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Orthopedic Massage Essentials

By Whitney Lowe, LMT

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Scapulohumeral Rhythm: Looking Beyond the Glenohumeral Joint

The shoulder has the greatest range of motion of any joint in the body. What is often overlooked in shoulder mechanics is that motion in the shoulder is not purely at the glenohumeral joint. The significant range of motion of the shoulder is made possible by the coordinated movement at the glenohumeral joint and the scapulocostal articulation. The scapulocostal articulation is where the scapula articulates with the posterior rib cage. It isn't a true synovial joint, but is considered an articular contact point where bones are meeting (scapula and ribs).

Movement Patterns That Are Coordinated

The scapula and humerus move in a coordinated pattern for all the primary movements of the shoulder. However, their combined motions are particularly important during shoulder abduction where they are both moving at near maximum range capability. Understanding how movement at the glenohumeral joint and scapulocostal articulation occur together is critical for awareness of the dysfunctional mechanics that occur in certain shoulder problems.

Scapulohumeral Rhythm: Looking Beyond the Glenohumeral Joint - Copyright – Stock Photo / Register Mark Maximum abduction of the shoulder is considered to be about 1800. Note that all the following descriptions about range of motion will be based on the assumption of 1800 of abduction. When the shoulder is abducted, motion is occurring at both the glenohumeral joint and the scapulocostal articulation. The combination of movement between the glenohumeral joint and the movement at the scapulocostal articulation is referred to as the scapulohumeral rhythm (also called the 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.

Understanding the Rhythm

There are two primary components of the scapulohumeral rhythm. The first is abduction of the glenohumeral joint, which is produced primarily by the supraspinatus and the deltoid muscles. The second part of the scapulothoracic rhythm is upward rotation of the scapula, and 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 1200 of glenohumeral abduction and 600 of scapular upward rotation.

Therefore there is roughly a 2:1 ratio of movement in the glenohumeral joint to that of the scapulocostal 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 scapular upward rotation will be occurring at the same time.

Scapulohumeral Rhythm: Looking Beyond the Glenohumeral Joint - Copyright – Stock Photo / Register Mark Figure 1 A common misunderstanding about glenohumeral abduction relates to supraspinatus activity during abduction. Many texts report that the supraspinatus muscle initiates abduction and is active for the first 25 to 40 degrees of abduction. It is then sometimes suggested that the deltoid takes over to perform the remainder of the abduction movement without further supraspinatus involvement, which is not accurate. The supraspinatus muscle remains active throughout the entire movement of abduction. It is most active during the early stages when there is very little deltoid activity, but it does not stop its contraction process throughout the entire abduction movement.

Figure 1 shows the humerus in 1200 of abduction. At this point you will notice that some of the motion has occurred at the glenohumeral joint and some at the scapulothoracic articulation. So, using the idea of our 2:1 ratio there has been about 800 of abduction at the glenohumeral joint and about 400 of upward rotation of the scapula in order to total 1200 of abduction.

Understanding how these movements at the glenohumeral and the scapulocostal articulations work together is very important when looking at certain shoulder complaints. When there is a disturbance to the scapulohumeral rhythm, it can play a significant role in shoulder pathologies. To see an example of how the scapulohumeral rhythm plays a part in shoulder pathologies, let's take a look at what happens with eventual results of long thoracic nerve entrapment.

Nerve Entrapment

The long thoracic nerve innervates the serratus anterior muscle. This nerve exits the spine very close to the brachial plexus and is often involved in brachial plexus injuries and can also be injured when people wear heavy backpacks or anything with heavy shoulder straps.

Scapulohumeral Rhythm: Looking Beyond the Glenohumeral Joint - Copyright – Stock Photo / Register Mark Figure 2 An injury to the long thoracic nerve may impair function in the serratus anterior muscle causing it to be weak. Serratus anterior weakness from long thoracic nerve compression is often evident with clients who display a winging scapula. When the serratus anterior is not working properly it doesn't pull the scapula into upward rotation far enough during abduction movements, and the humerus may hit the underside of the acromion process near the end of the abduction movement causing shoulder impingement and potential rotator cuff pathology.

Another common example of the importance of the scapulohumeral rhythm is illustrated with dysfunctional movement patterns that occur when a client has adhesive capsulitis of the shoulder (also called frozen shoulder). In this condition the glenohumeral joint capsule will adhere to itself and not allow full glenohumeral abduction. 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 Figure 2 (above) when an individual attempts abduction. There is a significant bind to glenohumeral abduction and the person is trying to compensate with additional upward scapular rotation and lateral flexion of the torso.

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.

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