Muscle Contributions to Propulsion and Support During Running

Understanding how muscles propel our body forward and support our body weight during running is challenging because important variables, such as muscle forces, are generally not measurable. Muscle-driven simulations of walking have been analyzed to gain insight into muscle actions; however, simulating running presents new challenges due to high accelerations and forces. PURPOSE: To create a 3-D, muscle-driven simulation of running and quantify how muscles contribute to propulsion and support (i.e., the forward and upward accelerations of the body mass center). METHODS: Motion and ground reaction force data were measured on an adult male subject running at 3.9 m/s on a treadmill. OpenSim, open-source dynamic simulation software, was used to determine the muscle activations and forces needed to drive a 31 degree-of-freedom musculoskeletal model to track the experimentally measured motion. Contributions of individual muscles to propulsion and support were examined by independently perturbing each muscle's force during the simulated movement to calculate its contribution to the acceleration of the subject's mass center. This analysis focused on the propulsive phase of stance, the time period when a foot is on the ground and the body mass center is accelerating forward. RESULTS: Gastrocnemius, soleus, and hamstrings contributed most to propulsion, while the quadriceps resisted forward propulsion. Soleus, gastrocnemius, and quadriceps contributed the most to vertical support, while hamstrings acted to pull the body mass center downward. CONCLUSIONS: The ankle plantarflexors are a key muscle group for both propulsion and support of the body mass center during the propulsive phase of running. Gastrocnemius contributes more to propulsion and soleus more to support, which is consistent with their actions during walking. We have used the first 3-D, muscle-driven simulation of a running gait cycle to answer the fundamental question of which muscles propel the body forward and support body weight during running. Supported by NSF and Stanford Fellowships, and NIH Grant GM63495.