SHOULDER MUSCLES AND THEIR FUNCTIONS - ANATOMY AND PHYSIOLOGY - 2025

The shoulder muscles are an intricate system of overlapping and intersecting 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 joint with the greatest range of motion in the entire body.

For this reason, many muscles that act synergistically are necessary to achieve such varied mobility. Although most of these muscles are small to medium-sized, by working synergistically they can exert a remarkable amount of force without compromising precision and finesse of movement.

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

Shoulder movements

It is impossible to understand the muscles of the shoulder without knowing the movements that this joint is capable of carrying out.

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

Abduction

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

Adduction

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

To avoid this, the adductors work together with the abductors to allow the arm to approach the trunk smoothly. In addition, the shoulder adductors allow pressure to be exerted between the inside of the arm and the trunk.

Flexion

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

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

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

Extension

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

Internal rotation

During internal rotation, the front of the arm is closer to the trunk while the back moves away. If the shoulder is viewed from above, it is a counterclockwise movement.

External rotation

Movement opposite to the previous one. In this case, the front side of the arm moves away from the trunk and the back side moves closer. Viewed from above it is a clockwise movement.

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 glenohumeral joint (between the scapula and the head of the humerus). When this movement is performed, practically all of the shoulder muscles are used in a coordinated and sequential manner.

Shoulder muscles and their functions

The different muscles of the shoulder function as primary motors in some movements, secondary motors in others, and antagonists in another group of movements. The muscles with their most prominent functions are listed below:

Deltoid

Source: wikimedia.org/wiki/File:Deltoideus.png. Author: sv: Användare: Chrizz

It is the largest and most visible muscle in the shoulder, being the one with the highest degree of development.

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

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 flexing the shoulder; whereas when the posterior belly does, it is a secondary motor in extension.

Subscapular

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

Supraspinatus

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

Infraspinous

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 external rotation of the shoulder.

Major round

Image source:

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

In addition, it has an important role in shoulder extension and functions as a secondary motor in its internal rotation.

Minor round

Source: ca.wikipedia.org/wiki/Fitxer:Teres_minor_muscle_back3.png. Author: Anatomography

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

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

Coracobrachial

It is not a proper shoulder muscle; in fact, it is part of the anterior brachial region. However, its insertion into 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 attachments and large size make it an important driver of various movements of the shoulder.

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

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

Latissimus dorsi

Source: commons.wikimedia.org/wiki/File:Latissimus_dorsi.png. Original by sv: Användare: Chrizz, 27 maj 2005

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

References

1. Lugo, R., Kung, P., & Ma, CB (2008). Shoulder biomechanics. European journal of radiology, 68 (1), 16-24.
2. Bradley, JP, & Tibone, JE (1991). Electromyographic analysis of muscle action about the shoulder. Clinics in sports medicine, 10 (4), 789-805.
3. Christopher, GA, & Ricard, MD (2001). Shoulder biomechanics in volleyball spiking: implications for injuries (Doctoral dissertation, Brigham Young University).
4. Scovazzo, ML, Browne, A., Pink, M., Jobe, FW, & Kerrigan, J. (1991). The painful shoulder during freestyle swimming: an electromyographic cinematographic analysis of twelve muscles. The American journal of sports medicine, 19 (6), 577-582.
5. Scovazzo, ML, Browne, A., Pink, M., Jobe, FW, & Kerrigan, J. (1991). The painful shoulder during freestyle swimming: an electromyographic cinematographic analysis of twelve muscles. The American journal of sports medicine, 19 (6), 577-582.
6. Terry, GC, & Chopp, TM (2000). Functional anatomy of the shoulder. Journal of athletic training, 35 (3), 248.
7. Perry, JACQUELIN (1983). Anatomy and biomechanics of the shoulder in throwing, swimming, gymnastics, and tennis. Clinics in sports medicine, 2 (2), 247-270.