A damped knee joint may be locked myoelectrically at any angle of flexing. A special hydraulic cylinder is pivoted above the knee center at one end and in the shank at the other end. This attachment strategy allows 110 degrees of knee flexion, permitting the normal range of sitting and standing positions. The cylinder assembly incorporates a closed loop, sealed hydraulic system and electro-mechanical valving to control piston position. The closed loop hydraulic system also prevents fluid leakage and contamination. Valving is accomplished by a microelectrically operated barrel or gate valve located in a hydraulic system. The use of a barrel or gate valve allows the microelectrically operated plunger to be physically isolated from the working fluid and to be operated reliably at the highest working pressure of the systems.