Muscles can do two things—they can contract and relax. A muscle gets shorter and thicker when it contracts. When the muscle relaxes, it gets longer and thinner.

Muscles make us move by working together in pairs. For example, the upper part of your arm has a biceps muscle and a triceps muscle (see the diagram). This pair of muscles work together when you bend your arm at the elbow. As your arm bends, two things happen—the biceps muscle contracts and the triceps muscle relaxes. As you unbend your arm, the triceps muscle contracts and the biceps muscle relaxes.

When you want to move in a certain way, your brain sends signals to the muscles you need to use. When you want to bend your arm at the elbow, your brain sends out two signals. One signal goes to the biceps muscle and tells it to contract. Another signal goes to the triceps muscle and tells it to relax. Both muscles work together to make you bend your arm.

The muscles that you can control are called voluntary muscles. All the muscles that make your body move are voluntary muscles. You tell your body that you want to ride a bike or throw a ball. Your brain sends signals to the voluntary muscles that you need to use. You control your voluntary muscles.

Your body has muscles that you do not control. These are called involuntary muscles. These muscles work on their own. For example, your heart is an involuntary muscle. It contracts and relaxes as it beats. You do not need to think about making your heartbeat—it beats on its own. That is why your heart keeps beating when you are asleep.