Real-Time Kinematics Measurement of Human Lower-Limb Underwater Motions Based on Inertial Units

Human motion measurement is important in underwater sports analysis and underwater equipment evaluation, even for wearable robot control. In this paper, a real-time kinematics measurement system for human lower-limb underwater motion was proposed. Seven inertial sensing units are placed on the waist and thighs, calves, feet of both legs. Euler angles are used to describe the attitude difference between units and, after some adjustments, as the three-dimensional angle of corresponding joint. According to the evaluation with optical motion capture system on land, the maximal dynamic measurement error was less than 5 degrees by the proposed system. Pool experiment was also carried out and stable results were obtained for breaststroke at different speeds. The results proved the feasibility of the method for underwater human motion measurement.