The scapula is a remarkable structure. It allows humans to do everything from shrugging to communicate uncertainty to throwing a 90-mph fastball. Because of the scapula’s complexity, it is also prone to dysfunction, sidelining athletes, weekend warriors, and gardeners alike.
Gaining a better understanding of the design of the scapula and the role it plays in movement will provide a strong foundation for learning how it functions and the many ways in which things can go wrong.
The Anatomy of the Scapula
Before diving deeper into scapula function and dysfunction, we must better understand how to view the scapula.
- The scapula has numerous muscles attaching to it, all of which contribute to movement and stability. More importantly, however, they routinely perform either action synchronously. Scapular motion is highly complex although we try to make it seem simple. Since muscles don’t work in isolation, we need to move away from thinking of individual muscle action and function.
- We must keep in mind that the thorax has an ellipsoid design. This is important because the shape of the thorax does not allow for single planar movement. Scapular rotations, although they move around an axis, actually cross planes of motion and do not occur in a linear fashion. This is due to the shape of the thorax as well as the fiber orientation of the muscles acting upon the scapula.
- Finally, the scapula is, at its core, a link within the kinetic chain. It has a number of crucial roles, but most importantly, it is the link that transfers energy from the large muscles of the legs and core to the smaller muscles of the arm. You may have heard the shoulder being described as a “funnel.” This description comes from the idea that the scapula directs energy from the core to the arm similar to how a funnel directs fluid into a specific container or reservoir.
The Many Roles of the Scapula
Optimal scapular function is a key component of all shoulder function. The scapula plays many roles in shoulder function. Anatomically, it is the “G” of the glenohumeral joint and the “A” of the acromioclavicular joint. Physiologically, it is the “S” of scapulohumeral rhythm (SHR), the coupled and coordinated movement between the scapula and arm that allows you to place your arm in the optimum position and motion to accomplish tasks.
These complex motions and translations are necessary to allow the scapula to function as part of SHR, the integrated coupled motion of the moving arm and scapula that is the basis for effective upper extremity use.
The scapula plays several roles to achieve efficient SHR:
- It allows a congruent ball-and-socket arrangement through the full ranges of arm motion by keeping the alignment of the humerus and glenoid within physiologic limits, which maximizes the concavity/compression capability of the joint.
- It creates a stable base for optimal activation of the scapular-based muscles. Studies in asymptomatic subjects have documented that maximal demonstrated rotator cuff strength can be developed when the scapula is stabilized in a position of neutral retraction.1, 2 Excessive protraction or retraction decreased the developed strength by 11 percent. In symptomatic subjects, the change was even greater: A retracted scapula increased the developed arm strength by 24 percent.3, 4
- It clears the acromion as the arm elevates. Most kinematic studies show that posterior tilt is necessary, in addition to upward rotation, to allow maximum arm flexion. This position allows optimal function in overhead activities and reduces the occurrence of external impingement symptoms.
- It allows for optimal force transfer from the site of largest force development (the core) to the most common force delivery site (the hand) as part of the kinetic chain of all integrated dynamic body activities. The shoulder acts as a funnel, transmitting and concentrating the developed forces. This function requires dynamic stability for the efficient transfer of energy. The dynamic stability is created by the actions of the scapular stabilizers, which are maximized when hip and trunk strength are maximized.
In future articles, we’ll look more closely at function and dysfunction of the scapula, including rotation and translation as well as scapular dyskinesis. In the meantime, you can gain more understanding of the shoulder’s anatomy by watching these MedBridge courses:
- Evaluation of Shoulder Impingement
- Evidence-Based Examination of the Shoulder: An Update
- Biomechanics of the Shoulder
You can also develop expertise in the shoulder by completing the Rehabilitation of the Shoulder MedBridge Certificate program.
- Smith, J., Kotajarvi, B. R., Padgett, D. J., & Eischen, J. J. (2002). Effect of scapular protraction and retraction on isometric shoulder elevation strength. Archives of Physical Medicine and Rehabilitation, 83(3): 367–370.
- Smith, J., Dietrich, C. T., Kotajarvi, B. R., & Kaufman, K. R. (2006). The effect of scapular protraction on isometric shoulder rotation strength in normal subjects. Journal of Shoulder and Elbow Surgery, 15(3): 339–343.
- Kibler, W. B., Sciascia, A., & Dome, D. (2006). Evaluation of apparent and absolute supraspinatus strength in patients with shoulder injury using the scapular retraction test. American Journal of Sports Medicine, 34(10): 1643–7.
- Tate, A. R., McClure, P. W., Kareha, S., & Irwin, D. (2008). Effect of the Scapula Reposition Test on shoulder impingement symptoms and elevation strength in overhead athletes. Journal of Orthopaedic and Sports Physical Therapy, 38(1): 4–11.