What it shows
Each heartbeat pushes a wave of blood through the arteries, and at the wrist this wave makes a tiny matchstick rock. Counting the rocks for a minute gives the pulse rate, which rises after exercise because the muscles need more oxygen.
The science
The heart is a muscular pump that beats about 70 to 100 times a minute at rest in a child. Each beat sends blood into the elastic arteries, which stretch and spring back, and you feel this as the pulse. During exercise the muscles use more oxygen, so the brain makes the heart beat faster and harder to deliver more blood. After rest the pulse slowly returns to its resting value.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A used matchstick or a thin toothpick
- A small ball of clay or sticky tape
- A watch or phone stopwatch
- A notebook and pencil
- A friend as a partner
Safety
For this project: Use a burnt or broom-stick matchstick only. Exercise gently and stop if anyone feels dizzy. A child with a heart or breathing problem should only watch, and an adult must supervise.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Sit quietly for five minutes so the pulse settles to its resting value.
Stick a small ball of clay on the end of the matchstick so it can stand upright.
Find your pulse on the thumb side of the inside of your wrist and press the clay end gently on the spot. Move slightly until the stick rocks in time with the beats.
Count the rocks for 30 seconds and double it to get beats per minute. Repeat three times and write the average.
Walk on the spot or climb stairs for two minutes, then sit down at once and count the pulse again for 30 seconds.
Count again after every minute of rest until the number comes back near the resting value, and note how many minutes it took.
Plot a graph with minutes of rest on the x axis and pulse per minute on the y axis. A common mistake is pressing too hard, which stops the pulse.
Repeat with a partner, then compare rates, remembering that fitter people usually recover faster.
What you should see
A resting pulse of about 80 beats a minute rises to about 110 to 130 after two minutes of exercise and falls back in 3 to 5 minutes, drawn as a smooth curve on the graph.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What exactly is the pulse?
It is the wave of pressure that travels along an artery each time the heart beats.
2Why does exercise raise the pulse?
Working muscles need more oxygen and sugar, so the heart pumps more blood each minute.
3Why not press hard on the wrist?
Pressing hard closes the artery and you lose the pulse.
4Why do we average three readings?
A single reading can be wrong by a beat or two, and the average is more reliable.
Take it further
Compare the recovery time of five friends and see whether those who play sport daily recover faster.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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