Hey welcome to another lesson on Proko. This lesson is about the types of joints in the

human body.

As artists we really only need to learn the synovial joints, like the shoulder and knee,

because they move. Joints that are fibrous and cartilaginous hardly move, and some, like

the connection of the two pubic bones, don't move at all. When you're learning how to draw,

pay attention to synovial joints, and keep your thoughts away from pubic bones.

If you know where the hinge joint is on this foot, you know that it won't do this or this

when you stand on your toes. It will do this. We need to know the position of that hinge

joint to make squash and stretch look like the real thing.

Synovial Joints

There are 6 types of synovial joints. They have varying shapes, but the important thing

about them is the movement they allow. Joints determine what positions our bodies can take.

We learn them to invent poses. And we learn the limits to stretch the limits.

The 6 types of synovial joints are: Hinge, Pivot, Ball & Socket, Ellipsoid, Saddle, and

Plane. Let's go through them one by one.

Hinge

The hinge is a very simple joint. It allows movement only on one axis. It's structure

prevents rotation this way, or this way. The head of one bone wraps around the cylindrical

head of the other, allowing a very stable rotation this way.

Going back to the terminology from last week, the hinge joint allows flexion and extension.

That's it! Thats all it does, but it does it well. Like the hinges on a door, allow

it only to open or close. The best example of it is the elbow. Here's the rotation on

a simplified skeleton. Flexion and extension.

So if the elbow only allows flexion and extension, how is it that we are able to twist the forearm?

Well, let's take a look at the next joint..

Pivot

The pivot joint also allows rotation at only one axis. However, it rotates along the long

axis. A cylindrical bone fits into a ring of bone and ligament, like with the radio-ulnar

joint just below the elbow. The cap on the radius bone fits nicely into this notch on

the ulna bone. Ligaments complete the ring, holding the bone in place and allow the radius

only to rotate inside of it.

The result on the forearm is what we call pronation and supination. During pronation,

the base of the radius rotates over and around the head of the ulna. The ulna stays relatively

still. Remember, the hinge joint at the elbow, prevents the ulna from twisting. So all of

that twisting happens at the radius.

And by the way, the distal joint of the ulna and radius is also a pivot joint. The combination

of the pivot at the top and at the bottom creates that twisting motion for pronation

and supination.

Ball & Socket

The ball & socket is the champion of all joints. Hooray for the ball and socket! It's structure

is just like how it sounds. A ball inside of a socket. This simple and effective structure

allows it to move in all axes - flexion, extension, abduction, adduction, rotation, and circumduction.

The two ball and socket joints of the body are at the hip and the shoulder. The hip has

a deep socket, which gives it stability, but limits some range of motion. The shoulder

joint has a shallower socket, which gives it greater range of motion, but takes away

some stability. Maybe that's why a dislocated shoulder is so common.

Ellipsoid

The ellipsoid joint is very similar to a ball & socket. However, the ligaments and its oval

shape prevent rotation. But it still has the ability to rotate on two axes, which allows

flexion/extension, abduction/adduction, circumduction. Circumduction is just a combination of all

the others in a circular motion.

The ball, or oval head also slides inside the socket. When it rotates along the wider

plane, you can see how it pops out too much from the socket. So, it slides in back to

center.

A great example of an ellipsoid joint is the wrist, aka radiocarpal joint. The group of

carpal bones rotate inside the socket of the radius.

Saddle

The saddle joint is similar to the ellipsoid, but the rotation is limited mostly because

of the bone structure. The structure of the saddle is very interesting. Both bones have

a concave and convex surface. Convex means the surface sticks out, like a hill. Concave

means the surface curves in, like a hole or a cave. The concave plane of one fits on the

convex plane of the other. It's like a 3D yin yang! Or a cowboy on a horse! The saddle

makes the bottom piece and the cowboy's legs make the top piece.

The legs of the top piece, which wrap around the body of the bottom piece allow a rotation

this way. The body of the top piece can glide inside of the legs of the bottom piece.

So, this unique structure allows the joint to flex, extend, abduct, adduct, circumduct,

and very slightly rotate.

An example of a saddle joint on the body is the carpometacarpal joint of the thumb.

Let's see that baby in action!

Plane

Finally, the plane joint. Not really as interesting as the others, but deserves our love anyway.

It's basically two flat-ish surfaces, one on top of the other. These surfaces can glide

or rotate.

They usually come in groups, like the carpals of the hand and the tarsals of the foot. Ligaments

hold these bones together, but might allow some rotation and gliding.

Another plane joint is the acromioclavicular joint. That's the one between the clavicle

and acromion process of the scapula. When we elevate the shoulder, the angle in here

will adjust to keep the scapula vertical.

The Spine

I mentioned in the beginning that cartilaginous joints are not important for artists, except

in one, big, important case: your spine. We'll talk about that, in the next lesson.

Assignments

For this lesson, there's two assignments.

The first assignment is to find these joints on yourself and study the movement.

I've posted 6 close-up images of Skelly's joints in the description under this video.

Your second assignment is to draw them as simple versions. When you try to put them

into perspective, you may find it difficult, but I will have Marshall help you understand

how to do it. Post your drawings in the facebook group, at facebook.com/groups/anatomy4artists.

Marshall, the perspective master will help to critique your assignments in the facebook

group. Later, Marshall will do a demo to show you how to simplify your forms and move them

around.

Last week we studied the language of anatomy. Just looking at a long list of terms can be

daunting and difficult to remember. That's why I put together a PDF ebook that visually

defines all these terms. This ebook is available as a premium feature at proko.com/anatomy.

This week I've included another PDF ebook showing which of the 6 types every joint in

the body is classified as. And again it's shown visually, rather than just a list, so

you can print these out as reference.

Another premium feature you'll see this week when you login to your account, is a 3D model

of Robo Skelly. This is a skeleton with simplified forms. Simplified forms are a lot easier to

understand, to remember and are more practical to apply to your drawings. So, you can zoom

in and rotate around Robo Skelly directly in your browser. This will help you with the

assignment this week and upcoming lessons when we study the skeleton in detail.

To get all the premium features, go to proko.com/anatomy