The human gait cycle is typically viewed as a periodic sequence of discrete events, starting with heel contact during initial stance and ending with knee extension during late swing. This convention has informed the design of control strategies for powered prosthetic legs, which almost universally switch between multiple distinct controllers through the gait cycle based on a finite state machine. Human locomotion is further discretized into a small set of task-specific finite state machines, e.g., one for uphill and one for downhill. However, this discrete methodology cannot synchronize to the continuous motions of the user or adapt to the continuum of user activities. Instead of discretely representing human locomotion, this talk will present a continuous parameterization of human gait based on 1) a phase variable that robustly represents the timing of human joint patterns, and 2) task variables that parameterize kinematic adaptations to ground slope and walking speed. A unifying prosthetic leg controller is then designed around this continuous parameterization to synchronize prosthetic joint patterns with the timing and activity of the human user. The viability of this approach is demonstrated by experiments with above-knee amputee subjects walking on a powered knee-ankle prosthesis at variable speeds and inclines. Volitional control is demonstrated through non-rhythmic activities such as forward/backward stepping and kicking a ball. Ongoing work in the design of high-performance prosthetic legs with low-impedance actuators will be also presented, followed by applications in powered orthoses for stroke gait.