What Voluntary Muscles Do

1Muscles work in pairs

A muscle pulls on a bone when it contracts, but it cannot push the bone back. So muscles work in pairs: when one contracts, its partner relaxes.

• To bend your arm: the biceps contracts (gets short and fat) and pulls your forearm up. The triceps relaxes.
• To straighten your arm: the triceps contracts and pulls your forearm down. The biceps relaxes.

Your thighs work the same way: the back thigh muscles bend the knee, and the front thigh muscles straighten it.

A muscle only contracts when a message from your brain arrives along a nerve. That is how you control your voluntary muscles.

Feel it: hold your left hand on your right upper arm. Bend and straighten your right arm. Feel the biceps bulge at the front, then the triceps harden at the back.

2Jobs of voluntary muscles

1. Movement: moving parts of the body, like writing or waving, and moving the whole body from place to place, like walking, running and dancing.
2. Posture and balance: holding your body upright when you sit and stand.
3. Face and mouth: smiling, frowning, speaking and chewing.
4. Protection: the tummy muscles guard the organs in your belly.
5. Heat: working muscles make heat. When you are very cold you shiver: your muscles shake quickly to warm you up.
6. Breathing: muscles between your ribs help your chest move when you breathe.

Try it yourself

Try each one first, then tap Show answer to check.