Hold your arm out straight, then bend it at the elbow. What is happening under your skin that makes that move possible?
There is a hidden team under your skin doing that job every time you pick up a pencil.
Today we will feel that teamwork on your own arm, then build and test a working model of a hinge joint and a muscle pair.
Bend your elbow again. That bend happens at a hinge joint: like a door hinge, it moves one way and back again.
A joint cannot move itself. Muscles move it, and a muscle can do only one thing: pull. When a muscle pulls, it gets shorter and fatter, and it tugs on the bone it is joined to. A muscle can never push.
So one muscle can move your forearm only one way. To move it back, a second muscle on the other side pulls the other way. Two muscles that take turns like this are a muscle pair.
Look at the picture. Which muscle pulls to bend your arm? Which muscle pulls to straighten it?
Place one hand on the front of your upper arm. Bend your elbow slowly and feel the muscle firm up. That is your biceps pulling.
Now place your hand on the back of the same upper arm. Straighten your elbow and feel the muscle firm up. That is your triceps pulling.
Notice: when one muscle pulls and shortens, the other softens and lengthens. They work as a pair.
In your group, build this elbow model. The two card strips are the bones, the split pin is the elbow hinge, and the two elastic bands are the muscle pair.
Follow the build guide below one step at a time, and check each step against its picture before you go on. The yellow band goes on the front of the forearm. The red band goes on the elbow point, the short end that sticks out past the pin.
When your model looks like the big picture, stop. Do not pull the bands yet: first we predict what each band will do.
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