Fig. 1.

a) the sagittal plane and b) transverse plane of the schematic of the eight-link, three-dimensional, model of the human body with leading heel at the point of (0, 0, 0). Solid thick and dashed line respectively represents the leading and trailing sides of the body. The model includes a lumped head, arms, and trunk (HAT) segment as well as both feet, legs, thighs, and a segment connecting both hip joints. This link segment between hips, which was mass-less, was for considering the effect of pelvic rotation on the step length. α is the angle formed by the line connecting ankle and heel and the sole. Its value is fixed for each individual subject, but may vary among subjects. The segmental length and mass and the position of each segment’s center of mass (COM) were calculated for each individual subject. The COM position (x_COM) is represented relative to the base of support (BOS, i.e. the leading heel). The step length (s) is calculated as the distance between two heels in the anteroposterior direction at the instant of touchdown. Joint angles θ_i (i = 1, 2, 3, …, 8) specify the angles of the leading foot, leading ankle, leading knee, pelvic rotation, leading hip, trailing hip, trailing knee, and trailing ankle, respectively. On the leading side, the positive direction of the rotation axes is along the negative Y-axis (laterally to the leading side), but for the trailing side of the body the joint axes were in the direction of the positive Y-axis (laterally to the trailing side). The positive direction of the pelvic rotation is along the positive Z-axis. The positive X-axis is the direction of forward progression, the positive Y-axis is leftward, and the positive Z-axis is upward.