What Exactly is the Rotator Cuff?

Shoulder pain is a very common reason for people to seek treatment from a physical therapist. It is not unusual for me to hear from a patient that his shoulder started hurting several months ago and he “just thought it would get better.” That patient ends up in my office because the pain did not go away on it’s own. So why is that? What is it about the shoulder that makes it prone to injury and less likely that it will get better without intervention?

The shoulder is composed of the humerus (or upper arm bone), scapula (shoulder blade), and clavicle (collar bone). The humerus and scapula come together to form the glenohumeral joint and are held in place by ligaments and a fibrous joint capsule. Outside of the joint capsule is a group of muscles collectively referred to as the rotator cuff. The rotator cuff is comprised of four muscles (supraspinatus, infraspinatus, subscapularis, and teres minor).

Image Source: medlineplus.gov - NIH

Image Source: medlineplus.gov - NIH

These muscles originate on the scapula and attach to the head (or top) of the humerus. The rotator cuff muscles have individual actions (lifting the arm and rotating the arm) and collective actions. Collectively, the rotator cuff holds the humerus tightly in the joint socket and depresses the humerus slightly so that when you raise your arm over your head, this action happens smoothly. The rotator cuff also has to work in a coordinated fashion with the larger muscles of the arm and upper back to allow for full movement and function of the arm and shoulder.

So why is the rotator cuff so commonly a source of pain? There are a few reasons. First, the shoulder is a joint that moves in a lot of directions and with many degrees of freedom. There is a lot of mobility in that joint and therefore a lot of demand on the muscles to support all of the actions and mobility. Second, the muscles of the rotator cuff are quite small. A muscle’s strength is directly related to the cross sectional size of the muscle. For example, your thigh muscles are quite strong because they are big, bulky muscles. The rotator cuff muscles are small and thin and therefore less strong and resilient. The third reason is that the blood supply to the shoulder joint is relatively poor. When a tissue (muscle, tendon or ligament) gets injured, it heals when blood brings healing cells to that tissue. So if the blood supply is limited, the automatic healing process will also be limited.

If you are having shoulder pain or have questions about preventing injury, please call your physical therapist. We can help!


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