INFRASPINATUS MUSCLE: ORIGIN, FUNCTIONS, DISORDERS - ANATOMY AND PHYSIOLOGY - 2025

The infraspinatus muscle, or musculus infraspinatus, forms together with the supraspinatus, the subscapularis and the teres minor the structure known as the rotator cuff. They all originate from the scapula or shoulder blade and insert into the outermost portion of the humerus trochiter. The infraspinatus muscle owes its name to the fact that it occupies a large part of the infraspinatus fossa of the scapula.

On the other hand, both the infraspinatus muscle, the supraspinatus and teres minor muscles are called external rotator muscles, while the subscapularis is called internal rotator.

Graphic representation of the location of the infraspinatus muscles (right and left). Source: File: Infraspinatus.PNG Wikipedia.org. Edited image.

Innervation and supply of the infraspinatus muscle is carried out by the suprascapular nerve and the suprascapular artery, respectively. The infraspinatus along with the rest of the muscles that make up the rotator cuff, provide great stability to the shoulder, keeping the structures in the correct position.

The joint action of the muscles prevents the head of the humerus from derailing from its place even when it is in motion, since together they push the humeral head towards the glenoid concavity. Likewise, they control the movement of the glenohumeral joint.

characteristics

The infraspinatus is a broad, flat muscle shaped like a triangle. It is a paired muscle, that is, there is one on each side of the body (right and left). Its location is deep.

Origin

It originates in two areas of the shoulder blade or scapula. The first area corresponds to the pit that bears the same name, the “infraspinatus” pit, covering two-thirds of it. The second area is the lower portion of the spine of the shoulder blade. It should be noted that the muscle fibers are arranged outwards and upwards.

Insertion

The infraspinatus muscle attaches to the middle of the humeral head by means of tendons. The muscle covers the capsule of the scapulohumeral joint. In general, the set of tendinous fibers of the rotator cuff also converge with the coracohumeral and glenohumeral ligaments.

Finally, the infraspinatus muscle implants behind the tendinous fibers of the supraspinatus, that is, in the humeral trochiter in its outermost portion.

Innervation

The suprascapular nerve is in charge of innervating the infraspinatus muscle.

Irrigation

The suprascapular artery is responsible for supplying the infraspinatus muscle.

Features

The function of the infraspinatus is to contribute to the external rotation movement of the arm and to participate to a lesser extent in the abduction movement together with the supraspinatus.

In the external rotation movement, it works synergistically with other muscles, such as: the teres minor and the posterior deltoid. While the muscles that antagonize the action of the infraspinatus are: the subscapularis muscle, the anterior deltoid, the latissimus dorsi, the pectoralis major and the teres major.

It also fulfills a joint function with the rest of the rotators and is to protect the joint of the humeral head inside the glenoid cavity.

In this aspect, the infraspinatus muscle acts together with the teres minor muscle, generating shear forces posteriorly. This influences the anterior stability of the shoulder joint.

Syndromes or related disorders

- Rotator cuff tendonitis

The most frequent syndrome in which the infraspinatus muscle is involved is the affectation called rotator cuff tendonitis or subacromial impingement.

Due to the fact that this affectation is common in certain athletes, it is sometimes referred to as the shoulder of a tennis player, swimmer or pitcher.

The injury occurs from overuse of the shoulder joint, with repetitive elevation of the arm.

During exercise, the head of the humerus rubs against the acromion and the coracoacromial ligament thickens and these in turn can rub against the supraspinatus tendon, generating chronic irritation and inflammation, up to the erosion of the tendons.

Other causes that can damage the rotator cuff are: decreased vascularity of the tendon, changes in collagen, anatomical changes in the subacromial space (narrowing), trauma, curved or hooked shape of the acromion, acromial thickness, among others.

Rotator cuff tear can also occur from excessive force.

If the problem is not treated in time and despite the pain the exercise is continued, the injury progresses to periostitis, with the destruction of the tendons in the humeral head.

In this involvement, most of the time the most affected muscle is the supraspinatus, while the infraspinatus is not affected.

However, the infraspinatus could atrophy because of the injury to the cuff itself, either because it directly affects the muscle or because there is damage to the suprascapular nerve.

Symptoms, signs

In rotator cuff tendonitis the pain is progressive. At first it is only when practicing sports, then the pain appears with the simple movement of placing the arm forward.

In infraspinatus tendonitis, there is pain and difficulty in external rotation of the arm. The pain usually increases at night.

Exploration

To palpate the rotator cuff, the patient is asked to extend the arm back and away from the body medially. The patient has pain on palpation of the affected tendons and also when raising the arm above the head. Whereas, with the arm at rest there is no pain.

Magnetic resonance imaging is only useful for complete rotator cuff tears, but not for a partial injury.

Treatment

Treatment will depend on the degree of injury. In minor injuries it is enough to comply with rest and strengthen the healthy shoulder. You can do pulling exercises if it doesn't cause pain.

In severe cases of complete tendon rupture or when there is no improvement within 6 months of treatment, surgery may be a good option.

- Infraspinatus tendinitis

If the pain is caused by direct inflammation of the infraspinatus tendon, the pathology is called infraspinatus tendonitis.

Exploration

If you want to specifically explore the function of the infraspinatus muscle, you can perform the Patte maneuver. The patient is placed in a sitting position and the clinician will stand behind him.

It helps to flex the patient's shoulder and elbow 90 °, with one hand the clinician fixes the shoulder and with the other grasps the patient's wrist. The patient is asked to try to move the arm up (external rotation), while the clinician resists the movement.

If the patient reports pain in the posterolateral portion of the acromion, the test is considered positive, that is, there is injury to the infraspinatus muscle.

Trigger points of the infraspinatus

Contractures in this muscle can cause painful and palpable points, called trigger points. The pain, in addition to affecting the shoulder and shoulder blade, can radiate towards the nape and arm.

A differential diagnosis should be made with other similar pathologies, such as: biceps tendonitis, arthritis of the shoulder joint, among others.

References

1. Frequent sports injuries. Acta pediátr. Costarric, 2003; 17 (2): 65-80. Available from: scielo.sa
2. Hoya J. Regenerative therapy of the supraspinatus tendon: study carried out in a murine model of chronic injury. 2014. Degree work to obtain the title of Doctor of Biology. Complutense University Madrid. Spain. Available at: eprints.ucm.e
3. Gutiérrez A. Impingement syndrome. Ortho-tips, 2006; 2 (2): 68-77. Available at: Medigraphic.
4. Takase K, Suzuki H, Matsuoka H. The Usefulness of Latissimus Dorsi Transfer for Reconstruction for Malignant Fibrous Histiocytoma in the Infraspinatus Muscle Region A Case Report. West Indian Med J 2010; 59 (1): 106-109. Available from: westindies.scielo.org
5. Laws M, Forriol F. Rotator cuff tear: etiology, exploration and treatment. Trauma Fund MAPFRE, 2012; 23 (1): 39-56 39. Available at: mapfre.com