Deltoid Muscle: Structure, Function & Diseases

The deltoid muscle is a large sheet-like skeletal muscle that resembles a triangular scarf when extended and spans the entire shoulder. It holds the head of the humerus in the socket and, together with other muscles, serves to elevate the humerus within a certain angular range.

What is the deltoid muscle?

The deltoid or deltoid muscle is part of the shoulder musculature and spans and wraps the shoulder joint in an anterior part (anterior deltoid), a lateral part (middle deltoid) and a posterior part (posterior deltoid). The anterior deltoid originates from the clavicle (collarbone), the middle from the acromion (shoulder roof), and the posterior deltoid originates from the scapula. Downward, all three parts of the muscle run pointedly toward the deltoid tuberosity, the point of attachment to the humerus lateral to the condyle, which is shared by all three parts of the deltoid muscle, which is about 2 cm thick. The expression of the deltoid muscle gives the shoulder its appearance. A trained deltoid gives the impression of “broad shoulders.” The deltoid muscle is motorically innervated by a branch of the axillary nerve, which innervates two other shoulder muscles. The axillary nerve originates from the brachial plexus, which connects to the spinal cord between the 5th and 6th cervical vertebrae (C5-C6).

Anatomy and structure

The deltoid muscle is a three-part sheet skeletal muscle that, like almost all skeletal muscles, is composed of striated muscle. The deltoid muscle is subject to volition and is motorically innervated by an efferent branch of the axillary nerve. Each part of the deltoid muscle is composed of muscle fiber bundles, each muscle fiber in turn consisting of several thousand filamentous myofibrils. They run through the muscle fibers along their entire length and are themselves composed of small units called myofilaments. Muscle contraction takes place in the myofilaments, which are made of special proteins. After receiving the command to contract, the individual myofilaments push into each other, forming the typical striated pattern that earned skeletal muscle the additional designation of striated. However, the deltoid muscle is not a pure command receiver via the efferent nerve fibers, which can only transmit the command “contract”. The muscle is also connected to the central nervous system via sensitive afferent fibers of the mixed axillary nerve and can influence the immune system and other body functions.

Function and structure

The deltoid muscle is responsible for most of the movements of the upper arm. In a complex interaction, the three parts of the deltoid muscle allow the upper arm to move in all conceivable directions and rotations. The anterior part of the flat muscle (pars clavicularis) allows the arm to be lifted forward (forward, upward), strained (adducted) against the body, and internally rotated. The middle part of the muscle (pars acromialis) can spread the arm (away from the body), and the posterior part (pars spinalis) is responsible for lifting back (backwards, upwards), for external rotation and also for spreading in a certain angular range. Combined lifting, spreading and rotating movements are only possible in a complex mutual support. In some cases, the three muscle parts act as synergists among themselves or as antagonists, such as the pars clavicularis and the pars acromialis. Whereas the former can act as an adductor (inducer), the pars acromialis acts as an abductor (abductor) and in this case is the antagonist to the pars clavicularis. In addition to its function as a “mover” of the upper arm, the deltoid also has a protective function. It envelops the shoulder joint over a large area and thus acts as mechanical protection for the shoulder joint against the impact of blows or blunt objects. The mechanical protective function is comparable to that of a flexible shoulder protector. It is also interesting to note that no major vessels or nerve tracts run in the area of the deltoid muscle, which is in an exposed position. This means that although mechanical force can injure the deltoid muscle, it is unlikely to cause secondary damage through nerve or vascular lesions.

Diseases and ailments

Functional impairment of the deltoid muscle may be caused by pathologic changes in or to the muscle itself or originate from damage to the axillary nerve. The most common type of impairment of the deltoid muscle is due to permanent tension, which can be caused by overuse or constantly repetitive incorrect loading. Permanent stress also favors the development of such tensions. Unfavorable postures at the PC, combined with stressful situations, enormously favor the development of muscle stiffness. A direct impairment of the deltoid muscle can be caused by so-called lipomas. These are benign tumors of the cells of the fatty tissue. Lipomas in the deltoid muscle are usually noticeable by small bumps on the surface. In rare cases, the muscle can be affected by venous thrombosis or inflammation of the arm veins. Shoulder pain in the deltoid area often does not involve the muscle itself, but an inflamed bursa that causes the discomfort. The most common nerve-related impairments of the muscle are due to (mechanical) compression of the axillary nerve, which can be triggered by constrictions in the “passage points” of the nerve and lead to neuritis. Another, nerve-related impairment, can be caused by a rather rare shoulder amyotrophy. It is an inflammation in the brachial plexus, from which the axillary nerve originates and innervates the deltoid muscle. Symptoms include sudden, tearing pain in the shoulder that may radiate into the arm. As the disease progresses, paralysis occurs, mainly affecting the deltoid muscle.