Comparison of two methods of determining patellofemoral joint stress during dynamic activities.

BACKGROUND Joint specific models rely on muscle force estimates to quantify tissue specific stresses. Traditionally, muscle forces have been estimated using inverse dynamics alone. Inverse dynamics coupled with static optimization techniques allow for an alternative method in estimating muscle forces. Differences between these two techniques have not been compared for determining the quadriceps force for estimating patellofemoral joint stress. METHODS Eleven female participants completed five squats and ten running trials. Motion capture and force platform data were processed using both solely inverse dynamics and inverse dynamics with static optimization to estimate the quadriceps force in a patellofemoral joint model. FINDINGS Patellofemoral joint stress calculations were consistently higher when using the combination of inverse dynamics and static optimization as compared to the inverse dynamics alone (p<0.05) yielding estimates that were 30-106% greater. INTERPRETATION When implementing joint models to estimate tissue specific stresses, the choice of technique used to estimate muscle forces plays an important role in determining the magnitude of estimated stresses in patellofemoral joint models.