According to the procedure described by Feltner and Dapena (4), the kicking leg was modeled as a three-link kinetic chain composed of the thigh, shank, and foot, in which the sum of all torques applied to each segment was set equal to the vector product of the segment’s moment inertia and angular acceleration. Segment mass, center of the mass
location, and moment of inertial values were derived from the data of young living Japanese athletes by Ae et al. (2) using a similar procedure first described by Jensen (7), which would be more appropriate for the participants in the present study. The mass of the shoe was 200 g, which was added to the foot mass, and its effect on the length of radius gyration of the foot segment was assumed to be small enough to be negligible. Using the methods similar to those reported by Sprigings et al. (15), the relative angular velocity vector of each joint was computed by subtracting its absolute angular velocity vector from that of the adjacent proximal segment. A reference frame was additionally defined at the center of trunk segment RTR to compute the relative angular velocity vector of the thigh. ZTR was a vector from the midpoint of a line segment between both hips to that between both shoulders, XTR was a vector from the left hip to the right hip, and YTR was the vector product of ZTR and XTR. All reference frame vectors were normalized to unit length. Thus, the absolute angular velocity vector of the trunk was calculated and then subtracted from that of the thigh.