---
title: "Muscles Work in Pairs"
description: "Explain that muscles work in antagonistic pairs — one contracts while the other relaxes — to produce movement, using the bicep and tricep as a key example"
canonical: https://lightmysky.com/learn/science/muscles-work-in-pairs-mt_lm0usBAF53
source: https://lightmysky.com/learn/science/muscles-work-in-pairs-mt_lm0usBAF53.md
retrieved: 2026-10-19
---

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# Muscles Work in Pairs

Explain that muscles work in antagonistic pairs — one contracts while the other relaxes — to produce movement, using the bicep and tricep as a key example

Subject: Science · Area: Organisms & Life Processes · Ages 11 to 13
Page: https://lightmysky.com/learn/science/muscles-work-in-pairs-mt_lm0usBAF53

## Ready when they can

- Explains why muscles can only pull, not push
- Describes what happens to the bicep and tricep when the arm is bent and straightened
- Gives another example of an antagonistic muscle pair

## Lesson: Muscles Work in Pairs

Muscles are made of fibers that shorten when they work. This shortening is called contracting. A muscle can only pull when it contracts. It can never push a bone the other way.

**Example.** Feel the front and back of your upper arm as you move it. The biceps is on the front, the triceps is on the back. Bend your elbow: the biceps contracts and pulls your forearm up, while the triceps relaxes and stretches out. Straighten your elbow: now the triceps contracts and pulls your forearm down, while the biceps relaxes.

*(drawing: The two muscles take turns. First the biceps contracts and the elbow bends. Then the triceps contracts and the elbow straightens. They never pull at the same time.)*

Because muscles only pull, most joints are moved by two muscles pulling in opposite directions. This kind of pair is called an antagonistic pair. To move the joint one way, one muscle in the pair contracts. The other one relaxes and stretches.

Your legs use the same trick. The quadriceps on the front of your thigh contracts to straighten your knee. The hamstrings on the back contract to bend it. Look for other antagonistic pairs in your own body.

**Recap.** Muscles can only pull, so they work in pairs: one contracts to move a joint one way, the other contracts to move it back.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Joints, Tendons & Ligaments](https://lightmysky.com/learn/science/joints-tendons-and-ligaments-mt_yefw2CQT4x)

## Opens up

- [Bone, Cartilage and the Mechanics of a Joint](https://lightmysky.com/learn/science/bone-cartilage-and-the-mechanics-of-a-joint-mt_1_q3aL7E5D)