The shoulder from the perspective of human anatomy, is actually referred to as a skeletal cage, or girdle, that consists of four separate joints; the sternoclavicular, acromioclavicular, glenohumeral, and scapulothoracic joints. When referring to the shoulder in colloquial terms, the focus is on the glenohumeral joint, the ball-and-socket joint located where the upper arm, or the humerus, meets a cavity on the side of the shoulder blade known as the glenoid fossa. This is the most mobile of all joints in the body, allowing freedom to use the shoulder to place the hand almost anywhere to perform activities as broad and powerful as swinging a club and as fine as manipulating a timepiece. In order to manage this range, however, the shoulder’s design is inherently unstable. It’s described as a ball-and-socket joint, but it’s more like a golf ball sitting on a tee, the top of the humerus sits on the shallow saucer-like cavity, called the glenoid fossa. Without the muscles, tendons, ligaments and joint capsule all serving as a sling to hold the unit together the shoulder joint would unravel.
There are certain movements in which the shoulder is particularly unstable. Lifting the shoulder above 90 degrees in the plane out toward the side of the body, known as abduction, creates a feeling of instability. That instability isn’t troublesome for routine activities like reaching for something in a cabinet, but when compounded with an unstable position with stress that accompanies overhead athletic movements such as pitching a baseball, striking overhead with a tennis racket, or lifting one’s bodyweight while rock climbing, the possibility of injury is greatly increased. The muscles most responsible for shoulder stability are the four muscle-tendon structures that comprise the shoulder’s rotator cuff. These muscles include the supraspinatus, infrapinatus, teres minor and subscapularis.
Since the soft tissue of the shoulder joint is responsible for stability more so than that of other joints the solution to preventing injuries is to maintain a balance of strength and flexibility in all muscles of the shoulder. These include the chest muscles as well; the pectoralis major and pectoralis minor; the scapular, or shoulder blade muscles, the trapezius, levator scapulae, rhomboids and seratus anterior as well as the deltoids, the rotator cuff muscles, the latissimus dorsi and muscles of the upper arm, the biceps and triceps.
The glenohumeral joint attaches the humerus to the glenoid fossa of the scapula, or shoulder blade. While the cavity is shallow, a fibrocartilage disc known as the glenoid labrum forms a rim around it deepening the socket and providing a more secure, although not foolproof, fit. The sternoclavicular joint attaches to the sternum which attaches to collarbone or clavicle, allowing for a small amount of rotation of the bones necessary when lifting the arm overhead. The acromioclavicular joint is on the edge of the shoulder, where the clavicle meets a part of the shoulder blade called the acromion. The scapulothoracic joint is not a true synovial joint, but rather the attachment by muscles of the shoulder blade to the trunk.
Most prominent along the slope between the neck and shoulder, the trapezius is a broad, flat muscle with three parts, an upper, middle and lower. The muscle extends all the way down to the center of the back and controls motion of the scapula during activities that require overhead motions such as swimming and tennis. The rhomboid muscles attach the scapula to the spine. They’re fundamental in controlling the position of the scapula, retracting it or pulling it back toward the spine during shoulder activities. The serratus anterior is a muscle that goes from the inner underside of the shoulder blade, around to the front of the trunk and attaches to the ribs in nine spots. It protracts, or pulls, the scapula forward and helps to tip it upward to position for overhead movements. The rotator cuff is a mass of four muscles and their tendons that move and stabilize the head of the humerus in the glenoid fossa. They keep the ball in the socket as the arm is raised, lowered and lifted through various arcs and angles around the joint. The cuff includes the supraspinatus, the subscapularis, the teres minor, and the infraspinatus, all of which blend into a single tendon. The deltoids are muscles which position the shoulder and cover the joint like epaulets. There are three aspects to the muscle, the anterior, middle and posterior, one each for lifting the arm forward, out to the side and backward.
The pectoralis major is the larger chest muscle, which is used in throwing and pushing. Strength of this muscle is directly associated with power and speed of throwing and pushing movements. A pitcher who wants to increase the speed of his pitches can do so by strengthening the pectoralis major and latissimus dorsi muscles. The major muscle of the back, the latissimus dorsi looks like a vest that covers almost the entire back. It is the muscle used to pull the body up when climbing and is also used in pulling, swimming, rowing and in swinging and throwing motions.
The biceps brachii muscle has two heads; the short head, which crosses only the elbow, and the long head, which functions at both the shoulder and the elbow. The long head of the biceps help stabilize the shoulder joint and may become more important in shoulders with a weak or torn rotator cuff. The triceps brachii are located in the back of the upper arm, the triceps is involved in throwing and pushing motions that move the arm downward and backward, as when pushing back on the poles during skiing, or in the pull-through phase of a freestyle swim stroke.
The subacromial bursa is a watery sac that allows the tendons of the rotator cuff to ride smoothly over bone within the shoulder joint. The glenoid labrum is a rim of fibrocartilage that surrounds the glenoid fossa, deepening the socket and imparting some extra stability to the joint. The joint capsule is a fibrous capsule surrounding the glenohumeral joint which is thin and, in a normal shoulder, remarkably loose. It is this looseness that contributes to the joint’s tremendous mobility. The glenohumeral ligaments are three ligaments that strengthen the front of the joint capsule. They appear as thickening of the joint capsule itself, rather than clearly distinct structures. If these ligaments become stretched out, a subluxation or dislocation of the joint may occur.
A full range of motion and poor stability characterize the shoulder and explain why athletic stresses take a heavy toll on the soft tissue of the shoulder. Careful warm-ups, stretches and strengthening exercises are essential preventative measures against injury.