Origin: Anterior head (front): Anterior surface of lateral clavicle
Lateral head (middle): Acromion process and spine of scapula
Posterior head (rear): Scapula, inferior edge
Insertion: All heads converge at deltoid tuberosity of humerus
Action:
Anterior Head(front): flexes and medially rotates the whole arm at the shoulder
Lateral Head (middle): Abducts the whole arm at the shoulder
Posterior Head (rear): Primarily hyper-extends shoulder, also involved in transverse extension