Deltoid Muscle

The deltoid muscle is a thick, flat triangular muscle which forms the round shape of the shoulder. One end is attached to the collar bone and shoulder blade, and the other to the humerus bone in the arm. The name derives from the shape of the Greek letter Delta (triangle). The deltoid is an antagonist muscle of the pectoralis major and latissimus dorsi during arm adduction.

The deltoid takes part in the movements of the upper arm, lifting the arm sideways away from the midline of the body. The deltoid main actions are flexion and horizontal adduction of the humerus (upper arm bone); those of the middle fibers are abduction and horizontal abduction; and those of the posterior fibers are extension and horizontal abduction.

The deltoid muscle has three sections, the front head, the side head and the rear head, and is innervated by the axillary nerve. Weight training exercises for strengthening the deltoid muscle include dumbbell curls, the dumbbell punch, and the military press.