How Your Muscles Work

Your body's movers

Every time you run, kick a ball, or even smile, your muscles are doing the work. Muscles are the tissues that move your body. You have more than 600 skeletal muscles attached to your bones, and they let you do everything from sprinting to picking up a pencil.

In this lesson you'll learn how muscles actually create movement, why they come in pairs, and how exercise makes them stronger.

How a muscle moves a bone

A muscle creates movement by contracting. To contract means to get shorter and tighter. When a muscle contracts, it pulls on the bone it is attached to, and the bone moves.

Muscles are attached to bones by tough, cord-like tissues called tendons. (Don't mix these up with ligaments, which connect bone to bone.) When the muscle contracts, the tendon tugs the bone like a rope pulling a lever.

Here is the key idea: a muscle can only pull, not push. It can shorten and tug, but it cannot lengthen itself to shove a bone away. So how does a bone move back to where it started?

Muscles work in pairs

Because a muscle can only pull, muscles work in opposing pairs. One muscle pulls a bone one way; its partner pulls it back the other way.

The classic example is your upper arm:

• The biceps is on the front. When it contracts, it bends your elbow and raises your forearm.
• The triceps is on the back. When it contracts, it straightens your elbow.

When you bend your elbow, the biceps contracts and the triceps relaxes and lengthens. When you straighten your arm, they swap jobs. They take turns, which is why your arm moves smoothly.

Movement	Muscle that contracts	Muscle that relaxes
Bending the elbow	Biceps	Triceps
Straightening the elbow	Triceps	Biceps

You'll find these pull-and-relax pairs all over your body, like in your legs (hamstrings and quadriceps).

The brain gives the orders

Muscles don't move on their own. Your brain sends a signal down your spinal cord and through nerves to the muscle. The signal tells the muscle fibers to contract. This happens in a fraction of a second, which is why you can react so fast when a ball comes toward you.

Inside each muscle are thousands of tiny fibers. Tiny protein threads inside them slide past each other to make the whole muscle shorten. The more fibers your brain switches on, the stronger the pull.

Three kinds of muscle

Not all muscles are the same:

1. Skeletal muscle moves your bones. You control these on purpose (voluntary).
2. Cardiac muscle is your heart. It pumps blood on its own, without you thinking about it.
3. Smooth muscle is inside organs like your stomach, moving food along automatically.

How muscles get stronger

When you exercise, you put a small, healthy amount of stress on your muscle fibers, causing tiny micro-changes. This is normal. Afterward, while you rest, your body repairs those fibers and builds them back a little stronger and more able. Over weeks, this is how training makes you fitter.

For this repair to happen well, muscles need:

• Rest and sleep, so the body can rebuild.
• Good food, especially protein to repair fibers and carbohydrates for energy.
• Water, since muscles are mostly water and need it to work.

This is why athletes don't train hard every single day, and why rest is part of getting stronger, not the opposite of it.

Keeping muscles safe

Warming up makes muscles warmer and stretchier so they are less likely to get pulled. Learn how in Why Warming Up Matters.

To fuel your muscles the right way, see Nutrition for Young Athletes.

Quick recap

• Muscles contract to pull bones; they cannot push.
• They work in pairs, like the biceps and triceps.
• The brain signals them through nerves.
• Exercise plus rest and good food makes them stronger.

Your muscles are an amazing team. Treat them well and they'll keep you moving!