INTRODUCTION: In alpine skiing, the determination of accurate orientations of a ski is the prerequisite for analyzing measurements of force plates fixed under the boots. Indeed, in order to decompose the forces encountered in a turn and to isolate the gravitational force, the orientation of the ski is required. Nowadays, the ski's orientation can be obtained from multi-camera Systems with a precision of five degrees for an object orientation (Richards 1999). However, this method requires an important infrastructure and is not adapted for everyday use and training purposes. The method described below proposes a new approach that is more accurate, faster, and easier to setup and is insensitive to the weather conditions. METHOD: The method relies on the trajectory determination provided by the Integration of low-cost GPS (Global Positioning System) and micro-electro-mechanical System (MEMS) inertial measurement units (IMU) (Waegli and Skaloud 2QQ7). As the skier's environment quickly alternates between open spaces and areas that block satellite Signals, the computation of GPS positions and velocities is sometimes difficult or even impossible to calculate. To overcome this lack of continuity in the GPS Signals and in order to observe accelerations (and hence forces) directly, MEMS-IMU sensors are integrated with GPS. In the presented experience, the GPS antenna is directly placed on the ski, whereas the inertial sensors are fixed on the athlete's boots. RESULTS: The GPS/MEMS-1MU Integration provides attitude results with an accuracy of 1-2° (1cj) (Waegli 2007). Figure 1 illustrates the orientation of a ski during a giant slalom. DISCUSSION: The closely coupled Integration strategy of satellite with the inertial measurements provides degree-level accuracy even for poor satellite-signal reception. Thus, the method proves to be sufficiently accurate to serve as a basis for the Interpretation of the force plate measurements. For a complete investigation, the right and the left foot must to be equipped with GPS and MEMS sensors to recover the orientation and Position of both skis. CONCLUSION: With an orientation accuracy of 1-2°, the method presented for the determination of the skis' orientation provides a substantial improvement over the existing techniques, and will serve as a basis for the evaluation of the force plate measurements.
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