Abstract
To obtain valid 3D joint angles with inertial sensors careful sensor-to-segment calibration (i.e. functional or anatomical calibration) is required and measured angular velocity at each sensor needs to be integrated to obtain segment and joint orientation (i.e. joint angles). Existing functional and anatomical calibration procedures were optimized for gait analysis and calibration movements were impractical to perform in outdoor settings. Thus, the aims of this study were 1) to propose and validate a set of calibration movements that were optimized for alpine skiing and could be performed outdoors and 2) to validate the 3D joint angles of the knee, hip, and trunk during alpine skiing. The proposed functional calibration movements consisted of squats, trunk rotations, hip ad/abductions, and upright standing. The joint drift correction previously proposed for alpine ski racing was improved by adding a second step to reduce separately azimuth drift. The system was validated indoors on a skiing carpet at the maximum belt speed of 21 km/h and for measurement durations of 120 seconds. Calibration repeatability was on average <2.7° (i.e. 3D joint angles changed on average <2.7° for two repeated sets of calibration movements) and all movements could be executed wearing ski-boots. Joint angle precision was <4.9° for all angles and accuracy ranged from -10.7° to 4.2° where the presence of an athlete-specific bias was observed especially for the flexion angle. The improved joint drift correction reduced azimuth drift from over 25° to less than 5°. In conclusion, the system was valid for measuring 3D joint angles during alpine skiing and could be used outdoors. Errors were similar to the values reported in other studies for gait. The system may be well suited for within-athlete analysis but care should be taken for between-athlete analysis because of a possible athlete-specific joint angle bias.
Highlights
Tracking of body segments and joints is traditionally performed with stereo-photogrammetric marker-based motion capture systems
The aim of this study was (1) to propose and validate an improved functional calibration which is fast and usable in-field, and uses available sports equipment components only; (2) to validate the 3D joint angles of the knee, hip, and trunk obtained by the use of this functional calibration for relatively long measurement durations (>30 seconds) in order to evaluate the impact of drift reduction
Functional calibration and joint angle validity could be assessed for all 11 athletes and trials, resulting in a total of 44 trials
Summary
Tracking of body segments and joints is traditionally performed with stereo-photogrammetric marker-based motion capture systems. Joint orientations can be computed by calculating the relative orientation between two adjacent segments following ISB recommendations [3,4] While such systems are well suited for in-lab measurements with relatively small capture volumes of a few cubic meters, they become unsuitable for larger volumes, such as often present in outdoor sports. For such sport applications inertial sensors have been proposed instead; e.g. to measure the kinematics of ski jumping [5], to estimate the instantaneous velocity for front-crawl swimming [6], to estimate spatio-temporal parameters in cross-country skiing [7], or to estimate temporal parameters during sprint running [8]. They are especially well suited for sports movement analysis because of their small size, possibility of being integrated into sports equipment or clothing, low dependence on environmental conditions (e.g. weather), and autonomy offering a pervasive monitoring
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