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There are three kinds of muscle. Skeletal muscles are joined to bones by tendons, for example in the arms, legs and face. Smooth muscle is in the walls of the stomach, intestines and blood vessels. Cardiac muscle makes up the walls of the heart.

Skeletal muscles are voluntary: you decide when to use them. Smooth and cardiac muscles are involuntary: the nervous system controls them automatically.

Muscles can only pull. They work in pairs: a flexor bends a joint and an extensor straightens it. The biceps is a flexor of the elbow and the triceps is an extensor.

Experiment: Clothes-peg challenge: do muscles get tired?

1. Question: does the number of squeezes you can do in 30 seconds change if you keep going without a rest? Write your prediction.
2. Hold the peg between thumb and first finger of your writing hand. One squeeze = open the peg fully and let it close.
3. Your partner times 30 seconds and counts your squeezes. Write the number down.
4. Straight away, without resting, do another 30 seconds. Repeat until you have four results.
5. Fair test: same peg, same hand, same fingers, same 30 seconds, and the same partner counting. Rest for 3 minutes, then do one more 30 seconds. Has the number gone back up?

30-second round	Squeezes	Did it feel harder?
Round 1	Predict: will round 4 have more or fewer squeezes than round 1?
Round 2	Predict: will round 4 have more or fewer squeezes than round 1?
Round 3	Predict: will round 4 have more or fewer squeezes than round 1?
Round 4	Predict: will round 4 have more or fewer squeezes than round 1?
After a 3-minute rest	Predict: will round 4 have more or fewer squeezes than round 1?

Questions

1. Name the three kinds of muscle and give one place where each is found.

2. Explain why muscles have to work in pairs. Use the words flexor and extensor.

3. Draw a bar chart of your squeezes in rounds 1 to 4. What pattern do you see?

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4. Why was it important to use the same peg and the same hand each time?

Sources for every fact on this sheet: echoesof.science/school/muscles/sources/

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