The shoulder joint - an introduction.
The shoulder is the most mobile joint in the body with many joint actions. Comprised of the head of the major bone of the arm (the humerus) and glenoid cavity (socket) of the scapula, this joint is frequently injured and a common source of dysfunction in people from all backgrounds. It is essential that the modern manual therapist have a solid understanding of the workings of the shoulder and treatment protocols for dealing with dysfunction. In this post we are not talking about the other structures surrounding the shoulder joint such as the clavicle, scapula etc, as these collectively make up what we know as the Shoulder Girdle. It should be noted however, that one needs a full understanding of all these areas in order to deliver effective treatment.
In total the shoulder (glenohumeral) joint is capable of 11 different actions. In no particular order these are flexion, extension, abduction, adduction, internal rotation and external rotation, horizontal adduction and abduction, horizontal internal and external rotation, and circumduction. Phew!
You can imagine that a highly mobile joint capable of this range of motion is susceptible to injury, especially when placed under great stress that many sports and activities demand. The shoulder joint is classified as a ball and socket joint, but in reality the socket part where the head of the humerus sits, is very shallow. This allows for considerable range of motion but at the cost of reduced stability. Static stability (that provided by harder structures like bones and ligaments) is limited in the shoulder area. Therefore a high amount of dynamic stability (that provided by muscles) is needed. This is provided by a group of muscles that most of us will have heard of - the Rotator Cuff muscles.
The rotator cuff is a group of 4 muscles that reside on the dorsal and costal surface of the scapula. On the dorsal surface from superior to inferior we have Supraspinatus, Teres Minor, and Infraspinatus. On the costal surface you will find Subscapularis. Like many muscles on the posterior side of the body, the RC group can often suffer from weakness and inhibition. The dorsal surface muscles are stablisers of the shoulder and also external rotators (supraspinatus also abducts the arm for the first 30 degrees). With a common postural defect being internally rotated shoulders as part of an upper crossed pattern, it's easy to see how these external rotators can become stretched, weak, and inhibited. Subscapularis, an internal rotator of the arm, on the other hand becomes potentially fibrotic, short, and tight. With these muscles required to stablise the shoulder during movement, proper function is of course highly desireable.
There are of course a number of problems that can occur with the shoulder joint. These include the head of the humerus being pulled up too high restricting the sub-acromial space, bursitis, ligament damage, rotator cuff dysfunction, capsular adhesion, and others. Often people have a high or low shoulder, but this can frequently be because of dysfunction elsewhere in the body.
In the next article we will see a brief introduction to the shoulder girdle.