The subscapularis muscle (Latin for lower shoulder blade muscle) refers to a larger skeletal muscle of the shoulder. The inside of the scapula is completely covered by the subscapularis muscle. Its primary function is internal rotation of the os humeri (Latin for humerus).
What is the subscapularis muscle?
An important component of the ventral group of shoulder muscles, the subscapularis muscle is a central muscle in the rotator cuff. It attaches to the inside of the scapula (shoulder blade). Together with the other rotator cuff muscles, the musculus infraspinatus (Latin for lower-bone muscle), musculus supraspinatus (Latin for upper-bone muscle), and the musculus teres minor ((Latin for small round muscle), it helps to hold and stabilize the head of the os humeri in the socket.
Anatomy and structure
The subscapularis muscle originates at the subscapular fossa, a bony depression on the ventral aspect of the scapula. It attaches to the tuberculum minus (Latin for small hump) of the humerus and to the bony structure that lies directly below it (crista tuberculis minoris). Here, some of the tendon fibers of the muscle may extend into the shoulder joint capsule. Of the rotator cuff muscles, the subscapularis muscle is the largest. Starting from there, it runs toward the caput humeri (Latin for head of the humerus). The upper part of the muscle (like that of the supraspinatus muscle) runs between the acromion and the caput humeri. The nerve supply of the muscle is given by the nervus subscapularis (lat. for subclavian nerve). This is one of the branches of the brachial plexus (Latin for brachial plexus).
Function and tasks
The primary function of the subscapularis muscle is to provide internal rotation of the upper arm in the shoulder. Another main function is adduction of the upper arm to the body, that is, bringing it closer. Likewise, the muscle can cause abduction of the upper arm, i.e., leading it away from the body. Due to the adhesion of some muscle fibers of the subscapularis muscle to the capsule of the shoulder joint, the joint capsule is tightened and thus stabilized. This muscle is an exceptionally strong shoulder muscle. It has a high physiological cross-section, which comes from its pronounced pinnation. For the internal rotation of the upper arm, the subscapularis muscle is therefore the most important actor. Adduction is supported by the upper region, while the lower region of the muscle provides abduction. Stabilizing the humerus in the glenoid fossa (Latin for shallow socket) prevents the bone from popping out of the socket. Similarly, entrapment of the capsule of the shoulder joint is prevented in this way. The end tendon of the muscle is very broad, which is why it plays an important role in protecting against anterior shoulder dislocation. Examples of the muscle’s function in everyday life are numerous wherever internal rotations are required. A typical task for this muscle, for example, is steering while driving a car, with arms crossed in front of the body. Internal rotation of the shoulder is also caused when the seat belt is fastened.
Diseases
Often, the subscapularis muscle, as a component of the rotator cuff, is involved in the event of a rupture of this cuff. Injury to the muscle can therefore produce the same symptoms of a common rotator cuff rupture. In addition to usually severe pain, an injury to this muscle also reveals a significantly impaired range of motion during internal rotation of the upper arm. However, it is also possible that simultaneous dislocation occurs, i.e. that the shoulder is colloquially dislocated. Particularly in certain types of accidents, a counter-impact is typical with an abducted arm, i.e., an arm that is moved away from the body. Such impacts with externally rotated arms often occur in sports such as handball or volleyball. The injury is associated with severe pain. In such cases, the affected person typically moves the arm slightly away from his body and supports it laterally to avoid pain. The mobility of the shoulder is very limited in this case, while the regular shoulder contour is missing. X-ray examinations, magnetic resonance imaging and magnetic resonance imaging are available for diagnosis.In X-ray images, the head of the humerus is usually no longer visible in the socket. In both methods of tomography, possible tears of the muscle stand out. A restriction of the function of the subscapularis muscle can also result from a paralysis of the subscapularis nerve, i.e. the nerve supplying the muscle. In this case, too, the restriction of internal rotation is a central symptom. In this case, the palm in particular can only be moved to the back with the help of other muscles. Because of its important stabilizing function with regard to the humeral head, damage to the subscapularis muscle also affects the stability of the humeral head. Damage to the subscapularis leads to significant destabilization of the humeral head. This can cause the humerus to slide forward until it rubs against the acromion or the coracoid process (Latin for raven’s beak process). This phenomenon is known as shoulder impingement. This should not be confused with the more common outlet impingement, which is caused by bony constriction. Partial lesions to this muscle are problematic. The loss of function often goes unnoticed initially because of the large tendon set and the direct contact of the muscle with the humerus below the tendon.
