The goal of my dissertation was to analyze muscle function and energetic cost during human locomotion across speed and gait. In my first chapter, I quantified the metabolic cost attributable to three important factors (propulsion, leg swing, stability) determining metabolic cost during slow, normal and fast walking. Using external assistive devices, I reduced the need for specific muscle actions related to each of these factors. This allowed me to partition the cost of each factor. I found that each factor contributed a different percentage to the net cost of walking for each speed.

As the speed of locomotion increases, a speed is reached when humans prefer to run rather than walk. Some have implicated local, muscle specific factors such as overexertion/fatigue or muscle force-velocity-length relationships as triggers of the walk-run transition. In my second chapter, I utilized external devices to alter the demand on specific trigger muscle groups. By changing demand in these groups, I significantly changed transition speed. My data support that all of these groups are proximate triggers for gait transition, but an underlying factor is ultimately determining gait transition speed.

Compared to other primates, humans have a distinct gluteus maximus. In my third chapter, I compared muscle activity in the gluteal group during walking, running, sprinting and climbing. The gluteus maximus was highly active during accelerations, sprinting and climbing. This could suggest that the primary role of the gluteus is for quick and evasive maneuvers.

Previously, I found no significant difference in cycling metabolic efficiency when non-amputees pedaled on their heels versus on the ball of their foot. I also found amputees and non-amputees to have similar pedaling efficiency at the same power outputs. In my last chapter, I determined lower leg muscle activity during two different foot-pedal positions (heel pedaling and pedaling on the ball of the foot). During cycling, the ankle joint musculature appears to primarily stabilize and permit power transfer from the hip and thigh muscles to the pedal. However, heel pedaling reduces the need for muscular stabilization of the ankle and thus has no net effect on efficiency.