How continuous is continuous passive motion of the knee?

In addition to its therapeutic benefits, continuous passive motion has recently found a novel application in inducing vibration of the patella which is a function of the surface characteristics of the patellofemoral articular cartilage. In order to simplify the interpretation of this vibration, it is important to maintain the relative motion of the two surfaces at a constant velocity. It has been found that, despite the highly complex geometry and kinematics of the knee joint, this condition may be satisfied by simply translating the heel from the buttocks at a constant linear velocity, due to the linear functions relating both knee angle and patellar displacement to heel translation.