The Inman concept of the mechanics of the deltoid describes a vertical upward oriented traction exerted on the upper end of the humerus at the beginning of arm elevation. However, Duchenne de Boulogne showed that the middle deltoid pushes the head downward. When the arm is at rest, the trajectory of the middle deltoid fibers changes by more than 90° so that the humeral 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 reconstructed shoulders. A new model, which includes the pulley effect, was designed 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 that the deltoid prevents upward migration of the humeral head and compresses it against the glenoid. This explains why many shoulders function well despite a massive cuff tear. This also implies that reeducation of the deltoid is a major aspect of the rehabilitation for patients with a rotator cuff tear.