High-performance lower-limb prostheses and orthoses could significantly improve the quality of life for nearly a million American amputees and even more stroke survivors, whose ambulation is slower, less stable, and less efficient than that of able-bodied persons. Although recent motorized prostheses and orthoses have the potential to restore mobility in impaired populations, critical barriers still limit their clinical viability. Current powered legs independently control different joints and time periods of the gait cycle, limiting robustness to environmental uncertainty and requiring clinicians to spend significant amounts of time tuning each control model to the individual. This sequential control methodology is a direct consequence of the current paradigm for viewing human gait patterns as functions of time. However, recent bipedal robots can stably walk, run, and climb stairs with one control model that drives joint patterns as functions of a mechanical variable, which continuously represents the robot's progression through the gait cycle, i.e., a sense of "phase." These new breakthroughs in robot control theory present an emerging opportunity to address a key roadblock in prosthetic technology with a paradigm shift in how the human gait cycle is viewed: as a function of a phase variable rather than time. Prosthetic legs could then be designed with a single control model that measures a biologically-inspired phase variable to match the human's volitional movement or respond to perturbations. Central to this challenge is a fundamental gap in knowledge about how the human neuromuscular system might maintain a sense of phase. This project aims to address this gap by 1) identifying a biomechanical phase variable used in human locomotion, and 2) designing a unifying control model for lower-limb prostheses and orthoses. I hypothesize that human joint patterns are driven by the heel-to-toe movement of the center of pressure (COP)-the point on the foot sole where the cumulative reaction force is imparted against the ground. I will test this hypothesis by observing the response of human joints to perturbations of the COP while walking over a robotic platform. I will then implement a novel control strategy using this sense of phase on a powered knee-ankle prosthesis, which will be validated with human amputee subjects. This investigation will be significant to our understanding of the neuromuscular system during locomotion, research methods for analyzing the gait cycle, and the design of clinically viable prosthetic control systems. The innovation of this work is encompassed in 1) a new phase-dependent paradigm of human locomotion that challenges the existing time-dependent paradigm, and 2) a novel control methodology that will accelerate the clinical adoption of powered prostheses and orthoses. The knowledge and concepts gained from this bold new paradigm will have a broad impact in physical medicine and rehabilitation, catalyzing technological advances for restoring mobility after stroke, spinal cord injury, and peripheral neuropathy. My expertise in robot control and postdoctoral training in prosthetics make me uniquely qualified to successfully execute this highly innovative work.