• Glenohumeral: “Gleno” refers to the glenoid fossa. This is the “socket” part of the ball and socket joint, which is the lateral most part of your shoulder blade. “Humeral” refers to the humeral head, which is the uppermost part of your upper arm, and forms the “ball” part of the ball and socket joint. Therefore, glenohumeral is a fancy term for “ball and socket.”
• Scapulothoracic: Scapula= shoulder blade. Thoracic = upper back and ribcage. This is the articulation formed by the shoulder blade moving on the ribcage when the arm moves overhead.

Overhead shoulder movement is one of the most foundational movements in circus and performing arts. Handstands and hanging all require full overhead shoulder range of motion, and any lack of full motion causes some serious compensations to occur in the rest of the body. So, what actually has to happen to achieve full overhead motion?

Bone and joint biomechanics: scapulohumeral rhythm

First, I want to talk about something called scapulohumeral rhythm: this refers to the NORMAL ratio of glenohumeral movement to the scapulothoracic movement during arm elevation.

Normal overhead shoulder motion is 180 degrees. This breaks down to roughly:

           120 degrees  from glenohumeral range of motion
​                        60 degrees  from scapulothoracic motion

This means that BOTH the arm AND the scapula must contribute to achieve optimal overhead alignment. If one of these is lacking, then the other must take up slack…OR, we attempt to compensate from other areas of our body (example: lacking full overhead motion in a handstand? Your body may compensate by arching low back to keep weight centered over shoulders to stay balanced…and thus, the banana handstand was born!) 

Several muscle groups are important in creating the individual movements that comprise scapulohumeral rhythm:

1. Upward rotators of the Scapula:  serratus anterior, lower trapezius, and upper trapezius are all responsible for moving the shoulder blade into upward rotation as our arm goes overhead. 
2. Glenohumeral Abductors: this is done predominantly by the deltoid, with a little bit of assist from the supraspinatus (one of the four muscles of the rotator cuff)
3. Rotator cuff:  These guys are the fine-tuning stabilizers that make small adjustments and tweaks as our arm goes overhead. The rotator cuff is made of four muscles that start on the shoulder blade and insert on the humerus. The job of the rotator cuff is to hug the “ball” (humeral head) safely into the “socket” (glenoid fossa) during all positions of overhead movement. ESPECIALLY during end range overhead motion! 

There are a few common places where dysfunction happens in scapulohumeral rhythm, especially in circus artists who commonly hang from our arms with our arms overhead, or stand on our hands.

 

1. Upward rotators don’t activate
2. Rotator cuff stops doing its job

Both of these are equally problematic, and typically dysfunction in one area leads to dysfunction in the other.

 

​Lets start with the rotator cuff (pictured above).  What happens when the rotator cuff doesn’t do its job with overhead movement? The “ball” drifts superiorly (UP), and bumps up against the top of the socket- the acromion process of our shoulder blade- which pinches our rotator cuff tendons and can lead to impingement or rotator cuff tearing. This is clearly an undesirable outcome.
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Back to the upward rotators (pictured above) – what happens if these don’t activate? This causes the shoulder blade to move LESS than it should into upward rotation as our arm goes overhead. When the shoulder blade doesn’t move into upward rotation, our body has to compensate elsewhere to get the full 180 degrees we require for many circus activities. In many people, the slack gets taken up by the glenohumeral joint. This seems like it wouldn’t be a big deal, but if the SOCKET stays down, and the BALL keeps going UP, we’re back to the previous scenario of shoulder impingement- the ball runs into the top of the socket, and wears on the rotator cuff tendons.
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Now, bringing these concepts together: lets think about the cue I often hear during hanging positions: “engage your shoulder blades, bring your shoulder blades down and back.” When you do this, you basically try to totally remove the “scapula” from scapulohumeral rhythm, and forcing the glenohumeral joint to take up more range of motion than it mechanically should. This ALSO moves the acromion process DOWN, much closer to our rotator cuff tendons.  In aerial arts and hand balancing, when we are supporting our entire body weight from this unstable position, it can lead to significant rotator cuff impingement and tearing over time.

These two photos are representative of what happens in each overhead arm position in a handstand. They’re screen shots taken from an AWESOME youtube video that the American Yoga School put out. If you have 10 minutes, this is NOT a waste of your time:  https://www.youtube.com/watch?v=_r20Tvrly_Q
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The take home message? Kill the “shoulders down and back” cue if your arms are overhead. Your rotator cuff will thank you.

I know that this can be an unpopular concept, especially for aerialists- the upwardly rotated scapula puts less space between the ears and the shoulders, and this isn’t the MOST aesthetically pleasing position…but you know what’s even MORE aesthetically icky? A shoulder fresh out of rotator cuff surgery, in a sling. There’s nothing pretty about THAT. 

Questions? Concerns?  Please comment below!