Continuous passive motion (CPM) is a mechanical device that keeps the knee moving after surgery. Passive means the machine does the work. All you have to do is relax and let it bend and straighten the knee. Muscle strength and endurance aren't needed.
In most hospitals the CPM is applied right in the operating room. You'll wake up in recovery already using the CPM. Each day the range of motion will be increased to help you regain full motion. The goal is to get 90-degrees of knee flexion before leaving the