The shoulder is a complex biomechanical region. Moving an upper extremity requires the coordinated effort of the glenohumeral, acromioclavicular and sternoclavicular joints, as well as the scapulothoracic articulation (which is not a true joint). The glenohumeral joint has the greatest range of motion of any joint in the body. However, achieving that range requires coordinated activity at multiple joints.

Shoulder pathologies often arise from poorly coordinated shoulder biomechanics. Coordination between the glenohumeral joint and scapulothoracic articulation is particularly important, especially for massage therapists.

What are Shoulder Impingements and How Do They Occur? 

A number of soft-tissue pathologies occur from compression under a region called the coracoacromial arch. The arch is created by the coracoid process, acromion process, and the coracoacromial ligament that spans between them.

The subacromial bursa, supraspinatus tendon, joint capsule, and biceps tendon long head are all susceptible to painful compression under the coracoacromial arch (Figure 1). Tissue compression under the arch is often referred to 

as shoulder impingement syndrome, although now it is also frequently called subacromial pain syndrome.

There are two types of shoulder impingement: primary and secondary. Primary impingement occurs from the position of the acromion process and the lack of space underneath it. For example, it is more commonly caused by an acromion process that tilts down at a bit of an angle and therefore decreases the space underneath it.

Secondary impingement is often the result of functional misuse or some other type of problematic shoulder biomechanics. For example, repetitive overuse of the arms during swimming is likely to cause secondary impingement because of the repeated overhead motions of the arm. This type of impingement is more likely to occur when an individual has laxity in the joint capsule because the head of the humerus moves around more in the glenoid fossa and will more easily bump up against the underside of the coracoacromial arch.

Any history of shoulder dislocations or subluxations should be considered as a contributing factor to secondary impingement problems. Secondary impingement also results from dysfunctional shoulder mechanics and improper coordination between upward scapular rotation and glenohumeral abduction.

Biomechanical Considerations: Rhythm and Weakness 

To understand how specific motions and shoulder anatomy mechanics lead to these issues, let’s take a more detailed look at shoulder abduction, which is the motion most often associated with subacromial compression. During shoulder abduction, there is significant movement at the glenohumeral joint, but the full range of motion cannot occur without upward rotation of the scapula as well. The coordinated motion of glenohumeral abduction and upward rotation of the scapula is called the scapulohumeral (or scapulothoracic) rhythm.

The purpose of this combined movement is two-fold. First, it allows the glenoid fossa to maintain a good position for the various roll, spin, and glide movements of the humeral head at the glenohumeral articulation. 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 motions.

The first component of the scapulohumeral rhythm is shoulder abduction and is produced primarily by the supraspinatus and the deltoid muscles. The second part, upward rotation of the scapula, is produced primarily by the upper and lower 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 scapular rotation for a full 180 degrees of shoulder abduction. Therefore, there is roughly a two to one ratio of movement in the glenohumeral joint to that of the scapulothoracic articulation. It is important to note that these motions are not sequential but almost concurrent. That means that most of the glenohumeral abduction and the upward scapular rotation occurs at the same time.

One of the main problems that lead to subacromial impingement is a disturbance to the scapulothoracic rhythm. Hypertonicity in the rhomboid or middle trapezius muscles is quite common. If these muscles are hypertonic, they may impair the ability of the scapula to move fully into upward rotation. Soft tissue treatments like massage can be helpful to reduce tightness in those muscles and ensure proper scapular rotation.

Weakness in the serratus anterior muscle is another factor that limits upward scapular rotation. The long thoracic nerve innervates the serratus anterior, and compression of this nerve may decrease motor signals to the muscle and cause serratus anterior weakness.

Long thoracic nerve compression often results from chronic weight placed on the shoulders, which occurs with backpacks or handbags. The straps press on the nerve on the superior aspect of the shoulder near the upper trapezius muscle. Also, note that the long thoracic nerve is very close to the brachial plexus, so in conditions like thoracic outlet syndrome, the long thoracic nerve may be compressed as well.

When the serratus anterior is functionally weak from nerve compression, there is inadequate upward scapular rotation. Without full upward scapular rotation, the glenohumeral head hits the underside of the acromion during abduction. Soft-tissue compression under the coracoacromial arch then results.

A common way to recognize potential serratus anterior weakness is noting, with the client prone on the table, if the vertebral border of their scapula lifts off he rib cage (often called scapular winging). The winged scapula occurs because the serratus anterior is unable to hold it close to the rib cage. There are numerous contributing factors to shoulder impingement that are not immediately apparent. Some factors exist in the absence of repetitive overhead motions, which are considered the most common cause of shoulder impingement. When noticing potential impingement problems, be sure to note the scapular motion’s accessory muscles to help encourage proper movement at the glenohumeral joint. 

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