What the Heck Is A Rotator Cuff, Anyway?

Earlier this week, I spoke to my brother, who informed me he may need rotator cuff surgery.
As a trainer, I am hearing this more and more.

In an attempt to reduce the freak-out factor, I am going to explain a bit about rotator cuffs.

The rotator cuff is made up of four muscles — subscapularis, infraspinatus, teres minor and the supraspinatus, — and their tendons. You can remember them by their acronym S.I.T.S

The main function of the rotator cuff is to keep the the shoulder muscles in place during high velocity movements (especially in the depression of the shoulder joint).

The subscapularis originates on the back side of the of the scapula. The connective tendon attaches to the chest. It is responsible for lateral rotation. (In real English: if you were to lift your arm in front of you, and move it inward, that's lateral rotation.) Depending what you read, it's a toss-up on whether the subscapularis or the supraspinatus is the most injured.
Pulling a band across the body can be very painful when this is injured. I see a lot of this in the gym when people bench press: they have a tendency to bring the weight all the way down to the chest, which could strain or tear the subscapularis tendon.

The supraspinatus is on top of the scapula. It raises the arm away from the body. Its tendons attach on top of the humerus. When this is injured, the pain is very often in the lifting of objects.

The infraspinatus takes up most of the scapula on the top side. The upper fibers abduct (raise) the arm while the lower fibers adduct (lower) the arm in a lateral motion. The tendons connect on the top back side of the humerus. When doing pull-ups, you must come down slowly — or risk injury.

Last, but not least, is the teres minor. It originates on the underside of the the scapula. Because it is underneath the infraspinatus, is the tendon connecting to the top backside of the humerus.
The teres minor externally rotates the arm outwardly. Lift your arm in front of you and move your arm toward the back of your body: that is external rotation.

An important fact to remember about the rotator cuff is that these muscles are layered underneath the three deltoid muscles (anterior, middle and posterior). When you move your shoulder and you get friction under the joint or you hear crackling, more times than not the rotators are getting hung up on the bone above them. This is called "impingement."
Impingements decrease range of motion in the shoulder and sometimes can cause pain.

Slowly, move your shoulder in all directions and see if you have pain or limited range of motion.
If you do, rest it and ice it. If pain persists see your doctor.

For a few good exercises, check out Body Result's Rotator Cuff Strengthening Exercises.