Collecting Kinematic Data on a Ski Track with Optoelectronic Stereophotogrammetry: A Methodological Study Assessing the Feasibility of Bringing the Biomechanics Lab to the Field

Abstract

In the laboratory, optoelectronic stereophotogrammetry is one of the most commonly used motion capture systems; particularly, when position- or orientation-related analyses of human movements are intended. However, for many applied research questions, field experiments are indispensable, and it is not a priori clear whether optoelectronic stereophotogrammetric systems can be expected to perform similarly to in-lab experiments. This study aimed to assess the instrumental errors of kinematic data collected on a ski track using optoelectronic stereophotogrammetry, and to investigate the magnitudes of additional skiing-specific errors and soft tissue/suit artifacts. During a field experiment, the kinematic data of different static and dynamic tasks were captured by the use of 24 infrared-cameras. The distances between three passive markers attached to a rigid bar were stereophotogrammetrically reconstructed and, subsequently, were compared to the manufacturer-specified exact values. While at rest or skiing at low speed, the optoelectronic stereophotogrammetric system’s accuracy and precision for determining inter-marker distances were found to be comparable to those known for in-lab experiments (< 1 mm). However, when measuring a skier’s kinematics under “typical” skiing conditions (i.e., high speeds, inclined/angulated postures and moderate snow spraying), additional errors were found to occur for distances between equipment-fixed markers (total measurement errors: 2.3 ± 2.2 mm). Moreover, for distances between skin-fixed markers, such as the anterior hip markers, additional artifacts were observed (total measurement errors: 8.3 ± 7.1 mm). In summary, these values can be considered sufficient for the detection of meaningful position- or orientation-related differences in alpine skiing. However, it must be emphasized that the use of optoelectronic stereophotogrammetry on a ski track is seriously constrained by limited practical usability, small-sized capture volumes and the occurrence of extensive snow spraying (which results in marker obscuration). The latter limitation possibly might be overcome by the use of more sophisticated cluster-based marker sets.

Are Existing Monocular Computer Vision-Based 3D Motion Capture Approaches Ready for Deployment? A Methodological Study on the Example of Alpine Skiing

“…However, a recent study demonstrated these errors to be marginally small; photogrammetric errors were found to be in the magnitude of ~2 cm or lower [2]. Moreover, the in-lab golden standard method of optoelectronic stereophotogrammetry was shown to be seriously constrained for collecting kinematic data on a ski track [1]. Accordingly, the reference method used can be argued to be the best option currently available.…”

“…Since the feasibility of collecting 3D kinematic data on a ski track by the use of marker-based optoelectronic stereophotogrammetry (golden standard under laboratory conditions) has been demonstrated to be strongly limited [1], recent biomechanical studies in alpine skiing have primarily incorporated systems with multiple panned/tilted/zoomed video cameras (i.e., video-based stereophotogrammetry) and/or wearable measurement technologies [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]. The major advantage of video-based stereophotogrammetry is its superior position determination accuracy and precision in connection with 3D human pose estimation [2]; however, their disadvantages are the complex multi-camera setup and extensive post processing (i.e., camera calibration and manual annotation) demands, and, compared to wearable measurement technologies, their limited capture volumes.…”

An Inertial Sensor-Based Method for Estimating the Athlete's Relative Joint Center Positions and Center of Mass Kinematics in Alpine Ski Racing

“…Earlier studies in alpine skiing primarily used video-based stereophotogrammetric systems to determine an athlete's kinematics on a ski track (Supej et al, 2003 ; Federolf, 2012 ; Spörri et al, 2012a , b , 2016b ; Hébert-Losier et al, 2014 ). Under such in-field conditions, photogrammetric errors of <1.5 cm were reported (Klous et al, 2010 ; Spörri et al, 2016c ). However, despite major advantages regarding accuracy, corresponding measurement setups are complex, capture volumes are limited to a few turns only, and post-processing is time consuming.…”

Experimental Evaluation of UWB Indoor Positioning for Indoor Track Cycling

“…Image processing systems do not require markers; the captured images are digitally analyzed to locate the athletes. In [18], an OMS was used to track an athlete during alpine skiing. They obtained sufficient accuracy, but the solution has limited practical usability because of the small capture volume and the obscuration of the markers due to snow spraying.…”