The deltoid muscle is a large, flat skeletal muscle that, when spread, resembles a triangular scarf and spans the entire shoulder. It holds the head of the humerus in the socket and, together with other muscles, serves to raise the upper arm in a certain angular range.

What is the deltoid muscle?

The deltoid or deltoid muscle is part of the shoulder musculature and surrounds the shoulder joint in a front part (anterior deltoid muscle), a lateral part (middle deltoid muscle) and a posterior part (posterior deltoid muscle).

The anterior deltoid muscle originates from the clavicle ( clavicle ), the middle one from the acromion, and the rear deltoid from the scapula. All three parts of the muscle point downwards towards the tuberosity deltoidea, to the point of attachment to the humerus at the side of the condyle, which is shared by all three parts of the approximately 2 cm thick deltoid muscle. The expression of the deltoid muscle gives the shoulder its appearance. A trained deltoid gives the impression of "broad shoulders". Motorically, the deltoid muscle is innervated by a branch of the axillary nerve (the axillary nerve ), which innervates two other shoulder muscles. The axillary nerve originates from the brachial plexus, which is connected to the spinal cord between the 5th and 6th cervical vertebrae (C5-C6).

Anatomy & Construction

The deltoid muscle is a three-part, flat skeletal muscle, which - like almost all skeletal muscles - is composed of striated muscle. The deltoid muscle is subject to the will and is motor 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 consists of several thousand thread-like myofibrils. They run through the muscle fibers over their entire length and are composed of small units, the myofilaments. In the myofilaments, which consist of special proteins, the muscle contraction occurs.

The individual myofilaments, upon receipt of the contraction command, merge into one another to form the typical stripe pattern that has given the skeleton musculature the additional designation cross-striped. However, the deltoid muscle is not a pure command receptor over the efferent nerve fibers, which can only transmit the command "contract". The muscle also communicates with the central nervous system via sensory afferent fibers of the mixed axillary nerve and may affect the immune system and other bodily functions.

Function & 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 allow movement of the upper arm in all imaginable directions and rotations.

The front part of the flat muscle ( Pars clavicularis ) allows the arm to be lifted forward (upwards), an adduction to the body and an internal rotation. The middle part of the muscle ( Pars acromialis ) can spread the arm (away from the body), and the back part ( Pars spinalis ) is responsible for back elevation (back up), for external rotation and also for the spread in a certain angle range.

Combined lifting, spreading and rotation movements are only possible in a complex mutual support. Some of the three muscle parts act as synergists or as antagonists, such as the pars clavicularis and the pars acromialis. While the former may act as an adductor, the pars acromialis appears as an abductor and is in this case the antagonist to the clavicular pars.

In addition to its function as a "mover" of the upper arm, the deltoid still has a protective function. It envelops the shoulder joint flatly and thus acts as a mechanical protection for the shoulder joint from the effects of violence by biting stabs or blows with blunt objects. The mechanical protection function is comparable to that of a flexible shoulder protector.

It is also interesting that in the area of ​​the deltoid muscle, which is located at an exposed point, no larger vessels or nerve tracts run. This means that while mechanical violence can injure the deltoid muscle, it can hardly cause secondary damage from nerve or vascular lesions.

Diseases & complaints

Deltoid muscle dysfunction may be caused by abnormal changes in or on the muscle itself or may originate from damage to the axillary nerve. The most common type of impairment of the deltoid muscle is a permanent tension, which can be caused by overloading or constantly repeating stress loads.

Also, long-term stress favors the formation of such tension. Unfavorable postures on the PC, combined with stressful situations favor the development of muscle hardening enormously. A direct impairment of the deltoid muscle can be caused by so-called lipomas.

These are benign tumors of the cells of adipose tissue. Lipomas in the deltoid are usually noticeable by small bumps on the surface. In rare cases, the muscle may be affected by venous thrombosis or inflammation of the arm veins. Shoulder pain in the area of ​​the deltoid muscle often does not affect the muscle itself, but an inflamed bursa that causes the discomfort.

The most common nerve-related impairments of the muscle are in a (mechanical) compression of the axillary nerve, which can be triggered by narrowing in the "passages" of the nerve and lead to neuritis. Another, nervous impairment, can be caused by a rarely occurring shoulder amyotrophy.

It is an inflammation in the neurovascular plexus brachialis, from which the axillary nerve originates and innervates the deltoid muscle. The symptoms are sudden, tearing pain in the shoulder that may radiate into the arm. As the disease progresses, it causes paralysis, which mainly affects the deltoid muscle.

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