A multidisciplinary team from EPFL, Salzburg University, Zürich University and ETHZ proposes an inertial sensor-based method to estimate the athlete’s relative joint center positions and center of mass (CoM) kinematics in alpine skiing.
Eleven inertial Physilog sensors were fixed to the lower and upper limbs, trunk, and head. The relative positions of the ankle, knee, hip, shoulder, elbow, and wrist joint centers, as well as the athlete’s CoM kinematics were validated against a marker-based optoelectronic motion capture system during indoor carpet skiing.
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Objectives: The purpose of this study was to expand the body model to obtain CoM and relative joint center positions from the inertial sensors and validate them against a video-based stereophotogrammetric reference system.
Background: For the purpose of gaining a deeper understanding of the relationship between external training load and health in competitive alpine skiing, an accurate and precise estimation of the athlete’s kinematics is an essential methodological prerequisite.
Method: Competitive alpine skiers measurements were conducted on an indoor skiing carpet. The inertial-based measured relative positions of the ankle, knee, hip, shoulder, elbow, and wrist joint centers, as well as the athlete’s CoM kinematics were validated against a marker-based optoelectronic motion capture system.
Results: For all joints centers analyzed, position accuracy (mean error) was below 110mm and precision (error standard deviation) was below 30mm. CoM position accuracy and precision were 25.7 and 6.7mm, respectively. Both the accuracy and precision of the system to estimate the distance between the ankle of the outside leg and CoM (measure quantifying the skier’s overall vertical motion) were found to be below 11mm. Some poorer accuracy and precision values (below 77mm) were observed for the athlete’s fore-aft position (i.e., the projection of the outer ankle-CoM vector onto the line corresponding to the projection of ski’s longitudinal axis on the snow surface). In addition, the system was found to be sensitive enough to distinguish between different types of turns (wide/narrow).
Conclusion: The method proposed in this papermay also provide a useful, pervasive way to monitor and control adverse external loading patterns that occur during regular on-snow training. Moreover, as demonstrated earlier, such an approach might have a certain potential to quantify competition time, movement repetitions and/or the accelerations acting on the different segments of the human body.
Reference: Fasel, B., Spörri, J., Schütz, P., Lorenzetti, S., and Aminian, K. “An inertial sensor-based method for estimating the athlete’s relative joint center positions and center of mass kinematics in alpine ski racing.” Frontiers in Physiology 8 (2017).