In an embodiment, a prosthetic leg including a modular knee element coupled to a modular ankle element. The modular knee element includes a knee motor rotatable about an axis of a shaft in the knee motor. The modular ankle element includes a drive portion coupled to a surface-contacting portion via a pylon having a top and bottom end. The top end is coupled to the drive portion and the bottom end is coupled to the surface-contacting portion. The drive portion includes an ankle motor coupled to a rotary series elastic actuator (RSEA) and a first pulley coupled to the RSEA. The surface-contacting portion includes a hinge and a second pulley with a belt engaging at least a portion of at least one of the first and second pulley, and a modular foot element coupled to the bottom of the hinge.