Why biofeedback would help this problem: The patella (knee cap) needs to move smoothly in a set direction along with the rest of the joint during almost every movement of the knee. Position and motion of the patella is nearly entirely controlled by the vastus lateralis and vastus medialis muscles which run from the patella to attachments in the thigh along the femur. The patella can come off track to one side and ride up onto the rest of the joint (types of subluxation) when there is an imbalance between the pulls exerted by the vastus lateralis and vastus medialis muscles. Patelofemoral pain can result from subluxation of the patella. This imbalance may occur due to an injury which weakens one of the muscles. Muscle tension biofeedback is used to measure the ratio of tensions between the vastus lateralis and medialis muscles in the non-painful leg under various amounts of stress in different positions. The ratios for the good leg are compared to ratios of those recorded in the same positions and stresses for the painful leg. People use the display of the tension patterns to match the signals from the bad knee to those from the good knee. When the patterns of movement are corrected through this process, the pain usually goes away.