Science
What do muscles do?
Quick answer
Muscles are bundles of stretchy tissue that pull to move your body, pump blood and push food along. They can only pull, so they often work in pairs.
In one sentence
Muscles are the parts of your body that contract, or get shorter, to make you move, to pump blood and to move food through you.
Explained
You have more than 600 muscles. They make up about a third to a half of your body weight. A muscle is made of many long, thin fibres that can shorten, or contract, and then relax. Muscles can only pull. They cannot push. That is why they work in pairs. When one contracts, the other relaxes.
Take your arm. To bend your elbow, the biceps on the front of your upper arm contracts and pulls your forearm up. To straighten it, the triceps at the back contracts and the biceps relaxes. Muscles are attached to bones by strong, stringy tendons. Your skeleton gives the muscles something firm to pull on.
hand
foot
shoe
There are three kinds of muscle:
- Skeletal muscle moves your bones. You control it, so it is called voluntary: biceps, calf, and the muscles that move your eyes.
- Smooth muscle is in the walls of your stomach, intestines and blood vessels. It works by itself, so it is involuntary. It squeezes food along your digestive system.
- Cardiac muscle is only in the heart. It beats without a rest, all your life.
Muscles need energy from the food you eat and oxygen from the air you breathe. Working muscles produce heat, which is why you feel warm when you run. Exercise makes muscles stronger. If you use them often, they grow and get better at work; if they are never used, they weaken. Warming up before exercise and stretching afterwards help keep them healthy.
Examples
| Muscle | Where | What it does |
|---|---|---|
| Biceps | front of upper arm | bends the elbow |
| Triceps | back of upper arm | straightens the elbow |
| Calf muscle | back of lower leg | lifts the heel |
| Heart | chest | pumps blood |
| Diaphragm | below the lungs | helps you breathe |
Common mistakes
- "Muscles push." They only pull.
- "Muscles turn into fat if you stop exercising." They shrink, and fat is a different tissue.
- "Your heart is not a muscle." It is a very special one.
Quick quiz
Muscles
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What do muscles do when they contract?
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Related topics
Skeleton
The skeleton is the frame of bones inside your body. It holds you up, protects soft organs like the brain and heart, and works with muscles so you can move. Adults have 206 bones.
The human body
The human body is made of trillions of cells that form organs, such as the heart, lungs and brain. Organs work together in systems that keep you moving, breathing, eating and thinking.
The heart
The heart is a muscle about the size of your fist that pumps blood around your body, carrying oxygen and food to every cell and taking waste away.
Blood
Blood is the red liquid pumped around your body by your heart. It carries oxygen and food to every cell, takes away waste and helps fight germs and heal wounds.
The brain
The brain is the control centre of your body. It sends and receives messages through your nerves so you can think, feel, move, remember and breathe.
Digestive system
The digestive system is the long tube and organs that break food down into tiny pieces your body can absorb for energy and growth, then get rid of what is left as waste.