Muscles Work in Pairs · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Muscles Work in Pairs

Science · Organisms & Life Processes · ages 11-13
Name ______________________   Date ____________

  1. When you bend your elbow, what happens to your biceps?

    • It pushes
    • It contracts
    • It relaxes and stretches

  2. What can a muscle do when it contracts?

    • Only pull
    • Push and pull
    • Only push

  3. One muscle can bend a joint and then straighten it back out all by itself.

    Circle one:   True   False

  4. Which pair of muscles works the same way as the biceps and triceps?

    • The quadriceps and hamstrings in the leg
    • Two biceps in the same arm
    • The heart and lungs

  5. You straighten a bent arm. What happens to the triceps?

    • It contracts and pulls the forearm up
    • It stays relaxed the whole time
    • It pulls the forearm down

  6. While the biceps contracts to bend the arm, the triceps contracts at the same time.

    Circle one:   True   False

  7. A crab's claw has two muscles: one opens it, one closes it. Why can't just one muscle do both jobs?

    • Because crabs only have small muscles
    • Because the claw does not need muscles at all
    • Because muscles only pull, so one pulls the claw open and the other pulls it shut

  8. Ravi says, "My triceps pushes my arm straight." What's the mistake in Ravi's thinking?

    • The triceps is in the leg, not the arm
    • Muscles can't push; the tricep contracts and pulls
    • Only the biceps can move the arm
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Muscles Work in Pairs W1-mt_lm0usBAF53-s1

  1. It contracts · Bending the elbow happens when the biceps contracts and pulls the forearm up.
  2. Only pull · Muscles contract, or shorten, and shortening can only pull. They can't push.
  3. False · A muscle can only pull one way. A second muscle is needed to pull the joint back.
  4. The quadriceps and hamstrings in the leg · The quadriceps and hamstrings are an antagonistic pair too: one straightens the knee, the other bends it.
  5. It pulls the forearm down · Straightening the elbow happens when the triceps contracts and pulls the forearm down, while the biceps relaxes.
  6. False · Only one muscle in the pair contracts at a time. The other one relaxes.
  7. Because muscles only pull, so one pulls the claw open and the other pulls it shut · Any pulling-only muscle needs a partner to reverse the movement, just like the biceps and triceps in your arm.
  8. Muscles can't push; the tricep contracts and pulls · Muscles only pull by contracting. The triceps pulls the forearm straight; it never pushes.
Worksheet · LightMySky