Corpus: Infraspinatus muscle

The infraspinatus muscle belongs to the dorsal group of shoulder muscles and is a strong external rotator of the humerus.

The infraspinatus muscle originates from the infraspinous fossa on the dorsal side of the scapula and largely fills it. It inserts at the greater tubercle of the humerus, thus being part of the so-called rotator cuff. The tendons blend into the area of insertion with the supraspinatus tendons, making a separation macroscopically impossible. Accordingly, the measurements for the sagittal extension of the infraspinatus tendon insertion vary between 13 and 25 mm.

The innervation of the infraspinatus muscle is provided by the suprascapular nerve from the brachial plexus (segments: C4-C6). Accessory innervation by the axillary nerve from the brachial plexus (segments C4 to C6) is also possible.

The infraspinatus muscle, along with the deltoid muscle and the supraspinatus muscle, is responsible for the external rotation of the upper arm, in which it is the most important player. In addition, its upper parts weakly support abduction with the arm lowered. With the arm raised, however, the muscle acts more adductively.

According to its main function in the shoulder joint, a failure of the infraspinatus muscle primarily leads to a limitation of external rotation. A clinical test to examine the function of the infraspinatus muscle is the infraspinatus test. An infraspinatus tendon rupture rarely occurs in isolation, but is most often a continuation of a supraspinatus tendon rupture posteriorly or as part of a massive rupture of the rotator cuff.