Bones and Joints: How We Bend

Why you can bend

Try touching your toes. Now wave, nod your head, and kick a leg. You can do all of these because your skeleton is not one solid lump of bone — it is made of more than 200 separate bones that meet at special bendy places called joints.

Without joints you would be as stiff as a statue. You could not sit, run, write or even chew. In this lesson we will look at where your bones meet, the different kinds of joint, and the clever parts that let them move so smoothly. First, it helps to know your bones — you can explore them in the human skeleton.

What is a joint?

A joint is simply a place where two bones meet. Most joints are designed to move, which is why you can bend your arm, twist your neck and wiggle your toes.

But moving bones rubbing together would wear out and hurt. So joints have some clever helpers:

• Cartilage is a smooth, slippery, rubbery covering on the ends of the bones. It cushions them and lets them slide without grinding.
• Ligaments are strong, stretchy bands that hold the two bones together so they do not slip apart.
• Joint fluid is a thin, oily liquid inside many joints that works like oil in a machine, keeping everything moving smoothly.

Joints and bones do not move by themselves, though. They need muscles and movement to pull them — muscles do the pulling, and joints let the bones bend.

Different joints for different jobs

Not all joints move in the same way. Your body has several types, each shaped for a special job.

1. Hinge joints. These work just like the hinge on a door, swinging back and forth in one direction. Your knee and elbow are hinge joints. That is why your elbow can bend and straighten, but cannot twist all the way round.

1. Ball-and-socket joints. Here, the round end of one bone sits inside a cup-shaped hollow in another, like a ball in a bowl. This lets the bone move in almost every direction — round in circles, up, down and side to side. Your shoulder and hip are ball-and-socket joints.

1. Pivot joints. These let one bone turn or rotate. The joint at the top of your neck is a pivot joint, which is how you can shake your head to say "no".

1. Gliding joints. Small bones that slide past each other, like the bones in your wrist and ankle, let those parts make many small movements.

1. Fixed joints. A few joints do not move at all. The bones in your skull are joined by fixed joints that lock tightly together to make a strong helmet around your brain.

Looking after your joints

Joints work hard for your whole life, so it pays to look after them.

• Move and play. Regular activity keeps joints flexible and the muscles around them strong.
• Warm up. Gentle stretches before sport prepare your joints and help prevent injuries.
• Eat well and drink water. Calcium-rich foods keep bones strong, and water helps your joints stay slippery.
• Rest after hard play. Joints, like muscles, need time to recover.

As people get older, the cartilage in some joints can slowly wear thin, which is why some older people get achy knees or hips. Staying active throughout life helps keep joints healthy for longer.

Try it: find your own joints

This activity helps you discover how many joints you have and how differently they move.

1. Hold one arm straight out. Now bend it at the elbow. Notice it only bends one way — that is a hinge joint.
2. Circle your whole arm round and round from the shoulder. See how it moves in every direction? That is a ball-and-socket joint.
3. Wiggle each finger. Count the small joints in just one finger — there are three! Tiny hinge joints let you grip and hold things.
4. Gently shake your head "no". The pivot joint at the top of your neck lets your head turn.
5. Now press your fingertips firmly on the top of your head. The bones do not move at all — those are fixed joints protecting your brain.

Why it works: By moving each part of your body, you can feel that joints come in different shapes for different jobs. Hinge joints bend one way, ball-and-socket joints swing all around, and fixed joints stay locked. Your skeleton is full of these clever meeting points, and together they let your stiff bones become a bendy, moving you.