Shoulder Muscles and their Functions

The shoulder muscles they constitute an intricate system of superimposed and interwoven muscle fibers that extend from the scapula, clavicle and ribs to the humerus, from all directions. This complex configuration is due to the fact that the shoulder is the articulation with the greatest range of movements of the whole organism.

For this reason, numerous muscles are required that act synergistically to achieve such varied mobility. Although most of these muscles are small or medium, by working synergistically they manage to exert a remarkable amount of force without compromising the precision and fineness of movement

This precision is due to the fact that each movement has agonist (effector) and antagonist (brakes) muscles. Each of these muscles allows a millimeter control of each movement made by the shoulder.

Shoulder movements

It is impossible to understand the muscles of the shoulder if the movements that this joint is able to perform are unknown.

In this sense and to facilitate the understanding of the biomechanics of the shoulder muscles, a brief review of the movements of this area is indispensable, so that the action of each muscle group can be understood:

Abduction

It is the separation of the trunk arm; that is, that movement that moves the arm and forearm away from the body.

Adduction

It is the movement opposed to abduction; that is, the one that brings the arm to the trunk. Although the cessation of the action of the shoulder abductors could cause the arm to fall by gravity, it would be an uncontrolled movement.

To avoid this, the adductors act together with the abductors to allow the arm to approach the trunk gently. In addition, the shoulder adductors allow to exert pressure between the inner side of the arm and the trunk.

Flexion

The flexion of the shoulder differs from the classic concept of flexion where one part of the limb approaches another, as occurs with the flexion of the elbow, when the forearm approaches the arm.

In the case of the shoulder, the flexion consists in the forward elevation of the arms, being possible to even reach the vertical.

That is, go from the natural position (arms extended to both sides of the body), go through the intermediate flexion (tip of the fingers pointing forward) and reach the maximum flexion of 180º, in which the fingers point to the sky.

Extension

It is a movement totally opposite to the previous one. In this case, the arm"extends"backwards. The range of the extension is much more limited, reaching no more than 50º.

Internal rotation

During internal rotation, the anterior aspect of the arm approaches the trunk while the posterior one moves away. If the shoulder is seen from above, it is a movement against the hands of the clock.

External rotation

Movement opposite to the previous one. In this case, the front face of the arm moves away from the trunk and the posterior face approaches. Seen from above it is a movement in a clockwise direction.

Circumduction

Some authors consider it a separate movement while for others it is the sequential combination of all shoulder movements.

During circumduction the arm draws a circle whose center is the gleno-humeral joint (between the scapula and the head of the humerus). When this movement is made, practically all the muscles of the shoulder are used in a coordinated and sequential manner.

Shoulder muscles and their functions

The different muscles of the shoulder function as main engines in some movements, secondary engines in others and antagonists in another group of movements. The following are the muscles with their most important functions:

Deltoid

It is the largest and most visible muscle of the shoulder, being the one that greater degree of development reaches.

Although it is a single muscle, the deltoid is composed of three parts or bellies: one anterior (forming the delto-pectoral groove in front), one medium (covering the shoulder above) and one posterior.

When acting in unison, the three bellies of the deltoid become the main abductor of the shoulder, since they are antagonists of adduction.

When the anterior belly of the deltoid is contracted, the muscle functions as a secondary motor in the flexion of the shoulder; whereas when the posterior belly does, it is a secondary motor in the extension.

Subscapular

This muscle is responsible for the internal rotation of the shoulder.

Supraspinatus

The main function of the supraspinatus is to be a shoulder abductor; therefore, it is an adduction antagonist.

Infraspinatus

Anatomically it is the natural antagonist of the supraspinatus and, therefore, it is considered an adductor of the shoulder, which works synergistically with the deltoid. In addition, it is a secondary motor in the external rotation of the shoulder.

Greater round

It is a polyvalent muscle involved in multiple movements. Its main function is to be a shoulder adductor; for this he works in unison with the supraspinatus.

In addition, it has an important role in the extension of the shoulder and works as a secondary motor in the internal rotation of the same.

Round smaller

Anatomically it is similar to the greater round but at the same time quite different. Given its location, it is an adductor arm, so it works synergistically with the greater round and enhances its effect.

However, when it comes to shoulder rotation, the minor round is an antagonist of the greater round, taking part in the external rotation of the shoulder.

Coracobrachial

It is not a shoulder muscle proper; in fact, it is part of the anterior brachial region. However, its insertion in the coracoid process of the scapula makes this muscle a notable adductor of the shoulder.

Pectoralis major

Like the previous one, it is not a muscle of the shoulder region. However, its humeral inserts and its large size make it an important motor of several shoulder movements.

The pectoralis major takes part in the extension of the shoulder, as well as in the internal rotation and adduction.

It is a very powerful muscle that, by working in an antagonistic manner with the pectoralis major, allows controlled and precise arm abduction. In addition, in forced adduction the breastplate generates a lot of force to keep the arms firmly attached to the trunk.

Dorsal width

This is a large back muscle that takes inserts in the humerus. Its anatomical position allows it to function as an extensor and adductor of the shoulder, when it takes fixed point in the insertions of the back and exerts the movement with its humeral portion. It is also a secondary agonist in the internal rotation of the shoulder.

References

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