The nervous system initiates
and controls every movement of the body, and the energy required for the
carrying out of physical exercises is developed in the muscles themselves, and
their power to transform potential into kinetic energy, which appears as work
or heat, and to renew their store of energy is the central] bet of muscular
exercise. For a long time, physiologists were content to study the causes of
the transformation of energy and the conditions which regulate the mechanical
efficiency of the muscles. Their investigations, carried out for the most part
on isolated muscle, yielded many valuable results, but the tendency to regard
muscular exercise as almost exclusively a muscular act led to a narrow and
imperfect conception of its true character. It has gradually become clear,
however, that the processes taking piece in the muscles, important though they
are, constitute only a fraction of the total activities of the body during
exercise, and that muscular movement is a very different thing from voluntary
exercise.
Although the setting free of energy, when a muscle contracts, is a
non-oxidative process, oxygen is necessary for the restoration of its potential
energy, and, in the absence of oxygen, mammalian muscle rapidly loses the power
of contraction. Further, since the energy of muscular work is ultimately
obtained from the oxidation of the food-stuffs, the enormous increase in the
amount of energy developed in the muscles during exercise involves a
corresponding rise in the intensity of their metabolic activities and in the
extent to which they consume oxygen. Indeed, it is now well known that the
amount of oxygen which a man consumes is a criterion of the degree of activity
of his muscles during exercise.
A
man, who is performing hard physical exercises, may use eight or ten times as
much oxygen as during rest, and the burden of meeting this demand for oxygen
falls upon the respiratory and circulatory systems, which, for this purpose,
are indissolubly linked together. Every increase in the requirements of the
body for oxygen is accompanied by adaptive changes in the circulation and the
respiration, which enable oxygen to be transferred more rapidly from the lungs
to the tissues; and the rapid, deep breathing, the powerfully beating heart,
the high blood-pressure, and the frequent pulse, present in the man who is
engaged in violent exercise, are just as much a part of the exercise, and just
as vital to its effective performance, as the movements of the muscles
themselves. Violent exercise taxes the resources of the circulatory and
respiratory systems equally with those of the muscles; and, partly because it
is called upon to maintain an adequate supply of oxygen to the brain as well as
to the muscles, partly, perhaps, owing to the larger number and greater
complexity of the adjustments required for this purpose, the heart, as a rule,
reaches the limit of its powers earlier than the skeletal muscles, and its
functional capacity determines a man's capability for exertion. It is clear,
then, that, apart from the changes taking place in the muscles themselves, the
activities of the rest of the body are largely directed during exercise to the
provision of an adequate supply of oxygen for the muscles, the heart, and the brain;
hence any picture of muscular exercise must include the whole range of these
activities.
Muscular
exercise thus presents three problems. The
first is the changes taking place in the skeletal muscles, whereby the
transformation of energy, which constitutes muscular movement, is effected; the
second is the nature of the adjustments occurring elsewhere in the body in
order to provide the muscles with the oxygen and food-stuffs required for this
purpose; and the third is the means by which these are interwoven and bound
together to produce the fabric of muscular exercise.
If
the body is to work efficiently and to develop its physical powers to their
fell extent, it is absolutely essential that the movements of the muscles on
the one hand, and the activities of the circulatory and respiratory systems on
the other hand, should be coordinated and integrated into a harmonious whole.
Perfect co-ordination brings about the maximum of work with the minimum of
effort. Incomplete co-ordination, on the contrary, inevitably spells
inefficiency. An exaggerated response on the part of the respiratory and
circulatory systems involves a useless expenditure of energy ; a deficient
response rapidly throws out of gear the controlling influence of the nervous
system, cripples the energies of the skeletal muscles, and renders exercise
ineffective or even impossible. In order to realize the completeness to which
this co-ordination can attain, and the effects of even the slightest disturbance
of the balance between the activities of the muscles and of the rest of the
body, it is only necessary to watch a trained and an untrained man running in a
race or engaged in some other form of exercise. In the former, every organ is
working smoothly, and is contributing its proper share towards bringing about
the perfect harmony of action which is requisite for efficient exercise. In the
latter, the leas perfect adjustment of the body is evident, both to the man
himself and to others, in the greater sense of effort, in the clumsiness of
movement, and in the more severe respiratory and circulatory distress.
