The stability of a robot subject to active constraints under the exertion of a human force is analyzed. Artificial potentials that are used to create a barrier on constraint surfaces are revisited and the robot's stability in the presence of active constraints is examined in terms of output passivity and state boundedness. It is shown that under input forces of finite energy the state is bounded and hence the motion is confined within the constraint region. However, the quality of response depends on the type of input and may include undesirable transients for physical human robot interaction (pHRI) applications.