(Or, why your scapula should move when your arm moves)
By Bernie Clark, February 10, 2022
Have you ever heard a yoga teacher tell students with arms overhead to “draw your shoulder blades back and down”? This is not always the right instructions, and indeed rarely is the best thing for a student to do. The shoulder complex consists of three bones and four joints. Movement at one joint causes and affects movement at the other joints. They are connected and coordinated. The most important coordination is called the scapulohumeral rhythm (also known as the glenohumeral rhythm), which is depicted in figure 2.
When the arm moves, the scapula moves. This is important and should not be resisted, because allowing the scapula to move as the arm rises maximizes the bony contact between the head of the humerus and the scapula (specifically, the shoulder socket or “glenoid fossa”). Scapular movement also minimizes the risk of impingement of soft tissues between the humerus and coracoacromial arch. Figure 2 illustrates an average case where, when the arm is fully elevated to 180° relative to the body, the scapula has upwardly rotated by 60°.[i] Notice how the rotation of the scapula directs the glenoid fossa upward, allowing it to form a base of support for the arm; the two bones are aligned. Were the scapula not rotated, the head of the humerus would be supported only by the inferior joint capsule and teres major tendon, which is not nearly as secure. This simple realization, that the scapula can and should upwardly rotate during arm elevation, will come as news to many yoga teachers.
Another benefit to allowing the scapula to upwardly rotate during arm elevation is the enhancement of the action of the deltoid muscle. If the deltoid muscle has maximized abduction of the arm at the glenohumeral joint and compression of the humerus against the scapula is preventing further arm elevation at the glenohumeral joint, further arm elevation relative to the torso can be achieved through scapular upward rotation.
FIGURE 2: The scapulohumeral rhythm. When the arm elevates at the glenohumeral joint, movement occurs at all the other joints of the shoulder complex.[ii] (GH = glenohumeral joint; AC = acromioclavicular joint; SC = sternoclavicular joint; ST = scapulothoracic joint.)
VARIATIONS IN THE SCAPULOHUMERAL RHYTHM
As far back as the 1940s it was noticed that for every 2° of arm elevation at the glenohumeral joint, the scapula rotates upward by 1°.[iii] The ratio of arm movement at the glenohumeral joint (GH) to the movement of the scapula at the scapulothoracic joint (ST) is called the scapulohumeral rhythm, which was initially determined to be 2:1.
The first 30° of arm elevation occurs purely at the glenohumeral joint; but after that, as the arm rises, the scapula rotates upward. However, later studies discovered that this ratio is not constant and there is considerable variation between people, and between healthy individuals versus those with shoulder pathologies. It is not clear whether a pathology causes a change in the rhythm, or whether a change in the rhythm leads to pathology. Variations in the average ratio of the scapulohumeral rhythm reported in studies range from 1:1 to 6:1.[iv] The higher the ratio (i.e., 6:1), the less the scapula moves as the arm elevates; the lower the ratio (i.e., 1:1), the more the scapula moves. However, these studies looked at a relatively small number of people. The full range of skeletal variation is likely much larger than this.
While upward rotation is the scapula’s most obvious movement during arm elevation, this is not the only movement, as illustrated in figure 2. During 180° of arm abduction in the frontal plane, the clavicle elevates about 25°, posteriorly rotates 25° and retracts a further 15–20° at the SC joint; the scapula also upwardly rotates 30° and posteriorly tilts 20° at the AC joint; and the arm externally rotates 35–40° at the GH joint, at least for an average person.[v] Whether you have the same amount of movement is not important; what is important is that your clavicle and scapula move and should be free to move as the arm moves. The clavicle and scapula are not one rigid frame; they have their own articulation at the AC joint, which allows the scapula to hug the contours of the upper back as it responds to movement of the arm.
The amount of movement cited illustrates an end-of-abduction position. During the process of getting the arm to that final position, the amount of movement of the scapula at the AC joint is complex. The peak amount of movement happens before the arm finishes elevating. Although highly variable by individual, the scapula may internally rotate at the AC joint slightly during elevation but will externally rotate slightly at full abduction.
Additionally, the amount of humeral external rotation ranges between 25° and 50°, but most of this occurs during the first 70–80° of abduction. How we raise the arms can be important, especially for people with shoulder pathologies or skeletal variations that are far from the norm. (See below.) Elevation of the arm in the scapular plane (scaption) or in the side plane (flexion) will change the end position values. Indeed, the scapula moves the least with flexion (scapulohumeral rhythm of 2.4:1), then scaption (2.2:1), and the most with abduction (2.1:1).[vi] So to generalize, which is always risky, the scapula will rotate upward more during elevation when the arm is to the side (abduction) than when the arm rises in front (flexion).
A Note to Teachers: your students’ scapulae will rotate the most when the arms are elevated by abduction to the side and the least when elevated by flexion to the front!
It is okay to let the shoulders rise during arm elevation
A 2016 study cited a large range of scapulohumeral rhythm, from 1:1 to 6:1.[vii] An earlier study cited a smaller range of 1.25:1 to 3.2:1.[viii] What do these ratios mean? With a 1.25:1 ratio, the arm and scapula almost equally contribute to 180° of arm abduction—100° glenohumeral arm elevation and 80° scapular upward rotation; a 3.2:1 ratio means that the arm moves a lot more than the scapula—138° versus 42°. In other words, some students need to move their scapula very little during arm elevation, while others need to move twice as much. If a yoga teacher happened to have a high scapulohumeral rhythm ratio, she could easily elevate her arm with very little scapular movement. Since that feels normal and natural to her, she would naturally be inclined to suggest her students also minimize the movement of their scapulae. That, however, would be unfortunate advice, because her scapulae are probably very different from her students’, as illustrated in figure 3.
Two anatomic features will dictate how much a scapula has to upwardly rotate in order to provide a solid base for the humerus:[ix]
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- the inclination angle of the glenoid fossa and
- the length (overhang) of the acromion process.
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The inclination of the glenoid fossa in the scapula on the left is greater than the one on the right. This means that the glenoid fossa on the left is already facing more upward than the right one, so the left scapula has to rotate less to serve as a platform of support for the head of the humerus. The scapula on the right will have to rotate more.
FIGURE 3 Two scapulae, posterior view. Each scapula has a different potential for arm elevation.
THE LATERAL LENGTH OF THE ACROMION PROCESS
The size of the acromion process overhang also affects arm elevation. When the shaft of the humerus contacts the lateral edge of the acromion process, abduction of the humerus with respect to the scapula ends. The scapula on the right in figure 3 has a very large and horizontally oriented acromion process, which overhangs the head of the humerus. Compression between the shaft of the humerus and the lateral edge of this acromion will occur sooner during abduction than for the left scapula. When impingement occurs here, regardless of glenoid inclination, the scapula will have to rotate more to elevate the arm. Comparatively, the acromion process on the left is very short and is angled upward, which will allow a lot more arm abduction before compression occurs. These bony characteristics conspire to require the right scapula to rotate much more than the left one in order to position the arm vertically.
WHAT IS YOUR INTENTION?
The cue to “drop the shoulders” for the student with the scapula on the right may change her aesthetic appearance, but functionally it will remove the necessary support of the glenoid fossa for the head of the humerus. Plus, for many students who have average-shaped bones, the only effect of dropping the shoulders will be to lower the arm, because the scapula had to rotate upward in order to elevate the arms to 180°. If the intention of a posture is to ensure that the scapula is positioned to fully support the head of the humerus, then it is necessary for an average student to allow the scapula to upwardly rotate about 60°. Insisting upon reduced scapular upward rotation can be detrimental to the stability of the arm at the glenohumeral joint.[x]
A Note to Teachers: Please let your students rotate their scapulae! There is no need to automatically insist that students draw their shoulder blades down and back. A better cue would be to ask, “What are you feeling in your shoulders?” to make sure they are not experiencing discomfort. And if they are not, allow them to find the position where they feel safe, solid and secure.
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[i] Elevation refers to raising the arms, however you wish to do it. Flexion is raising the arms in a forward direction. Abduction is raising the arms from the sides.
[ii] The cited values are from Donald A. Neumann, Kinesiology of the Musculoskeletal System, 3rd ed. (Maryland Heights, MO: 2017), 144.
[iii] See V.T. Inman, M. Saunders, and L.C. Abbott, “Observations on the Function of the Shoulder Joint,” Journal of Bone Joint Surgery 26.1 (1944): 1–30. Ernest Amory Codman originally coined the term scapulohumeral rhythm in 1934; see E.A. Codman, The Shoulder (Boston: G. Miller & Company, 1934).
[iv] Many factors can affect the scapulohumeral rhythm ratio, including how the arm is elevated, the plane of elevation, the speed, the load, and the presence of pain, shoulder tightness, and fatigue. See K.J. McQuade, J. Borstad, and A.S. de Oliveira, “Critical and Theoretical Perspective on Scapular Stabilization: What Does It Really Mean, and Are We on the Right Track?” Physical Therapy 96.8 (2016): 1162–9, doi:10.2522/ ptj.20140230. Other researchers have reported a lower but still significant range of 1.25:1 to 3.2:1; see McClure et al., “Direct 3-Dimensional Measurement.”
[v] See Donald A. Neumann, Kinesiology of the Musculoskeletal System.
[vi] See P.M. Ludewig et al., “Motion of the Shoulder Complex During Multiplanar Humeral Elevation,” Journal of Bone and Joint Surgery 91 (2009). Another study found 2.2:1 for flexion and 1.9:1 for abduction; See P.W. McClure, L.A. Michener, B.J. Sennett, and A.R. Karduna, “Direct 3-Dimensional Measurement of Scapular Kinematics during Dynamic Movements In Vivo,” Journal of Shoulder and Elbow Surgery 10.3 (2001).
[vii] See McQuade et al., “Critical and Theoretical Perspective on Scapular Stabilization.”
[viii] See P.W. McClure, et. al., “Direct 3-Dimensional Measurement of Scapular Kinematics during Dynamic Movements In Vivo.”
[ix] To be more complete, the humeral neck-shaft angle can also play a role. Let’s set the neck-shaft angle to be equal to A. Let’s set the inclination angle of the glenoid fossa to be B. We can then determine the amount of scapular rotation needed to elevate the arm so the humeral shaft is straight up (180°). Let’s call this value C. The formula we need to solve is C = A–B–90°. This equation can be derived geometrically, but I won’t go into the derivation here. The implication of the equation is that the lower the humeral neck-shaft angle, the less scapular rotation is required to fully elevation the arm! A scapula that has 15° of inclination with a humerus with a neck-shaft angle of 135° degrees will have to rotate 30°. However, a scapula with the same glenoid inclination but a humeral neck-shaft angle of 150° will need to rotate 45° to fully elevate the arm. This equation also shows that the lower the inclination value, the more scapular rotation will be required. Of course, this equation ignores the size of the acromion process, which can restrict the arm’s elevation and can render moot this whole process of determining the neck-shaft angle’s effect.
[x] See E. Itoi, N.E. Motzkin, B.F. Morrey, and K.N. An, “Scapular Inclination and Inferior Stability of the Shoulder,” Journal of Shoulder and Elbow Surgery 1 (1992): 131–9.