FORCE AND MOMENT MEASUREMENTS DURING ALPINE SKIING DEPENDING ON HEIGHT POSITION

Abstract

INTRODUCTION: When a ski is set on edge a lever arm is produced by the force FS, which is applied through the skier’s leg and boot midline, and the ground reaction force FR, which acts on the ski edge. A moment is necessary to keep the ski in its position (see figure and compare with Lind). It is hypothesized that the magnitude of this moment is mainly determined by the width-height proportion of ski and binding. In order to adjust this moment, the skier has to rotate his knee inwards or angle his hip in the lateral direction (Lind, Howe). The objective of this study was to clarify whether the height of the binding plate has any influence on the generated moment. METHODS: A professional ski racer (A-Kader DSV) descended a giant slalom course (at 25° steepness) nine times consecutively. For every run the equipment was identical (skis: ATOMIC ARC RS, binding: ESS 10.28) except for the adjusted height of the binding plate. Three different height positions were used. System A was comprised without a plate between ski and binding, system B with a plate of 1 cm height and system C with a plate of 2 cm height. Using a previously described measuring boot (Wimmer), the ground reaction forces were determined at four distinct locations underneath the boot soles. The specific set-up of the force sensors (two at every edge of the skis) allowed us to calculate the generated moments by known lever-arms. RESULTS: Out of nine runs, seven runs differed in elapsed time by less than 0.3 sec, and the average duration was 20 sec. For this reason a good comparability can be derived. The three fastest runs were made with the 2 cm binding plate, the three slowest runs without the binding plate. The magnitude of ground reaction force ranged from 2000 to 3500 N. The calculated moment was approx. 40-70 Nm and was independent of height position at all turns. Indeed, the moment variation was more affected by the specific turns of the course than by height position. However, the force readings during turns (and thus the moments) were smoother when a higher plate was used. CONCLUSIONS: Because no moment differences could be assigned to the different height positions, the varying width-height-proportions of the three systems may have resulted in three different edging angles. For system C a smaller edging angle would be necessary than for system B, whereas system A would need the largest edging angle. This might be important for the skier, since a smaller body angle in the lateral direction would be necessary to maintain equilibrium using a binding plate.