The exercises to strengthen the muscles around the knee are designed to improve its stability. During these exercises, co-contraction of the hamstrings occur depending on the level of quadriceps contraction. The purpose of this study is to develop a three-dimensional knee model that includes tibia, femur and patella and allows to predict knee response due to quadriceps and hamstrings co-contractions. As an application, results quantifying the changes in knee response are obtained following a simulated posterior cruciate ligament (PCL) injury. This is motivated by the continuing debate whether surgical repair is the method of choice for individuals with such an injury. It has been reported that following a knee injury, a large number of patients with PCL deficiency develop significant disability with Osteoarthritis (OA). The conditions tested simulate the straight-leg-raise exercise, during which the quadriceps muscles contract isometrically along with hamstrings co-contractions.