Humans tend to value economy of locomotion, often choosing movement strategies that help minimize how hard their bodies must work to perform a task. In this thesis I explore passive mechanisms humans use to reduce the metabolic energy consumed by their muscles, specifically due to positive muscle work, which is metabolically expensive compared to negative work or force production. I use a combination of modeling and human movement analysis to investigate how work performed passively by the ankle and by distributed soft tissues can save energy.

During normal walking, ankle push-off work provides an economical way to transition between steps. Push-off prior to collision redirects the body's velocity upward, which reduces the energy dissipated by collision, and the positive muscle work that must be performed to compensate for these losses. Through computational modeling and an experimental study of amputees walking on a variable-stiffness prosthetic foot, I demonstrate that elastic energy storage and return at the ankle can passively perform this energy-saving push-off function.

Active muscle work can also be reduced by passive soft tissues, which can perform mechanical work without the metabolic cost. I found that during walking and jump landings people choose to reduce demands on muscles by performing negative soft tissue work during collisions, and through a damped-elastic rebound of soft tissues after collisions.

While passive ankle work may seem entirely distinct from wobbling soft tissues, I demonstrate that similar biomechanical principles underlie the benefits of each. To save energy during locomotion one should avoid negative work that is not freely returned, otherwise it requires extra positive muscle work to compensate. Alternatively, passive mechanisms may provide a means to reduce both negative and positive muscle work.

However, since economy is not the only factor influencing movement, I also present a jump landing experiment to demonstrate how the amount of work people perform may reflect their subjective valuation of active muscle effort vs. other difficult-tomeasure costs, such as pain. Ultimately, the long-term goal is to use these fundamental energy-saving principles elicited through simulation and human movement analysis to inform the design of assistive technology for individuals with locomotor impairments.