Deltoid muscle

The deltoid muscle is the muscle forming the rounded contour of the human shoulder.

It was earlier called as the Deltoideus and the name is still used by some anatomists. It is called so because it is in the shape of the Greek letter Delta (triangle).

The deltoid is a frequent site to administer intra-muscular injections.

Table of contents

1 Origin
2 Insertion
3 Innervation
4 Action
5 Structure

Origin

It arises in three distinct sets of fibres:-

Anterior fibres : from the anterior border and upper surface of the lateral third of the clavicle
Middle fibres : from the lateral margin and upper surface of the acromion
Posterior fibres : from the lower lip of the posterior border of the spine of the scapula, as far back as the triangular surface at its medial end

Insertion

From this extensive origin the fibers converge toward their insertion, the middle passing vertically, the anterior obliquely backward and lateralward, the posterior obliquely forward and lateralward; they unite in a thick tendon, which is inserted into the V-shaped deltoid tubercle on the middle of the lateral aspect of the shaft of the humerus. At its insertion the muscle gives off an expansion to the deep fascia of the arm.

Action

The anterior fibres assist Pectoralis Major to flex the shoulder. The posterior fibres assist Latissimus Dorsi to flex the shoulder. The middle fibres abduct the shoulder from 15 degrees to 90 degrees.

Structure

The Deltoid is a classical example of a multipennate muscle.

The middle fibres of the muscle arise in a bipenniform manner (like a bird's feather) from the sides of the tendinous intersections, generally four in number, which are attached above to the acromion and pass downward parallel to one another in the substance of the muscle. The oblique fibers thus formed are inserted into similar tendinous intersections, generally three in number, which pass upward from the insertion of the muscle and alternate with the descending septa. The portions of the muscle arising from the clavicle and spine of the scapula are not arranged in this manner, but are inserted into the margins of the inferior tendon.