Muscles that cause two bones on two different sides of the joint to close, resembling a bending action, are called flexor muscles. However, each muscle has the ability only to pull, not to push, so muscles need to work in pairs.
While one muscle contracts to bend a joint, the other muscle contracts to straighten it. The one doing the bending is called the flexor, while the one doing the straightening is called the extensor. One example of a flexor muscle is the bicep, and its complementary extensor muscle is the tricep. When the bicep contracts, the forearm bends, and when the tricep contracts, the forearm straightens.