CHAPTER V

FATIGUE AND ITS PSYCHOLOGY

MUSCULAR and mental activity are always accompanied by liberation of energy and disintegration of tissue. A man gives out more than three and one-half times as much carbon dioxide per minute when walking at the rate of two miles an hour than when asleep. If his pace is quickened to three miles an hour he discharges more than five times as much, and when working in a treadmill nine times as much is given out as when asleep.

This making and remaking of the tissues of the body is a continuous process during life. "Did we possess some optic aid," says Foster, "which should overcome the grossness of our vision, so that we might watch the dance of atoms in this double process of making and unmaking in the living body, we should see the commonplace, lifeless things which are brought by the blood, and which we call food, caught up into and made part of the muscular whorls of the living muscle, linked together for a while in the intricate figures of the dance of life, giving and taking energy as they dance, and then we should see how, loosing hands, they slipped back into the blood as dead, inert, used-up matter. In every tiny block of muscle there is a part which is really alive, there are parts which are becoming alive, there are parts which have been alive but are now dying or dead; there is an upward rush from the lifeless to the living, a downward rush from the living to the dead.

"This is always going on, whether the muscle be quiet and at rest or whether it be active and moving. Whether the muscle be at rest or be moving, some of the capital of living material is always being spent, changed into dead waste, some of the new food is always being raised into

living capital. But when the muscle is called upon to do work, when it is put into movement, the expenditure is quickened, there is a run upon the living capital, the greater, the more urgent the call for action. Moreover, under ordinary circumstances, the capital is spent so quickly, during the action, that it cannot be renewed at the same rate; the movement leaves the muscle with an impoverished capital of potential stuff, and a period of rest is needed in order that the dance of atoms of which I just now spoke may make good the loss of capital and restore the muscle to its former power.'

Drawing on the capital of living matter, if the expenditure be in excess of production, brings fatigue. The causes of fatigue are chemical conditions resulting from changes within the organism. Certain substances essential to the activity of the protoplasm have been consumed in this activity, and waste products have accumulated. Oxygen and carbohydrates have been consumed. The chief source of the energy of muscles is carbohydrates. Experimentally, it has been shown that removal of most of the carbohydrates from the body of an animal produces symptoms of fatigue without exercise. Recovery may be brought about by feeding sugar. It is well known that mountaineers, and soldiers on a long march, are greatly strengthened by eating chocolate. Oxidation of carbohydrates results in the production of certain substances which act as poisons to the body. They are spoken of as "fatigue substances." Two of these substances, carbon dioxide and lactic acid, investigation has shown, are hostile to protoplasmic action. When present in considerable quantity they weaken the sensibility of muscle and diminish its response. A muscle so affected requires more prodding for a given piece of work. Indeed, a muscle treated with either of these substances is "fatigued" without having done any work.

1 Michael Foster, Nineteenth Century, vol. 34, p. 337.

Fatigue, according to Starling,1 probably depends upon two factors: First, upon "the consumption of the contractile material" [of the muscle] "or of the substances available for the supply of potential energy to this material,” and second, upon "the accumulation of waste products of contraction." 2 Although "a state which cannot be distinguished from fatigue can be produced in fresh muscles, by the injection of aqueous extract of the fatigued muscles of another animal," and although "fatigue may be artificially induced in a muscle by 'feeding' it with a dilute solution of lactic acid, and again removed by washing out the muscle with normal saline solution containing a small percentage of alkali," fatigue is not primarily in the muscles. This is proved by the fact "that direct stimulation of muscle will cause contraction after the synapse between nerve and muscle has lost its excitability."5 It has also been observed that a motor centre which has been fatigued for one reflex may be unaffected for another. "This state of fatigue is, accordingly, situated in some synapse, not in the efferent neurone itself." Further, it has been as clearly shown that it is not in the nerves as that it is not in muscle. "In fact it is not possible to demonstrate any phenomena of fatigue in the nervetrunk."7

Considerable evidence has been offered for locating fatigue in the synapse. "The lines of junction of nerves with other parts seem to be more readily fatigued. Nerve

1 Principles of Human Physiology, p. 209.

'Starling seems unmindful of Verworn and Dolley's "fatigue of excitation" and "fatigue of depression”; i. e., fatigue caused by the consumption of reserve material from which the energy of the nerve-cell is derived, and the accumulation of waste products more rapidly than the nerve-cell can eliminate them.

A. E. Schäfer, Text-Book of Physiology, vol. II, p. 389.

Ernest H. Starling, op. cit., p. 209.

5 William M. Bayliss, Principles of General Physiology, 1915, p. 451. William M. Bayliss, op. cit., p. 475.

"Ernest H. Starling, op. cit., p. 259.

cells, or the fields of conjunction in the central nervous system, seem to be markedly susceptible to fatigue," so "we may assume that the seat of the fatigue is to be sought either in the central nerve-cells or in the nerve network, 'synapses,' in relation to them." Evidence that the nerve-cell is the seat of fatigue has been given by Piper,2 who has shown that fatigue causes a decrease in the rate of oscillatory discharge of the nerve-cell. It has been known for some time that fatigue substances act in a stimulating manner upon the respiratory centres, quickening the rate and increasing the depth of respiratory movements.

The primary seat of fatigue, then, appears to be either in the nerve-cell alone, or in both the nerve-cell and synapse. However this may be, originating in one tissue, it spreads through the blood circulation to other tissues and quickly becomes a more or less general condition of the body, affecting all organs. That fatigue products become a part of the general circulation is indicated by the fact that the introduction of blood from a fatigued dog into the circulation of one that is fresh will produce all the symptoms of fatigue. "There is every reason to believe," says Lee, "that the main principles of muscular fatigue are demonstrable in other tissues and organs of the bodythat in them also fatigue is characterized physically by a diminution in working power, and chemically by the destruction of energy-yielding substances and the appearance of toxic metabolic products. Diminution of working power is manifested in very different ways by diverse tissues. Glands in fatigue seem to secrete less than when fresh, and it may be that the action of digestive juices is diminished. The kidneys may be deranged, so that their epithelium is unable wholly to prevent the passage of al

1 William Stirling, Outlines of Practical Physiology, 1902, pp. 300, 303. H. Piper, Elektrophysiologie menschlicher Muskeln, 1912, pp. 124 ff. 8 Popular Science Monthly, vol. 76, p. 182.

bumin from the blood to the urine. A fatigued heart is dilated, its beats are quickened and may become irregular, and its diastole, or resting period, may become abbreviated."

Since fatigue expresses itself in the tissues and organs of the body, the vigor of these organs is evidently an important matter; and any means of increasing their endurance has psychological significance. No apology, therefore, is necessary for referring to muscular exercise in the psychology of the day's work. The intimate relation between the mental and physical has long been recognized. Intelligence in animals is correlated with the range in muscular co-ordination, and the more complicated the neuromuscular apparatus the greater the intelligence of the animal. In man, intellect, feelings, and will are closely related to alterations in the circulation of the blood in the body as a whole, as well as in the brain; and it is only under the influence of exercise that the circulation maintains its highest degree of vigor. The so-called power the will is dependent in some degree upon the firmness of muscle, or, at any rate, upon the bodily tone; and sensitive mental response bears a direct relation to vigorous bodily reactions. So far as fatigue itself is concerned, this relation is even more clearly discernible. Weariness, however, whether mental or physical, is a relative matter. Some men are never fatigued, though they do a prodigious amount of work.

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The explanation of indifference to fatigue is found in the relation between expenditure of energy and the amount on deposit. Incessant change in bodily processes is characteristic of life. This change may be sluggish or it may be rapid-if too slow, the tissues and organs of the body are not properly renovated; and if too rapid, tissues are broken down faster than they can be rebuilt and bodily deterioration ensues. The chemical processes of the body should proceed as rapidly as an orderly reconstruction of