A CPM machine is a device fitted to the leg, usually after surgery, to keep the limb slowly bending and straightening.
Continuous passive motion machine
Continuous Passive Motion, Early Weight Bearing, and Active Motion following Knee Articular Cartilage Repair. Howard JS, Mattacola CG, Romine SE and Lattermann C. Cartilage. 2010 Oct; 1(4): 276–286.