A semi-actuated prosthetic knee device, configured to be coupled to an above-knee remaining lower limb of an amputee and operable in both actuated and un-actuated modes, comprising: an artificial foot having a toe and a heel; a shank link coupled to the artificial foot; a thigh link configured to be attached to an above-knee remaining lower limb of an amputee; a knee mechanism interconnecting said thigh link and said shank link, said knee mechanism allowing flexion and extension movements of said thigh link and said shank link relative to each other; a hydraulic torque generator interposed between said shank link and said thigh link; a hydraulic power unit including a hydraulic valve circuit hydraulically coupled to said torque generator, a hydraulic pump, and an electric motor mechanically coupled to said hydraulic pump; a power source for providing electric power to said electric motor; and a signal processor connected to the power source; characterized by a stance sensor capable of identifying which part of the artificial foot is in contact with ground; a knee angle sensor arranged to create a knee angle signal representing the angle between said shank link and said thigh link; and said signal processor being further connected to said knee angle and stance sensors (120, 124) and the hydraulic power unit for controlling operation of the hydraulic power unit based on signals (155, 234) received from the knee angle and stance sensors (120, 124) wherein, when said semi-actuated prosthetic knee operates in its actuated mode, electric power is transferred from said power source to said electric motor to drive the hydraulic pump to provide working fluid to said torque generator under control of the signal processor in order to create a torque between said shank link and said thigh link and, when said semi-actuated prosthetic knee operates in its un-actuated mode, said hydraulic valve circuit modulates a resistance of fluid flow in said torque generator such that said semi-actuated prosthetic knee operates without any transfer of electric power from said electric power source to said electric motor. an artificial foot having a toe and a heel; a shank link coupled to the artificial foot; a thigh link configured to be attached to an above-knee remaining lower limb of an amputee; a knee mechanism interconnecting said thigh link and said shank link, said knee mechanism allowing flexion and extension movements of said thigh link and said shank link relative to each other; a hydraulic torque generator interposed between said shank link and said thigh link; a hydraulic power unit including a hydraulic valve circuit hydraulically coupled to said torque generator, a hydraulic pump, and an electric motor mechanically coupled to said hydraulic pump; a power source for providing electric power to said electric motor; and a signal processor connected to the power source; a stance sensor capable of identifying which part of the artificial foot is in contact with ground; a knee angle sensor arranged to create a knee angle signal representing the angle between said shank link and said thigh link; and said signal processor being further connected to said knee angle and stance sensors (120, 124) and the hydraulic power unit for controlling operation of the hydraulic power unit based on signals (155, 234) received from the knee angle and stance sensors (120, 124) wherein, when said semi-actuated prosthetic knee operates in its actuated mode, electric power is transferred from said power source to said electric motor to drive the hydraulic pump to provide working fluid to said torque generator under control of the signal processor in order to create a torque between said shank link and said thigh link and, when said semi-actuated prosthetic knee operates in its un-actuated mode, said hydraulic valve circuit modulates a resistance of fluid flow in said torque generator such that said semi-actuated prosthetic knee operates without any transfer of electric power from said electric power source to said electric motor.