Computational biomechanics of human knee joint in maximum voluntary isometric extension with focus on the role of joint center positioning.

Maximum voluntary isometric contraction (MVIC) extension assesses knee quadriceps function, strength, neuromuscular recovery and rehabilitation programs. We use a musculoskeletal (MS) model of the lower limb that incorporates a finite element (FE) of a cadaver knee. Muscle/ligament/contact forces, tissue stresses/strains and passive reaction moments are computed while simulating MVIC extension in seated posture at three knee flexion angles (KFA) (30°, 60°, 90°). Three input parameters of MVIC extension moment, lever arm of the posteriorly-directed external force, and coactivation in knee flexors were each varied at four levels using a Taguchi orthogonal design. Sensitivity of output parameters to these input variables were estimated. The location of the joint center where moment equations are verified was also varied. Results demonstrated a significant increase in quadriceps, patellar tendon, and patellofemoral contact forces with KFA (p < 0.001) and MVIC moment (p < 0.001). Greater lever arm of the external force increased ACL forces. In contrast to OpenSim simulation of the same MVIC, changes in the joint center location affected only the passive reaction moments that varied to maintain equilibrium while the computed muscle forces and internal ligament/contact loads remained unchanged. Findings highlight inherent limitations in MS models that idealize the entire knee as a joint located at its center of rotation.