ABSTRACT
A sagittal-plane model of the knee is used to determine the relationships between the forces developed by the muscles and the forces induced in the cruciate ligaments during activity. The geometry of the model bones is adapted from cadaver data. Eleven elastic elements describe the geometric and mechanical properties of the knee ligaments. The model is actuated by eleven musculotendinous units, each unit represented as a three-element muscle in series with tendon. For isometric contractions of the quadriceps, ACL force increases as quadriceps force increases; for isotonic contractions, ACL force decreases monotonically as knee-flexion angle increases. The relationships between ACL force, quadriceps force, and knee-flexion angle are explained by the geometry of the knee-extensor mechanism and by the changing orientation of the ACL in the sagittal plane. For isometric contractions of the hamstrings, PCL force increases as hamstrings force increases; for isotonic contractions, PCL force increases monotonically with increasing flexion. The relationships between PCL force, hamstrings force, and knee-flexion angle are explained by the geometry of the hamstrings and by the changing orientation of the PCL in the sagittal plane. Hamstrings co-contraction is an effective means of reducing ACL force at all flexion angles, except near full extension.
