Deltoid

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..

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.

When all its fibers contract simultaneously, the deltoid is the prime mover of arm abduction along the frontal plane. The arm must be internally rotated. This makes the deltoid an antagonist muscle of the pectoralis major and latissimus dorsi during arm adduction.

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).

Axillary nerve

Primarily posterior circumflex humeral artery

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The deltoid muscle has three sections, the front head, the side head and the rear head. These heads have different actions and so need different strength training exercises to completely train the whole of the muscle.

• The Front deltoid is used in all pressing movements, such as the bench press, and can be trained adequately without any isolation movements. However for isolation front dumbbell raises are the most commonly used.

• The middle head of the deltoid is the main abductor at the gleno-humeral joint (shoulder) and is commonly trained using side lateral raises (also known as the shoulder fly). The middle head also benefits from overhead pressing movements.

• The rear head of the deltoid is best trained with bent-over dumbbell laterals (an inverted fly)

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 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.