Mechanics of the deltoid muscle

The Inman concept of the mechanics of the deltoid describes a vertical upwa rd oriented traction exerted on the upper end of the humerus at the beginni ng of arm elevation. However, Duchenne de Boulogne showed that the middle d eltoid pushes the head downward. When the arm is at rest, the trajectory of the middle deltoid fibers changes by more than 90 degrees so that the hume ral head is enveloped by the muscle; this suggests that the deltoid acts on the humeral head like a cable on a pulley. The authors studied the area of contact between the deltoid and the humeral head in three-dimensional reco nstructed shoulders. A new model, which includes the pulley effect, was des igned to explore the resultant total force applied by the deltoid onto the humerus, In most cases the resultant vertical force was oriented downward. Thus, the conventional model is not complete. The current model indicates t hat the deltoid prevents upward migration of the humeral head and compresse s it against the glenoid, This explains why many shoulders function well de spite a massive cuff tear. This also implies that reeducation of the deltoi d is a major aspect of the rehabilitation for patients with a rotator cuff tear.