The rotator cuff is comprised of the subscapularis, supraspinatus, infraspinatus, & teres minor. All of these attach into to a common tendon that impersonates a shirt “cuff” over top of the round head of the humerus. Unfortunately, because these muscles are so heavily utilized in our daily activities, tears in the rotator cuff muscle group are quite common.
The subscapularis is a powerful, triangle-shaped muscle that fits inside the scapula. When contracted, it will medially (internally) rotate the humerus, the only rotator cuff muscle capable of doing so. The supraspinatus fills in the depression above the spine of the scapula & then runs laterally to the head of the humerus, just under the acromion process. This muscle initiates abduction of the humerus to about ten degrees or so, until the much larger deltoid muscle takes over to continue the abduction. The infraspinatus is a broad muscle that fills the surface of the bottom two-thirds of the scapula. It then runs laterally to attach to the back of the top of the humerus. It functions along with teres minor & the posterior deltoids to laterally (externally) rotate the shoulder. Teres minor attaches to the bottom of the scapula & just below infraspinatus.