Abstract
In this study we present and validate a method to correct velocity and position drift for inertial sensor-based measurements in the context of alpine ski racing. Magnets were placed at each gate and their position determined using a land surveying method. The time point of gate crossings of the athlete were detected with a magnetometer attached to the athlete’s lower back. A full body inertial sensor setup allowed to track the athlete’s posture, and the magnet positions were used as anchor points to correct position and velocity drift from the integration of the acceleration. Center of mass (CoM) position errors (mean ± standard deviation) were 0.24 m ± 0.09 m and CoM velocity errors were 0.00 m/s ± 0.18 m/s. For extracted turn entrance and exit speeds the 95% limits of agreements (LoAs) were between −0.19 and 0.33 m/s. LoA for the total path length of a turn were between −0.06 and 0.16 m. The proposed setup and processing allowed estimating the CoM kinematics with similar errors than known for differential global navigation satellite systems (GNSS), even though the athlete’s movement was measured with inertial and magnetic sensors only. Moreover, as the gate positions can also be obtained with non-GNSS based land surveying methods, CoM kinematics may be estimated in areas with reduced or no GNSS signal reception, such as in forests or indoors.
Highlights
In the development of World class athletes’ monitoring, assessment of their training quantity and quality and evaluation of their performance plays a substantial role in snow sports
For applications where overall body posture remains relatively constant, it can be assumed that the 3D center of mass (CoM) kinematics can be approximated by the global navigation satellite systems (GNSS) antenna kinematics with sufficient precision (Terrier and Schutz, 2005; Townshend et al, 2008; Waldron et al, 2011; Scott et al, 2016)
When body posture is changing significantly during motion cycles, and when instantaneous CoM kinematics are the variables of interest, an approximation of CoM by the kinematics of a GNSS antenna cannot be considered to be sufficiently valid
Summary
In the development of World class athletes’ monitoring, assessment of their training quantity and quality and evaluation of their performance plays a substantial role in snow sports. Video and body worn sensor-based systems have been proposed to assess performance (Supej et al, 2005; Reid, 2010; Spörri et al, 2012) and training load (Spörri et al, 2015, 2017; Gilgien et al, 2018). To measure human body displacement in alpine ski racing using body worn sensors, differential global navigation satellite systems (GNSS) are recognized to be well suited. They allow obtaining the three dimensional (3D) antenna trajectory at a reasonably high sampling frequency with sub 5-cm accuracy for good GNSS conditions (Gilgien et al, 2014). An alternative solution needs to be found to track the athlete’s CoM 3D position relative to the GNSS antenna position
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