One of the most important factors potentially affecting the walking motion is the lower limb muscle mass. It is possible the mass of the lower limb muscles is correlated with walking motion. We investigated the relationships between walking motion by 3D motion analysis and the mass of the thigh muscle with ultrasound imaging. Our data demonstrated significant correlations between the mass of the thigh muscles and some of the joint kinematics measured at the hip, knee and ankle. These results suggest the knee extensor muscles mass regulates knee joint in association with the lower extremity movements at the onset of walking in free speed.