Biomechanics

 THE SHOULDER & THE OVERHEAD ATHELETE

The shoulder, being the most mobile joint of the human body, is highly susceptible to injury [1].  Athletes often incur injuries of the shoulder as they subject the joint to extreme ranges of motion and force, as well as excessive repetitions of these motions [2]. 

Overhead athletes; those playing racquet sports, sports requiring throwing (baseball, cricket) and swimmers (in particular front crawl, back stroke and butterfly swimmers) commonly report shoulder pain.  It has been shown that more than 40% of elite swimmers complain of shoulder pain at some point [3] with most of their pain being due to instability [4].  Overhead athletes are prone to static stabiliser stress (ligamentous labral complex) and dynamic stabiliser stress (rotator cuff musculature)1.

Common to all theses sports are the repetitive, propulsive and explosive movements of the glenohumeral joint during throwing, swimming and hitting a ball with a raquet[3].  The clinician must note however that there is considerable variance in the range of motion and strength adaptations of elite versus amateur sport players and so these two sub-groups should to some extent be considered as separate entities in terms of treatment, even within the same sport [5]. The phase of movement involved in throwing a ball or taking a forward stroke when swimming can be divided into stages [3]:

Cocking
This involves abduction, external rotation and extension.  The muscles involved are the posterior deltoid, infraspinatus and teres minor.  In this position the joint is at risk of anterior subluxation because the anterior capsule is stretched.  There is also a risk of infraspinatus overactivity.

Acceleration
This is the main phase; it is propulsive in throwing/ racket sports and, along with the follow-through is involved in the pull through in swimming.  The muscles involved are the anterior deltoid, pectoralis major, latissimus dorsi, subscapularis and teres major.  Repetitions of this phase can lead to rotator cuff attrition (mainly the supraspinatus and biceps brachii tendons) attributable to impingement taking place under the coracoacromial arch. 

Overuse injuries can occur by way of coracoid process stress fractures, in younger atheletes strress fractures of the humerus can occur.  Tendinitis of the conjoint tendon of the short head of the biceps brachii and coracobrachiallis is also seen.  Repeated elbow extension in swimmers has been noted to result in tendinitis of the long head of the triceps.

Follow through
During this phase the infraspinatus has a restrictive role.  Shoulder soreness can occur due to further activity of the biceps brachii.  Posterior strain occurs at this time and the joint is at risk of posterior subluxation.

Clearly keen athletes are at risk of the problems outlined above due to their high training/ playing regimes.  The injuries they sustain are termed ‘overuse injuries’ [6].  The most common injuries to the shoulder occurring in athletes are instability, rotator cuff injuries and superior labrum lesions [7].  It has been shown that the primary cause of shoulder pathology in the overhead athlete is a superior labrum lesion with internal impingement [8].  Impingement results because of hypovascularity, fatigue, poor stroke mechanism and progressive instability of the hypermobile joint [2].  Instability occurs because overhead athletes develop anterior capsular laxity and posterior capsule contracture [10]. Internal impingement can result secondary to this [11].