Subscapularis muscle

Origin and insertion

Subscapularis is a strong triangular muscle that fills the subscapular fossa of scapula. Lying posterolateral to the thoracic cage, it starts as a wide muscle whose medial two-thirds originate from the subscapular fossa of scapula and from several tendinous intramuscular septa at the ridges of the fossa. The remaining fibers arise from an aponeurosis that covers the posterior surface of the lateral third of the muscle.  

From the costal surface of the scapula, the muscle fibers course laterally, gradually narrowing towards a round tendon that courses towards the proximal humerus. The large subscapular bursa that communicates with the shoulder joint separates the subscapularis tendon from the neck of the scapula. The tendon then inserts to the lesser tubercle of humerus and anterior part of the articular capsule of the glenohumeral joint. As it approaches the humerus, the tendon is continuous with the tendon of the teres major muscle. In fact, these two muscles share the same innervation and actions, and are often considered as a functional muscular unit.

Relations

The subscapularis muscle forms the majority of the posterior wall of the axilla, facing the contents of axilla with its anterior surface. Serratus anterior covers its superolateral part, while he coracobrachialis and biceps brachii muscles lie over its inferomedial part. Fascial spaces between the subscapularis, serratus anterior muscles and the thoracic cage are filled with loose connective tissue that facilitates the gliding movements of the scapulothoracic joint. The central part of the muscle is crossed by the cords of the brachial plexus and their branches, the axillary nerve, axillary artery and vein. 

The posterior surface of the muscle’s tendon is blended with the fibrous capsule of the glenohumeral joint, and is continuous with the tendons of supraspinatus, infraspinatus, teres minor and long head of triceps brachii. These tendons together comprise the rotator capsule around the shoulder joint. 

The subscapularis muscle participates in formation of the three intermuscular axillary spaces;

• Quadrangular space; subscapularis forms the anterior boundary of this space, while the shoulder joint capsule and teres minor bound it superiorly, teres major inferiorly and the surgical neck of the humerus laterally. This space is traversed by the axillary nerve and posterior circumflex humeral artery and vein.
• Upper triangular space, in which subscapularis comprises the anterior border. The space is completed by the teres major inferiorly, teres minor posteriorly and long head of triceps brachii laterally. The circumflex scapular artery traverses this space.
• Lower triangular space (triangular interval), here the subscapularis makes up the anterior border. Teres major, long head of triceps brachii and the humerus comprise the posterior, medial and lateral borders of this space, respectively. This space serves as a passageway for the radial nerve and deep brachial artery.

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Innervation

Subscapularis is innervated by the upper and lower subscapular nerves (C5-C6), that stem from the superior and posterior cords of the brachial plexus, respectively.

Blood supply

Blood supply to the subscapularis muscle comes from a series of branches of the subclavian artery;

• Axillary artery, a continuation of the subclavian artery
• Subscapular artery, a branch of the axillary artery
• Suprascapular artery, which branches off the subclavian artery via the thyrocervical trunk

Function

The subscapularis muscle is the only medial (internal) rotator of all the rotator cuff muscles. Due to its unique axis of pull, from the proximal humerus to the costal scapular surface, subscapularis medially rotates (internal rotation) the humeral head within the glenoid fossa. To a lesser extent, subscapularis also adducts the arm. The sequence of these two movements is seen in walking when your arms swing.

Along with the rest of the rotator cuff muscles, subscapularis contributes to the creation of concavity compression, a stabilizing mechanism in which compression of the humerus into the concavity of glenoid fossa prevents its dislocation by translating forces. Subscapularis is an important stabilizer of the shoulder joint when the deltoid, pectoralis major, biceps and triceps brachii muscles are active. These muscles are responsible for several over-head movements of the arm (e.g. throwing, pulling downwards), during which the force of subscapularis prevents upward shoulder dislocation. However during these particular movements, subscapularis is also the most prone to impingement.

The subscapularis muscle also promotes coordination between the movements of the glenohumeral and scapulothoracic joints. Although the muscles of the back predominantly act on the scapulothoracic joint, subscapularis contributes by ensuring that scapula movements follow the shoulder, in actions such as reaching for a high object.

Consolidate your knowledge about the subscapularis and other muscles of the shoulder with our quiz!