Rehab Line has 5530 leg Extension / Curl Rehab with dual function unit adds for a safe and effective rehabilitation of the extensor and flexor muscle of the knee joint. It can also balance the weight of the leg itself in order to start the rehabilitative process almost immediately after surgery. Important Factors Range Limiters: The range limiters cut out dangerous areas of motion and by using the performance recorder you can test and follow up the progress of the person.