Movement: skeleton and muscles · Model
Joints
Shoulders dislocate far more often than elbows. What makes the two joints different?
Start here
Two joints in the same arm.
The shoulder is the most frequently dislocated joint in the body. The elbow is one of the least. They are joined by the same kind of tissue, in the same arm, in the same person.
If you had to guess, why does the shoulder come out of place so much more than the elbow?
Good guess. Every direction a joint can move in is a direction it cannot hold against. The shoulder swings almost anywhere, so almost anything can push it out.
Fair guess. Strength is not the limit here. It is what the shape of the joint allows.
Next: bend and twist four joint shapes on a bench and see which ones refuse.
A joint is a place where two bones meet. Ligaments strap them together, and a layer of smooth cartilage keeps the ends from grinding. What the joint can do after that is decided by one thing: the shape of the two ends.
The model · drive the joint
Four shapes, four sets of rules
0 of 4 joints tried
Bend each one as far as it goes, then try to twist it. Two of them will refuse, and the refusal is the point.
Commit first. Your knee and your shoulder are both joints. How do they compare?
20°
It will not. The end of one bone sits in a groove in the other, and a groove only lets you go one way.
Directions it moves in
1
Where you have one
Elbow, knee, fingers
What holds it together
Ligaments down both sides
The trade: One direction, and a very strong refusal of every other one. You can hang your whole body weight from a bent elbow and it will not fold sideways.
Key fact
The shape of the bone ends decides what a joint can do. Every direction it can move in is a direction it cannot resist.
Your turn · four places in the body
Which of the four is this?
0 of 4 decided
A model is worth having when it makes the right prediction, and worth understanding when it fails. One of these four is not any of the types.
Your knee: it bends behind you and straightens, and it does not turn.
Hinge. One direction only, held by strong ligaments at each side. It is also why a twisting tackle damages a knee so badly — the twist is the one thing it cannot give way to.
Your hip: it swings your leg forwards, backwards, out to the side, and lets you turn your foot outwards.
Ball and socket. Same shape as the shoulder, but with a much deeper socket. Deeper socket, less range, far fewer dislocations — the same trade made differently.
The seams across the top of an adult skull, where the plates meet along a jagged line.
Fixed. They move in a baby, while the skull is still growing, and then lock. A protective box is worth more than a movable one.
The joint at the base of your thumb, which lets you swing the thumb across your palm and press it against every finger.
None of these. It is a saddle joint: both bone ends are curved one way and hollow the other, like two saddles crossed. It moves in two directions but not three, and no other joint in the body is quite like it.
Three fitted and one did not. That is not a fault in the model — a model that fits everything has stopped telling you anything. Four types cover most of the skeleton and then hand you a short list of joints worth looking at properly.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“Muscles hold the bones together at a joint.”
The Achilles tendon is the thick cord you can feel at the back of your ankle. Commit before you read on.
A tendon joins muscle to bone. It is how the pull gets from the muscle to the thing being moved, and the Achilles is the biggest one you have. A ligament joins bone to bone, straps the joint together and stops it moving in directions it should not.
The two words are easy to swap and the consequences are not. Tear a tendon and the muscle can no longer move that bone at all. Tear a ligament and the joint still moves — it moves too much, in a direction it was never meant to, and that is why a bad ankle sprain takes longer to trust than a broken bone.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Which of these joints allows movement in more than one direction?
Rung 2 · The one that catches people
Why can your shoulder rotate when your elbow cannot?
Rung 3 · Explain
Explain why the shoulder is worth having even though it dislocates, and why the knee is built the opposite way. Use both joints in your answer.
Rung 4 · Take it somewhere new
You are designing a robot arm that has to lift a kettle off a worktop and pour it into a cup. Choose a joint type for the shoulder, the elbow and the wrist, justify each one, and then say one thing your robot will not be able to do that a person can.
Key note
A joint is where two bones meet, strapped together by ligaments and faced with cartilage. Hinge joints move one way; ball-and-socket joints move many; pivot joints turn; fixed joints do not move at all. Range and stability are bought from each other.
Going further
The smooth cartilage on the end of a bone has no blood supply of its own. It is fed by fluid squeezed through it every time the joint is loaded and unloaded — which means the only way to feed it is to use it, and the only way to starve it is to stop. It also means that when it is damaged it heals slowly and often not at all, because there is no bloodstream delivering material to repair it with. That single fact is behind most of what a physiotherapist tells people to do.
Before this lesson
Next in this unit
At GCSE this becomes
- Joint injury and repair, and the mechanics of levers in the body.
Where to next
Ask MrBadmus
Mixing up tendons and ligaments?
Lesson content © MrBadmus.