with contributions by Keith Wittenstein. Special thanks to Sarah Court, Dinneen Viggiano and Trina Altman for editing and feedback

This article is Part 2 in a series on shoulder biomechanics.

shoulder bonesIn the first part of this article, we discussed cues and teachings that are passed down via teachers and the potential problem that can create with cueing – specifically, the cue “move your shoulder blades away from your ears”. Let’s begin with one of the most important things to know about the shoulder joint complex. It’s not just one joint. It’s four. When you peel back the skin that curtains the shoulder stage, all four of these joints need to work together to allow for the arm’s broad range of 180 degrees of flexion when the arms are alongside the ears. What happens “on stage” appears exclusively like movement of one bone, the arm bone. Meanwhile, what’s happening backstage is a coordinated co-movement of several bones at several joints. Three bones fit together to make up these four joints. They are the arm bone (humerus), the shoulder blade (scapula), and the lesser known and frequently forgotten about collarbone (clavicle). We will refer to these bones by both their Latin and English names throughout the rest of this blog.

Let’s learn how these bones articulate at these joints.

The first and most famous joint we’ll talk about is the glenohumeral joint. This is where the head of the humerus and the scapula (via it’s articulation with the glenoid fossa, or the shoulder socket) move relative to one another. In layman’s terms: the shoulder joint is where the arm bone attaches to the shoulder socket which is located on the shoulder blade! This joint tends to command most of our attention since it allows for movement that is very accessible to us perceptively. When we look at ourselves in the mirror, the humerus appears to hang right off the side of our trunk and when we go to lift it up overhead, its movement is very noticeable and seems to take place separately from the torso.

However all 3 bones – the humerus, the scapula and the clavicle – participate in positioning the humerus overhead. The perceptual difference being, the humerus is on stage (a comedic actor, perhaps), while the other two work backstage behind a curtain of upper torso tissues that mask their cooperation. Just as when an actor performs on stage, we do not see her lighting and costume designer at work, but rather the evidence of their work in her appearance, so too, the movement of the humerus is the result of a coordinated effort whose support crew goes unseen. Past the first 30 degrees of the humerus’ upward movement from the trunk, the scapula and clavicle move too. Their co-movement facilitates continued upward movement of the humerus. Simply put, without this backstage support, the humerus show would not go on.

Another of these crucial, behind the scenes shoulder joint players is the acromioclavicular joint, or the AC joint for short. This is where the acromion process of the scapula articulates with the lateral end of the clavicle. The acromion process is a witchy-like finger projecting out of the scapula and pointing from the back of the shoulder to the front. It is the highest point of the shoulder. Acromion comes from the Greek roots akros, “highest” and omos,” shoulders.” This joint does not allow for a great deal of movement between the scapula and the clavicle. In fact, it is most often injured when forced to allow for movement that it isn’t structurally designed to permit. Therefore, due to its limited mobility as a joint, when the clavicle moves, for the most part, we are best to let the scapula move with it. This is a key point that will come up often in our discussion of a more skillful approach to positioning the scapulae when the humeri are overhead.

Another of these crucial, behind the scenes shoulder joint players, is the sternoclavicular joint, or the SC joint. This is where the top of the breastbone (manubrium), provides a fulcrum point for the clavicle to rise when the arms reach overhead. This movement of the clavicle is called elevation. We will explain why later in this blog post, but for now, know that when the arms are overhead, the clavicle must elevate a full 30 degrees. Remember that the clavicle attaches to the scapula at the AC joint – that relatively immobile joint that we said we’d be mentioning a lot. This means that when the clavicle elevates, the acromion process part of the scapula must move upwards (toward the head) along with it.

Upward rotation of the scapula.
Upward rotation of the scapula.

In the image to the left, the scapula has spun a few degrees, like a turntable, to allow the acromion process (the witchy finger projection on the scapula that forms the AC joint with the clavicle) to move upward. This movement of the scapula is called upward rotation. During upward rotation, the bottom angle (inferior angle) of the scapula rotates upward. Remember that the AC joint does not structurally allow for one of its bones (the clavicle) to move up while the other (the scapula, specifically the acromion process of the scapula) moves down.

Unfortunately, this is what you ask those two bones to do when you work scapular depression (pull your shoulder blades down) while the clavicle elevates to assist in taking the arms overhead. Doing this creates a stressful dissonance in the soft tissues surrounding the AC joint. These tissues find themselves caught in the middle of two biomechanically conflicting goals: the upward effort of the end of the clavicle and the downward effort the practitioner makes on the scapula.

Keep tuning in this month for the rest of this series, Uplifting News for Depressed Shoulders!


 Enjoyed this article? Read Building Balanced Shoulders.

Comments (59)

Leave a Reply

Your email address will not be published. Required fields are marked *