I have been a big fan and admirer of the shoulder ever since I was in physical therapy school 21 years ago. It lead to my focus on my doctoral thesis and further writing and research to better understand its mechanics/anatomy and physiology. I have had the unique opportunity to work/teach with some of the clinical and research leaders in my field such as Dr. Robert Donatelli PT, PhD, OCS. One of the things that amazes me most is how neglected this vital region of our body is exercised and understood. When I go to the gym rarely do I see someone training the all-important rotator cuff (RTC) group or its synergists -- the scapular rotator/stabilizer group.
"Shoulder pain is one of the most common musculoskeletal symptoms seen in primary care, with a lifetime prevalence rate of up to 67 percent."  The underlying etiology or cause of injury to the RTC group are multifactorial, but my hope is if you understand how important these muscles are and begin to give them some attention in your workouts you can stay out of the that 67 percent. I would like to give you a brief anatomy lesson on these muscles and the joints of the shoulder and then take you through a video to help educate you on which are the most accepted and evidenced-based ways to specifically train your RTC group and scapular muscles.
Most of us think of the shoulder as simply the "ball-and-socket" connection from the head of the humerus (the ball) and the scapular (shoulder blade) glenoid (socket), respectively. This part of the shoulder is analogous to a golf ball sitting on a tee. The humeral head is four times the size of this little dish, or glenoid fossa, that it sits and rotates over. Thus, the shoulder provides us with an amazing amount of mobility; however, it lacks inherent stability. This is where the ever-important group of muscles called the rotator cuff (RTC) come in to play. They are aptly termed the dynamic stabilizers of the glenohumeral joint (ball-and-socket).  However, the shoulder is actually comprised of three other joints that work synergistically with the ball-and-socket. They are the scapulothoracic joint (how the shoulder blade moves over the ribs), acromioclavicular joint (the collar bones connection/articulation with the shoulder blades acromion process), and the sternoclavicular joint (the sternum/breastplate articulation with the collar bone).
In order for you to simply raise your arm overhead, all four of these joints must work together in a coordinated fashion that is mainly directed by the balance of strength and neuromuscular control between your RTC and scapular stabilizers vs. your deltoid muscle. This is kind of like a constant tug-of-war between the deltoid and the RTC and scapular muscles. The RTC are made up of four muscles including the subscapularis, which is the largest of the four and attaches in the front of the scapula and inserts into the humerus. It is the main internal rotator for the glenohumeral joint and also acts a dynamic stabilizer for preventing anterior dislocations and acts as a secondary elevator for the shoulder. The three RTC in the back of the shoulder are the supraspinatus, infraspinatus and the teres minor. The supraspinatus helps center the humerus bone into the socket upon lifting your arm up. The infraspinatus and teres minor are the main external rotators of the shoulder and help with dynamic stability especially with the arm in elevated positions such as during the follow-through phase of throwing a baseball. All of the RTC have their origin from the scapula. Thus, proper scapular position and movement is essential to help give the RTC their most optimal "length-tension relationship". , ,  This means giving the muscle its best position to generate force that produces a compressive effect on the ball and socket and simultaneously reduces tension-type stress on the more inert tissues surrounding the joint such as your labrum and ligaments.  Thus weakness, in the RTC muscles results in a loss of the force couple at the glenohumeral joint (ball-and-socket). 
Morphologically, the difference is quite great between the deltoid and the RTC. The deltoid group is a type I muscle fiber, which means it is fatigue-resistant and also makes up close to 42 percent of the shoulder mass. The deltoid rarely gets injured from an orthopedic standpoint, but the RTC are constantly at risk for injury in the weekend athlete and gym-goer, competitive professional, or the unfortunate fall on an elderly arm. The RTC begin to lose their vascularity as we age and a steady diminution begins around age 30. If you don't train these muscles and you are doing exercises that have become in vogue now such as Kettlebells, Trx training, CrossFit, or simply doing heavy bench-press, dips, or overhead behind the neck presses -- all of which includes many exercises that require a heavy force on the shoulder -- it may just be a matter of time before your shoulder becomes a problem.
I invite you to take a look at the video below to learn some basic ways to protect your shoulder. If you already have pain in your shoulder you should seek out an orthopedic or sports medicine doctor for a diagnosis. It's possible they may prescribe physical therapy which in many cases will hasten your recovery and help you avoid more invasive procedures such as surgery. Remember, it is a "No Pain, All Gain" theory with strengthening your RTC and scapular muscles. If you work into pain you may be making your condition worse so remember to keep it a pain-free regimen.
1) Wassinger C, Sole G, Osborne H. Clinical Measurement of Scapular Upward Rotation in Response to Acute Subacromial Pain. J Orthop Sports Phys Ther. 2013; 43:199-203.
2) Saha AK. Dynamic Stability of the glenohumeral joint. Acta Orthop Scand. 1971;42:491-505.
3) Oyama S, Myers J, Wassinger C, Lephart S. Three-Dimensional Scapular and Clavicular Kinematics and Scapular Muscle Activity During Retraction Exercise. J Orthop & Sports Phys Ther. 2010; 40:169-179.
4) Labriola JE, Lee TQ, Debski RE, McMahon PJ. Stability and Instability of the glenohumeral joint: the role of the shoulder muscles. J Shoulder Elbow Surg. 2005;14:32S-38S.
5) Lee SB, Kim KJ, O'Driscoll SW, Morey BF, An KN. Dynamic glenohumeral stability provided by the rotator cuff muscles in the mid-range and end-range of motion. A study in cadavera. J Bone Joint Surg Am. 2000;82:849-857.
6) Zazzali M, Donatelli R, Wilkes J. Subscapularis plays unsung role in shoulder pain. J Biomechanics, July 2006.
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