Brushing your teeth, combing your hair and changing a light bulb may seem like relatively simple common household tasks. However, simplicity can largely be contributed to the intricate relationship of the scapula and the shoulder complex. Although large attention is placed on the trunk (core stability) in training, rehabilitation and performance in other regions, especially the shoulder complex, often get overlooked. The shoulder complex plays an integral role in upper quarter function and deserves more attention. A healthy shoulder complex will allow ease in functional tasks, better performance in sporting events, and prevention of injuries. In this Part I series on the Upper Core, essential anatomical features will be explained. Also, muscles actively contributing to the mechanics of the upper quarter region will be considered. The importance will be reinforced through the pathomechanics of the upper core. The Part II series to the Upper Core will look at activation and maintenance, through strengthening to provide stability for upper quarter function.

What is the Shoulder Complex?
The shoulder complex includes the clavicle, scapula, and humerus. The bones are united through the glenohumeral (GH) and acromioclavicular (AC), and the sternoclavicular (SC) joints. The scapulothroacic joint is also included within anatomical descriptions of the upper quarter(1).

Movement stemming from the upper extremities involves a coordinated interaction from all the joints. The upper core has unique mechanics that allow tremendous mobility requiring coordinated and synchronous motion of all the joints. However, the range of motion is compromised by its lack of bony or ligamentous attachments from the scapula to the axial skeleton(2). Due to anatomical architecture, scapulothoracic motion requires a combination of clavicle motion on the thorax (SC Joint), and/or further scapula motion relative to the clavicle at the AC joint(3).

Scapula
The scapula, often discounted in training programs, bears significant responsibility in upper quarter function. With numerous muscular attachments, the scapula work synchronously with the humerus to allow full range of motion known as scapulohumeral rhythm(4). Through precise coordination with arm movement, stability is provided for distal mobility. Another important function is its ability to regulate force transmission. Through the large range of motion, the scapula prevents impingement and still allows high force transmission.

Scapula at Rest
The triangular scapula is a flat bone that lies over the posterior thorax from ribs two to seven. At rest, scapular positioning is important in a normal functioning upper core. Altered cervical and thoracic spinal alignment has been postulated in disrupting the resting position resulting in muscular weakness, dysfunction, and decreased range of motion(3). One study demonstrated these changes through measuring spinal posture and its relationship to angulation of the scapula. Results showed increased forward angulation of the scapula with the increased curvature of the thoracic spine (kyphotic curve)(3). Consequently, it is important to assess scapular position during rest.

Coordination of movement through force couples (synergistic cocontractions) is another important function of the muscles that attach and control the scapula(4). The intrinsic rotator cuff muscles, known as dynamic stabilizers, include the supraspinatus, infraspinatus, teres minor, and subscapularis all have origins on the scapula. Opposing include the static stabilizer muscles that insert on the scapula and consist of the upper and lower trapezius, rhomboids, and serratus anterior(3). The oppositional interplay between static and dynamic stabilizer muscles creates a force couple relationship. Concentric and eccentric activity of the muscles have been postulated as anchoring the scapula in lieu of the absence of bony articulations(5). Therefore, length-tension relationships for scapular musculature are paramount for efficient upper core function. Weakness or poor length-tension relationships can result in faulty positioning and mechanics. Understanding the muscular functions controlling the relationship of the scapulothoracic joint can prevent upper extremity pathomechanics and proper exercise can be prescribed.

The illustration of father and son raising a heavy ladder can be similar to the relationship of the muscles of the shoulder. The stronger father will lift and move the heavy ladder while the weaker son will guide it as it is raised and prevent it from lifting off the ground (stabilize). The father represents the static stabilizing muscles (serratus anterior, trapezius, rhomboids) acting as primary movers and the son acts as the dynamic stabilizing rotator cuff muscles through stabilization. At any point the father can generate too much power that will overcome the resistance of the son resulting in instability(1).

Summary
The four joints including the glenohumeral (GH), acromioclavicular (AC), sternoclavicular (SC), and scapulothoracic make up the upper core. Normal upper core function requires coordination between joints. Muscular attachments including the static stabilizers (rhomboids, trapezius, and serratus anterior) that provide the movement while the opposing rotator cuff muscles (dynamic stabilizers) maintain stability. The force couple relationships properly align the bony architecture through maintaining posture. Any weakness in the kinetic chain will create a faulty alignment and predispose injury. Therefore, a static scapular analysis is vital prior to assessing any dynamic movements.

 

REFERENCES

1. Terry GC, Chopp TM. Functional anatomy of the shoulder. Journal of athletic training. 2000;35(3):248.
   
2. Paine R, Voight M. The role of the scapula. The Journal of orthopaedic and sports physical therapy. 1993;18(1):386.
   
3. Culham E, Peat M. Functional anatomy of the shoulder complex. Journal of Orthopaedic and Sports Physical Therapy. 1993;18:342-342.
   
4. Voight ML, Thomson BC. The role of the scapula in the rehabilitation of shoulder injuries. Journal of athletic training. 2000;35(3):364.
   
5. Kibler WB. Role of the scapula in the overhead throwing motion. Contemp Orthop. 1991;22(5):525-532.