This exercise is for those who can't do other quad movements without knee pain. In conjunction with investing in sessions with a physical therapist, a good exercise to help re-establish the range of motion, connective tissue strength, and quad strength directly surrounding the knee is the Peterson step up. As seen in the video, start with a low elevation and allow the knee to track forward over the toe on the working leg. You can lift the heel off the step. Disallow the supporting leg from helping out by "pushing off" of the floor. Once you can perform this with no problem, don't think of adding weight to the lift – think of adding range of motion. Use a higher box and start again.