When a unilateral transfemoral amputee performs sprinting, alternation between the sound limb and the prosthetic limb causes asymmetrical motion. One of the causes of the asymmetrical sprinting performance is that the knee joint of the prosthetic limb does not flex easily due to strong flexion resistance in order to prevent giving way during the landing by extension delay of the shank after flexion in the swing phase. This study examined whether the prosthetic limb performs the same motion as the sound limb working with the movements of the hip joint and other areas by setting the knee joint to allow easier flexion when relaxing the flexion-and-extension resistance of the knee joint in the swing phase. Moreover, when possible, we also examined whether this allows more rational motion than during sprinting when the flexion resistance of the knee joint is set strongly. The composition of the prosthetic limb used for the trial was an IRC (Ischial Ramal Containment) suction socket, high-activity hydraulic knee joint (model 3R55; Otto Bock, Duderstadt, Germany) in which the flexion-and-extension resistance was relaxed, in combination with an energy-storing prosthetic foot for running (model Sprinter; Otto Bock, Duderstadt, Germany). Sprinting was performed by a unilateral transfemoral amputee using this prosthetic limb after the subject had learned how to sprint while using it. Sprinting performance was filmed (30 fps) with a video tape recorder (DCR-PC101 NTSC, Sony Corporation, Tokyo, Japan), and we compared the motion of the prosthetic foot and the knee joint angle with the motion of the sound limb. Moreover, the motion was also compared with that during sprinting when the flexion-and-extension resistance of the knee joint was set strongly. As a result of the relaxed setting of the prosthetic knee joint, it was shown that the prosthetic limb performed the same motion as a sound limb without giving way at landing by extension delay of the shank of the prosthetic limb during the swing phase. Moreover, it was shown that this setting facilitates more rational motion than that during sprinting with a stronger setting for the flexion resistance of the knee joint.