So far, various kinds of artificial knee joint have been developed, and their performances have been evaluated for practical use. Unfortunately, there have been no practical methods to measure the force acting on the knee joint in vivo. We have developed a new type of artificial knee joint, designated a CFK (Complete Flexion Knee) prosthesis, which enabled us to make a deep squatting or even a sedentary sitting necessary for a japanese style. Yet the model analyses or the experimental studies concerning about the deep squatting or the sedentary sitting are quite few and the data necessary for evaluating the performance of the CFK are lacking. Thus, using a 2D mathematical model, we calculated the force acting on the knee joint and muscles’ forces of the lower limb subject to sitting to or rising from the deep squatting or the sedentary posotion respectively. We could introduce the reasonable results from our quasi-dynamic models. Now, we are ready to assess the CFK performances using these results.