Fitness exercise: Why can each group effectively stimulate muscle growth when they are exhausted?

The so-called exhaustion means that until the end, if you don't rest, you can't do it anymore. Not all muscle fibers are involved in training. You only used part of the muscle fiber. Exhaustion will make these muscle fibers tired, and at rest, the body will let other muscle fibers replace the tired muscle fibers. Repeated many times will make the whole muscle group get full exercise. This is the importance of fatigue.