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Deltoids - Anterior, Medial & Posterior

In human anatomy, the deltoid muscle is the muscle forming the rounded contour of the 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 fibers:

Anterior fiber: from the anterior border and upper surface of the lateral clavicle.

Middle fibers: from the lateral margin and upper surface of the acromion.

Posterior fibers: 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 laterally, the posterior obliquely forward and laterally; they unite in a thick tendon, which is inserted into the V-shaped deltoid tuberosity 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 fibers 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 fibers 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 fibers 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).

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