The shoulder joint has the greatest range of motion of any joint in the human body. Because of this level of mobility, it has much less structural stability than any other joint. Hinge joints such as the knee or elbow are very stable, since they have only one plane of movement. Other ball-and-socket joints, such as the hip and ankle have a smaller range that the joint can move, so the “ball” can sit more deeply in the “socket”.
Since the shoulder does have such a great range of motion, the “ball” (the head of the humerus), sits very loosely in the “socket” (the glenoid cavity of the shoulder blade). Along with the labrum, which will be covered in the third article of this series, the only thing holding the shoulder joint together is the rotator cuff.
The rotator cuff (not “rotary cuff”, or “rotator cup”, as it is often called) comprises four muscles: the supraspinatus, the infraspinatus, the teres minor, and the subscapularis. The initials of these muscles are sometimes made into an acronym, giving the rotator cuff the nickname of the SITS muscles.
Between these four muscles, with the supraspinatus abducting the arm (raising to the side), the subscapularis internally rotating, and the teres minor and infraspinatus externally rotating, the rotator cuff not only holds the humerus securely in the glenoid cavity, but also aligns the entire shoulder joint to make sure it moves correctly.
One of the most common causes of shoulder pain occurs when the rotator cuff is imbalanced with the subscapularis being overly tight and the infraspinatus and teres minor being weak and/or loose. The subscapularis as the primary internal rotator of the shoulder, is used extensively in both fitness and sports; pushups and bench presses, throwing a ball or throwing a punch, all include internal rotation. The opposing movement, external rotation, needs to be worked as well, but doesn’t have as many day-to-day activities that utilize it. For that reason, care must be taken to include movements such as band pull-aparts, reverse flyes, and even specific external rotation movements (with a band or a dumbbell) to ensure these muscles stay balanced and the shoulder stays healthy.
The next article in this series will go over some of the best exercises to ensure the rotator cuff stays healthy and balanced to help the overall functionality and pain-free movement of the shoulder joint.