Muscles and Movement

The body's movement machines

Wiggle your fingers. Blink your eyes. Take a deep breath. Every single move you make happens because of muscles. Muscles are stretchy, bundled fibres that can squeeze shorter to create movement. Without them you could not walk, talk, smile or even pump blood around your body.

You have around 600 muscles, and together they make up almost half of your body weight. Some you control on purpose, like the muscles in your legs. Others work all on their own, like the muscle in your heart. Let's find out how these amazing machines work.

How muscles move bones

Your muscles team up with your bones. You can learn about the bones themselves in the human skeleton. Most muscles are joined to bones by tough, rope-like cords called tendons. When your brain sends a signal, the muscle contracts — it tightens and gets shorter. As it shortens, it pulls on the bone, and the bone moves.

Here is the key secret: a muscle can only pull. It can never push. Think about pulling a door open with a rope — you can tug it towards you, but you cannot push the door shut with the same rope. Muscles have exactly this problem.

So how does a bone get pulled back again? With a partner muscle.

Why muscles work in pairs

Because muscles can only pull, they are arranged in pairs that pull in opposite directions. The best example is in your upper arm.

• Your biceps muscle is on the front of your upper arm. When it contracts, it pulls your forearm up, bending your elbow.
• Your triceps muscle is on the back. When it contracts, it pulls your forearm down, straightening your elbow.

When the biceps pulls, the triceps relaxes, and the other way around. The two take turns. This clever push-me, pull-me teamwork lets your elbow bend and straighten smoothly. Nearly every joint in your body works this way, with one muscle to move it and another to move it back.

Three kinds of muscle

Not all muscles are the same. There are three types, each with a special job.

1. Skeletal muscle. These are the muscles attached to your bones, like the biceps, leg and tummy muscles. You control them on purpose to move, run and lift. They are the muscles you can flex and feel.

1. Smooth muscle. These line the inside of organs like your stomach and gut. They work automatically, without you thinking. For example, smooth muscles squeeze your food along during digestion, even while you sleep.

1. Cardiac muscle. This special muscle is found in only one place: your heart. It is incredibly strong and tireless. It never stops contracting and relaxing, beating about once a second your whole life to pump blood around your body. Your muscles are one part of how all the systems of the human body cooperate.

Keeping your muscles healthy

Muscles love to be used. When you exercise — running, climbing, swimming, dancing — your muscles get a little stronger each time. Here is how to take care of them:

• Move and play often. Exercise builds stronger, healthier muscles.
• Eat good food. Foods with protein, like eggs, beans, fish and nuts, give muscles the building blocks they need to grow and repair.
• Drink water. Muscles work better when your body has enough water.
• Warm up and rest. Stretching before sport and resting afterwards helps muscles avoid injury and repair themselves.

When muscles work hard, they sometimes feel sore the next day. That soreness is just your body repairing tiny strains and making the muscle a bit stronger than before.

Try it: feel your muscles in action

This activity lets you actually feel a muscle pair at work.

1. Sit down and rest your right arm on a table, palm facing up.
2. Place your left hand gently on the front of your upper arm (your biceps).
3. Now bend your elbow to lift your hand towards your shoulder. Can you feel the biceps bunch up and get harder? That is it contracting.
4. Slowly straighten your arm again. Feel the biceps soften as it relaxes — and feel the triceps on the back of your arm tighten instead.
5. Try lifting a light book and repeat. The muscle works harder, so you can feel it more.

Why it works: When you bend your elbow, the biceps shortens and pulls the bone up, so it bulges and feels firm. When you straighten, the biceps relaxes and its partner the triceps does the pulling. You are feeling the push-me, pull-me muscle pair that lets your arm move — proof that muscles really do move you by pulling on your bones.