Computational models are needed to estimate soft tissue loads during movement. It would be ideal to perform such estimates on a subject-specific basis, where the information could be used clinically for assessing injury risk, planning treatment and monitoring rehabilitation outcomes. Musculoskeletal simulation software has evolved to the point that it is now relatively straight forward to estimate muscle forces needed to emulate subject-specific joint kinematics and kinetics. These muscle forces can subsequently be used as boundary conditions in a knee mechanics model to estimate the associated ligament and cartilage loads. However, this serial simulation approach may ignore inherent interactions between musculoskeletal dynamics and internal joint mechanics. That is, cartilage contact forces and ligament tension can potentially contribute to joint moment equilibrium. Further, ligament stretch may allow joint kinematics to vary in a way that affects muscle moment arms and lines of action about the joint. Thus, it would be preferable if muscle, ligament and cartilage contact loads were estimated simultaneously so that these interactions are accounted for. The objective of this study was to incorporate a six degree of freedom tibiofemoral model into an existing subject-specific gait simulation framework. In this study, we introduce the computational model, and then use it to track measured gait dynamics of a subject with an instrumented knee joint replacement. A comparison of the model-predicted and measured tibiofemoral contact forces provides a basis for assessing the validity of this novel co-simulation framework.
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