The shoulder has the greatest range of motion of any joint in the body and shoulder abduction is an essential component of many upper extremity activities. Shoulder abduction is one of the few joint motions that has a normal range of at least 180 degrees.
One of the reasons for this extensive range is that shoulder abduction is much more complex than movement at the glenohumeral joint alone. Abduction requires coordinated movement of the glenohumeral joint and the scapulothoracic articulation and this is called the scapulohumeral (or scapulothoracic) rhythm.
The purpose of the scapulohumeral rhythm is two-fold. First, it allows the glenoid fossa to maintain a good position for the various movements of the head of the humerus. Second, the changing position of the glenoid fossa allows for a better length-tension relationship in the muscles acting across the glenohumeral joint to produce shoulder motion.
There are two key motion components of the scapulohumeral rhythm. The first is abduction of the glenohumeral joint. Abduction of the glenohumeral joint is produced primarily by the supraspinatus and deltoid muscles. The second part of the scapulothoracic rhythm is upward rotation of the scapula. Upward rotation of the scapula is produced primarily by the upper and lower fibers of the trapezius as well as the serratus anterior muscle.
If the scapulothoracic rhythm is properly coordinated, an individual will have approximately 120 degrees of glenohumeral abduction and 60 degrees of upward rotation of the scapula. There is about a 2:1 ratio of movement in the glenohumeral joint to that of the scapulothoracic articulation. These motions are not sequential, but mostly concurrent. That means that most of the glenohumeral abduction and the scapular upward rotation will be occurring at the same time.
The figure below shows the humerus in 120 degrees of abduction. At this point part of the motion has occurred at the glenohumeral joint and part has occurred at the scapulothoracic articulation. So, using the idea of the 2:1 ratio there has been about 80 degrees of abduction at the glenohumeral joint and about 40 degrees of upward rotation of the scapula in order to total 120 degrees of abduction.
Understanding how these movements at the glenohumeral joint and the scapulothoracic articulation work together is very important when looking at shoulder complaints. For example, an injury to the long thoracic nerve that innervates the serratus anterior muscle, such as from heavy backpack straps, may impair function in the serratus anterior muscle. If the serratus anterior is not functioning properly, the scapula will not upward rotate enough. If it doesn’t upwardly rotate enough, the humerus may compress the underside of the acromion process during attempted abduction and cause shoulder impingement or rotator cuff disorders.
Another common example of the importance of the scapulohumeral rhythm is evident with adhesive capsulitis of the shoulder (also called frozen shoulder). In this condition the glenohumeral joint capsule will adhere to itself and not allow full motion. When this occurs there will be a very evident disturbance in the scapulohumeral rhythm. Any attempts at abduction will usually require significant substitution and you will often see a motion like that pictured in the figure below, when an individual attempts abduction.
Identifying proper and dysfunctional movement patterns in various joints will be a great help in understanding the nature of certain pathologies affecting those joints. In order for us to understand the best way to treat various shoulder problems, we must be able to recognize and evaluate both correct and dysfunctional movement patterns.