When a girl bends her arm she uses different muscle to bring it back the original position Why cant use same both bend and unbend arm?

The muscles that are used to bend the arm (biceps) are different from the muscles that are used to straighten the arm (triceps). This is because the biceps and triceps are located on opposite sides of the arm. When the biceps contract, they pull the arm up, while when the triceps contract, they pull the arm down.

If the same muscle were used to both bend and straighten the arm, then it would have to contract and relax at the same time. This is impossible, as a muscle can only contract or relax at one time.

Therefore, the body has two sets of muscles for each limb - one set to bend the limb and one set to straighten it. This allows the body to move smoothly and efficiently.