We developed the sensor system and method by using wireless motion sensor (WMS) installing inertial and magnetic field sensors to obtain the trunk joint torque for the hitting action by the rugby-football player. This system consists of inertial and magnetic field sensors and force plate. We defined the 3D rigid link model of the rugby player, and constructed motion equations based on the Newton-Euler method. We measured the accelerations and angular velocities by using WMS sensors attached to upper body and lumber segments, and impact force by using force plate. The WMS and the force plate outputs were converted into the lumber-spine joint torque. The behavior of the lumber-spine joint torque for keeping the posture of the rugby player in the hitting action was expressed qualitatively.
