Understanding Shoulder Anatomy and Function

The shoulder complex is so named because of both the large area that it covers and the complexity of its anatomy and biomechanics. It includes most of the upper trunk from front to back. If you look at where the muscles involved in upper-body movement originate and attach, the bones that form the entire shoulder complex are the sternum (breastbone), clavicle (collarbone), scapula (shoulder blade), humerus (arm), cervical and thoracic vertebrae (neck and upper spine), parts of the skull, and the ribs.

Figure 9.1 Anatomy of the Shoulder Complex

The muscles that are involved in upper-body movement flex (move the arm up), extend (move the arm backward), abduct (move the arm away from the side of the body), adduct (move the arm toward the center of the body), internally or externally rotate the arm, protract (move the shoulder joint forward), retract (move the shoulder joint backward), elevate (move the shoulder joint up) and depress (move the shoulder joint down).

The design of the shoulder joint allows great degrees of freedom. It has the largest range of motion (ROM) of any joint in the body. Because of this mobility, though, it sacrifices some stability. The primary stabilization of the shoulder joint happens as the muscles that surround the entire joint do their job correctly. Without the forces that these muscles produce, the shoulder becomes an unstable joint that is at risk for injury, especially when you perform overhead and rotational movements. The key to having healthy shoulders and super upper-body strength is to balance the upper-body muscles so that they can work synergistically.

The scapula is a flat, triangular bone that has two prominent projections on the top called the acromion process and the coracoid process. You can find your acromion process by following your clavicle out to the end of your shoulder. That bony projection is the acromion process, and the joint located there is the acromioclavicular (AC) joint. Just below and to the inside of the AC joint is the coracoid process. On the outside of the scapula is a slight cavity where the head of the humerus is located. A bony projection on the side of the humerus, called the greater tuberosity, is a site for muscular attachment. The front of the humerus contains a groove where the head of the biceps tendon rests.

Ligaments are strong bands of tissue that connect bones. Many ligaments add degrees of stability to the shoulder, but the coracoacromial ligament is the most talked about because it is the one primarily involved in shoulder impingement (which is discussed in detail later). The coracoacromial ligament connects the acromion process to the coracoid process and makes a roof inside the shoulder. The shoulder capsule is a fibrous but loose sleeve that surrounds the humeral head and is reinforced with ligaments.

The space beneath the acromion is called the subacromial space. The coracoacromial ligament is the roof of the space and the top of the humeral head is the floor. The subacromial space available varies with shoulder movement and steadily decreases as the arm is elevated and as the joint ages. Located inside this space are the subacromial bursa (a small fluid-filled sac that protects tendons from the hard surfaces of bones, allows them to glide smoothly during movement, and provides nutrition), the rotator cuff tendons, and the long head of the biceps tendon.

We live in a sedentary and flexion-biased society. For the majority of the day, we are sitting-whether in the car, at a desk at work, or at home on the couch. We usually do things in front of our body as we lean forward, ever so slightly. Even as you read this book right now, quickly check your posture. Of course, the minute I mentioned posture you probably straightened right up. But before that, more than likely your head was forward, your shoulders were rounded, and your arms were out in front of you. Don't feel bad-such posture is a natural and typical part of our lifestyle. But because of this lifestyle the muscles on the front of our upper body are contracted and shortened and the muscles on our upper back are relaxed and lengthened. Also, such posture reduces the already small space that we have in between the humeral head and the coracoacromial ligament, where all of our rotator cuff tendons and bursa are found, which in turn leads to many common complaints of neck and shoulder pain. When we throw in all of the repetitive motions we do during the day and then add sports and exercise to the mix, we can exacerbate the problem and cause a clinical condition like shoulder impingement. In addition, many sports are flexion-biased and most people concentrate on their fronts (the so-called beach muscles) at the gym-the chest muscles, shoulders, biceps, and abdominal muscles.

Shoulder impingement is the trapping of the rotator cuff tendons, the subacromial bursa, or the biceps tendon in the subacromial space. It progresses in stages and starts with inflammation that is reversible, but that can lead to irreversible thickening of the bursa, tendinitis of the rotator cuff, rotator cuff tears, and bony changes like spurs on the acromion process. Signs and symptoms of shoulder impingement are anterior shoulder pain during movements of flexion, abduction, or rotation; weakness of the rotator cuff muscles; and limited range of motion in the shoulder.

As with many syndromes, the causes of shoulder impingement can be multifactorial (because poor posture increases muscular weakness) and one problem frequently leads to another (because muscular weakness can result in muscular tears). The causative factors can be either intrinsic, pertaining to the muscles of the shoulder joint, or extrinsic, applying to other structures like bones, ligaments, and capsules. Intrinsic factors are muscular weakness, muscular overuse, and muscular degeneration. For example, poor posture contributes to muscular weakness or imbalance, which destabilizes the humerus. This instability can cause the top of the humeral head to bump into the coracoacromial ligament and put pressure on the tendons and bursa in the subacromial space. Muscular overuse that leads to inflammation of tendons also decreases this already small space. In late-stage impingements the constant inflammation leads to degeneration and tendon tears. These problems further weaken the joint and allow excessive movement of the humeral head, which then leads to more trauma.

Extrinsic factors are acromion shape, degeneration of the acromioclavicular joint, and tightness or laxity of the ligaments or the capsule. For example, acromion processes can have three different shapes in different people: flat, curved, or hooked. The hooked acromions are associated with a higher prevalence of rotator cuff tears, because they decrease the subacromial space and can slice at the tendon. Arthritis that causes degenerative spurs to form on the underside of the acromion leaves even less space available for movement without impingement. When the ligaments and capsule of the shoulder are lax, the humerus is unstable. Just as weak muscles do, this instability allows the humeral head to move excessively and damage the rotator cuff tendons and bursa. On the other hand, if the capsule and ligaments are too tight, there won't be enough space available for all of the structures to move without compression.

Finding out what is causing the problem through a physical therapist is crucial for treating the impingement appropriately. A case of tight musculature not letting natural glide occur might mean stretching, whereas a problem caused by joint instability and weak musculature might call for some specific strengthening exercise.