Full Deltoid Muscle Description
[Continued from above] . . .
The deltoid has three origins: the lateral end of the clavicle, the acromion of the scapula at the top of the shoulder, and the spine of the scapula. Each origin gives rise to its own band of muscle fibers with the anterior band forming at the clavicle, the lateral fibers forming at the acromion, and the posterior fibers forming at the spine of the scapula. The bands merge together as they approach the insertion point on the deltoid tuberosity of the humerus.
The deltoid has three distinct functions that correspond to the three bands of muscle fibers. Contraction of the anterior fibers flexes and medially rotates the arm by pulling the humerus towards the clavicle. Flexion and medial rotation of the arm moves the arm anteriorly, as in reaching forward or throwing a ball underhand. The lateral fibers abduct the arm by pulling the humerus toward the acromion. Abduction of the arm results in the arm moving away from the body, as in reaching out to the side. Contraction of the posterior fibers extends and laterally rotates the arm by pulling the humerus toward the spine of the scapula. Extension and lateral rotation moves the arm posteriorly, as in reaching backwards or winding up to throw a ball underhand.
Prepared by Tim Taylor, Anatomy and Physiology Instructor