The infraspinatus muscle is a thick triangular muscle located on top of the shoulder blade and covers more than two-thirds of the shoulder blad or scapula. The infraspinatus muscle coveres most of the infraspinatous fossa.
The infraspinatus is one of the four muscles of the rotator cuff, and the main function of the infraspinatus is to externally rotate the humerus and stabilize the shoulder joint. The muscle function is also commonly known as causing external rotation of the shoulder, but it is actually the upper arm that is rotating externally, caused by the external rotation of the long bone of the upper arm — the humerus.
The infraspinatus is located underneath the posterior deltoid muscle and underneath the trapezius muscle.
Origin: infraspinous fossa of the scapula
Insertion: middle facet of greater tubercle of the humerus