A: Bodybuilders lift very heavy weights. When a person lifts heavy weight, it places a great demand on their muscles. The muscles break down (little microscopic tears in the muscles) and then repair. When they repair, they grow stronger and bigger, then they lift again and they grow stronger and bigger again. The size and strength are responding to the demand that is placed on them. Just like if you are running, and you do it consistantly and work hard at it, your body reacts to the demands by strengthening and getting stronger. I hope this helps.

Trainernyc