A new type of knee prosthesis capable of making deep knee flexion has been long awaited for Asian and Muslim people. Our research group has developed a prosthesis for deep flexion and designated it as CFK (Complete Flexion Knee). In order to assess the performance of CFK, we have set up various kinds of simulation/experimental projects, such as a cadaveric study, a mathematical model analysis, a photoelastic analysis and FEM analysis. For carrying out the above-mentioned projects, we faced the most fundamental problem; the information about the muscles' forces and the forces acting on the joints is limited for the activities with low and middle flexion of knee angle such as waking, but not for deep flexion such as squatting or sedentary sitting. The objective of this study is to introduce the muscle and the joint forces at deep knee flexion by using a 2D mathematical model. Double leg ascending and single leg ascending from deep squatting were studied for 10 healthy male subjects. The results demonstrated that the knee joint force was largest for the legs during double leg ascending, followed the dominant leg during single leg ascending and it was smallest for the supporting leg. The critical state for the prosthesis for deep flexion was found as the maximum joint force with F_<nmax>=4.6±0.6 (4.3〜5.2) times Body Weight at knee flexion angle of β=142±8 (134〜147)°. Ascending speeds did not affect the values of the muscle and joint forces as long as the motion was not strenuous like jumping.