Chapter 3 of 8

All 8 chapters
  1. 1The skeleton, readunderstood
  2. 2Joints, readunderstood
  3. 3Muscles move bones, readunderstood
  4. 4Teeth, readunderstood
  5. 5Food groups, readunderstood
  6. 6A balanced diet, readunderstood
  7. 7Exercise and rest, readunderstood
  8. 8Keeping food safe, readunderstood

S2.3.3 Chapter 3

Muscles move bones

How does my arm bend and straighten?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Feel a muscle get hard as it pulls on a bone
  • Explain why muscles work in pairs to bend and straighten an arm

Hold the front of one upper arm with your other hand. Now bend that arm, so your fist comes up to your shoulder. What do you feel?

A lump gets hard. That lump is a muscle. A muscle gets short and hard. It pulls on a bone.

A tendon is a strong cord. It joins a muscle to a bone. The muscle pulls, and the tendon pulls the bone.

A muscle that is not working. Two bones stand at each end. A long thin muscle lies between them. A short tendon joins the muscle to each bone. Labels: bone, tendon, muscle, soft.bonemuscletendonsoft
The same muscle when it pulls. It is shorter and fatter and hatched. The bone on the right has moved closer, with an arrow marked pulls. Labels: bone, tendon, muscle, hard, pulls.bonemuscletendonhardpulls
A muscle pulls by getting short and fat. It pulls the bone with it.

The muscle at the front of your upper arm is the biceps. The muscle at the back is the triceps.

Draw it first

Draw an arm and its two muscles

  1. An arm seen from the side, drawn in two steps. First the arm is bent. The muscle at the front is short and fat, labelled biceps. The muscle at the back is long and thin, labelled triceps. Then the arm is straight. Now the muscle at the front is long and thin. The muscle at the back is shorter and fatter than before. It pulled the arm straight. Labels: biceps, triceps.tricepsbiceps

    1 Draw a bent arm. Write biceps at the front and triceps at the back. The biceps is short and fat.

  2. An arm seen from the side, drawn in two steps. First the arm is bent. The muscle at the front is short and fat, labelled biceps. The muscle at the back is long and thin, labelled triceps. Then the arm is straight. Now the muscle at the front is long and thin. The muscle at the back is shorter and fatter than before. It pulled the arm straight. Labels: biceps, triceps.tricepsbiceps

    2 Draw the arm straight. Now the biceps is long and thin. The triceps pulled the arm straight.

To bend your arm, the biceps gets short and hard. It pulls your lower arm up. The triceps goes soft.

A bent arm from the side. The biceps at the front has the word pulls over its name. The triceps at the back has the word soft under its name. Labels: biceps, triceps, pulls, soft.tricepsbicepspullssoft

To straighten your arm, the triceps gets short and hard. It pulls your lower arm back down. The biceps goes soft.

A straight arm from the side. The triceps at the back has the word pulls under its name. The biceps at the front has the word soft under its name. Labels: biceps, triceps, pulls, soft.tricepsbicepssoftpulls

A muscle can only pull. It can never push. When it stops pulling, it goes soft and long again. It does not push the bone back.

So muscles work in pairs. Each one pulls one way. Its partner pulls the other way.

See it three ways

As a picture

A table with three columns, You, Front and Back. Row one: bend, with hard and soft. Row two: push down, with soft and hard. Front means the front of the upper arm. Back means the back of the upper arm.YouFrontBackbendhardsoftpush downsofthard

In words

Front and back mean the front and back of your upper arm. When you bend your arm, the front muscle is hard. When you push down and straighten it, the back muscle is hard. The other one goes soft.

Try it

Feel the pair of muscles

You need: your own arm, a table.

Be safe: Press down only as hard as feels comfortable. Stop if anything hurts.

  1. Sit at a table. Rest your right hand flat on the table top.
  2. Hold the front of your right upper arm with your left hand.
  3. Lift your right fist toward your shoulder. Feel the front of the arm. Is it hard or soft?
  4. Put your right hand flat on the table again. Press it down hard, as if to straighten your arm. Feel the front. Is it hard or soft?
  5. Move your left hand to the back of the upper arm. Press down again. Feel the back.

What you will see: When you lift your fist, the front muscle goes hard. When you press down, the front goes soft. The back goes hard. Each muscle only pulls. Its partner pulls the other way.

If you cannot try it, this is what happens.
A table with three columns, You, Front and Back. Row one: lift fist, with hard and soft. Row two: press down, with soft and hard. Front means the front of the upper arm. Back means the back of the upper arm.YouFrontBacklift fisthardsoftpress downsofthard

A wrong idea many people have

A muscle pushes your arm straight.

Why it feels right: Pressing on a table feels like pushing with your whole arm.

The test that breaks it: Hold the front of your upper arm. Press your hand hard on a table. Your arm tries to straighten. The front muscle goes soft. It is not pushing. Now feel the back of the arm. That muscle is hard. It is pulling.

What is true: A muscle can only pull. The back muscle pulls your arm straight. The front muscle goes soft.

Key words

muscle
it gets short and hard, and pulls on a bone.
tendon
a strong cord that joins a muscle to a bone.
biceps
the muscle at the front of your upper arm. It bends your arm.
triceps
the muscle at the back of your upper arm. It straightens your arm.

Back to your guess

The answer is A: Muscles pull on the bones.

Muscles pull on bones. A muscle can only pull. It gets shorter and harder.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. What can a muscle do?

    Tap your answer.

    1. Not this one. A muscle cannot push. When it stops pulling, it goes soft. Try another.

    2. Not this one. A muscle cannot push. The back muscle pulls your arm straight. Try another.

    3. Yes, that is right. A muscle pulls by getting shorter and harder. It cannot push.

    See the answer

    It can only pull.

    A muscle pulls by getting shorter and harder. It cannot push.

    • If you chose A: A muscle cannot push. When it stops pulling, it goes soft.
    • If you chose B: A muscle cannot push. The back muscle pulls your arm straight.

  2. Look at the picture. The arm is bent. Which muscle is hard and pulling?

    A bent arm seen from the side. The muscle at the front is short and fat, labelled biceps. The muscle at the back is long and thin, labelled triceps. Labels: biceps, triceps.tricepsbiceps
    Tap your answer.

    1. Not this one. The triceps is long and thin here. It is soft. It pulls when you straighten the arm. Try another.

    2. Yes, that is right. To bend the arm, the biceps gets short and hard. The triceps goes soft.

    3. Not this one. In a pair, one muscle pulls while its partner goes soft. That is how the arm moves. Try another.

    See the answer

    biceps

    To bend the arm, the biceps gets short and hard. The triceps goes soft.

    • If you chose A: The triceps is long and thin here. It is soft. It pulls when you straighten the arm.
    • If you chose C: In a pair, one muscle pulls while its partner goes soft. That is how the arm moves.

  3. What joins a muscle to a bone?

    Tap your answer.

    1. Yes, that is right. The muscle pulls the tendon. The tendon pulls the bone.

    2. Not this one. A joint is where two bones meet. A tendon joins a muscle to a bone. Try another.

    3. Not this one. A rib is a bone. A tendon is the cord that joins a muscle to a bone. Try another.

    See the answer

    A tendon

    The muscle pulls the tendon. The tendon pulls the bone.

    • If you chose B: A joint is where two bones meet. A tendon joins a muscle to a bone.
    • If you chose C: A rib is a bone. A tendon is the cord that joins a muscle to a bone.

  4. Why do muscles work in pairs?

    Tap your answer.

    1. Not this one. No muscle pushes. Each one only pulls. Try another.

    2. Yes, that is right. The biceps bends the arm. The triceps pulls the other way to straighten it.

    3. Not this one. The point is direction. One muscle pulls the arm up and the other pulls it down. Try another.

    See the answer

    Each muscle can only pull one way, so a partner pulls back.

    The biceps bends the arm. The triceps pulls the other way to straighten it.

    • If you chose A: No muscle pushes. Each one only pulls.
    • If you chose C: The point is direction. One muscle pulls the arm up and the other pulls it down.

  5. From an earlier chapter

    Your elbow bends one way, like a door. What kind of joint is it?

    Tap your answer.

    1. Not this one. A ball and socket joint turns many ways, like the shoulder. The elbow bends one way. Go to S2.3.2. Try another.

    2. Not this one. The elbow moves. Only some joints, such as those in the skull, are joined tight. Go to S2.3.2. Try another.

    3. Yes, that is right. A hinge joint bends one way. The elbow, knee and finger joints are hinge joints.

    See the answer

    A hinge joint

    A hinge joint bends one way. The elbow, knee and finger joints are hinge joints.

    • If you chose A: A ball and socket joint turns many ways, like the shoulder. The elbow bends one way. Go to S2.3.2.
    • If you chose B: The elbow moves. Only some joints, such as those in the skull, are joined tight. Go to S2.3.2.

  6. Draw a straight arm. Label the muscle that pulled it straight.

    See the answer

    A straight arm. The triceps at the back is labelled.

    To straighten the arm, the triceps pulls. It gets short and hard as it pulls.

  7. Your arm is bent. You straighten it. What does the biceps do?

    Tap your answer.

    1. Not this one. A muscle cannot push. The triceps pulls the arm straight. Try another.

    2. Not this one. The biceps pulled to bend the arm. To straighten it, the triceps pulls and the biceps goes soft. Try another.

    3. Not this one. The biceps is at the front. It pulls the arm bent. The triceps at the back pulls it straight. Try another.

    4. Yes, that is right. The triceps pulls the arm straight. The biceps goes soft and long, ready to pull again.

    See the answer

    It goes soft and long.

    The triceps pulls the arm straight. The biceps goes soft and long, ready to pull again.

    • If you chose A: A muscle cannot push. The triceps pulls the arm straight.
    • If you chose B: The biceps pulled to bend the arm. To straighten it, the triceps pulls and the biceps goes soft.
    • If you chose C: The biceps is at the front. It pulls the arm bent. The triceps at the back pulls it straight.

  8. You bend your arm. What do you feel at the front of your upper arm?

    Tap your answer.

    1. Yes, that is right. The biceps gets short and hard. It pulls your lower arm up. You can feel it.

    2. Not this one. A muscle goes hard when it pulls. It never pushes. Try another.

    3. Not this one. Bones cannot move by themselves. Muscles pull on them, and you can feel the muscle go hard. Try another.

    See the answer

    A hard lump. The biceps is pulling.

    The biceps gets short and hard. It pulls your lower arm up. You can feel it.

    • If you chose B: A muscle goes hard when it pulls. It never pushes.
    • If you chose C: Bones cannot move by themselves. Muscles pull on them, and you can feel the muscle go hard.

Keep it

Copy this into your exercise book.

A muscle can only pull. It gets short and hard. A tendon joins it to a bone. The biceps bends your arm. The triceps straightens it. Muscles work in pairs.

Draw: A bent arm. Label the biceps and triceps. Colour the hard one.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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