A semi-actuated above knee prosthetic system, which is mostly passive in nature and includes a shank link coupled to an artificial foot, a knee mechanism connected to the shank link and a thigh link attached to an above-knee remaining lower limb of an amputee, is operable in either an actuated mode or an un-actuated mode controlled by a signal processor linked to various prosthetic mounted sensors which may include combinations of knee angle, stance, thigh angle and shank angle sensors. Power is delivered through an electric motor connected to a battery source and employed to drive a hydraulic pump which is part of an overall hydraulic power unit including the torque generator. A signal processor selects a swing state from at least forward, combination forward and descent, combination forward and ascent, reverse, combination reverse and descent, and combination reverse and ascent swing states.