What it shows
Two rubber bands or balloons with pull threads on a hinged cardboard arm show that muscles work in pairs: when the biceps is pulled short the arm bends, and when the triceps is pulled short the arm straightens. Muscles can only pull, never push.
The science
A muscle is attached to two bones across a joint by tendons. When it contracts it gets shorter and pulls the bones closer, bending the joint. It cannot push the joint back, so a second muscle on the other side pulls the joint straight again. The biceps on the front of the upper arm and the triceps on the back are such an antagonistic pair: when one contracts, the other relaxes and is stretched.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Thick cardboard (an old carton)
- A split pin or a bolt and nut for the elbow
- 2 long balloons or 2 strong rubber bands, for the muscle bellies
- Thread for the tendons, 2 paper clips, tape and glue
- Scissors (adult) and a marker
Safety
For this project: An adult must cut the cardboard and make the elbow hole. Do not snap rubber bands near eyes.
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.
Step 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.
Cut two cardboard strips about 25 cm long for the upper arm and forearm, giving the forearm a short 3 cm tail beyond the elbow hole, and a small square for the hand.
Join the two strips at the elbow with the split pin so the forearm swings freely.
Tape the first balloon or rubber band (the biceps) along the inner side of the upper arm; tie a thread (its tendon) to the forearm a few centimetres past the elbow on the inner side and run it up through a paper-clip loop taped near the shoulder.
Do the same on the outer side for the triceps, but tie its thread to the short tail behind the elbow, because the real triceps pulls on the bone behind the elbow.
Pull the biceps thread, as if the muscle were contracting, while the triceps thread stays loose, and watch the forearm bend upwards.
Pull the triceps thread while the biceps thread goes slack and watch the arm straighten.
Label the biceps, triceps, humerus, radius and ulna, elbow joint and tendons.
Feel your own upper arm as you bend and straighten it to compare.
What you should see
The arm bends when the biceps shortens and straightens when the triceps shortens. It is clear that each muscle only pulls, and that two muscles are needed to move the joint both ways.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Why do muscles work in pairs?
A muscle can only pull by contracting, so a partner is needed to pull the joint back.
02What joins a muscle to a bone?
A tendon.
03Which muscle bends the elbow?
The biceps, on the front of the upper arm.
04What happens to the triceps when the biceps contracts?
It relaxes and is stretched.
Take it further
Build a knee with hamstring and quadriceps bands, or add a string pulley to lift a small weight with the hand.
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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