An accessory movement is one which occurs as part of a physiological movement but which the patient cannot perform himself in isolation. For example, when you extend the knee the tibia glides anteriorly on the femur as part of the movement but the patient is not able to do this in isolation without also extending the knee. The therapist can however do a passive anterior glide of the tibia (an anterior draw) with the knee in partial flexion in order to mobilise this part of the movement and thus help to increase extension in a knee that is lacking the last few degrees of extension. Another example would be in the shoulder where the action of putting the hand behind the back (medial rotation) results in an anterior movement of the head of the Humerus in its socket, so doing a Maitland PA glide can help to mobilise medial rotation--the patient however is again not able to do this movement except as part of the full physiological movement of medial rotation.