The rotator cuff is a group of 4 muscles surrounding the gleno-humeral joint that act with differing vector forces to vacuum/suck the head of the humerus into the glenoid fossa. It then continues to act upon the orientation of the head of the humerus within the fossa as the humerus is moved and the shoulder is mobilized.
Without the vacuum action and balance of these 4 intrinsic muscles, the often stronger global muscles can easily pull on the humerus and shift it anteriorly (most commonly) into a sub optimal position. Hence we have then lost the balance that allows the shoulder to work optimally.
Once shoulder movement occurs, this can, over a period of time, cause wear and tear on the lining of the fossa or cause impingement on the surrounding ligamentous structures. This isn't always a bad thing, this will not always cause pain and impingment shouldn't be demonized.
The acronym used for remembering the rotator cuff is SITS. Supraspinatus, Infraspintus, Teres Minor and Subscapularis. I'm sure there are many more, so use what ever works for you.
The supraspinatus muscle lies above (superior) to the spine of the scapula. It causes abduction and external rotation when in an abducted position. It lies beneath the larger global muscle known as the deltoid. Both these muscles are active when performing exercises such as lateral raises or simply lifting your arm overhead.
The infraspinatus lies directly inferior to the supraspinatus and below (inferior) to the spine of the scapula. Its main action is to externally rotate the gleno-humeral joint. You can see this by the orientation the muscle fibers. They run in a traverse arrangement, meaning when they contract concentrically (get shorter) they will pull the humerus into external rotation.
The Teres minor originates on the lateral boarder of the scapula and attaches onto the humerus on the posterior side making it an external rotator of the gleno-humeral joint. Its fibers run in a similar arrangement to the Infraspintus. It also acts as a anchor to pull down the head of the humerus against the upward pull of the supraspinatus. The Teres minor is therefore of upmost importance and concern when impingement injuries present themselves.
The sub scapularis lies in between the anterior surface of the scapula and the back of the rib cage. Because of its orientation and attachment sites it is a strong internal rotator of the gleno-humeral joint. Don’t be fooled, it doesn’t do all the heavy lifting itself, there are some other large and therefore strong internal rotators of the shoulder joint. Global mobilizes such as Latissimus Dorsi, Teres Major and Pectoralis Major also internally rotate the shoulder.
As we know, no one muscle can or should be isolated. So although we have reviewed the muscles one by one in this article, we should work them together in all movements and in all planes of motion.
To learn more about assessing, positioning and working the entire shoulder girdle optimally, join the upcoming shoulder workshop.