Antagonistic muscles:
Muscles often work in pairs which work against each other. These are called antagonistic pairs.
The muscles in the upper arm control the bending and straightening of the arm.
The two muscles, the biceps and triceps are working against each other.
When the biceps contracts the lower arm is raised and the arm bends. In this position the triceps muscle is relaxed. To straighten the arm the reverse happens. The triceps contracts straightening the arm, while the biceps relaxes.