It is simply the case of the automobile over again. To run smoothly and continuously there must be fuel steadily supplied and the exhaust――the waste products――cast off, leaving no residue to clog up the engine. But you watch your fuel tank and see that it is well supplied, for you know the consequences if the fuel runs out――if you run your engine up to the last drop. To use up no more fuel in the body than the body will replace, is the secret of athletic success. To go beyond this point means injury――often injury that a life time can never repair. This is one reason why sprinters who have made records succumb early in life to disease or ill health. They have over-heated their human machinery, and the worn parts can never run completely smooth again. How is this fuel that is being used up in the body replaced? The muscle is continually undergoing change of material. The minute substances which make up the muscle, and whose very actions keep it alive, are being constantly cast off, fresh substances taking their place. The cast-off material is the fatigue poison. And it is a poison. Inject it into a dog and he will soon die. Without muscle rest, this dead poisonous stuff cannot be replaced fast enough by the new products, and the result is an impoverished capital of force elements. This does not apply only to the muscle in active use up to this point, but to all muscles of the body. The energy products of the food are delivered up to the muscle by the blood, and this fluid picks up and carries away the cast-off substances of the muscle. These fatigue products are only _gradually_ eliminated from the blood. In the youth these poisonous products are cast off much more rapidly than in the full grown man. Also they are not so rapidly manufactured. But right here lies a danger point――the youth is not warned in time and often goes beyond the point where the off-castings are entirely eliminated and some remain stored up in his system. Then his nervous system suffers and the trouble goes on unrecognized until it is too late. Another thing may happen. In the youth the fatigue poison may be thrown off quicker than the good material can take its place, hence such an athlete――generally a runner――will fall from sheer exhaustion of the muscular mechanism――perhaps ruined for life. To keep on the right side of the danger-line in exercise, the muscle must have short intervals of rest. Nature so well understood the proneness of man not to heed advice that she placed the action of one muscle beyond his control. This muscle is so constructed, internally adjusted, that it has its regular periods of rest, and only in disorder of the body can its expenditure be raised beyond its means. This great and wonderful muscle is the heart.