Deltoid Muscle

The deltoids are a fan shaped muscle originated from the lateral 1/3 of the spine of the scapula (posterior), the acromion (middle) and the lateral portion of the clavicle (anterior). Because the fibers of each section are identical as far as where they attache on the upper arm, the muscular actions actually overlap considerably. It would be best to consider the deltoids as a singular fan-shaped muscle, rather than the traditional three separated "heads"