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® 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.