This study predicts muscle forces acting on the lower extremity when the knee joint is in deep flexion. The whole body was approximated as a link model, and then the moment equilibrium equations at the lower extremity joints were derived far given reaction farces against the ground. Measurement of deep flexion was carried out by placing ten markers on the body. This study calculated the moment acting at each Joint from the equations of force and moment, classified the complicated muscles around the knee joint, and then predicted the muscle forces to balance the joint moment. Two models were proposed in this study: the simpler one that consists of three groups of muscle and the more detailed one of nine groups of muscle.