When humans and other animals run, they literally bounce along the ground. The combined action of the integrated musculoskeletal system results in the overall leg behaving like a single mechanical spring during stance. This dissertation examines how humans adjust their leg stiffness and muscle activity in response to changes in environmental forces. The first three chapters focus on leg stiffness adjustments to accommodate surfaces of different stiffnesses. I find that humans increase leg stiffness for lower surface stiffnesses when they hop or run. This adjustment allows humans to perfectly offset decreases in surface stiffness. As a result, humans run with similar biomechanics (e.g., stride frequency, ground contact time, total vertical displacement of the center of mass, and peak ground reaction force) regardless of surface stiffness. When runners encounter abrupt changes in the running surface, they immediately adjust leg stiffness for their first step on the new surface. These results provide important insight into the mechanics and control of animal locomotion, and suggest that incorporating an adjustable leg stiffness may increase the capabilities of robots and prostheses on varied terrain. The last chapter of this thesis investigates how humans adjust muscle activity to accommodate simulated reduced gravity. The results show that reduced gravity has fundamentally different effects on muscle recruitment patterns during walking and running. In addition, the finding that vastus lateralis activity during walking is independent of gravity level helps explain why the metabolic cost of walking decreases only slightly under reduced gravity.