Muscles actuate walking by providing vertical support and forward progression of the mass center. To quantify muscle contributions to vertical support and forward progression (i.e., vertical and fore-aft accelerations of the mass center) over a range of walking speeds, three-dimensional muscle-actuated simulations of gait were generated and analyzed for eight subjects walking overground at very slow, slow, free, and fast speeds. We found that gluteus maximus, gluteus medius, vasti, hamstrings, gastrocnemius, and soleus were the primary contributors to support and progression at all speeds. With the exception of gluteus medius, contributions from these muscles generally increased with walking speed. During very slow and slow walking speeds, vertical support in early stance was primarily provided by a straighter limb, such that skeletal alignment, rather than muscles, provided resistance to gravity. When walking speed increased from slow to free, contributions to support from vasti and soleus increased dramatically. Greater stance-phase knee flexion during free and fast walking speeds caused increased vasti force, which provided support but also slowed progression, while contralateral soleus simultaneously provided increased propulsion. This study provides reference data for muscle contributions to support and progression over a wide range of walking speeds and highlights the importance of walking speed when evaluating muscle function.
Bibliographical noteFunding Information:
We gratefully thank Allison Arnold, Darryl Thelen, Chand John, Eran Guendelman, Ayman Habib, and Ilse Jonkers for many productive discussions and for their assistance in generating the simulations. This work was supported by the National Institutes of Health through the Roadmap for Medical Research U54 GM072970 and NIH R01 HD046814. We also acknowledge NSF award CNS-0619926 for computer resources.
- Forward dynamic simulation
- Muscle function
- Pediatric gait
- Walking speed