Shoulder joints & movements – a guide for artists
In this post I want to look at how the bones of the shoulder move together, and hopefully demystify the mechanics of it all. If we can learn what’s going on in the shoulder and why, we should be able to draw or sculpt it in any position, and from any angle (with proper reference of course!).
We will start with some simpler movements to help solidify some concepts, look at flexion of the shoulder, and finish with a breakdown and 3D animation of the scapulohumeral rhythm.
If you haven’t already, you can check out part I which describes each of the bones of the shoulder.
Joints
To start let’s look at the joints between the bones.
We have:
- One between the sternum and the clavicle known as the sternoclavicular joint.
- One between the acromion process and the clavicle called the acromioclavicular joint.
- One between the glenoid cavity and the humerus called the glenohumeral joint.
Hopefully you can see the pattern there…if not it might be time for bed.
Movements
Moving on! (See what I did there?)
As we have seen, the sternoclavicular joint (1) is the only joint between the arm and the rest of the body. Because of this it’s where all the movement of the arms start.
Elevation and depression
In the scene above we can see the shoulder moving up and down (we call this ‘elevation and depression’). The clavicle is hinging at the sternoclavicular joint (1). At the other end of the clavicle the scapula hinges at the acromioclavicular joint (2) to stay more or less upright despite the rotation of the clavicle. Look from the back and compare the clavicle to the spine of the scapula to see how the two bones are moving relative to each other.
Protraction and retraction
In this scene we can see the shoulder moving forward and back (‘protraction and retraction’). Again most of the movement is coming from the clavicle. This time, the acromioclavicular joint is allowing the scapula to slide around the curve of the ribs as it moves forward and back (this is easiest to see from above).
I’d suggest you take a few minuets to understand the movements and relationships before we move on.
Flexion
Okay, now let’s try raising the arm up for a high five.
First we will colour the scapula, humerus and clavicle to keep track of them.
If we move the humerus without moving the other bones, we can only get it up so far before the flat end of the clavicle and the acromion get in our way.
If we are going to go any further we need to move and rotate the acromion and the clavicle up and out of the way.
You might remember that the clavicle is cylindrical where it attaches to the sternum (sternoclavicular joint); this allows it to rotate. When we rotate the clavicle back this also rotates our scapula, tilting it back and sliding it around the ribs. This gives us room to move the humerus up and deliver that high five: great!
I know it’s hard to see this in 2D, so now that you understand the principles, let’s take a look at it in 3D (ohh, exciting!).
When we look at this in 3D you can see how the clavicle both lifts and rotates back to accommodate the humerus.
We can see that that movement is passed on to the scapula. This is most obvious looking at the acromion process which starts with its flat top pointing straight up and ends with it pointing back.
Like in protraction and retraction, we can see how the acromioclavicular joint allows the scapula to flair out as it slides around the ribs.
Drawing a line from the glenohumeral joint (hi Glen!) to the lowest point on the scapula, you can see it begins the movement at an ~45° angle and ends pointing almost straight down.
Lateral flexion
Okay, so we know what to do for high fives. But! What about if we are doing the YMCA dance and we want to be the Y?
Again we will colour our scapula, humerus and clavicle so we can keep track of them.
If we bring the humerus up without moving the other bones we run into the same problem as before: the acromion and clavicle are getting in our way.
The solution is actually the same as before: we need to rotate and raise the clavicle, which in turn will slide the scapula around the ribs and raise the acromion out of our way. Unlike in flexion, this movement happens in stages; you may have come across this before, it’s known as the scapulohumeral rhythm.
There are three distinct phases to this movement:
- 1st phase the arm moves 0°- 30°
- 2nd phase the arm moves 30°- 90°
- 3rd phase is 90°- 180°
As an artist I find it easier to relate the angles on the different bones in these phases to a clock face, the way you might in a life drawing class.
Let’s add a clock face and give it 3 hands, again colour coding them for the scapula, humerus and clavicle.
0°- 30°: In the first phase, the arm moves 30°.
On our clock face this brings the humerus to between 24 and 25 minuets past.
The clavicle lifts an almost imperceptible 5°. This movment shifts both our clavicle and scapula.
30°- 90°: In the second phase, the arm moves another 60°.
In the literature there is a 2-1 relationship between the humerus and scapula, but I havent found that to be the case exactly.
As the humerus moves from 24 minutes past to 15 min past the clavicle lifts and tilts backwards to make room for the humerus.
The clavicle moves to about 12 min past, this movment is passed on to the scapula which shifts about 2 minutes to ~27 min past on our clock face.
It’s not obvious here but the tilting of the clavicle is passed on to the scapula, the bottom of the scapula starts to slide forward along the ribs. This movment is easier to see in the animation toward the end of this post.
90°- 180°: In the third phase, the arm moves another 90°.
The humerus lifts from 15 minutes past to 1 minute past.
The clavicle moves from 12 minutes past to 10 minutes past.
The clavicle continues to twist backward (around 25° from it’s starting position) this is evident in the picture as we can see more of it’s flat top.
The clavicle is also retracting now (moving towards the back of the body) It’s not obvious in the picture but can be seen in the animation below.
As before these movements are passed on to the scapula which moves from 27 minutes psat to 23 minutes past.
The scapula continues to slide around the ribs flaring out slightly.
You can see how much the scapula has moved by comparing the spine of the scapula and the acromion to where they sit in the previous images.
It’s important to note that in this phase the humerus also twists medially (towards the midline of the body) along its length around 90°. You can see this if you try the movement yourself and watch the inside of your elbow. It will begin facing forward and end pointing over your head. I would advise doing this when you are in a room by yourself so people don’t think you are strange.
Okay great, let’s see how that all looks together in 3D.
To build up a good idea of what’s going on here, start by watching the acromion process. It follows the end of the clavicle closely, and it will show you how much the clavicle and scapula are rotating through each part of the movement.
When watching the lowest point of the scapula notice how it starts at the back of the ribs and slides around the ribs to end almost directly below the head of the humerus. It’s amazing how far around the ribs it comes – look at it from the back, the side and the front.
Don’t forget that the clavicle moves backwards as well as rotating and lifting to make room for the humerus; this is easiest to see from the side.
1st phase 00:00 – 01:00
2nd phase 01:01 – 02:80
3rd phase 02:81 – 05:20
That’s it! Well done for making it to the end. Hopefully this has helped you get a better understanding of what’s going on in the shoulder. In order to make it stick I recommend picking a few views of the above animations and sketching them out to ensure that it all makes sense in your head.
If you have any feedback please leave a comment as I am hoping to continue this series and I would like to make it as useful as I can.
References
Goldfinger, Elliot – Human Anatomy for Artists: The Elements of Form. Oxford University Press Inc (2011)
Hogarth, Burne – Dynamic Anatomy: Revised and Expanded Edition. Watson-Guptill Publications (2003)
Neumann, Donald A. – Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation. Mosby Elsevier (2010) p.132-135
Netter, Frank H. – Atlas of Human Anatomy, Professional Edition. Elsevier (2018)
Zarins, Uldis – Anatomy For Sculptors: Understanding the Human Figure. Anatomy Next, Inc. (2014)
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