Skeleton & muscles, but bitesize.

Part 1Why we have a skeleton

Imagine taking all the bones out of your body. You'd collapse into a soft heap on the floor — like a tent with no poles. Your skeleton is the frame of around 206 bones that holds you up and lets you move.

But holding you up is only one of its jobs. The skeleton does four important things, and you need to know all of them.

Part 2The four jobs of the skeleton

Here are the four functions. A neat way to remember them: support, protection, movement, and making blood.

1. Support. The skeleton holds the body up and gives it its shape. Without a rigid frame you couldn't stand.

2. Protection. Bones form a hard case around your softest, most important organs. The skull protects the brain. The ribs protect the heart and lungs. The backbone (spine) protects the spinal cord.

3. Movement. Bones give your muscles something firm to pull on. Muscles are attached to bones, so when a muscle pulls, the bone moves.

4. Making blood cells. Inside some of your bigger bones is a soft tissue called bone marrow. This is where new blood cells are made — proof that bones are very much alive.

Part 1What is a joint?

If your skeleton were a single solid piece, you couldn't bend at all. Luckily it isn't. A joint is a place where two bones meet. Joints are what let you bend, twist and turn.

Not all joints move the same way. Two types you need to know are the hinge joint and the ball-and-socket joint.

A hinge joint works like the hinge on a door — it bends one way only. Your elbow and your knee are hinge joints. They let you bend and straighten, but you can't rotate your forearm at the elbow.

A ball-and-socket joint has a rounded "ball" on one bone that sits in a cup-shaped "socket" on another. This lets the bone rotate in almost any direction. Your shoulder and your hip are ball-and-socket joints — that's why you can swing your arm round in a full circle.

Part 2The parts inside a joint

If bones simply rubbed together, they would wear away and it would be painful. So joints have some clever parts that hold the bones together and keep them moving smoothly.

Ligaments are tough, slightly stretchy bands that join bone to bone. They hold the joint together so the bones don't pull apart.

Cartilage is a smooth, rubbery layer that covers the ends of the bones. It cushions them and stops them grinding against each other.

Synovial fluid is a slippery liquid inside the joint. It works like oil, lubricating the joint so the bones slide easily.

Part 1How a muscle moves a bone

A muscle is a bundle of tissue that can change length. When a muscle gets a signal from the nerves, it contracts — meaning it gets shorter and fatter. As it shortens, it pulls on the bone it's attached to, and the bone moves.

Muscles aren't joined straight onto bone. They're connected by a tendon — a tough cord that links the muscle to the bone. When the muscle contracts, it pulls the tendon, and the tendon pulls the bone.

Part 2Muscles can only pull

Here's the most important idea in this whole unit, and the one most pupils get wrong:

A muscle can only PULL. It can never push.

When a muscle contracts, it pulls a bone towards it. But a muscle cannot push a bone back. To relax, all a muscle can do is stop pulling and go limp — and going limp doesn't push anything.

So how do you move a bone back again? You need a second muscle on the other side to pull it the other way. That's the big idea we'll meet in the next topic: muscles have to work in pairs.

Part 1Why muscles work in pairs

From the last topic, you know a muscle can only pull. One muscle on its own could pull a bone one way — but nothing would pull it back. That's the problem a pair solves.

Muscles are arranged in antagonistic pairs: two muscles on opposite sides of a joint that pull in opposite directions. The word "antagonistic" means "working against each other" — when one pulls, the other relaxes, and vice versa.

Part 2The biceps and triceps

The classic example is in your upper arm. The biceps is on the front; the triceps is on the back. They form an antagonistic pair across the elbow joint.

To bend the arm (raise the forearm): the biceps contracts and pulls the forearm up, while the triceps relaxes.

To straighten the arm (lower the forearm): the triceps contracts and pulls the forearm back down, while the biceps relaxes.

Notice the pattern: in each movement, one muscle contracts while its partner relaxes. They never both pull hard at the same time.

Part 1The muscle-strength practical

How do you measure how strong a muscle is? A common school experiment uses a hand-grip dynamometer — a device you squeeze as hard as you can, which reads off the force in newtons (N). The bigger the squeeze, the bigger the number.

Suppose a class investigates whether older pupils have a stronger grip. They measure the grip strength of pupils of different ages and write the results in a table.

A table is tidy, but it's hard to spot the pattern just by reading numbers. That's why we draw a graph — a picture of the data makes the trend jump out.

Part 2Plotting a graph & the line of best fit

To turn the table into a graph, plot each pair of numbers as a point: age along the bottom (the x-axis), grip strength up the side (the y-axis). Each pupil becomes one cross on the grid.

The points don't sit in a perfectly straight line — real measurements never do. So instead of joining the dots, you draw a single straight line of best fit: a line that passes as close to as many points as possible, with roughly the same number of points above and below it.

The line of best fit slopes upwards. That tells us the pattern: as age increases, grip strength tends to increase too. Older pupils generally have a stronger grip.

Part 3Reading values off the graph

Once you've drawn the line of best fit, you can use it to predict values you didn't measure. To find the likely grip strength of a 12-year-old, go up from 12 on the age axis until you hit the line, then go across to read the grip strength.

This is reading between points we measured — the answer is an estimate, but the line of best fit gives us a sensible one because it follows the overall trend.