Abstract A primary requirement of an above-knee leg prosthesis is that it enables the amputee to walk with a normal appearing walking pattern. The leg motion during the swing and stance phase of level walking should approximate that of a non-amputee. To accomplish this, a control mechanism is added to the conventional leg prosthesis. This study is concerned with the performance of the mechanism during the swing phase. The normal walking pattern for a normal man was obtained from the literature for average, tall, medium, and short men. The data, consisting of tabulated experimental data for the linear and angular displacements of the normal leg, were smoothed and differentiated twice to obtain velocities and accelerations. A mathematical model was chosen that typifies the current mechanical-hydraulic mechanisms. The physical constants were calculated from data collected from many sources. The force, velocity, and power were calculated for the control mechanism.