Most clinicians identify the posterior deltoid as a prime mover in shoulder extension.
The prone position, with the arm at the side and then lifted off the table or from the arm off the side of the table and up generally offers the highest amplitude scores seen in testing this muscle. One reason for the success in this position is that the posture of the exercise has some constraints applied to it, limiting the arm from moving into abduction, or rotation, where another muscle might have the opportunity to provide assistance in performing shoulder extension. Assistance might reduce the activity of the posterior deltoid. Two major factors influence whether or not a muscle fires as the prime mover per the expectation of the clinician. The first is the degree to which postural constraints are imposed on the posture of the exercise to enhance recruitment of the desired muscle.