In this work a 3D musculoskeletal model of the knee joint, that can simulate a squat movement, has been developed. Such a model would be useful to understand the biomechanics of the knee. The model consisted of all the the bones with their articular surfaces, all relevant ligaments, the patella tendon and the quadriceps muscle. The squat movement is only regulated through velocity elongation of the quadriceps only. A squat movement between knee angles of 9° and 47° could be performed before element distortion occurred. The model outcome was compared with other numerical results, obtained from the literature. An osteochondral defect of 1 square mm in the femoral cartilage was mimicked and replaced with cartilage having stiffer, softer and normal properties. The softer defect reached 5.5% more in compression than the normal defect. In the stiffer defect 4% less compression occurred compared to the normal defect.