Abstract: An apparatus for use in bracing or exercising a knee joint in a manner that allows bending of the joint only along a predetermined path which approximates the natural bending of the joint. On one side of the joint are upper and lower struts that run along and are attached to the upper and lower leg and are attached to one another near the joint by a connecting mechanism which includes a set of linkage arms and telescoping elements that allow the struts to pivot relative to one another along the prescribed pivot line. On the other side of the joint are other upper and lower struts that run along and are attached to the upper and lower leg and are attached to one another near the joint by a ball and socket connector. The apparatus may be used as a brace or, alternatively, may be used with a cycling mechanism to cyclically flex and extend the joint or with a force-applying mechanism to apply a constant force urging the extension or flexing of the joint.