AUTOMATIC AND MANUAL ESTIMATES OF PASSIVE DRAG DIFFER
Kjendlie, P.-L., & Olstad, B. H. (2012). Automatic 3D motion capture of swimming: marker resistance. Presentation 1939 at the 59th Annual Meeting of the American College of Sports Medicine, San Francisco, California; May 29-June 2, 2012.
The aim of this study was to pioneer a 3D automatic motion-capture system used on human swimmers and to evaluate if there was increased drag due to motion-capture markers attached to a swimmer's body. Ss (N = 2) performed a total of 80 push offs with an 8 m underwater gliding phase. During each test, the velocity of the swimmer's body was recorded using a Qualisys automatic tracking 3D motion capture system. Six underwater motion capture cameras recorded spherical markers attached to the swimmer. Using inverse dynamics the drag coefficient was estimated in 40 tests wearing only 3 markers and compared to 40 tests where a total of 24 markers were attached to the swimmer. All markers had a diameter of 19mm.
The passive drag coefficient when gliding with 24 markers was ~30.49 and ~26.37 for the two Ss respectively. Without markers the drag coefficients were ~28.49 and ~23.87 respectively. There was a statistically significant higher drag when wearing markers compared to without, the difference averaging to 7% and 10% for the Ss respectively, and the effect sizes were large (1.17 and 2.57 respectively).
Implication. Although spherical markers at a sufficient size are necessary for accurate motion capture underwater, these markers create a significant increase in the passive drag of swimmers. This should be taken into account when making kinematical analyses with this system in the aquatic space. The results of this paper were limited to passive drag, and it should be expected that different results would exist with active drag.
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