Abstract: A knee joint which locks when the weight of an amputee is imposed upon the prosthesis, such as when taking a step. The joint includes a housing which is rotatably mounted relative to a shaft. A plurality of metal discs are mounted on the shaft so as to be axially moveable but non-rotatable relative thereto. Interleafed with the metal plates, a plurality of fiber discs may be mounted so as to be both rotatable and axially moveable relative to the shaft, but fixed against rotation relative to the housing. A pair of wedge-shaped reaction discs may be mounted on the shaft at approximately the center of the housing. A double-wedge-shaped actuating disc may be mounted between the two reaction discs for radial movement relative to the shaft. An actuator rod connected to the stump-receiving portion of the prosthesis presses against the actuator when the amputee puts weight on the prosthesis. This causes the interleafed discs to engage in friction contact to prevent rotation of the housing and the shaft.