A lower limb prosthesis includes a thigh component 2, a shin component (3, Figure 1) and a knee joint (5, Figure 1) disposed between the thigh component 2 and the shin component (3) to permit angular movement between an extended position and a flexed position. A knee flexion control unit 6 (Figure1) is arranged to lock the prosthesis in the extended position for supporting the weight of an amputee during a stance phase, and to unlock the prosthesis in response to a hyperextension moment. An extension stop 20, 21 is positioned to limit the extension of the knee joint. The stop 21 has a resilient characteristic and is movable to adjust the extent at which it limits the extension of the knee joint. Preferably the stop 21 is positioned between the thigh component 2 and the shin component (3). Movement of the stop 21 preferably adjusts the magnitude of the hyperextension moment required to unlock the prosthesis. Optionally, the stop 21 comprises a screw with a plurality of longitudinal grooves 24 and a screw retention assembly 26 engages with these grooves 24, allowing the screw to be turned in steps. Preferably the extension stop comprises a resilient fixed stop 20 and an opposite adjustable stop 21.
