Final answer:
Muscles can only contract, not actively extend. They work in pairs to move bones in different directions.
Step-by-step explanation:
Muscles can only contract. They cannot actively extend, though they can move or relax back into the non-contracted neutral position. Therefore, to move bones in opposite directions, pairs of muscles must work in opposition. Each muscle in the pair works against the other to move bones at the joints of the body. The muscle that contracts to cause a joint to bend is called the flexor. The muscle that contracts to cause the joint to straighten is called the extensor. When one muscle is contracted, the other muscle from the pair is always elongated.