The invention is directed to controlling therapy delivery based on a relative motion between a first and second activity sensor. The relative motion between the activity sensors is representative of the relative motion between the locations of the body of the patient at which the respective activity sensors are located. The use of relative motion, however, may substantially remove motion experienced by both the activity sensors, e.g., motion caused by the environment in which patient is located, thus providing a new reference frame from which to analyze the motion measurements. The relative motion may be used to detect a condition of a movement disorder and/or control delivery of the therapy delivered to patient to treat or reduce the condition.