The Deltoid originates from both elements of the pectoral girdle. It arises from the acromion, the spine of the scapula, and the superior surface of the lateral 1/3 of the clavicle. It inserts on the deltoid tuberosity of the humerus.

Injury to the axillary nerve, resulting in paralysis of the deltoid muscle, presents as flattening of the shoulder cap.