The effect of light reflections from the snow on kinematic data collected using stereo-photogrammetry with passive markers
This study aimed to compare kinematic data collected during ski-cross starts outdoors on snow in daylight (high albedo) to similar data collected indoors with infiltrating sunlight but without light reflections from the snow (low albedo) using a video-based motion capture system with the active filtering function enabled. A 12-camera 3D motion capture system (Qualisys AB, Sweden) was used to measure test objects and eight skiers performing a ski-cross start on a slope outdoors and on a wooden start ramp indoors. The average residuals and standard deviations of the length of the calibration wand calculated indoors and outdoors by the calibration software were compared using descriptive statistics. Static and moving fixed length measures and thigh length measures were compared using Bland–Altman plots. Calibration residuals were slightly increased outdoors (1.77 mm) compared to indoors (1.54 mm), while wand length varied by 3.63 and 1.51 mm, respectively. Fixed static lengths differed by −8.65 ± 4.94 mm (shorter indoors), whereas fixed moving lengths differed by 0.85 ± 1.05 mm (longer indoors). A randomly chosen marker pair on one segment (Thigh) showed a mean difference of 1.19 ± 22.05 mm (longer indoors). It is concluded that 3D motion capture outdoors on snow in daylight is feasible, provides kinematic data comparable to indoors, and could be used to research biomechanics in snow sports.
KeywordsActive filtering 3D motion capture system Outdoor Ski-cross Snow
We thank Mr. Tommy Eliasson, Sports Director of the Swedish ski-cross national team, for his cooperation during the study and Mr. Mikael Therell, Technician at Swedish Winter Sports Research Centre, for his assistance in building the indoor start ramp. We also acknowledge Mr. Leif Jepsen and Mr. Jan Stavnshøj, Aalborg University, for their assistance with the instrumentation of the start gate. Our gratitude is extended to the team at Qualisys AB for their logistic support and provision of additional cameras and, in particular, Mr. Patrik Almstrøm and Mr. Reimund Trost for their technical help during the testing sessions.
- 1.Abdel-Aziz YI, Karara HM (1971) Direct linear transformation from comparator coordinates into object space coordinates in close-range photogrammetry. In: Proc. of the Symposium on Close-Range Photogrammetry. Falls Church, VA: American Society of Photogrammetry, pp 1–18Google Scholar
- 3.Qualisys (2011) Oqus 100, 300+, 400 and 500 series––Qualisys motion capture camera with high-speed video. Qualisys AB 2011-04-01, Accessed 27/07/2013 http://www.qualisys.com/wp-content/uploads/2011/09/PI_Oqus.pdf
- 4.Golf video Qualisys Qualisys (2010) Outdoor mocap outdoor in sunlight. Qualisys AB 18.10.2010, Accessed 27/07/2013. http://person.hst.aau.dk/uwek/OtherLinks/Golf_motion_outdoors_720p.mp4 (also available through youtube.com)
- 5.Motion Analysis Inc (2012), Movement analysis videos: outdoor baseball and golf. Web resource Uploaded on Sep 6, 2011, Accessed 27/07/2013 http://www.youtube.com/watch?v=Rfi4evDa3YU