The purpuse of this study was to clarify the mechanism of movement of the lower limbs as to the increase of running speed. Six subjects (3 trained and 3 untrained) were employed in this study. In order to analyze the mechanism, Fourier transform method was used. The results were summarized as follows: 1) As the running speed increased, values of constant and amplitude of the thigh angle were increased. This means that the center of swinging movement of the thigh shifted foward and its amplitude increased as the running speed increased. 2) As the running speed increased, the amplitude of the hip torque increased and the initial phase of the hip torque shifted foward. This result suggested that in accordance with the increasing of running speed, the hip joint must generate not only big torque but also change the pattern of the torque. 3) In relation of torque to angular velocity and angle at the hip joint, it was indicated that angular velocity reached to the maximum just before torque became 0, and that the torque showed maximum negative when the angle became maximum positive. 4) Amplitude of the hip torque and the knee torque were increased as the running speed in. creased, but the increase rate of the hip torque was bigger than that of the knee torque.