It is sufficiently obvious that it is through muscular activity that we do many necessary, useful, or otherwise desirable things; and it is also a matter of common experience that muscular activity is required in order to build up strong muscles. A very considerable amount of it is required in order that the laborer may do his work, and a similar amount is necessary in order that one may become an athlete. But the effects of muscular activity on the body as a whole are not so obvious; while a large number of people think that it is “a good thing” and a smaller number are convinced that it is absolutely necessary to the best of health, yet we not infrequently hear men and women seriously question the latter proposition and even venture to doubt the truth of the former. Now there is nothing in hygiene more clearly established than that muscular activity is absolutely essential to healthy living. The effects of a sedentary life may not show themselves at once, but almost without exception they will assert themselves in the end.
Muscular work, in other words, not only enables us to influence our surroundings, not only builds up strong muscles, but in other and equally important though unseen ways ministers to the health of the body as a whole. It is the purpose of this section to present this, the most important hygienic side of our subject, by describing some of the physiological effects which muscular activity produces in the body, and the hygienic value of each of these effects.
The Present Use of the Term ‘‘Muscular Activity.” — In the present chapter the term *muscular activity” is used in a somewhat general sense, and without attempting to set sharp limitations upon it. Strictly speaking, of course, muscular activity would include all work done by the muscles of the body, and this is of various kinds. Even those persons who do no manual labor unconsciously perform muscular work; the heart works on, the breath comes and goes through orderly muscular contractions; sitting and standing, speech, gestures, mastication, — all these things involve muscular activity, and do, as a matter of fact, contribute something to the maintenance of the healthful conditions of the body. It is not improbable that they are the physical salvation of thousands of people leading sedentary lives.
The Physiological Effects of Muscular Activity and their Hygienic Value.— We may now turn to the hygienic value of the more important physiological effects of these general muscular activities, leaving for subsequent consideration exercises designed for special purposes, such as much of our gymnasium work. (a) The physical and chemical changes in the working organ are greater than those accompanying any other bodily activity. The output of carbon dioxide by the body per minute is increased at once from three- to tenfold with what would be termed moderate or vigorous exertion, while digestion seldom increases it more than one fifth, and mental work shows practically no effect upon it. Large quantities of heat are likewise liberated and the temperature of the muscle rises several degrees.
These physical and chemical changes are mentioned first because the hygienic effects upon the body as a whole are to be traced to them as the primary cause. (b) As the result of these changes in the muscles new physical and chemical conditions are introduced into the blood and lymph. The excess of carbon dioxide is entirely excreted by the lungs, so that the blood carried to the other organs by the arteries shows no increase in this substance; but other waste products (such as salts of sarcolactic acid), whose elimination requires the cooperation of other organs than the lungs, are found in the arterial blood in larger quantities than during rest. The chemical and physical characteristics of the immediate environment of every cell of the body is thus changed, and profoundly changed.
Let us now consider the reaction of other organs to these changes in the muscles and in the blood and lymph. (c) Some of the most striking effects of muscular work are those which are connected with the heat-regulating mechanism. The large liberation of heat by the working muscle necessitates active measures to get rid of that heat and maintain the constant temperature. The small arteries of the skin dilate, while those of internal organs constrict, perspiration is secreted, and all these processes are carried out in a coordinated manner.
The nervous mechanism of heat regulation is given a new form of activity, and thus receives valuable training in adjusting itself to the changing conditions with which it has to cope in daily life. <Callout type=