The biomechanics of swimming cause considerable stress on the shoulder joint which may be accentuated by improper stretching or training techniques. The rotator cuff, and particularly the supraspinatus tendon, is at risk in repetitive overhead stroke activity. Arthritis in the shoulder is primarily centered at the acromioclavicular joint; degeneration may occur as a result of overuse or leverage of the scapuloclavicular mechanism, or from motion related to upward pressure at the undersurface of the acromion due to subluxation or instability of the glenohumeral joint. Instability of the glenohumeral joint is a major problem which may occur in itself or in combination with rotator cuff tendinitis. The glenohumeral joint is stabilised superiorly by a posterior superior sling consisting of the long biceps tendon, the superior joint capsule, and the coracoacromial and coracohumeral ligaments. An anterior inferior sling mechanism consisting of the inferior glenohumeral ligament and subscapularis musculotendinous unit provides significant stability if uninjured. Fragments of labral tissue may mechanically wedge into the joint also leading to symptoms of subluxation. If the humeral head is wedged or allowed to slip out of joint due to capsular incompetency, secondary rotator cuff 'impingement' may occur; this is particularly difficult to manage. Prevention of injury is best accomplished through a programme of flexibility and strengthening avoiding overuse.