Muscles work by getting shorter. We say that they contract, and the process is called contraction.

Muscles are attached to bones by strong tendons. When a muscle contracts, it pulls on the bone, and the bone can move if it is part of a joint.

Muscles can only pull and cannot push. This would be a problem if a joint was controlled by just one muscle. As soon as the muscle had contracted and pulled on a bone, that would be it, with no way to move the bone back again. The problem is solved by having muscles in pairs, called antagonistic muscles.

Biceps and triceps

The elbow joint lets our forearm move up or down. It is controlled by two muscles, the biceps on the front of the upper arm, and the triceps on the back of the upper arm. The biceps and the triceps are antagonistic muscles.

• when the biceps muscle contracts, the forearm moves up

• when the triceps muscle contracts, the forearm moves down.

This solves the problem. To lift the forearm, the biceps contracts and the triceps relaxes. To lower the forearm again, the triceps contracts and the biceps relaxes.