Beyond the Glenohumeral Joint: A Holistic Approach to Shoulder Function

The shoulder complex is a highly intricate system made up of 4 joints: the glenohumeral (GH), acromioclavicular (AC), sternoclavicular (SC) and scapulothoracic joint (ST) with the role of the shoulder girdle to effectively and fluidly position the hand in space in all 3 planes of motion (sagittal, frontal, and transverse), requiring synergistic interplay between all 4 joints. This synergistic interaction between these 4 joints allows for the glenoid fossa (socket) on the scapula (shoulder blade) to be orientated in such a way that the humeral head is constantly in contact with the glenoid fossa. This is what is often referred to as congruency. The shoulder, more specifically the glenohumeral joint, is an amazing joint having the ability to display huge degrees of freedom due to the humeral head on average being 3 times larger than the glenoid fossa that it sits in (Saha, 1971). Furthermore, the shoulder blades have a concave shape that complements the convex structure of the rib cage. This anatomical design allows for the scapula to glide smoothly over the rib cage, ensuring efficient movement and stability. Already, I think one can clearly gauge that the shoulder was designed to bias mobility over stability! 

However, the most intriguing joint to me is the scapulothoracic joint, described as a functional rather than a true anatomical joint purely due to a lack of articular cartilage, synovial fluid, and joint capsule consistent with other anatomical joints. To put it more simply, the surrounding musculature provides stability to the scapula by controlling and coordinating motion, known as force couples, as the bone floats across the top of the rib cage. Now there are plenty of conversations surrounding this musculature that influences the scapula, such as the lower traps, posterior deltoid, serratus, and rhomboids, for example. Many training approaches jump straight to improving kinetic, or more trainable, characteristics of these muscles to dissolve shoulder pain and improve shoulder function. However, for some reason there does not seem to be nearly as much concern about the musculature responsible for controlling the other half of the scapulothoracic joint, the rib cage. It seems puzzling to me that so much focus is placed on improving only one half of a joint, so let’s consider some numbers to perhaps understand what juice is worth the squeeze. A total of 17 muscles directly influence the shoulder complex, along with 28 distinct joint motions occurring at all 4 joints. However, an additional 15 muscles act upon the rib cage. Taking the total to 32 muscles effectively involved in all 4 joints. So when considering specifically the scapulothoracic joint, almost 50% of all muscles involved with the entire shoulder complex act strictly upon one half of this joint. Now, if half the muscles involved in the shoulder complex are responsible for movement of the rib cage, surely there should be a more even 50/50 split of motion coming from the rib cage to facilitate optimal scapula motion versus movement coming strictly and solely from the glenohumeral joint? Possibly! But furthermore, we also know that the spine, more specifically, the thoracic spine and rib cage, was designed to move, and for more information on this topic, consider reading my blog post To Move or Not to Move? That is the question! This relationship between rib cage and scapula has been cited in numerous papers for years, but shoulder surgeon Ernest Amory Codman’s (1934) early work defined this relationship as scapulohumeral rhythm. 

A cross-section of the abdomen displaying some of the musculature involved with the rib cage and spine.

Consequently, both these appreciations of the muscles or forces couples (kinetics) as well as the structural orientation of joints and bones (kinematics) can be considered under the concept of form and force closure pioneered by Vleeming and colleagues (1995) to further understand joint mechanics. Force closure requires the active stabilisation joint force to be produced to keep joint surfaces together, whereas form closure is described as congruency created by the orientation of structures. So in relation to the shoulder, the orientation of the humeral head on the glenoid fossa, but this concept is applicable to all joints! With this understanding, we can now appreciate that there should be equal importance between kinematics and kinetics when creating stability within a joint, and this is emphasised with the shoulder due to the structural design of the joint, where there is a compromise of stability for mobility. However, I am arguing that perhaps not enough focus and emphasis is placed on the importance of the articulation of the rib cage to facilitate congruency, or the marrying up of the humeral head on the socket, to promote a pain-free, high-performing shoulder. Historically, the likes of Codman (1934) and Inman and colleagues (1944) placed importance on the scapulothoracic joint in overall shoulder function, with more recently Clayton Thompson (RS3 Sports) stating that “rib cage function pretty much dictates everything else." Furthering this, I personally believe that due to the prevalence of kyphotic spinal positions (flexion) arising from postures, there is a greater demand to address the rib cage prior to kinetic physical capacities. 

Force couples at the shoulder complex.

What is the issue with overly kyphotic spinal positions, you may ask? To answer this, it is important to note that up until this point we have only appreciated and understood the influence of the scapula in relation to the rib cage, but let’s also take some time to consider how the position of the spine and rib cage can influence the scapula. Gary Ward’s (2013) work appreciates how 3D spinal movement can directly influence positioning of the scapula. For example, looking in a 1D sagittal plane, flexion of the spine and anterior tilt (forward) of the rib cage will result in protraction and elevation of the shoulder blades. On the other hand, extension of the spine and a posterior tilt (backwards) of the rib cage will be accompanied by retraction and depression of the shoulder blades—now consider the other two planes of movement! Consequently, this changes length-tension relationships of the surrounding musculature, which, as we previously established, plays a key role in providing appropriate stability to the joint. Andrew and Hugh Huxley’s (1954) sliding filament theory explains that muscles produce the greatest force when their actin and myosin filaments are positioned optimally for cross-bridge formation. Therefore, when muscles are being pulled away from these optimal length-tension relationships, they lose their ability to produce sufficient levels of force for joint stabilisation—an issue that can be particularly problematic for the prone unstable shoulder joint. 

Building upon our understanding of the interdependent relationship between the shoulder complex, rib cage, and spine, let’s now try applying this to a sporting action like throwing a javelin or even a tennis serve to see where problems may arise. Both activities rely on similar joint actions—specifically, shoulder flexion and external rotation of the GHJ to allow the arm to move in the most efficient manner and produce high levels of force rapidly but safely. This is because the pecs and lats are the two biggest muscles responsible for accelerating the arm during throwing motions, and when entering shoulder flexion and external rotation, the muscles are wound up storing elastic potential energy to be released in the concentric phase of the throw. However, movement at the GHJ is only one piece of the puzzle. To execute these actions efficiently time after time, we must also consider the role of the scapula. What is it doing—or what should it be doing—to support and facilitate those joint motions at the GHJ? 

Novak Djokovic serve - notice how we see the posteior tilt of the rib cage coupled with spinal extension in the thorax during the late cocking and early acceleration stage paired with external rotaiton and shoulder flexion. Versus an anteriorly tilted rib cage coupled with spinal flexion and internal rotation of the shoulder during the deceleration phase.

Johannes Vetter javelin athlete - again notice how huge amounts of external rotation are paired with posterior tilt of the rib cage and spinal extension during the acceleration phase of the throw. Verus a more anteriorly tilted rib cage coupled with spinal flexion during the follow through.

Two major joint actions that occur at the scapula and that are often talked about are posterior tilt and upward rotation. These two actions are crucial in order to place the glenoid fossa in optimal positions for the most congruency between the ball on the socket when the hand is overhead. Currently, we have understood that both shoulder flexion and external rotation at the GHJ are accompanied by posterior tilt and an element of upward rotation of the scapula. However, if an individual is trying to achieve all these positions at the shoulder blade in a highly flexed, kyphotic spine, this can become problematic. If the rib cage is convex and the shoulder blades are concave, how is one meant to achieve posterior tilt and an element of upward rotation of the scapula while remaining in a highly flexed, kyphotic thorax without creating a discongruency or disconnect between the scapula and rib cage? (See far right image below). In order to achieve optimal levels of congruency between the rib cage and shoulder blade and consequently between the ball and socket, the rib cage must posteriorly tilt and the thoracic spine must extend to facilitate posterior tilt and appropriate levels of upward rotation of the scapula to position the glenoid optimally for the humeral head. Therefore, if an individual has an inability to extend the spine and posteriorly tilt the rib cage, how can the shoulder blade posteriorly tilt and orientate the glenoid fossa into a stable and appropriate position onto the rib cage to facilitate shoulder flexion and external rotation of the shoulder during sporting activities like a tennis serve or javelin throw? In my opinion, it will have a very hard time! 

Images showcasing how the orientation of the rib cage impacts congruency between the rib cage and shoulder blades when accessing shoulder flexion.

Ultimately, the shoulder complex requires a comprehensive understanding of multiple factors to design effective training programmes. Too often, there is seen to be an overemphasis on solely the glenohumeral joint (which many deem to be “the shoulder”), while the role of the spine and rib cage - structures that fundamentally dictate how the rest of the shoulder complex functions -  are heavily overlooked. As Austin Einhorn puts it, “when you do reductionist training, you get reductionist results. ” My interpretation of this is that we should not limit our perspectives when addressing and solving problems around the complexity of the shoulder. I find it hard to envision scenarios where asking individuals to perform certain joint motions at the shoulder - such as flexion, extension, internal or external rotation during sporting tasks - can be done effectively, with high and rapid force outputs repeatedly, without consideration of the rib cage and spine. While there can be exceptions where tissue isolation is necessary (early rehab settings), most global movements and activities involving the arm I believe demand a more integrated approach. That said, I do believe there is a time and place to purely focus on more kinetic, trainable physical capacities, such as peak force or rate of force development (RFD), without consideration of kinematics or structural alignment. However, I am stating that appreciation of the kinematics of the entire shoulder complex - including the rib cage and spine - can help facilitate optimal joint congruency, providing individuals with a stable base to maximise performance and minimise injury risk. This broader perspective presents, in my opinion, the lower hanging fruit to go after but let’s also consider that this is only one lens to understand the shoulders complexities.

References / Sources:

Austin Einhorn - Co-Founder of Apiros & The Evolved Coach

Clayton Thompson - Founder of RS3 Sports & Integrated Performance Coach for LA Dodgers

Codman, E. A. (1934). The Shoulder: Rupture of the Supraspinatus Tendon and Other Lesions in Or about the Subacromial Bursa. United States: T. Todd Company, printers.

Huxley, A. F., & Niedergerke, R. (1954). Structural changes in muscle during contraction: interference microscopy of living muscle fibres. Nature, 173(4412), 971-973.

Huxley, H., & Hanson, J. (1954). Changes in the cross-striations of muscle during contraction and stretch and their structural interpretation. Nature, 173(4412), 973-976.

Inman, V. T., Saunders, D.M., and Abbott, L.C. (1944). Observations on the Function of the Shoulder Joint. Journal of Bone and Joint Surgery, 26-A, 1-30.

Saha, A. K. (1971). Dynamic stability of the glenohumeral joint. Acta Orthopaedica Scandinavica, 42(6), 491-505.

Vleeming, A., Pool-Goudzwaard, A. L., Stoeckart, R., van Wingerden, J. P., & Snijders, C. J. (1995). The posterior layer of the thoracolumbar fascia. Spine, 20(7), 753-758.

Ward, G. (2013) What the Foot? A Game Changing Philosophy of Human Movement To Eliminate Pain and Maximise Human Potential. London: Soap Box Books.