Terrestrial animals move in predictable ways. At each speed, they consistently choose a particular stride frequency and gait This thesis examines the roles of springs, pendulums and musculoskeletal forces in determining stride frequency, gait and the rate of energy consumption.

The first part ofthis thesis focuses on how the muscle-tendon spring system determines stride frequency in bouncing gaits. It is well established thatrunning animals save energy by using their muscles and tendons as springs which store and return elastic energy. I find that when humans hop or run, the preferred frequency is determined by a combination ofthe spring-like behavior of the body and the cost of generating muscular force to operate the springs.

The second part of this thesis uses simulated reduced gravity as a tool to address the question of what determines the costs of walking and running. When gravity is reduced, the cost ofrunning decreases in proportion to body weight but the cost of walking only decreases slightly. This finding supports the idea that the cost of running is determined by the cost of generating muscular force to support the weight of the body. However, it does not appear that this is true for walking. Walking involves a pendulum-like exchange of gravitational potential energy and forward kinetic energy, and this exchange may be reduced at low gravities. Thus, although the muscles have to support less weight during low gravity walking, they may have to do more mechanical work because ofineffective pendulum exchange.

In the final part of this thesis, I test whether peak musculoskeletal forces trigger the trot-gallop transition in quadrupeds. In dogs, goats and horses, peak skeletal stresses increase with trotting speed and are reduced when they switch to the gallop. I find that when horses carry weights, they switch from a trot to a gallop at the same critical level of force but at a lower speed. In addition, the gait transition does not occur at the speed which would minimize energetic cost Thus, I conclude that musculoskeletal forces are the trigger for the gait transition.