When they work, muscles break fibers. The more intensely you work out, the more they break. So you have to rest for them to be rebuilt (and more fit to the exer

When they work, muscles break fibers. The more intensely you work out, the more they break. So you have to rest for them to be rebuilt (and more fit to the exercise), otherwise at some point they give in. The more frequently you train, the more they can withstand.