Muscles work in pairs
Biology · Key Stage 2 · The School
A muscle can only pull
A muscle can do one thing. It can pull. It cannot push at all. This sounds strange at first. Think about a rope. A rope can pull something towards you. It cannot push something away. A muscle works the same way. It needs a partner to move a bone back the other way. Muscles are attached to bone by tough cords called tendons. The tendon is what actually gets pulled.
Two muscles, one job each
Look at your upper arm. The BICEP sits on the front. The TRICEP sits on the back. They are a pair. When the bicep pulls, your arm bends. When the tricep pulls, your arm straightens. Only one of the pair pulls at a time. The other one relaxes and goes soft. This kind of pair is called an antagonistic pair. The word just means the two work against each other.
Pairs all over the body
Arms are not the only place this happens. Legs have muscle pairs too. So does your back. Every joint that bends needs at least one pair. One muscle bends the joint. The other muscle straightens it again. Neither one could do the job alone. Even your face uses muscle pairs, to smile and to frown.
Hold your arm out straight. Feel the top of your upper arm with your other hand. Now bend your arm slowly. Feel the bicep go hard and short. It is pulling. Now straighten your arm again. Feel the back of your arm instead. The tricep goes hard this time. The bicep goes soft. Each muscle only ever pulls. It is the pair, working in turn, that lets the arm move both ways.