Final answer:
The muscle that straightens a joint is called the extensor. Muscles work in pairs, with one contracting and the other relaxing. For example, the biceps muscle is a flexor that closes the limb, while the triceps muscle is an extensor that opens the limb.
Step-by-step explanation:
When a joint needs to be straightened, the muscle that contracts to cause the straightening is called the extensor. Muscles work in pairs, with one contracting and the other relaxing. For example, in the arm, the biceps muscle is a flexor that closes the limb, while the triceps muscle is an extensor that opens the limb.