Inertial Measurement Units (IMUs) increasingly gain scientific interest because they are less time consuming and priceless than traditional methods, i.e. video analysis to analyze performance-related parameters. Only few studies in swimming have addressed the underwater dolphin kick (UDK), known to have an important influence on overall swimming performance. The investigations of the UDK were limited to video analysis. Various factors were identified which have an influence on the UDK performance, such as the identical duration of up- and downbeat and constant frequency, resulting in a high toe speed and a large angular velocity of the hip. The present study compares IMU data with video data of a kick cycle and the up and downbeat phase of the feet and hips. 11 national and international top athletes participated during regular diagonstic in the Olympic Training Center Hamburg. Kick cycles (110) were measured via video (50-100 Hz) and IMU (400 Hz, +/- 16g, +/- 2000°/s) and both measuring methods compared using the Bland-Altman Plot. The results of the hip-foot comparison within one method showed no significant difference, while the comparison of both methods showed a significant difference. We explain this by the inherent error in the detection of key positions from the video. From a practical viewpoint, the absolute difference (max. 0.07 s) is negligible. Future efforts is on software development, which automatically analyses the UDK and supports daily work from coaches and scientists.
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