A detailed dynamic model of the stump-prosthesis system for an above knee amputee was developed. The model was used to examine the influence of controls and design parameters on the limb system performance during the swing phase of gait. The model duplicated the clinically known fact that hydraulic knee controllers allow the amputee to change walking speed while mechanical knee controllers limit the amputee to a single walking speed. Contrary to current practice, the simulations suggest that light weight prosthesis designs do not perform as well as heavier designs. A simple design based on a constant friction knee is shown to yield good overall performance.