EXERCISE AND GROWTH Fatigue as a Danger Signal.

The chief use of exercise in childhood, whether of body or mind, is to make us grow; but it can do this only by being kept within limits.

Within these limits it will increase the vigor of the heart, expand the lungs, clear the brain, deepen sleep, and improve the appetite.

Beyond these limits it stunts the body, dulls the brain, overstrains the heart, and spoils the appetite.

How are we going to tell when these limits are being reached?
Nature has provided a danger signal--fatigue, or "tiredness." Fatigue is due, not to complete exhaustion, but to poisoning of the muscle, or nerve, by its own waste substances.