To achieve a better understanding of the mechanics of the intact and reconstructed human knee joint, a sagittal plane mathematical model of the human knee and lower extremity was developed. This model was used to investigate the forces transmitted by bone, muscle, and ligament during static and dynamic activity. Currently, no practical methods exist to directly measure forces in the muscles, ligaments, and bones at the knee in vivo. Understanding the mechanics of the knee is motivated by the high incidence of injury and degenerative disease at the knee, and the frequency of its surgical treatment.

The hip, ankle, and toes were each modelled as a simple hinge joint. Relative displacements of the femur, tibia, and patella in the sagittal plane were described using a more detailed representation of the knee. The geometry of the model bones was adapted from cadaver data. Eleven elastic elements described the geometrical and mechanical properties of the knee ligaments and joint capsule. The model was actuated by twenty-two musculotendinous units, each unit represented as a three-element muscle in series with tendon. Validation of the model was achieved by comparison of model results to published in vitro and in vivo experimental data.

The pattern of ligament loading at the knee is determined by the interactions between the forces developed by the muscles, the external loads applied to the leg, and the geometry of the ligaments and bones at the knee. The calculations show that cruciate-ligament forces vary nonlinearly with flexion angle and muscle force. For extension exercise, anterior cruciate ligament (ACL) forces are most sensitive to quadriceps force at small flexion angles, and become less sensitive as flexion angle increases. For flexion exercise, posterior cruciate ligament (PCL) forces are most sensitive to force in the hamstrings at large flexion angles, and become less sensitive as flexion angle increases. Provided that the forces applied by the muscles are equal during weightbearing and non-weightbearing exercise, the forces induced in the ligaments remain approximately the same. The calculations incorporating total knee replacement (TKR) show that the articular geometry of the TKR components contribute to dysfunction of the PCL.