Muscle fibers are rebuilt during rest. Consider the way that you work out at the gym; you have specific days that you work arms and then legs etc. You space out the workouts of each group of muscles. Your body needs time to rebuild the muscle that was broken down with the work. If you are working very hard then you may need to move to one session a day to see how your body responds. Watch your protein intake to ensure your body has enough to fuel muscle growth.