What is an example of a prime mover?

Muscles produce movement by contracting. When a muscle contracts, it pulls on the bone to which it is attached, causing the bone to move. Muscles can only pull, they cannot push. Therefore, muscles must work in pairs to move a bone in opposite directions. For example, when you bend your arm, the biceps muscle contracts to pull the forearm up, and the triceps muscle relaxes to allow the forearm to bend.