unctional stability, are at increased risk of injury to the rotator cuff. Tennis players and throwing athletes, activities that are essentially asymmetrical, tend to develop greater shoulder external rotation in their dominant shoulder and this is commonly associated with functional instability. Shoulder-injury prevention strategies need to focus on improving shoulder stability.
Impingement and the rotator cuff
The bony anatomy of the glenohumeral joint comprises a large ball (the head of the humerus) and small socket (the glenoid of the scapula) with the muscles of the rotator cuff and scapular rotating (stabilising) muscles acting as the most important dynamic stabilisers of the joint. The muscles of the rotator cuff envelop the glenohumeral joint itself, and comprise the supraspinatus, infraspinatus, teres minor and subscapularis muscles. Supraspinatus abducts the arm (moves it laterally away from the side of the body), infraspinatus and teres minor externally rotate the shoulder, and subscapularis is prin-cipally an internal rotator of the shoulder. Sitting above the cuff is the coracoacromial arch, made up of the coracoid and acromion bony processes of the scapula and a ligament connecting the two processes. As the arm is abducted away from the body or flexed (brought forward), 'impingement' or squeezing of the rotator cuff between the head of the humerus below and the coracoacromial arch above can occur. The healthy, conditioned rotator cuff functions effectively as an integrated unit to stabilise and depress the head of the humerus, opposing the action of the large deltoid muscle and thereby preventing impingement. Any overhead activity that involves the arm being taken often enough from below shoulder level to above shoulder level has the capacity to damage the rotator cuff. With repeated impingement, a poorly conditioned cuff can become damaged, and a cycle of cuff damage, impaired function, further impingement and worsening cuff damage is initiated.
This form of primary impingement is most commonly seen in weight trainers who over-emphasise the development of their 'prime moving muscles' (pectoralis major, latissimus dorsi and deltoid) at the expense of their rotator cuff. It seems to be increasingly common in athletes as they reach their thirties. Primary impingement is preventable and, if the cuff is appropriately conditioned, exercises such as behind-the-neck press, incline bench press and extended front laterals, will not lead to pain. Differences in the shape and bony configuration of the undersurface of the acromion may dispose an athlete to this particular injury. A Type II (curved) or Type III (hooked) acromion will reduce the effective space through which the supraspinatus tendon slides during abduction. Plain X-rays should enable these two variations to be identified.
Secondary impingement refers to impingement secondary to underlying glenohumeral instability, when the rotator cuff is fatigued by its efforts to keep the humerus centred on the glenoid and consequently allows the head of the humerus to ride up, reducing the subacromial space. This is probably the most common mechanism of cuff injury seen in younger athletes, particularly those with increased joint laxity, and is seen frequently in swimmers and throwers. The primary problem here is instability and, unless this is treated, pain will be ongoing and progressive.
A strong and healthy rotator cuff is essential to the overhead athlete. In recent years, the role of the scapula-stabilising muscles in positioning the glenohumeral joint for optimal rotator-cuff function has been increasingly emphasised. Co-ordinated action of this group of muscles is needed to provide a stable base for pain-free overhead activity. The overly simplistic 'ball and socket' model of the shoulder joint has been superceded by a model similar to the performing seal that can balance a ball on its nose. The seal equates to the scapula, and constant small adjustments by the seal (scapula) are needed to prevent the ball falling off its nose (glenoid). Overhead athletes must 