Three energy-based phases of gait. Normalized knee power versus time graph (solid blue curve) is shown for a prosthetic leg with a typical inertial configuration (upper leg mass = 50% of able-bodied value, and lower leg and foot mass = 33% of able-bodied values [20,25]) moving with normative kinematics. Symbol designates knee power normalized to body mass. Phase 1 is a negative and positive work phase (), which can be partially replicated with a spring element, and phases 2 and 3 are purely negative work phases (Wpos = 0), which can be accurately replicated with dampers. Normalized knee power versus time graph (dashed red curve) is also shown for able-bodied values of segment masses [22] (upper leg mass = 100% of able-bodied value, and lower leg and foot mass = 100% of able-bodied values).