Abstract: An apparatus for exercising muscles of the leg comprising a frame, a padded sling for engaging the back of the knee, a load and a cable running from the load to the sling over one or more pulleys hung from the frame so that the load loading is applied perpendicularly to the back of the knee as it is straightened. A foot rest aligned with the sling assures that the foot remains planted throughout the movement of the leg except for flexing at the ankle. At least one of the pulleys is adjustable so that legs of different length can be accommodated. The type of load includes weights, an hydraulic cylinder, extension or compression springs and isokinetic mechanisms.