Arm bones & arm movements – a guide for artists
If you want to sculpt or draw a convincing arm you need to understand what’s going on under the surface. The arm has two movements: bending the elbow (also known as flexion and extension) and rotating the wrist (also known as pronation and supination).
For each movement we will look at the bones involved, how they move together and the bony landmarks you can use to find them when sculpting or drawing. Once we understand the theory we can look at how they work together in 3D – ohh exciting!
(I have a similar guide for the shoulder bones here.)
You ready?
Okay, let’s go!
Bones
First let’s colour the three bones for clarity. We have the humerus, ulna and radius.
Elbow flexion and extension
For flexion and extension we only need the humerus and ulna. Let’s start with just those two.
When we look at these two bones together we can see that, surprisingly, they do not form a straight line; there is a few degrees’ difference between them.
This is known as the the carrying angle. It’s usually around 5° to 15° and it’s more pronounced in females.
If we separate these two bones we can see how they fit together. The ulna has a notch in it (called the trochlear notch) which fits snugly with the cylindrical shape on the end of the humerus (the trochlear).
This creates a simple hinge joint. These two bones fit so snugly together the ulna can only rotate in one plane.
While we are here let’s look at the bony landmarks of the elbow. The two extrusions on the humerus are known as medial and lateral epicondyles.
Medial means closer to the midline of the body and lateral means closer to the edge of the body. Keep in mind that we are looking at the bones of the right arm here.
These epicondyles are the insertion point for a lot of the muscles in the forearm and give us two of the three bony landmarks in the elbow.
The third bony landmark in the elbow is created by the top of the ulna (the olecranon) which gives the elbow its distinctive bump.
These 3 bony landmarks form a triangle.
Which changes shape as the elbow is bent.
Let’s see how the humerus and ulna work together in 3D. These movements are known as flexion (bending the arm) and extension (straightening the arm).
We will add the other bones for context.
Great, we know how the elbow works and are now familiar with its bony landmarks. Next we will look at rotating the wrist.
Pronation and supination
Look at the back of your hand (or a friend’s if you don’t have a hand). On the pinkie side of the wrist there is a small bump; this is the distal end of your ulna. Turn your hand to face you. If you put your fingers against that bump you can follow the ulna, along the arm, all the way back to the olecranon at your elbow. This long bony landmark is known as the ‘ulnar furrow’ – it separates the flexor and extensor muscles in the forearm and it’s the easiest way to find the wrist when you are drawing or sculpting.
Okay, we can find the landmarks to place the wrist but in order to properly represent it we need to understand how it works. Let’s bring back the radius and see how everything fits together.
As you can see, the ulna starts thick and thins out towards the wrist.
The radius is the opposite: large at the wrist and thinner at the elbow.
Also note the shape of the radius at the elbow. It has a cylindrical head.
If we remove the hand (don’t worry we will put it back when we are finished) and look at the wrist from below, it becomes quite clear how the two bones move together.
The radius slides around the circular head of the ulna.
Supinated
Pronated
Looking at the other end of the radius we can see how it sits against the humerus.
The radius’s cylindrical shape allows it to rotate around its axes.
The domed shape on the end of the humerus (the capitulum) allows the radius to rotate (for pronation and supination), and to glide over its surface (when the elbow is flexed).
Don’t worry, this is much easier to see when looking at the movment in 3D.
In pronation, when the radius moves to the medial side of the arm, the ‘mass’ of the arm moves medially (closer to the body) and counters the carrying angle of the ulna.
This gives the appearance of a straight arm.
One last thing I wanted to mention is: sometimes it might look like the ulna moves as you twist your wrist but this movement is coming from the humerus. Put your elbow on the table in front of you and hold your hand up at a 45 degree angle. Now put your finger and thumb against the medial and lateral epicondyles of your humerus. If you twist your wrist you should feel your humerus moving ever so slightly as the top of your ulna shifts back and forth. Cool hu?
So, when drawing or sculpting the arm: first you place the elbow; then from the olecranon, follow the ulnar furrow to find the distal end of the ulna; then you put down the radius in relation to the ulna.
That’s it, well done for getting through the whole thing. If you found this useful or you have any suggestions for what I can improve please leave a comment below.
Happy arting!
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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