Course setting and skier mechanics in competitive alpine skiing.

The projects aim is to enrich the understanding of how course setting manipulation affects skier kinematic and kinetic characteristics and their significance for injury prevention.

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Time:

Place: Norges Idrettshøgskole

Formal title

Characterisation of Skiers' Mechanics, Course Setting and Terrain Geomorphology in World Cup Alpine Skiing using Global Navigation Satellite Systems

Description

The course setting and skier mechanics investigation is part of a FIS injury prevention project held by the Department of Sport Science and Kinesiology/USI, University Salzburg, Austria. In collaboration with the Ecole Polytechnique Fédérale (EPFL), Switzerland and the WSL Institute for Snow and Avalanche Research SLF, Switzerland the reasons for injuries in competitive alpine skiing to occur are elaborated from a biomechanical perspective. The Department of Physical Performance at NSSS is contributing to this multicentre project with the module course setting manipulation and its influence on skier mechanics. The skier kinetic and kinematic properties are captured both in World Cup race situations and experimental setups by means of Global Navigation Satellite Systems (GNSS)- and inertial technology.

Result

Since there was no existing appropriate methodology to capture skier mechanics across large capture volumes under WC racing conditions in speed disciplines, a novel method was invented. The method was based on a global navigation satellite system (GNSS) and was tailored to assess position, velocity, acceleration and external forces while causing minimal interference to the athlete. The method was assessed in a field study against an independent reference system and found valid to assess skier position, velocity, acceleration and external forces in WC alpine skiing. Furthermore, the position results of five different geodetic GNSS methods were compared to an independent reference system. The comparison revealed that differential GNSS applying GPS and GLONASS satellite systems and the satellite signal frequencies L1 and L2 was the only configuration which consistently yielded postion results that were accurate enough to capture alpine skiing under WC conditions. The motion capture method was applied in male WC alpine skiing competitions in the disciplines giant slalom (GS), Super-G (SG) and downhill (DH). Seven GS, five SG and five DH races were assessed in this study. The GNSS device was carried by one forerunner per race and collected data for entire runs. Prior to the races, course setting and terrain geomorphology were captured using static differential GNSS. The captured positions were used to compute digital terrain models (DTM) of the race courses including gate positions. The DTM, the skiers’ GNSS trajectory and the method developed in the first part of the study were used to compute the position, velocity, acceleration and external forces of the forerunner. The captured data from WC races were used to comprehensively and quantitatively characterize skier mechanics, course setting and terrain geomorphology for the disciplines GS, SG and DH in male WC alpine skiing.
The study revealed that variability in course setting was introduced by the horizontal gate distance and that the horizontal gate distance tended to decrease with decreasing terrain inclination in GS. Gates were set close to terrain transitions. Terrain was on average steepest in GS followed by SG and DH. Extreme terrain inclination changes along the skiers’ trajectory per unit time skiing were overrepresented in DH, while extreme changes per unit distance were overrepresented in GS. Mean speed was found to be 17.7 m/s in GS, 23.8 m/s in SG and 25.6 m/s in DH. Skiers skied straight (turn radius > 125m) for approximately 45% (DH), 20% (SG) and 7% (GS) of the time. The median ground reaction force was found to be 1.46 BW in GS, 1.42 BW in SG and 1.21 BW in DH. The median air drag force was 0.07 BW in GS, 0.09 BW in SG and 0.13 BW in DH. Ski–snow friction was the main contributor to energy dissipation in GS and SG, while in DH the contribution of air drag and ski–snow friction was approximately equal. The data on skier mechanics were used to assess if the differences in injuries per 1000 runs between disciplines could be explained by differences in skier mechanics between disciplines.

This investigation showed that WC alpine skiing is approximately equally dangerous, per unit of time, for all disciplines. In contrast, the skiers’ mechanical characteristics were significantly different between disciplines. Therefore, it is likely that the causes and mechanisms of injury are different for the specific disciplines. In SG and DH, injuries might be mainly related to higher speed and jumps, while injuries in the technical disciplines might be related to a combination of turn speed and turn radius resulting in high loads.