The deltoid muscle is divided into three parts: anterior, middle and posterior, with the anterior fascicle starting at the lateral half of the clavicle, the middle fascicle at the acromion, the posterior fascicle at the acromion, and the deltoid muscle stopping at the humeral deltoid ramus. The deltoid is a triangular-shaped muscle with the base up and the tip down that lies beneath the skin of the shoulder and wraps around the shoulder joint from the anterior and posterior lateral sides. The anterior deltoid muscle allows the upper arm to flex and rotate inward at the shoulder joint, the middle muscle bundle allows the upper arm to abduct, and the posterior muscle bundle allows the upper arm to extend or rotate outward at the shoulder joint. In addition, the deltoid muscles play a role in strengthening and stabilizing the shoulder joint.