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
What a learner can do afterwards
- 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
The lesson
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.
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.
Because muscles only pull, most joints are moved by two muscles pulling in opposite directions. This kind of pair is called an antagonistic pair. Only one muscle in the pair contracts at a time. 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.
Muscles can only pull, so they work in pairs: one contracts to move a joint one way, the other contracts to move it back.
Watch it
Where it sits
Learn first
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Nothing builds on it yet.
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.