Dynamic and static properties of the human knee joint in axial rotation

Joint laxity, muscle strength, elastic stiffness, viscosity and limb inertia about the internal-external rotation axis of the human knee joint were studied in vivo for normal and anterior cruciate ligament (ACL) injured subjects. A joint driving device was developed to perturb the knee joint about the axial rotation axis in various patterns while the subject kept the knee relaxed or contracted knee muscles at certain preset levels about the axial rotation axis. Compared to the flexion-extension axis, the knee joint showed much lower but still significant axial rotation muscle strength, substantially smaller limb inertia, lower viscosity, and comparable joint stiffness. Active muscle contraction increased joint stiffness and viscosity in axial rotation substantially. It also reduced knee axial laxity considerably. Cocontraction was more likely to occur in axial rotation muscle contraction than in flexion-extension, which tended to make active axial rotation joint stiffness higher. An ACL injured knee tended to show larger axial rotation laxity and lower joint stiffness but muscle contraction reduced its differences from uninjured knees. The passive joint stiffness in axial rotation was the lowest among the three rotational axes.