The infraspinatous muscle is one of the four muscles that comprise the rotator cuff. The other muscles are the supraspinatus, teres minor, and subscapularis. It is a triangular shoulder muscle that rises above and is attached to the infraspinatous fossa, which is more commonly known as the shoulder blade. This muscle aids in the lateral motion and horizontal extension of the arm as well as overall joint stability. This thin shoulder muscle is attached to the suprascapular nerve and is therefore associated with the fifth and sixth cervical vertebrae. Two thirds of this muscle is composed of fleshy fibers, while the other third is devoted to tendinous fibers that become the infraspinatous tendon. The infraspinatous muscle is the second most injured muscle of the shoulder. Motions required to throw a ball or swing a racket can cause strain within the muscle and referred pain throughout the shoulder area. Combined with stress from the infraspinatous tendon, chronic pain in this area may be diagnosed as infraspinatous tendinitis.