In this study, we present the conceptual design of a fully-passive transfemoral prosthesis. The design is inspired by the power flow in human gait in order to have an energy efficient device. The working principle of the conceptual mechanism is based on three storage elements, which are responsible of the energetic coupling between the knee and the ankle joints. Design parameters of the prosthesis have been determined according to the energy absorption intervals of the human gait. Simulation results shows that the power flow of the system is comparable with human data. Finally, an initial prototype is presented as proof of concept.