Which muscle is primarily responsible for shoulder abduction?

The deltoid muscle is primarily responsible for shoulder abduction due to its anatomical placement and functional characteristics. The deltoid is a large, triangular muscle that forms the rounded contour of the shoulder. Its fibers are organized into three distinct parts: the anterior, lateral, and posterior fibers. Among these, the lateral fibers are particularly active in the abduction movement, especially when the arm is raised away from the body.

Shoulder abduction is the movement that occurs when the arm moves laterally away from the body, typically occurring in the first 15 degrees of abduction under the control of the supraspinatus (a muscle of the rotator cuff) followed by the continued contribution of the deltoid. The deltoid muscle becomes the prime mover for shoulder abduction as the arm is raised above shoulder level, effectively lifting the arm in an upward arc.

Other muscles listed, such as the latissimus dorsi, trapezius, and pectoralis major, have different primary functions. The latissimus dorsi is mainly involved in actions like shoulder extension and internal rotation; the trapezius primarily assists with movements of the scapula, including elevation and retraction; and the pectoralis major primarily functions in shoulder