An electromyographic study of various muscles in the region of the shoulder joint in twenty-two healthy men combined with a study of dissections shows that, contrary to expectations, downward dislocation of the shoulder joint is not prevented by the vertically running muscles, for example, the deltoid, biceps, and triceps. Normally it is prevented by a locking mechanism dependent on three factors: (1) the slope of the glenoid fossa, leading to (2) the tightening of the superior part of the capsule (including the coracohumeral ligament), and (3) the activity of the supraspinatus muscle (and, to a lesser extent, the posterior fibers of the deltoid).