The objective of this study was to investigate our hypothesis involving treadmill walking as revealed by time series of ultrasound images. The hypothesis is that the changes in the length of muscle fiber (MFL) of the triceps surae muscle depend on the position of the center of mass (COM) relative to the ankle joint center of rotation. Seven healthy subjects participated in this experiment. The time series of ultrasound images in MG and the joint angular displacements in the hip, knee, and ankle were recorded while the subjects walked on a treadmill at speeds of 3.0 and 4.0 km/h. The MFL in MG was lengthened at the early stance phase and shortened at the late stance phase. Namely, the changes in the MFL of MG while treadmill walking followed our hypothesis, which was demonstrated while standing upright. These results suggest that the propulsive torque around the ankle joint is controlled by the shortening of the MFL in MG, whereas the MG muscle was in the eccentric contraction phase.