CHAPTER V.

EXERCISES OF ENDURANCE.

Conditions of Exercise of Endurance; Moderation of Efforts; Slow Repetition-Fractional Work-Conditions Inherent in the Worker. Men and Animals who have "Staying Power "-Need for perfect Equilibrium between the Intensity of the Work and the Power of Resistance of the System-Importance of Respiration in regulating Exercise of Endurance Exercise of Endurance makes the Functions more Active without Fatiguing the Organs-Association of the Great Functions with Moderate and Prolonged Muscular Work; Respiration more Active without Breathlessness; Circulation Quickened without Palpitation - Indications and Contra-indications for Exercises of Endurance-Parallel with Exercises of Speed-Why Children bear Exercises of Endurance badly-Veterans and Conscripts.

I.

WE call exercises of endurance those in which the work must be continued for a long time.

In these exercises the expenditure of force is determined less by the intensity and rapid succession of efforts, than by their duration. It is necessary that the muscular effort shall not be too considerable, and the movements not too rapid, in order that fatigue under its various forms may not interrupt them too soon. So that an exercise of endurance is only moderate exercise if it lasts a short time, while it may become forced exercise if it be continued too long.

In these exercises the quantity of work done after a long time, at the end of a day, for instance, may be very considerable, but the expenditure of force is made in such small fractions that there is no painful muscular effort, nor any marked disturbance in the organic functions. So that a man performing an exercise of endurance may pass, almost without noticing it, to strong doses of muscular work.

The animal machine is made in such a manner as to be able to perform without fatigue movements of a determined intensity and speed. When these limits are not exceeded, no appreciable disturbance is produced in the system, and the work is done amidst complete tranquillity of the vita' functions. Thanks to the perfect equilibrium between the muscular exertion and the power of resistance of the subject, he is able in exercises of endurance to go on working for a long time, and let the useful effects of work insensibly accumulate, without causing any disturbance to the various parts concerned in its performance.

We see at once the importance and usefulness of exercises of endurance when we have to do with a feeble system, with a person of low resisting power, to whom we wish to give the benefits of muscular work, while enabling him to avoid the dangers of fatigue. Similarly we are sometimes able to give a sick man a very energetic remedy by administering it to him in "fractional doses."

The division of work into fractional quantities sufficiently small to enable the system to support each one without disturbing its normal functions, such is the essential condition of exercises of endurance.

Another condition is necessary to constitute an exercise of endurance; the muscular efforts must be at intervals sufficiently long that the effect of a second may not be added to that of a first. Between two successive doses of work there must be a sufficient time for repose.

There are organs in the human body which perform a considerable work continuously throughout life. It is surprising, for instance, to think how the hollow muscle we call the heart, goes on contracting from birth till death, without ever suspending or slackening its work. This is because the cardiac muscle performs a work of endurance. The expenditure of force at each beat is well balanced with the power of resistance of the system of which it forms a part, and the interval between two beats is a time just long enough to rest the fibres.

But if some circumstance occurs which increases the work of the organ, as we see in constriction of the orifices for instance, or if the contractions become immoderately frequent, as is the case in palpitation, the conditions of work are changed. The heart, instead

of having to do a simple work of endurance, has to do a work of speed or strength incompatible with continued work; the muscle becomes fatigued, its fibres lose their elasticity and their energy, there is overwork of the heart, and a condition of asystole comes on, of which death is the inevitable consequence.

Similarly in the muscles of animal life, increased energy, or more rapid succession of movements, tends to make the exercise of endurance pass into one of speed or of strength.

In an exercise of strength there is accumulation of work, because each muscular effort is very intense. In an exercise of speed there is multiplication of work, for the movements have little energy, but the rapid succession of efforts of small intensity leads in the end to an accumulation of work. In an-exercise of endurance, on the contrary, the efforts being repeated at sufficient intervals, the work is fractional, for at any moment the quantity of work performed by the organism does not exceed its power of resistance.

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Among exercises ordinarily practised, which are those call exercise of endurance? This question raises a first difficulty, for the same exercise may represent in turn a work of speed, a work of strength, or a work of endurance, according to the conditions in which it is performed.

Rowing, for example, is a work of speed in a rowingmatch, and a work of endurance in a long course. Walking, which is the type of exercises of endurance, may present the characters of an exercise of strength when it is performed on a very steep slope. Thus in certain ascents in which it is necessary to climb almost perpendicular slopes, each step represents a great expenditure of muscular force, and the tourist is obliged to interrupt his work as

frequently as if he were walking in the plain with a heavy burthen on his shoulders.

The conditions under which the man performing the exercise is placed have no less importance than the exercise itself in determining its character as one of endurance.

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Exercise of endurance is characterised by the necessity for perfect equilibrium between the intensity of muscular effort and the power of resistance of the system. Now there is nothing so variable as the power of resistance of each individual. So that which is for one man exercise of strength or of speed, becomes for another, stronger or better trained, a simple exercise of endurance. A canter is an exercise of speed for a cart horse, used only to walk; it is an exercise of endurance for a thoroughbred, which can sustain this pace for an entire day without stopping. Rowing seems an exercise of strength to a man who is learning; after a quarter of an hour he is out of breath. For a waterman it is an exercise which he can perhaps keep up a whole day without any fatigue.

There are then two conditions necessary to constitute an exercise of endurance: (1) a certain moderation in the violence of the exercise, (2) a certain power of resistance on the part of the system.

This is why the word "staying-power," which conveys the idea of length of time, applies rather to the qualities of the man or the animal, than to the nature of the work they perform. A work of endurance is one whose mode of performance enables it to be long continued; and a man or animal with "staying power are those whose system is fit to support prolonged work.

Certain persons are unable to perform the most moderate exercise without showing, after a very short time, the signs of extreme fatigue. There are others who keep up with surprising power of resistance the most violent exercises, and for them exercises of strength and of speed become exercises of endurance.

Generally these differences in power of resistance, in the staying power of different people, are due to differences in their respiratory powers.

We may say that the respiratory fitness of the individual is the true regulator of a work of endurance.

In order that an exercise may be long continued the first condition is that it does not lead to breathlessness. We can go on walking in spite of weary legs and sore feet; but we cannot go on running when we are out of breath. We saw in the chapter on Breathlessness that this form of fatigue is due to an intoxication of the blood by an excess of carbonic acid. To escape from this intoxication, which renders the continuance of the work impossible, a man must eliminate the excess of carbonic acid as fast as it is formed, and, the formation of carbonic acid being in proportion to the quantity of work done in a given time, we may definitely conclude that in an exercise of endurance, the work of the muscles must be subordinated to the power of the lungs. Thus all the conditions which increase the respiratory power increase the fitness for keeping up intense work for a long time, and a man has "staying power" when he has "wind."

II.

The effects of exercise of endurance may be exactly deduced from the conditions in which this exercise is performed. Evidently an exercise which is incompatible with breathlessness will lead to none of the accidents of forced respiration. During such an exercise there will be no fear of rupture of tendons, of laceration, or of excessive shaking of muscular fibres, for the movements must never become so violent as to exceed the power of resistance of the organs. Further, exercise of endurance does not sensibly disturb the working of the organs, hence in it there will be no very energetic association of the great functions of the economy with the muscular work. In a man walking, for instance, there will not be the rapid rise of temperature, the copious perspiration, the excessive quickening of the pulse and the violent panting which we observe in a runner.

Still we must not imagine that even the most moderate