This is the reason why, during exercise, we breathe faster and deeper than at other times, and why our skin begins first to glow and then to perspire.
If these waste-materials form in the muscles faster than the blood can wash them out, they poison the muscle-cells and we begin to feel tired, or fatigued.

This is why our muscle-cells are often so stiff and sore next morning after a long tramp, or a hard day's work, or a football game.

A hot bath or a good rub-down takes the soreness out of the muscles by helping them to get these poisonous wastes out of their cells.
Thus when we play or run or work, we are not only exercising our muscles and making them gain strength and skill, but we are stirring up, or stimulating, almost every part of our body to more vigorous and healthful action.
Indeed, as our muscles alone, of all our body stuffs, are under the control of the will, our only means of deliberately improving our appetites, or strengthening our hearts or circulation, or invigorating our lungs, or causing a large part of our brains and minds to grow and develop, is through muscular exercise.

This is why nature has taken care to make us all so exceedingly fond of play, games, and sports of all sorts, in the open air, when we are young; and, as we grow older, to enjoy working hard and fighting and "hustling," as we say; and that is the reason, also, why we are now making muscular exercise such an important part of education.
FOOTNOTES: [22] The muscle does not get any bigger when it contracts, as was at one time supposed; if you were to plunge it into a bath of water, and then cause it to contract, you would find that it did not raise the level of the water, showing that it was of exactly the same size as before, having lost as much in length as it gained in thickness.
[23] In the leg below the knee, and in the forearm, we have two groups of "benders" or _flexors_, and "straighteners" or _extensors_, as in the upper arm and leg, only slenderer and more numerous.

They taper down into cord-like tendons at the wrist and ankle to fasten and to pull the hands and feet "open" and "shut," just as do the strings in the legs and arms of a puppet or mechanical doll, or the sinews in the foot of a chicken..