Abstract: An inverse dynamics computer simulation was carried out to investigate the functionality of lower extremity muscles in the swing phase of a transfemoral amputee gait. With each muscle as an ideal force generator, the lower extremity was simulated as a two-degree of freedom linkage with the hip and knee as its joints. Kinematic data of hip and knee joints were recorded by a motion analysis system. Through a static optimization approach, the forces exerted by muscles were determined so that recorded hip and knee joint angles were produced. Simulation results showed that in comparison to a healthy model in a transfemoral amputee model, the hip flexors force exertion duration is longer and hip extensors exert smaller force. These results correspond to experimental records of electromyography activity of leg muscles. In addition, results showed that as the mass and moment of inertia of prosthetic shank is increased individual muscle forces will increase. This is in accord with the previous theoretical and experimental studies that showed an increase in both biological and prosthetic shank mass and moment of inertia is a cause to increased muscle efforts and electromyography activity and energy cost of walking.