Musculoskeletal models allow us to study muscle coordination and joint injuries in ways that in vivo experiments cannot. Models and simulations can compute internal joint contact forces, explore unsafe conditions, and simulate injuries without risk of harming experimental subjects. Models also enable systematic variation of muscle activity to evaluate its effect on joint loading and injury. The goals of this dissertation were to systematically quantify the effects of varied muscle activity in three applications:​ (1) to decrease knee forces during walking, (2) to estimate increased knee forces due to crouch gait in subjects with cerebral palsy, and (3) to prevent ankle sprains during landing.

Muscles induce large forces in the tibiofemoral joint during walking and thereby influence the health or degradation of tissues like articular cartilage and menisci. It is possible to walk with a wide variety of muscle coordination patterns, but the effect of varied muscle coordination on tibiofemoral contact forces remains unclear. The first goal of this dissertation was to determine the effect of varied muscle coordination on tibiofemoral contact forces. We developed a musculoskeletal model of a subject walking with an instrumented knee implant. Using an optimization framework, we calculated the tibiofemoral forces resulting from muscle coordination that reproduced the subject’s walking dynamics. We performed a large set of optimizations in which we systematically varied the coordination of muscles to determine the influence on tibiofemoral force. Peak tibiofemoral forces during late stance could be reduced by increasing the activation of the gluteus medius, uniarticular hip flexors, and soleus and by decreasing the activation of the gastrocnemius and rectus femoris. These results suggest that retraining of muscle coordination could substantially reduce tibiofemoral forces during late stance.

Muscle coordination and the resulting tibiofemoral forces may vary dramatically due to changes in walking kinematics, especially for individuals with gait pathologies. Crouch gait, a common walking pattern in individuals with cerebral palsy, is characterized by excessive flexion of the hip and knee. Many subjects with crouch gait experience knee pain, perhaps because of elevated muscle forces and joint loading. The second goal of this dissertation was to examine how compressive tibiofemoral force change with the increasing knee flexion associated with crouch gait. Using our musculoskeletal model, muscle forces and tibiofemoral force were computed for three unimpaired children and nine children with cerebral palsy who walked with varying degrees of knee flexion. Compressive tibiofemoral force increased quadratically with average stance phase knee flexion (i.e., crouch severity) during the stance phase of walking, primarily due to concomitant increases in quadriceps forces. These results revealed that walking in crouch generates increased knee loading which may contribute to knee pain in individuals with crouch gait.

Muscle coordination and pose are suspected causes and predictors of ankle inversion sprains. Interventions that retrain muscle coordination have helped rehabilitate injured ankles, but it is unclear which muscle coordination strategies, if any, can prevent ankle sprains. The third goal of this dissertation was to determine whether coordinated activity of the ankle muscles could prevent excessive ankle inversion during a simulated landing on a 30–degree incline. We used musculoskeletal simulations to evaluate two strategies for coordinating the ankle evertor and invertor muscles during simulated landing scenarios:​ planned co-activation and stretch reflex activation with physiologic latency (60-millisecond delay). Our simulations revealed that strong preparatory co-activation of the ankle evertors and invertors prior to ground contact prevented ankle inversion from exceeding injury thresholds by rapidly generating eversion moments after initial contact. Conversely, stretch reflexes were too slow to generate eversion moments before the simulations reached the threshold for inversion injury. These results suggest that training interventions to protect the ankle should focus on stiffening the ankle with muscle co-activation instead of increasing the speed or intensity of the evertor reflexes.

This dissertation examines the effects of varied muscle coordination on two of the most common musculoskeletal injuries:​ chronic degradation of the knee and acute ankle inversion sprains. Our results revealed key connections between specific changes in muscle coordination and improved function of the knee and ankle, suggesting exciting future research areas for designing and testing interventions that protect knee and ankle function. Additionally, this dissertation provides a computational foundation for systematically exploring muscle coordination in musculoskeletal models, and provides them, free and open source, to the broader research community.