Deltoid is a powerful muscle. It arises from a long strip of bone and converges triangularly to its insertion on the deltoid tuberosity. It covers the shoulder joint like a cape and its convex shape is due to the underlying upper end of the humerus. The anterior and posterior fibers are long, and in parallel bundles. The inter­mediate, or acromial fibers are multipennate. The muscle arises from the anterior border and upper surface of the flattened lateral one-third of the clavicle, from the whole of the lateral border of the acromion and from the inferior lip of the crest of the scapular spine as far medially as the ‘deltoid’ tubercle. On the lateral border of the acromion four ridges may be seen; from them four fibrous septa pass down into the muscle. The deltoid tuberosity on the humerus is U-shaped, with a central vertical ridge. From the ridge and limbs of the U three fibrous septa pass upwards between the four septa from the acromion. The spaces between the septa are filled with a fleshy mass of oblique muscle fibers which are attached to contiguous septa. The multipen­nate centre of the deltoid so formed has a diminished range of contraction, but a correspondingly increased force of pull. The anterior and posterior fibers, arising from the clavicle and the scapular spine, are not multi­pennate. They converge on the limbs of the U-shaped deltoid tuberosity of the humerus. Their range of move­ment is greater but the force of their pull is less than that of the multipennate central portion of the muscle.

Nerve supply of deltoid:

By the axillary nerve (C5 and 6), from the posterior cord of the brachial plexus. The nerve runs transversely round the back and lateral side of the surgical neck of the humerus giving off numerous branches that enter the muscle in radial directions; splitting the muscle vertically does not damage the nerve supply. The surface marking of the nerve is along a transverse line about 5 cm below the tip of the acromion.

Action of deltoid:

Working with supraspinatus, deltoid abducts the arm. Most of this work is done by the multipennate acromial fibers. The anterior and posterior fibers act like guy ropes to steady the arm in the abducted position. The anterior fibers acting alone assist pectoralis major in flexing the arm; the posterior fibers assist latissimus dorsi in extending the arm.