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Instep Dance Magazine Articles

Reprints of monthly column as first appearing in Instep Dance Magazine.

June 1999

Shoulder Injuries - Part 1 - Functional Anatomy

By Rick Allen, DC

"Better health leads to better dancing."

Although injuries in dancers occur more frequently in the lower extremities, the upper extremities can also be involved in both chronic and acute conditions. Clearly the most frequently injured joint in the upper extremity is the shoulder, which is typically involved in spins, overhead lifts and drops. This month let's review the functional anatomy of the shoulder. Next month we'll dive into treatment and case studies.

Shoulder Diagram

The shoulder is a very complex joint. It is composed of a ball, known as the humeral head, and a very shallow socket, known as the glenoid. The ball is located at the upper end of the arm bone (humerus) and is generally slightly smaller than a tennis ball. The socket is saucer-shaped and measures approximately 1 1/2 inches in diameter. The other joint of the shoulder is the acromioclavicular joint, composed of the outer end of the clavicle (collar bone) and the acromion, which is the most upper outer portion of the scapula (wing bone) (Figure 1). Because the shoulder moves through such large and varied ranges of motion -- that is, it can move forward, backward, up, and around in various combinations -- it is typically a very unstable joint. However, it has such versatility and wide range of motion that we may perform such activities as dancing, swimming and baseball.

Rotator Cuff/Shoulder Diagram

Holding the joint together are various ligaments and the capsule. On top of these ligaments runs a group of muscles called the rotator cuff. They begin on the scapula and attach to the humeral head just outside the joint (Figure 2). These muscles control the rotation of the shoulder; they also hold the shoulder in place, by providing an adequate positioning of the humeral head in the glenoid, which allows the other muscles around the shoulder to function with better mechanical advantage. The rotator cuff is composed of three muscles: supraspinatus, infraspinatus, and teres minor. A fourth muscle, the subscapularis, arises from underneath the scapula and attaches on the humeral head as well. The first three are external rotators; the fourth is an internal rotator.

Another structure often involved with injuries in the shoulder is the bursa that lies between the acromion and the rotator cuff. Also sometimes affected is the biceps tendon, which is an extension of the biceps muscle on the upper arm that attaches inside the shoulder joint itself. There it becomes intimately involved with the rotator cuff and susceptible to shoulder injury.

Part of the shoulder's motion also takes place where the scapula attaches to the chest. As the arm is raised, the scapula rotates up and forward; this action allows the arm an extreme range of motion. The muscles that perform this particular motion are very strong and provide raw power to the shoulder, as opposed to the fine-tuning that is more the function of the rotator cuff. The power muscles of the shoulders include the pectoralis, latissimus dorsi, and teres major.

The rotator cuff muscles have a very difficult job. Not only must they rotate the humerus under various circumstances, but they also act as stabilizers and must work in a static, as well as dynamic, way. Not only are these muscles used vigorously during athletic activities, but also extensively in carrying out routine jobs of the arm and shoulder. Because these muscles and their related tendons pass underneath the acromion, they may become pinched and irritated by that bone, causing rotator cuff tendinitis and related problems and even "frozen shoulder".

Next article: Let's move on to the treatment of shoulder injuries, including some real-life examples from our dance community.

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