Comparison of the Turn Switch Time Points Measured by Portable Force Platforms and Pressure Insoles

Abstract

Several methods to determine turn switch points during alpine skiing using the vertical GRF exist in the literature. Although comparative studies between pressure insoles (PI) and force platforms (FP) have been conducted, there are no reports comparing the detected time points. Yet, these sensors and methods have been used interchangeably. This study aims to compare the turn switch time points with both sensors and various methods. Twenty skiers performed turns with FP and PI for two different ski styles (high and low dynamic turns). Three different assessment methodologies were compared: minima, functional minima, and crossings. Bland Altman and repeated measures ANOVA were used to assess statistical differences. Main effects of sensor and method were observed (p < 0.001). Although there was a low effect size ( ${\text{η}}_p^2$ = 0.013) between FP and PI, the 95% CI yielded values representing >30% of the turn duration. A large effect size (η² = 0.153) was found between the crossing method and the minima and functional minima methods. This indicates that those methods assess different events during the turn switch phase. In conclusion, the sensors and assessment methodologies compared in this study are not interchangeable with the possible exception of the minima and functional minima assessed with FP.

Keywords: GRF; event detection; force binding; pressure; sensor; ski.

Figure 1

Set up of the FP between the bindings and the skis.

Figure 2

Example of the signals during a turn: the right foot (dashed line), the left foot (dotted line), and the sum (solid line). In the upper graph GRF measured with the FP placed under the bindings. In the lower graph force calculated from the pressure values from the PI between the boot and the foot. The vertical lines represent the turn switch points assessed with the different methods: minima (dashed line), functional minima (solid line) and crossing (dotted line).

Figure 3

Example of the different moments in time detected by FP (in black) and PI (in gray) represented as the mean ± SD for the 3 different assessing methodologies at 6 Hz with all the turns pooled together.