Locomotion requires coordination of leg joints to maintain stability and to maneuver. We studied leg joint function during constant-average-velocity running and the sagittal-plane maneuvers of step ascent and descent. We tested two hypotheses: (1) that leg joints perform distinct functions during locomotion; and (2) that humans select functional parameters to maximize intrinsic dynamic stability. We recorded whole-body kinematics and forces when participants stepped up or down a single vertical step, and found that leg joints show functional differences during both constant-average-velocity locomotion and maneuvers. The hip, knee and ankle function as a motor, damper, and spring, respectively. We therefore constructed a simplified computational model of a human leg with a motor, damper, and spring in series (MDS). The intrinsic dynamics of the model resulted in sustained locomotion on level ground within narrow parameter ranges. However, using parameters experimentally derived from humans, the model showed only short-term stability. Humans may not optimize intrinsic dynamic stability alone, but may instead choose mechanical and behavioral parameters appropriate for both constant-average-velocity locomotion and maneuvers. Understanding joint-level mechanical function during unsteady locomotion helps to understand how differential joint function contributes to whole-body performance, and could lead to improvements in rehabilitation, prosthetic and robotic design.