Muscles enable humans to walk, yet the mechanisms by which the forces generated by dozens of muscles give rise to walking are not well understood. An improved knowledge of muscle function during normal human walking has applications in many fields, from improving treatments for individuals with walking abnormalities to designing bipedal robots and prosthetic devices. Furthermore, characterizing how normal muscle function changes with walking speed could be helpful in determining why some patients with impairments have limited walking speed. Quantifying muscle function is challenging because the dynamic processes that link muscle coordination to observed motion are complex. Muscle-actuated dynamic simulations provide a means to investigate cause-effect relationships between individual muscle activity and movement of the body. The goal of this dissertation was to identify the muscles that provide vertical support of body weight and forward progression of the body mass center during walking. We examined muscle function over a range of walking speeds, from very slow to fast, in unimpaired humans.

We first identified muscle contributions to support and progression by developing and applying a new perturbation analysis method to a three-dimensional simulation of walking at a single speed. We then applied this method to identify muscle contributions to support and progression over a range of walking speeds for multiple unimpaired subjects. To do this, we generated and analyzed three-dimensional muscle-actuated simulations of gait for eight subjects walking overground at very slow, slow, free, and fast speeds. After identifying peak muscle contributions to support and progression for each subject at all speeds, we performed a repeated-measures analysis of variance to examine the effects of walking speed on these contributions. Our findings suggest that a relatively small group of muscles provides most of the forward progression and support needed for normal walking. Gluteus maximus and vasti provided vertical support during early stance, with some assistance from gluteus medius. Gluteus medius continued to support the body during midstance. Gastrocemius and soleus generated support during late stance. Vasti and gluteus maximus resisted progression during early stance, while gastrocnemius and soleus assisted progression during late stance.

Walking speed had statistically significant effects on peak support and progression contributions from several muscle groups. The influence of walking speed was most apparent as speed increased from slow to free. Vasti's support contributions increased dramatically (p < .001), and support contributions from gluteus maximus (p = .006) and soleus (p = .001) also increased. Contributions from vasti to resist progression increased significantly (p < .001) and from soleus to assist progression increased significantly (p < .001). These results demonstrate the importance of walking speed when evaluating muscle function.

This dissertation provides new insights into how muscle function changes with walking speed and contributes a set of reference data with 32 simulations that are available for use by others.