Shoulder extension is the action of moving the arm backward at the glenohumeral joint. The muscles that actively produce this motion are those that pull the humerus posteriorly: latissimus dorsi provides powerful extension and adduction; the posterior fibers of the deltoid assist in extending the arm when it’s elevated or abducted; the long head of the triceps brachii crosses the shoulder and contributes to extension, especially as the elbow is flexed; and the sternocostal (lower) portion of the pectoralis major can help extend the shoulder from a flexed position, aiding the backward pull of the humerus. Together they form the agonist group for this movement. In contrast, the anterior deltoid and the clavicular fibers of the pectoralis major primarily flex the shoulder, while the supraspinatus and infraspinatus are more about stabilization and external rotation, not primary extension.