We describe the preliminary optimal design of an electromechanical above-knee active prosthesis with energy storage and regeneration. A DC motor-generator applies a positive or negative torque to the knee. The control system regulates the exchange of energy between the motor-generator and a supercapacitor. The central idea of the design is motivated by the mechanics, energy management, and sensor-based control that constitute human movement. We use biogeography-based optimization, which is an evolutionary algorithm, to optimize the system parameters, and we evaluate its performance with Simulink (R) models. We optimize three alternative prosthesis designs. Simulation results indicate that the prosthesis can be optimized to achieve knee angle tracking with an RMS error on the order of 0.2 degrees.