Muscles: three kinds, pairs and tiredness
Skeletal, smooth and cardiac muscle, voluntary and involuntary, muscle pairs, tendons and what happens when a muscle gets tired. With a clothes-peg fatigue fair test.
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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.[1][2]
Skeletal muscles are voluntary: you decide when to use them. Smooth and cardiac muscles are involuntary: the nervous system controls them automatically.[1]
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.[1][6]
When the biceps lifts a cup it is the main muscle (the prime mover); the triceps, which works the other way, is its antagonist. Bones act as levers and the joints as the turning points.[5]
Skeletal muscles do more than move us: they hold our posture against gravity, keep joints steady, protect the organs in the belly and make heat, for example when we exercise or shiver.[3]
Muscles need energy to contract. If a muscle can no longer contract when the nerves signal it, it has fatigue: it is tired.[4]
Try it: Clothes-peg challenge: do muscles get tired?
You need
- One spring clothes peg per pair (all the same kind)
- A stopwatch or classroom clock with a seconds hand
- A pencil and the results table
Safety
Use ordinary wooden or plastic pegs, not strong clips. Squeeze only your own fingers' peg, never someone's skin. Stop if your hand hurts (tired is fine, pain is not). Pupils with a hand injury can be the timer and counter.
What to do
- Question: does the number of squeezes you can do in 30 seconds change if you keep going without a rest? Write your prediction.
- Hold the peg between thumb and first finger of your writing hand. One squeeze = open the peg fully and let it close.
- Your partner times 30 seconds and counts your squeezes. Write the number down.
- Straight away, without resting, do another 30 seconds. Repeat until you have four results.
- 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?
What happens, and why?
Most people manage fewer squeezes in each new 30 seconds. The finger muscles are getting fatigued: they find it harder to keep contracting when the nerves signal them. After a rest the number usually goes up again. Muscles need energy to contract, and when they work hard without a break they cannot keep up.[4]
Play: Which kind of muscle?
Sort each muscle into skeletal, smooth or cardiac muscle.
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 15–16