The desire and the need for an above knee amputee athlete to have a recreational prosthesis is evident when an estimated 10,000 young amputees with an active sports interest are considered. Conventional prostheses function well for slow controlled gait, but fail to meet the demands of rapid walking and jogging. Tests on an active 22 year old above knee amputee showed that stride length and stride frequency could be increased by changing the inertial properties of the lower portion of the prosthesis. A four camera, three dimensional motion analysis system was used to further investigate the effects of changing inertial properties on an above knee bench prosthesis. By adding either a one or 1.5 pound weight to various locations on the shank of the bench prosthesis, the effects during free swing were studied. Results indicated that the frequency of swing was increased with the 1.5 pound weight located ten centimeters below the knee joint of the above knee bench prosthesis. This condition also created less rebound effect and greater knee flexion of the prosthesis. All of these results indicated that changing the inertial properties of the lower portion of the prosthesis created the conditions necessary to permit rapid gait for an above knee amputee.