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.
However, there are certain conditions having their origin in the will and the excitement of mental forces, where the heart is pushed beyond its self-control. Such cases sometimes occur in college boat races――the four-mile races. It is not so much the mere muscular efforts of the contestants as it is the intense mental excitement accompanying the effort. No youth of a highly-strung, nervous temperament should row in these races――no matter what his muscular strength is.
The heart, though making contractions at the rate of seventy-two times a minute, is able to continue its work without fatigue throughout the life of the individual. Each contraction of this wonderful muscle is followed by an interval of rest, during which the cells recuperate. Push the heart-beats to a very rapid rate and we approach the danger-line at which fatigue products cannot be replaced by fresh cells; the intervals of rest are not sufficient. The same conditions exist in every muscle. This is the reason why we often see immediate or ultimate collapse at the finish of a four-mile boat race or a quarter-mile run. Such a collapse may be followed by irremediable loss of health.