The deltoid is the main muscle that makes up the shoulder. It originates on the clavicle, acromion, and spine of the scapula. It inserts on the deltoid tuberosity of the humerus. The deltoid acts at the shoulder to produce abduction, and assist with flexion and extension. The deltoid is especially prone to developing trigger points. These posterior style will refer pain into the anterior and lateral shoulder. Trigger points in the posterior delt will refer pain into the posterior shoulder with spillover down the lateral arm.