When a girl bends her arm, she uses a different muscle to bring it back to the original position. why can't she use the same muscle to both bend and unbend her arm?

For the answer to the question above, Muscles can only contract, not push 

"Muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten or remain the same. Though the term 'contraction' implies shortening, when referring to the muscular system it means muscle fibers generating tension with the help of motor neurons (the terms twitch tension, twitch force, and fiber contraction are also used)."