The rotator cuff is actually a series of four muscles that cross the shoulder (glenohumeral) joint:
3. Teres Minor
All four of these muscles originate and insert in different areas throughout the scapula and humerus. For those visual learners, check out the picture below:
This particular anatomy has pros and cons. A significant advantage of having this loose ball-and-socket configuration is that it provides a significant range of motion. This motion, obviously, is what allows us to move our arm into numerous positions. However, all this range of motion comes at a price. Such a shallow ball-and-socket joint provides a great deal of instability. A golf ball can be easily knocked off the tee, and that same concept can be applied to the shoulder. Ultimately, we what what's called a good ball-and-socket congruency between the humerus and the scapula. Ball-and-socket congruency, in other words, means keeping the head of the humerus together with the scapula. As we use our arm through activities of daily living, throwing, and lifting there must be some restraints on the shoulder to keep the humerus and scapula together.
And here is where we enter with the rotator cuff. Although the rotator cuff is supported by connective tissue such as the labrum, the rotator cuff has the primary role of maintaining the aforementioned ball-and-socket congruency of the shoulder. As the humerus tends to shift forward, back, up, down, or away, the rotator cuff activates to keep the shoulder and arm together. Think of it as a seat belt for your shoulders!
Now when the rotator cuff (among other things) fails to do its job properly, that will result in excess movement of the head of the humerus on the scapula. This is most common to see in throwing athletes. When the athlete accelerates through a throw, the arm externally rotates and will want to ride forward in relation to the body, a term called anterior glide. If the rotator cuff is not functioning correctly and at the right time, this anterior glide can cause issues such as SLAP lesions, biceps tendon irritation, or even dislocation.
So now that we've established that we want to address rotator cuff strength, how do we train it? The muscles of the rotator cuff are small, and thus we don't want to use a significant amount of weight for fear that the shoulder will recruit larger muscle groups and leave the rotator cuff behind. Additionally, the rotator cuff is primarily composed of Type I fibers, which are fatigue-resistant. Thus, we can use a high rep range of 15 reps or so per set. A few exercises we use here at SPS include the standing dumbbell external rotation, TRX I, Y, T complex, and the single-arm prone shoulder raise. These sample exercises are a few that we use to help ensure optimal shoulder health for the athlete and to mitigate chances of injury.
Andrew Kossow, CSCS