This paper describes a measurement method of three-dimensional (3D) forearm movement during pitching motion using an elbow-mounted sensor (3D sensor). The 3D sensor comprises accelerometers of two kinds with dynamic range of 4 [G] and 100 [G], and two kinds of gyroscopes with dynamic range of 300 [deg/s] and 4000 [deg/s], respectively, because the sensors used in measurement of sports activities require a wide dynamic range. The 3D sensor, attached on the forearm, measures 3D acceleration and angular velocity. The 3D trajectory of the forearm is estimated through double integration of the measured acceleration, which is transformed from the acceleration based on the system of moving coordinate on the forearm to that on the fixed system of coordinates. Because the estimated trajectory of the forearm is affected by the numerical integration of the measured data including errors, the 3D trajectory error is reduced by determining the position and posture of the forearm at the end of the pitching motion. Results of the pitching experiment show that the 3D trajectory and angle of the forearm estimated by the 3D sensor agree with those measured from a video camera image with an error margin of around 10 %.

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