Fusion motion capture: A prototype system using inertial measurement units and GPS for the biomechanical analysis of ski racing

Abstract

In this pilot study fusion motion capture (FMC) has been used to capture 3‐D kinetics and kinematics of alpine ski racing. The new technology has overcome the technological difficulties associated with athlete performance monitoring in an alpine environment. FMC is a general term to describe motion capture when several different streams of data are fused to measure athlete motion. In this article inertial measurement units (IMU), global positioning system (GPS) pressure sensitive insoles, video and theodolite measurements have been combined. The core of the FMC is the fusion of IMU and GPS data. IMU may contain accelerometers, gyroscopes, magnetometers and a thermometer, and they track local orientation and acceleration of each limb segment of interest. GPS data are fused with local acceleration data to track the global trajectory of the athlete. Fusion integration algorithms designed by the authors [1] were used to improve the accuracy of the independent Kalman filter solutions provided by the vendors of both the GPS and IMU. The GPS accuracy was improved from a dilution of precision of ±5m (meaning 50% of the measurements will be within 5 m of the true value) to a maximum error of ±1.5 m over the race course, while the IMU orientation error was reduced from over 20° to less than 5°. The reader is invited to assess the validity of these results by comparing videos of the motion to the fusion motion capture output in the electronic version of this manuscript. Accuracy in laboratory situations has been validated, [2,3] but because such systems are becoming more popular, this system needs to be validated on the snow. As more accurate dual frequency GPS systems become less expensive this type of system will become more accurate and affordable. A biomechanical analysis was undertaken of a New Zealand Alpine Ski Racing Team member negotiating a 10‐gate giant slalom course over 300 m in length. The abundant data in the results were used to create new tools for measuring alpine ski racing technique, such as colour‐coded force vector analysis. The new parameters introduced in this article, such as effective inclination and ground reaction force power, are independent of the stylistic constraints often imposed by the coach or athlete. Two ski runs have been compared. Although the difference between the two run times was only 0.14 s or 1%, FMC and force vector analysis were able to pick up the subtle changes in technique between the two runs. It is believed the analyses will provide useful design parameters to ski equipment engineers and will allow athlete feedback through augmented reality animations about variables, including limb dynamics, centre of mass (CoM) trajectory, CoM velocity, and external forces. In‐depth analysis of the changes in net joint torques with changes in athlete posture may be useful for coaching athlete specific technique changes to improve performance and reduce injury potential. In addition, it is possible to extract key performance indicators about the athlete's physical and physiological limits, such as the mean coefficient of wind drag and the maximum inclination angle while turning, which may be used to optimise race strategies. There are tentative plans to use an improved version of a similar motion capture system to analyse forerunners on the FIS world cup race circuit. The purpose is to reduce knee anterior cruciate ligament (ACL) injuries and provide a visual biomechanical analysis of an athlete running the course to enhance the experience of the television audience. In alpine ski racing, forerunners ski the course before the first athlete to set ski tracks through the gates and check the safety of the course.

Keywords:

• GPS
• ski racing
• fusion motion capture
• biomechanical analysis
• inertial measurement units

Notes

Institute of Food Nutrition and Human Health, Massey University, Private Box 756, Wellington, New Zealand. E‐mail: [email protected]