There are lots of muscles that help to move your arm in all the many ways it can move. The bicep and tricep muscles are two of the most noticeable, partly because of how big they get on some bodybuilders. These paired muscles flex and extend the elbow joint, allowing you to bring a fork to your mouth or throw a frisbee. When one flexes, the other relaxes.
In this muscle machine, you’ll be mimicking the action of a muscle by pulling a string. This is actually pretty similar to how your muscles work. Muscles are made up of long fibers of cells, kind of like bundles of string. The cells in a relaxed muscle fiber are long and skinny. When a muscle flexes, the cells yank themselves together, going from long and skinny to short and round. This shortens the muscle fiber too, so anything the muscle fiber is attached to will move in the direction of its contraction. The string mirrors this by changing its length when you pull on it.
Download the printable here.