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 ill
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.
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.
At the other extreme are those who perform severe manual labor,
or who engage in vigorous exercises or purposely cultivate exceptional
We are not, however, directly concerned at present with either
of these extremes, nor with those forms of muscular activity
so common today in workshops where, hour after hour, the workman
performs the same task over and over again. We are rather concerned
with those forms of muscular work which are seen in a lumber
camp or on the farm; which present the characteristic of variety
and involve the use of the musculature of the body as a whole;
in short, those forms of activity by which until very recently
the human race has supported itself in its daily life. Such things
as brisk walking, running, rowing, wood chopping, swimming, tennis
playing, would thus be placed in the same class, since they involve
a use of the muscles similar to those which we have mentioned.
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 sarco-lactic 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.
(d) Closely connected with the foregoing is the (temporary)
relief afforded to any congestion of blood in the internal organs.
Sedentary occupations usually involve more or less overfilling
of the blood vessels of the stomach and intestine, the pancreas,
the liver, the spleen, and the kidneys; they also involve the
absence of those movements of the trunk whose pumping action
affords a marked assistance to the flow of blood through the
abdominal organs. The congestion thus caused is not a good thing;
it almost certainly renders the organs concerned more liable
to inflammatory processes, and, if there has been established
any tendency to catarrhal conditions, it aggravates that tendency.
Popular experience has long associated with health a good color
of the skin; and, while it is not safe to make such an inference
in all cases, pallor very frequently means internal congestion,
unhealthy digestive functions, and greater liability to cold
in the head or the chest.
(e) Muscular activity is the only thing which can be depended
upon to increase the work of the heart. While this fact makes
caution and moderation necessary for persons having certain forms
of heart disease, yet for the vast majority of people it is of
the greatest hygienic importance to accustom the heart to reasonably
hard work. Only in this way does it receive the training necessary
for its proper development and for the maintenance of its strength.
Emergencies will arise when the heart is called upon for severe
effort, brief or prolonged. The familiar example of the sudden
"sprint" for a car is a case in point; and there are
times, as in pneumonia, when the issue in sickness is largely
determined by the endurance of the heart. In too many such cases,
if the patient escapes the fatal issue, it is only with a permanently
It is important not only that the heart should be kept ready
for emergencies but also that it be kept in condition for vigorous
work as a regular duty of daily life. One of the worst of "vicious
circles," as physicians call them, is the acquirement of
a weakened heart by abstention from proper muscular exertion,
and, as a consequence of this weakened heart, increasing disinclination
to exertion of any kind whatever. The failure to take proper
exercise leads to deterioration in strength and endurance on
the part of the heart; and this cardiac deterioration, with the
resulting discomfort of breathlessness, leads in turn to abstention
from muscular activity.
(f) Muscular exercise is the one agent which increases the
depth and frequency of the respiratory movements. The hygienic
importance of this does not lie in the better oxidation of wastes,
since, so far as we have any accurate knowledge on the subject,
it would seem that the processes of respiration during sedentary
life more than supply the existing demands of the tissues for
oxygen. The increased respiration is rather of importance because
of the secondary effects of the respiratory movements in promoting
the flow of blood, and especially the flow of lymph.
It is probable that the" freshening effects" of
muscular exercise are to it very large extent attributable to
the improved lymph circulation in the tissues, and this effect,
it will be remembered, is felt in the immediate environment of
almost every cell in the body. The suction action of inspiration
quickens the lymph flow from all organs outside the thorax, and
the increased pumping action of the respiratory movements themselves
aids the lymph flow from the lungs and other organs within the
thorax. Waste products are more completely removed from the lymph
spaces surrounding all cells, and thus one of the most important
of fatigue conditions is relieved. Where lymphatics are subject
to the pumping action of contracting muscles and of the alternate
flexion and extension of joints, the suction action of the respiratory
movements is reenforced. This pumping action especially affects
the lymphatics of the arms and legs, and those of the abdominal
cavity (through the action of the diaphragm and the trunk movements).
The increased respiratory movements also contribute to greater
mobility of the ribs and to the better ventilation of the lungs.
During vigorous exercise all lobes of the lungs are used, and
the dangers attendant upon disuse of the apical lobes are largely
(g) Moderate exercise exerts a favorable effect upon the digestive
organs, although the precise action involved is very complicated.
Here also it improves the lymph flow, thus promoting absorption
and producing better conditions in all digestive glands and in
the muscular apparatus of the digestive tract; it prevents continued
congestion and the unfavorable attendant conditions. It is probably
also a direct stimulus to peristalsis, for unquestionably the
exercises which involve movements of the trunk often prove a
peculiarly efficient remedy for constipation.
The above summary is very far from a complete enumeration
of the effects of muscular exercise upon the organism, but it
will suffice to show how essential an element such exercise is
in the life of the body. The training of the heat-regulating
mechanism, the training of the heart, the improved lymphatic
environment of every cell resulting from increased breathing
movements and from the pumping action of mechanical motion, the
relief of internal congestions and the favorable influence upon
digestive functions, - all these things are fundamental to healthful