INTRODUCTION: Selected aspects of the design and validity testing of the Fusion Motion Capture System for alpine ski racing are the topics of this paper. Fusion Motion Capture is a wearable motion capture System utilizing GPS and inertial measurement units (IMUs) developed by the author. Other Instruments can also be included such as force platforms, pressure sensitive insoles, and cameras. Fusion Motion Capture may provide valuable information to sports such as alpine ski racing, where the motion of interest covers a large volume because it allows the whole performance to be captured automatically. Several papers incorporating IMUs were presented at the recent ISB Conference. The wide range of accuracy reported from IMU measurements suggests that that the accuracy of the sensors may be dependant on the particular application. An IMU contains accelerometers, gyroscopes, and magnetometers. They can provide estimates of 3D orientation, angular velocity, and linear acceleration. If the athlete accelerates through global space fusion motion capture accuracy may be improved by additional information from GPS, camera, timing gates, or some other global measurement. METHOD: Three different experiments were conducted to determine the accuracy of inertial measurement units in statfc1, dynamic2' and free movement situations. Two additional experiments were conducted to test the accuracy of the Fusion Motion Capture system for large scale motion capture, both indoor (inline skating) and outdoor (giant slalom ski racing) situations were tested. Fusion Motion capture and subsequent analysis consists of MATLAB algorithms written by the author. Visually the large scale motion capture was also qualitatively validated against recordings from a handheld video camera. RESULTS: Some results from the dynamic accuracy experiments are presented. The motion of a simple pendulum was captured. Both the manufacturers supplied Kalman filter algorithm and the author's own Fusion algorithms3 were tested against Video motion capture to determine the accuracy, the reliability, and the measurement characteristics of the system. DISCUSSION: Accurate orientation results are possible from IMUs in dynamic situations, with the author's fusion algorithms, error 0.9° RMS. One should exercise caution when using the rnanufacturer's Kalman filter algorithm, as it is known to perform well only for "certain types of motion," and it performed poorly in the pendulum experiment, error 11.7° RMS. The fusion algorithms developed by the authors are specific to skiing, and have not been tested on general motion. Therefore, there the validity for general motion is unknown.
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