Deltoid muscle

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

It was previously called 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.

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.

Innervation

Axillary nerve (C5,C6)

Action

The anterior fibres are involved in shoulder abduction when the shoulder is externally rotated. The anterior deltoid is weak in strict transverse flexion but assists the pectoralis major during shoulder transverse flexion / shoulder flexion (elbow slightly inferior to shoulders).

The posterior fibres are strongly involved in transverse extension particularly since the latissimus dorsi is very weak in strict transverse extension. The posterior deltoid is also the primary shoulder hyperextensor.

The lateral fibres are involved in shoulder abduction when the shoulder is internally rotated, are involved in shoulder flexion when the shoulder is internally rotated, and are involved in shoulder transverse abduction (shoulder externally rotated) -- but are not utilized significantly during strict transverse extension (shoulder internally rotated).

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.

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