The Infraspinatus Muscle is a thick, triangular-shaped muscle that occupies most of the infraspinous fossa located on the posterior surface of the scapula. It serves as one of the four muscles of the rotator cuff, and its primary functions are external rotation of the humerus and stabilization of the shoulder joint.
Anatomy
Morphology and Connection Points
The Infraspinatus Muscle originates from the medial two-thirds of the infraspinous fossa of the scapula and the overlying infraspinatus fascia. After crossing the lateral border of the scapular spine, the muscle fibers extend to the medial aspect of the greater tubercle of the humerus, inserting via a tendon. This wide insertion area both enhances the muscle's force-generating capacity and contributes to shoulder stability. The tendon inserts into the posterior portion of the shoulder joint capsule, directly contributing to the posterior stability of the glenohumeral joint. Its larger insertion area compared to the supraspinatus muscle makes it more resistant to rotator cuff tears. Furthermore, a bursa may occasionally be present between the tendon and the joint capsule, and this bursa may connect to the shoulder joint space in some individuals.
Embryological Origin and Classification
The Infraspinatus Muscle is a muscle belonging to the scapulohumeral muscle group, developing from the dorsal muscle mass of the upper extremity. Embryologically, it originates from somitic mesenchyme derived from paraxial mesoderm. This muscle is one of the posterior components of the rotator cuff, contributing both to motor movements and providing passive stability by supporting the joint capsule.
Nerve Innervation and Vascular Supply
The muscle is innervated by the suprascapular nerve (C5–C6). This nerve passes through the suprascapular notch at the superior border of the scapula to reach the muscle fibers. The same nerve also innervates the supraspinatus muscle, providing coordinated neural control of shoulder movements. Vascularly, branches of the suprascapular artery and the scapular anastomotic network supply the muscle.
Function and Clinical Relationships
The primary function of the infraspinatus is lateral rotation of the humerus, or external rotation of the arm. This movement is used in functions such as turning the palm up or rotating the back of the hand upward. Furthermore, the muscle helps stabilize the shoulder joint by keeping the humeral head stable in the glenoid cavity. It protects against subluxation and dislocation by supporting the joint capsule posteriorly. When the arm is stationary, it also indirectly supports scapular movements by pulling the inferior angle of the scapula inward. Functionally, it works synergistically with the teres minor and is particularly active in external rotation movements. It also supports shoulder extension with the posterior fibers of the deltoid muscle. Clinically, the infraspinatus muscle is often evaluated as a cause of rotator cuff tears. It is not torn as frequently as the supraspinatus; however, it can cause pain as a result of chronic overuse, poor posture, and trauma. Compression of the suprascapular nerve, which innervates the muscle, can result in decreased muscle strength and pain. This condition is associated with impingement at the suprascapular notch. Furthermore, decreased external rotation ability can lead to shoulder instability and an increased risk of dislocation.
During the diagnostic process, MRI and ultrasonography are effective methods for evaluating tendon structure. An external rotation strength test performed during the physical examination is helpful in assessing muscle strength. As an anatomical variation, fusion of the infraspinatus muscle with the teres minor is a common occurrence in clinical practice and is considered a normal variation in some individuals.
