Infraspinatus Muscle Anatomy

Location & Overview

The infraspinatus muscle is a thick and triangular muscle of the shoulder. It is one of the four rotator cuff muscles of the shoulder and occupies a large part of the infraspinous fossa (which is located on the posterior surface of the scapula). The remaining three rotator cuff muscles are the supraspinatus, teres minor and subscapularis ^{[1] [2]}.

The infraspinatus muscle is located on the posterior surface of the scapula. It’s located inferior to the supraspinatus and superior to the teres minor (situated between these two muscles). The infraspinatus originates medially at the infraspinatus fossa of the scapula and inserts laterally at the middle facet of the greater tubercle of the humerus. The supraspinatus and infraspinatus are closely related anatomically to one another due to only be separated by the spine of the scapula. Therefore, the supraspinatus and infraspinatus are considered as one functional unit. Moreover, the infraspinatus muscle is commonly fused together with the teres minor muscle ^{[3] [4] [5] [6]}.

The fibres of the infraspinatus muscle run almost parallel to the teres major and minor. The fibres are directed towards the shoulder. The infraspinatus tendon is sometimes separated by bursa from the capsule of the shoulder joint, which may communicate with the joint cavity ^{[7] [8] [9] [10]}.

Origin & Insertion

The infraspinatus originates at the infraspinous fossa of the scapula and then inserts at the middle facet of the greater tubercle of the humerus ^{[11] [12] [13] [14]}.

Actions

The primary action of the infraspinatus muscle is lateral rotation of the shoulder. Lateral rotation of the shoulder is when the humerus/arm rotates clockwise (e.g rotating in a direction towards the back/posterior of the body when in the anatomical position) ^{[15] [16]}.

In addition to external rotation, the infraspinatus also plays a role in stabilising the humerus in the glenohumeral joint. The head of the humerus sits within the glenohumeral joint and stabilising it is important. This stabilisation prevents damage to the surrounding structures. Someone who has an inability to maintain the humeral head in the glenoid fossa may be referred to as having ‘shoulder instability’ ^{[17] [18]}.

Innervation

Both the infraspinatus and supraspinatus are innervated by the suprascapular nerve. This nerve comes from the upper portion of the brachial plexus from the nerve roots C5 and C6 ^[19].

Blood Supply

Blood is supplied to the infraspinatus from the suprascapular and circumflex scapular arteries ^{[20] [21]}.