Instantaneous velocity estimation for the four swimming strokes using a 3-axis accelerometer: Validation on...

Inertial measurement unit systems (IMU) are increasingly used in sports. However, no algorithm evaluating the instantaneous swimming velocity has been validated on the four swimming strokes, or on dive start. Our objective is to develop and validate a new method to measure instantaneous swimming velocity on athletes presenting various musculoskeletal and neurological impairments and swimming one of the four swimming strokes. Seven Paralympic athletes were involved and performed a total of 18 trials of 50 m in real conditions. All trials were recorded with a 3-axis accelerometer, a tethered device and a video camera. The instantaneous velocity was computed by drift-free integration of the forward acceleration. For all trials, Bland-Altman analyses showed a bias of 0.03-0.06 m.s^-1 with the tethered device, with 95% LOA lower than 0.31-0.80 m.s^-1, RMSE was 0.14-0.39 m.s^-1, and ICC amounted 0.494-0.941. No significant difference was found for the mean 50 m velocities between the accelerometer, the tethered device and the video camera. The results obtained for freestyle were comparable to those obtained with GPS in outdoor pools. Those obtained for backstroke, breaststroke and butterfly were very encouraging. Our method is simple, reliable, advantageous compared to tethered devices, and could be easily used in the field by coaches.