll towards the force plate from a resting start position and push-off the plate at a preferred comfortable speed over two surface conditions. The first condition was the normal force platform surface which was shown to have similar frictional characteristics as skate park concrete in pilot testing. For the second condition a strip of skateboard griptape was applied to the force plate to provide a higher friction surface. Each subject performed push-offs on both surfaces and the starting order was randomized across all subjects. GRF and in-shoe pressure data were collected for ten push- offs within each condition. All GRF and moment data were filtered at 100 Hz using a 4th order low pass Butterworth filter. The coefficient of friction (COF) was defined as the ratio of the normal force (VGRF) to the horizontal force (APGRF) required to produce movement between two surfaces (Frederick, 1993). Means and standard deviations (SD) of the APGRF, VGRF, and COF values were calculated across all 10 trials for each subject during both conditions (griptape, no griptape) to yield representative data. To test for differences between the two conditions, a repeated measure one-way ANOVA was conducted on the 15 mean values for each GRF and COF parameter. Significance level was set at 0.05 for both tests and p-values were reported. Results The results of the force plate and in-shoe pressure data revealed three distinct skateboard push-off styles: heel-toe, mid-foot, and forefoot. Summary results of the COF, APGRF, and VGRF parameters evaluated across all subjects in this study are provided in Table 1. Five out of fifteen subjects exhibited a heel to toe push-off, which was similar to the GRF's exhibited in walking. Five out of fifteen subjects exhibited mid-foot to toe push-off. The remaining five subjects exhibited forefoot only push-offs which were similar to the GRF's seen in sprinting.