Infraspinatus Muscle Anatomy

The infraspinatus is a muscle of the scapulohumeral region that is also part of the SITS muscles or the Rotator cuff muscles. The infraspinatus located medially in the infraspinous fossa of the posterior aspect of the scapula and occupying three quarters of the infraspinous fossa. The infraspinatus is partly covered by the deltoid and trapezius muscles and it is enclosed in a thick sheath (fascia). The fibrous intramuscular septa help to give added attachment to the multipennate fibers of the infraspinatus muscle and these fibers converge to slide freely over the bare area of the scapula at its lateral angle. The infraspinatus tendon replaces the muscle fibers and is blended with the capsule of the shoulder joint to increase its strength.

Action/Function of the Infraspinatus

The function of the infraspinatus is in helping to stabilize the glenohumeral joint as well as acting as a powerful lateral rotator of the humerus a function common to rotator cuff muscles.

Infraspinatus Origin and Insertion

The muscle arises from the infraspinous fossa (medial three fourth) and also from the deep surface of the infraspinous fascia that covers it. The subacromial bursa lies between the remaining one third (bare area) of the scapula and the muscle and sometimes communicates with the shoulder joint. The infraspinatus tendon blends with the capsule of the shoulder joint. The infraspinatus inserts in the smooth area on the middle facet of the greater tubercle of the humerus between the supraspinatus and teres minor.

Infraspinatus Innervation

The infraspinatus gets its nerve supply from the suprascapular nerve (through C5 and C6 spinal segments ). The suprascapular nerve arises from the upper trunk of the brachial plexus. After giving branches to the supraspinatus muscle, the suprascapular nerve passes around the lateral border of the scapular spine and enters the infraspinous fossa to give two branches to the infraspinatus muscle.

Infraspinatus Blood Supply

The blood supply of the infraspinatus is from the suprascapular artery which is the largest branch of the axillary artery . The circumflex scapular arteries also supply the infraspinatus. The venous drainage of the infraspinatus is from the subscapular vein that is formed by the joining of thoracodorsal and circumflex scapular veins and accompanying the subscapular artery to drain into the axillary vein.

Infraspinatus Test

To test the infraspinatus, the elbow is flexed and the arm is adducted and then laterally rotated against resistance. When the infraspinatus is acting normally, it can be palpated inferior to the scapular spine. To test the function of the supra-scapular nerve that supplies both the supraspinatus and infraspinatus, both muscles should be tested as described, if they can be palpated against resistance, it shows the nerve is functional.