Name the joint.
Your body bends in five different kinds of way. Max bends in only one. Find all five on yourself, then walk across the kitchen like him.
- How long
- About fifteen minutes.
- You need
- Your own body · A pencil and paper · Some room to walk
- Messy
- No.
Step 1. Meet the five kinds.
Look at the drawing and the five cards under it. Each kind of joint moves like something already in your house. Read how many ways each one moves. That number matters at the end.
Step 2. Test the eight places on yourself.
Write the numbers 1 to 8 down the side of your paper. Do each test in the list below, slowly, and write which of the five kinds you think it is. Nobody is marking this. Your body is the answer key.
Step 3. Open the answers.
Under the list there is a fold that says Open the answers. Check yours. If one surprised you, do that test again and feel for it.
Step 4. Walk across the kitchen like Max.
Max has eight joints, and every one is the same kind: a hinge that turns on one screw. So his shoulders and hips swing forward and back and nothing else, and his ankles do not move at all. Walk across the room moving only like that. No circles, no twisting, stiff ankles.
Step 5. Count the ways.
Add up the ways for your shoulder, elbow, hip and knee on one side. Then do Max's same four. The card at the bottom has both sums.
The five kinds.
Hinge
1way
Like a door hinge
Bends one way and back, like a door. It cannot go sideways.
Pivot
1way
Like a doorknob
Turns round on itself, like a doorknob. It cannot bend.
Ball and socket
3ways
Like a ball in a cup
Goes any direction, round in a full circle, and twists too.
Saddle
2ways
Like a horse saddle
Rocks forward and back and side to side, like sitting in a saddle. It does not twist.
Condyloid
2ways
Like an egg in a spoon
An oval in an oval dip. Forward and back, side to side, never a twist.
There is a sixth kind, called plane: the little bones inside your wrist sliding past each other. It is real, but you cannot move it on purpose, so it is not in the test.
The eight places.
- 1
Your elbow
Bend it. Now try to bend it sideways.
- 2
Your knee
Sit down and swing your foot. Try to swing it sideways.
- 3
Your shoulder
Draw a big circle in the air with your straight arm.
- 4
Your hip
Stand on one leg and draw a circle with your other foot.
- 5
The top of your neck
Shake your head no.
- 6
Your forearm, near the elbow
Hold your elbow still and turn your palm up, then down.
- 7
Your wrist
Wave it up and down, then side to side. Now try to twist only the hand.
- 8
The bottom of your thumb
Touch your thumb to each fingertip in turn.
Open the answers
- 1Your elbow: hinge.
- 2Your knee: hinge.
- 3Your shoulder: ball and socket.
- 4Your hip: ball and socket.
- 5The top of your neck: pivot.
- 6Your forearm, near the elbow: pivot.
- 7Your wrist: condyloid.
- 8The bottom of your thumb: saddle.
From OpenStax Anatomy and Physiology, the chapter on synovial joints. The knee is a hinge that can turn a tiny bit when it is bent, which is why some books call it a modified hinge.
Count the ways
You: 8. Max: 4.
Shoulder 3, elbow 1, hip 3, knee 1. That is 8 ways for one arm and one leg. Max has a hinge in each of the same four places, one way each, so 4.
Each of his joints turns on one brass screw or bolt, with springy washers squeezed in between so a limb stays wherever you put it. One screw can only turn one way. His ankles are glued, so they are not joints at all.
That is the end of this one
The word for what you just did is degrees of freedom.
A degree of freedom is one separate way a thing can move. A hinge has one. A ball and socket has three: forward and back, side to side, and a twist. Engineers count them on every robot arm, every steering wheel and every door, because each one is a way the thing can move and a way it can go wrong.
Where to go now