Fatigue And Its Psychology
( Originally Published 1921 )
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.
Fatigue, according to Starling,' 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." 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," a 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."
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." 6 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 nerve-trunk."
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-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, 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 body—that 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 albumin 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 coordination, 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 of 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.
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 the tissues permits. Fatigue is a sign that one is going too fast. But it should be observed that the rate of activity, again, is a relative matter. The important question is not how much one is spending but what is the ratio of expense to one's bank-account. Though the average amount of a man's work is about two million foot-pounds per day, Miller, the winner of the 1898 six-day bicycle-race, performed more than fifteen million foot-pounds of work on the first day, as calculated by Carpenter, and he averaged on each of the six days more than nine and a half million foot-pounds, which is approximately five times the daily average of men. But more than this, during the month following this tremendous expenditure of energy Miller again competed, this time in a twenty-four-hour race, and two months later he broke his previous record, winning another six-day race.
Of course few engage in competitive contests of just this sort, but well-developed organs mean much for endurance in other fields than those of the race-track. "Voluntary and involuntary muscles should possess size, toughness, and contractile power sufficient for both ordinary and extraordinary demands. The heart should be able to resist a high blood-pressure without detriment to its muscle-fibres or valves. The capillary bed should be capacious. Vasomotor response should be ready. Respiratory organs should be capable of quickly bringing in oxygen and quickly eliminating carbon dioxide. Osmotic exchange should be rapid. Secreting and excreting organs should be quickly supplied with blood and lymph and capable of quickly supplying their products."
Preparedness for possible emergencies is important here as in everything else, since preparation for the exceptional is excellent equipment for the usual. To almost every one come days of strain. Demands are suddenly put upon us, and the ease and success with which we accomplish them depend upon our readiness. The reconstructive adaptation of the human body to the calls made upon it is one of the remarkable biological facts. But there should always be a margin of safety, and this requires that strength be kept at a high level of vigor. "Moderate but increasing amounts of exercise, producing moderate but increasing amounts of fatigue substances, put the tissues into a state of tolerance or resistance, such that when the supreme effort is demanded of them they do not succumb. He who wins is he who can best resist the poisons of fatigue." Training consists, among other things, in producing resistance to fatigue products.
It has long been known that organisms may become immune to toxic substances by growing accustomed to gradually increasing quantities. Illustrations of physiological immunity are abundant, and they are important because they show the extent to which resistance, in the matter of which we are speaking, may be developed. A man may gradually increase the dose of arsenic until he can take enough to kill ten men. His adaptation to this drug is now so perfect that if he suddenly stops taking it he will die. In other words, to accustom him again to the normal conditions of human life the dose must be gradually decreased until his organs have readjusted themselves to its absence. Adaptation to the poisons of venomous snakes may also be artificially produced by gradually in-creasing the amount given. Such a man may then live among these poisonous snakes without danger to his life.
In the same way immunity to the poisons of fatigue sub-stances may be developed. Excess in anything is debauchery, but in the matter of physical exercise excess varies with individuals. What tires one is insufficient for the well-being of another. The physical strength of some is abundant and of others insufficient. One should endeavor to extend the boundaries of one's endurance, always stopping before exhaustion is reached, but doing a little more the next day. The muscles of the heart should be strengthened by exercise, just as those of the arm should be hardened. A man with unhypertrophied heart and muscles is a mollycoddle who cannot stand strain when it is put upon him.
The test of "fitness" is the capacity to continue active unfatigued. The curve of fatigue for any particular day is to a certain extent a gauge of the bodily and mental condition at the time. The night's rest, digestion, emotions, all of these influences and others that are not so obvious, greatly influence the ability to do work and largely deter-mine the sort of work done, its accuracy and value. The most important factor in producing fitness, however, because of its far-reaching effect upon the body and mind, is probably exercise. With moderate exercise there is no evidence that any other substance than carbon dioxide is formed, but with violent exercise lactic acid is also produced. A variety of experiments indicate that lactic acid appears with a relative deficiency in the supply of oxygen to the contracting muscle. The copious intake of oxygen that accompanies exercise would tend, therefore, to reduce the lactic-acid formation. Respiration is essentially a gaseous exchange and takes place primarily in the cell. For these reasons, and since muscle in an atmosphere of pure oxygen does not fatigue as soon as in an atmosphere of nitrogen, and a muscle stimulated to exhaustion in an atmosphere of nitrogen recovers quickly in pure oxygen, it would appear that the presence of oxygen tends to push back the fatigue-point. All of this suggests the value of volumes of fresh air in delaying fatigue and of furthering recovery when fatigue has set in.
"The practical importance of muscular exercise upon the processes of oxidation in the body cannot be over-estimated in these days when overcivilization tends to weaken the physique and moral fibre of man. There is no other condition, whether it be physiological or pathological, which will produce such a great increase in the absorption of oxygen and the discharge of carbon dioxide, such a wide-spread effect upon the exchange of material in the body. Man in a primitive condition is forced to muscular exercise in order that he may obtain food or protect him-self from wild animals, or more often from his fellow men. Civilization cannot override in a generation or two the characteristics impressed through countless ages, and the need of exercise becomes imperative and finds satisfaction in sport, when, owing to the process of division of labor, some classes become differentiated for work necessitating but little muscular activity. . . One of the marked characteristics of life is oxidation, but the benefits of muscular work are not to be attributed to that alone. Muscular activity is not a simple increase of oxidation; the body is not a machine from which work can be obtained simply at the expense of more fuel and increased wear and tear. The co-ordination of all the systems of the body is necessary, and all parts are affected; the growth and vitality of the body are favored by the work performed. In these respects muscular exercise is of the utmost importance, and one may see in the training for warfare among the highly civilized and especially the manufacturing nations a blessing in disguise. Should the danger of war ever be entirely removed the only safeguard against degeneration would be outdoor sport."
Unfortunately, physical activity is too commonly regarded as the luxury of play. The belief is somewhat wide-spread, especially in the United States, that muscular exercise is unnecessary and perhaps undesirable after a man has reached the sixth and seventh decade of his life. To be sure, the popularity of golf has, to a certain extent, changed this opinion, but, after all, a comparatively small proportion of these older men play the game. The expense, time, and output of energy needed are resistances to which those advanced in years readily yield, and walking, as well as gymnasium exercise, are felt to be uninteresting. Consequently, these men give themselves up to the physical and mental disintegrating processes of age. The effect of regular exercise in developing the bodily organs and in promoting resistance to fatigue, even in those advanced in years, is seen in an instance cited by Sargent.' "Mr. Smith Robertson, of Eau Claire, Wisconsin, a man 5 feet 8 inches in height, and weighing 140 pounds, began systematic exercise with 10-pound dumb-bells and a horizontal bar when sixty-nine years of age. He worked with this apparatus for about ten minutes a day, and walked from four to six miles a day regularly for a period of three years. At the end of this time he found that his weight had increased from 140 to 160 pounds, his chest measurement had increased from 36 to 40 inches, and all the other muscles of the body proportionately; and at eighty-three years of age he could walk or run almost as easily and with apparently the same elasticity as fifty or sixty years before."
Another striking illustration of the value of muscular exercise is Edward Payson Weston. In childhood he was weak and sickly, and, as he grew older he displayed no particular athletic ability and had little endurance. At about eighteen years of age fear for his health caused him to turn to walking. A few years ago, at seventy-two years of age, he walked from New York to San Francisco, a distance of four thousand miles, in a few hours more than one hundred and four days. Not satisfied with this record, he walked back from Los Angeles to New York, about three thousand six hundred miles, in ninety-five days.
One of the valuable contributions of the entry of the United States into the present war is the discovery of the value of strenuous exercise. The young men come home on their furloughs so completely made over as to be hardly recognized by their friends or themselves. Thin boys have taken on the right amount of flesh, and the fat have be-come normal. And with it all has come a hardening of the muscles that has altered their feelings, thoughts, and acts. They have gained a little sweetness and some light.
Physical exercise as a means of acquiring resistance to fatigue has received too little attention, and the value of medicine has been overestimated. "It is well known that a man `in training' has greater endurance than one who attempts exertion without previous systematic exercise or training. In general, it may be said that a person in the `pink of condition' is fit not only for physical but also for mental exertion. The great majority of adults are far from being `in condition,' suffering either from lack of exercise or from too much exercise. The ordinary man errs either in one direction or the other. The brain worker lives too sedentary a life, while the manual worker, through fatigue caused by long hours, is in a continual state of over-exertion. Could these conditions be remedied, endurance, as measured by capacity to withstand prolonged strains, might be greatly increased.
"Experiments have shown that physical endurance can be doubled by dietetic causes alone, or doubled by exercise alone. By both together it is not unlikely that it could be tripled or quadrupled. But when it is said that the endurance, or capacity for exertion, of the ordinary healthy man could be thus multiplied, it is not meant that the hours of his daily work, or even his daily output of work, could be increased in such a ratio. What it does mean is the removal of the fatigue limit, a freer and more buoyant life, and a visible increase in the quantity and quality of work per hour. In an ideal life fatigue would be seldom experienced. But in most lives, unfortunately, fatigue is a daily experience."
Probably few men actively engaged in earning their living know the feeling of this buoyant life. The stress of business and of professional problems to be solved, and the worries of making ends meet, keep them at best balancing on the edge of fatigue. They are unacquainted with the freedom that clears away the mental haze and gives constructive visions and insights which cannot be obtained when the fatigue limit is approached. They are content with living in the conventional state of "good health."
The difference between being "well" and thoroughly "fit" has been splendidly characterized by Fisher. "When a person is free from all specific ailments, both serious and minor, he usually calls himself `well.' There is, however, a vast difference between such a `well' man and one in ideally robust health. The difference is one of endurance or susceptibility to fatigue. Many `well' men cannot run a block for a street-car or climb more than one flight of stairs without feeling completely tired out, while another `well' man will run twenty-five miles or climb the Matter-horn from pure love of sport. The Swiss guides, through-out the summer season, day after day, spend their entire time in climbing. A Chinese cooly will run for hours at a stretch. That the world regards such performances as `marvellous feats of endurance' only shows how marvellously out of training the world as a whole really is. In mental work some persons are unable to apply themselves more than an hour at a time, while others, like Humboldt or Mommsen, can work almost continuously through fifteen hours of the day."
The. source of muscular energy is, of course, important in connection with ability to work and to avoid fatigue. The evidence here is quite clear. Fisher and Fisk report' an experiment made to test the popular idea that meat is especially "strengthening." "The experiments consisted of endurance tests made on forty-nine persons representing the two types of dietary habits. . . The experiments furnished a severe test of the claims of the flesh abstainers. Two comparisons were planned, one between flesh-eating athletes and flesh-abstaining athletes, and the other between flesh-eating athletes and flesh-abstaining sedentary workers. The results would indicate that the users of low protein and non-flesh dietaries have far greater endurance than those who are accustomed to the ordinary American diet. . . . Similar results have been found in other investigations. It is probable that the inferiority of meat-eaters in staying power is due primarily to high protein and not to meat per se."
Experiments on vegetarians and meat-eaters in the University of Brussels gave similar results. The tests were largely comparisons of strength and endurance. As regards strength, little difference was discovered between vegetarians and " carnivores." In endurance, however, a striking difference was found, the vegetarians surpassing the carnivores from 50 to 200 per cent.
Though meat does not give the strength and endurance that it has been thought to furnish, it is doubtful whether its sudden exclusion from man's dietary is desirable. As Pavlov has shown, meat is one of the most peptogenic foods. It seems to stimulate the stomach, and while this excitation may be to a certain extent artificial, some stomachs seem to require it at least occasionally. Perhaps the safer course is to indulge when the craving is felt. The mistake is in regarding it as an essential article of daily diet.
Chittenden believes that the waste products of foods containing an excessive quantity of protein, conspicuously meat, is one of the causes of fatigue. His dietetic tests made on men in various occupations are remarkably suggestive. "Nowhere in the literature of nutrition do we find an experiment so painstaking and accurate, covering so long a period, with a diet of so low a protein content following a normal diet," says Benedict.' "But," continues the same writer, "it has not been incontrovertibly proven that such a diet could be advantageously adopted for all time, nor that the apparent improvements were due entirely to the low protein. Dietary studies all over the world show that in communities where productive power, enterprise, and civilization are at their highest, man has instinctively and independently selected liberal rather than small quantities of protein." Lusk also takes a similar view. He quotes Rubner approvingly, who, he says, "believes that there should always be an excess of protein constructive material, so that if after physical exhaustion there is depletion of the glycogen reserves, under which circumstances the wear and tear on the cell protein is increased, there may be building units in reserve to quickly restore the tissue destroyed." This question of diet, however, seems not yet closed.
The evidence indicates that, while muscular work can be done at the expense of any of the foodstuffs, carbohydrates are the most available source of muscular energy, and the brain requires the same food as the other cells of the body. Carbohydrates include starches and sugars. Each plant has its own kind of starch, and the grains and potatoes are starchy foods. Protein, found in lean meat, fish, milk, eggs, and cheese, supplies building material. Corn and the cereal grains, with the exception of rice, also contain protein matter. It would be possible to eat nothing but protein, but the system would be overloaded with protein waste. Enough protein should be eaten for building material, and carbohydrates and fats should be taken for energy foods. Fats produce more than twice as much heat as the carbohydrates, so the Eskimo and others who live in cold climates eat chiefly protein and animal fat. Those in warm climates, on the other hand, live largely on carbohydrates, vegetable proteins, and fruits. Theoretically, either one of the energy foods with protein would sustain life, but for practical reasons, such as digestion, it is better to take both carbohydrates and fats. Fat is also valuable as a food, because man is more prone to certain diseases, among them tuberculosis, if the amount of fat in the diet is too limited.
The weight of opinion seems to be increasingly favorable to protein reduction, but the purposes of nutrition must not be forgotten. These purposes Lusk has put in a striking way. "The workshops of life," he says, "require fuel to maintain them, and a necessary function of nutrition is to furnish fuel to the organism that the motions of life continue. Furthermore, the workshops of life are in a constant state of partial breaking-down, and materials must be furnished to repair the worn-out parts. In the fuel factor and the repair factor lie the essence of the science of nutrition."l But fuel and repair do not exhaust the subject of nutrition.
"The statement that a diet composed of the five classes of foodstuffs—proteins, fats, carbohydrates, salt, and water —is all that is necessary for the maintenance of life is not strictly true. . . . It seems that in addition to the five classes of foodstuffs, minimal quantities of certain other substances are necessary in order that the processes of life may proceed normally. How these substances act we do not know, but we must imagine that they have a drug-like effect on some organs of the body, and take the place of or give rise to some of the hormones which are essential for the orderly working of the different organs of the body." hese vitamines, as they are called, are present in the juices of fruits, fresh vegetables, eggs, peas, beans, milk, and in the outer layer of grains. Since they are destroyed by excessive heat, the best sources from which to obtain them are uncooked vegetables and fruits.
"Men and horses work best when they are well fed, and feed best when they are well worked. Work creates a craving for food, and thus assists digestion and absorption. Here is a natural stimulus to the appetite, more potent than any drug; it is known to some, but many will not be cured of their ailments by muscular work; they seek some miracle-working drug, or the waters of a fashionable health-resort, where they are unconsciously made to take exercise and lead a more natural life."
We have been considering some of the conditions of physical and mental vigor. Let us now turn to the effect of normally continuous work. At first one's power of work increases, then continues at its maximum, and if the work is continued one's efficiency gradually diminishes. Sometimes the efficiency may be maintained beyond the normal period by increasing the stimulus. We may drive ourselves in one way or another. But even under stimulus the time finally comes when we are no longer able to keep at a high level of production. The first delay is observable in the time required to get "warmed up" to the work. This preliminary warming up is noticed alike in physical and mental work. It is well known "that at the commencement of exercise or muscular work there is often absent even in the trained man that smoothness in the coordination of the muscular contractions, respiration, and circulation, which comes after a short time, and is expressed in the colloquial language of the laborer as `getting into the swing of the work.'"
Practice and skill also reduce fatigue in intellectual activities. Mental work of a very simple nature frequently produces exhaustion in those unaccustomed to using their minds in such a manner. MacCauley, who investigated the Seminole Indians of Florida,' has given some interesting information regarding the mental fatigue of these people. It should be noted, first of all, that in MacCauley's opinion the Seminoles rank in the first class of American aborigines. "They seem to be mentally active. . . . In their intercourse with one another they are, as a rule, voluble, vivacious, showing the possession of relatively active brains and mental fertility. Certainly, most of the Seminoles I met cannot justly be called either stupid or intellectually sluggish." To test the effect of continuous moderate mental activity on people unaccustomed to it, one of the men was kept busy for the greater part of the day answering questions. "Occupying our time with inquiries not very interesting to him, about the language and life of his people, I could see how much I wearied him. Often I found by his answers that his brain was to a degree paralyzed by the long-continued tension to which it was subjected."
Mosso, with the effect of practice and skill in mind, investigated the fatigue of soldiers who were learning to read and write. The result of the study of these men was of great importance to them because moderate success shortened the time of military service. One of the officers to whom Mosso wrote for information replied: "At the class examination at which soldiers have to give proof that they are not illiterate in order to obtain their discharge, I have often seen great, strong men perspire until drops of sweat fell upon the paper. At Lecco I saw one faint during the examination, then, feeling better, demand another trial; but on the threshold, at sight of paper and book, he turned pale and fell into a fresh faint."
Activity, even of a practised and habitual sort, however, tends to produce fatigue, when the work is done under unfavorable conditions. Walking is automatic, but if shoes or boots are uncomfortable, effort to avoid the pain brings into action muscles commonly unused. "Work which is performed painfully by man or beast is uneconomical. The truth of these statements has been fully demonstrated by the successes and failures of forced marches. Pain is beneficent; it is a warning, a natural safeguard, an incentive to rest; the sensations of pain may be neglected, or may be deadened by drugs, and work can be performed, but it is extravagant work, and the penalty has to be paid sooner or later."
"Fatigue is also accompanied by an extravagant metabolism; from this cause the output of carbon dioxide may be increased even as much as 21 per cent. An abnormal rise in the temperature of the body is produced by excessive and prolonged work under unfavorable conditions, and apparently causes a further extravagant combustion."
The amount of fatigue, however, bears no direct relation to the work done. Much depends upon the condition of the muscle. It was shown in Mosso's laboratory that a fatigued muscle suffers more harm from a light task than an unfatigued one from much more severe work, and, in the unfatigued muscle at least, the expenditure of energy during the first half of a period of work is considerably less than during the second half. Vigorous, unfatigued muscle has a certain amount of available energy which may be drawn upon, but a fatigued muscle uses energy needed for other purposes. The difference is much like using one's bank-account instead of withdrawing money from one's business. But, in addition to the muscle exhausting its re-serve of energy when work is continued beyond the fatigue-point, the nervous system performs its part with a more intense and wasteful expenditure. Under these conditions, however, nervous energy is not merely squandered. The irritation that accompanies fatigue leads to a breaking-down of nervous structures. The energy utilized in the contraction of muscle is derived from chemical changes occurring in the muscle, and muscular exercise increases the intake of oxygen as well as the output of carbon dioxide. Since no correspondingly significant changes are observed in the nitrogenous metabolism, it appears that the energy for muscular contraction is derived largely from the oxidation of carbohydrates. "In excessive work, as opposed to normal vigorous work, there is evidence of a certain amount of nitrogenous breakdown of the structure itself." This is due to the wear and tear of the machinery. Overwork tends to destroy muscle-substance.
We said that the expenditure of energy during the first half of a period of work is less than during the second half. This is true even when more work is done during the first part of the period than during the second half. Mosso found in testing muscles that, if the work is reduced by one-half, the time needed for rest is only a quarter of the time required for recovery when the full amount of work is done. Further: "From this experiment . . . ," he says, "it appears that if the energy of the muscle is not completely exhausted, that is to say, if the final contractions are not made, the fatigue is much less, and the muscle is able to perform more than double the amount of mechanical work which it would do if it worked to the point of exhaustion with the most favorable conditions for repose." Probably this is approximately true of the mind as well as of muscles. That is, if work is not carried to the point of exhaustion, the necessary periods of rest are shortened, and the amount of work finally accomplished is much greater than when the work is carried to the point of extreme fatigue, with longer periods of rest under exceptionally favorable conditions for repose. Wimms, for instance, found that short rest-pauses are more favorable in hard mental tasks than in easier ones.
Vigorous, unfatigued muscle is indifferent to the work that it does, provided the work is within the limit of its capacity. When, on the other hand, the energy of a muscle is depleted, release from even a small part of the work is a relief. Amateur mountain-climbers well know the anguish of the last half-hour, though the preceding four hours, requiring perhaps more labor per mile, brought only joy. This is also true of the mind. Experience shows that a robust mind is dauntless. But let one's energy be-come diminished, and escape from small matters seems like release from an unbearable burden. In other words, the intensity of fatigue bears no definite relation to the amount of work done.
We have spoken of the striking effect of very slight additions to the stimulus when fatigued, and reference was made to the strain of the last part of a mountain-climb. On the mental side this is especially noticeable when one is recovering from sickness. Under these conditions even conversation is fatiguing. Cerebral activity decreases, and one is slow to get the meaning of what one reads or hears. But irritability increases. Sounds and noises ordinarily unnoticed are annoying. Francis Galton relates an interesting experience of his own.
"A few years ago I foolishly overworked myself, as many others have done, misled by a perverted instinct, which goaded to increased exertion instead of dictating rest. The consequence was that I fairly broke down, and could not for some days even look at a book or any sort of writing. I went abroad, and, though I grew much bet-ter and could amuse myself with books, the first town where I experienced real repose was Rome. There was no doubt of the influence of the place—it was strongly marked; and for a long time I sought in vain for the reason of it. At length, what I accept as a full and adequate ex-planation occurred to me—simply, that there were no advertisements on the walls. There was a picturesqueness and grandeur in its streets which sufficed to fill the mind, and there were no petty distractions to fret a wearied eye and brain."
There are several causes for the lack of correlation between fatigue and the quantity of work done. First of all, as has already been said, comes the effect of the emotions. It makes an immense difference whether a man enjoys his work or not. "A man with no interest is rapidly fagged. Prisoners are well nourished and cared for, but they can-not perform the task of an ill-fed and ill-housed laborer. Whenever they are forced to do more than their usual small amount they show all the symptoms of being over-taxed and sicken. An army in retreat suffers in every way, while one in the advance, being full of hope, may perform prodigious feats."
The effect of the emotions upon fatigue shows that the vocation which a young man selects means much more than the financial returns. His ability to put his work through depends upon his working capacity, and the latter draws its energy in no small degree from his enjoyment of what he is doing. In the matter of exercise, again, the value is not gained merely from using the muscles, but the service is greatly enhanced by the bodily thrills that come only with zest for the activity. Just as food should be palatable to serve best its purposes, so exercise needs to be enjoyable.
We have said that fatigue dulls comprehension. It also weakens the memory. The writer has observed that his memory of facts and names deteriorates markedly after a mountain-climb of four or five hours' duration. Associations will not respond under the influence of fatigue that comes over an amateur who must break in his climbing-muscles each summer. Mosso quotes alpinists as saying that the events of the last part of an ascent are least distinctly remembered, and one "was obliged to take notes during an ascent, because on his return in the evening he remembered almost nothing." This probably refers to a condition of complete exhaustion. At any rate the writer has not observed it in himself, not even after a severe six hours' ascent of Long's Peak. Memory was not normal, but its deficiency lay rather in failure to recall the details of what had been read or heard previous to making the ascent.
The effect of fatigue upon memory raises the larger question of the relation between different sorts of fatigue. The belief is quite general that, when one is mentally tired, change of occupation gives sufficient rest. This is probably true only when fatigue is not fatigue, but ennui. Work becomes monotonous and we think that we are tired. Indolence is often taken for fatigue. In such cases change of occupation is refreshing. Even another book treating of a different subject brightens the mind. Mere change, with its new sights, and sounds, and people, breaks the dead level of monotony. One grows tired of seeing the same persons and hearing them say the same things. So one finds that "fatigue" vanishes in another town or city, though the conditions for work may be no better in other respects. For this reason it is mental economy occasion-ally to pack one's grip and begin anew in another place. Change of scenery clears the mind. The writer invariably finds that the "fatigue" of the year disappears and that work can be continued with renewed vigor when vacation permits a change of habitat. Sometimes, again, one has sat so long that the muscles are tired. Exercise or a brisk walk in the open is then a relief. Moderate muscular exercise is also stimulating. The abundant oxygen of the fresher air also plays its part..
The work-cure for those with whom fatigue is chronic is closely related to this matter of change. The "invalid" who waits till he feels able to work will never begin. The writer has known of young women afflicted with chronic invalidism who were cured by activity. In several cases it was social-settlement work, in which they were so busy and stood on their feet so long that strong men would have been exhausted. Yet at night they were only pleasantly tired. One young woman who could not walk half a mile without exhaustion and whom the slightest exertion sent to bed, became so interested in a secretaryship which she secured that the day's work brought no fatigue, though typewriting is not a restful occupation. A part of the value of the "work-cure" consists, of course, in keeping the thoughts from one's "afflictions" and in dissipating worry; and worry is the most efficient promoter of fatigue. Work or activity of any sort, in which one finds pleasure and success, is almost certain to be unattended by fatigue. The usual periods of rest and vacation will be sufficient to meet the demands for recuperation.
But none of these states is fatigue in the proper sense. When one is tired from concentrated attention and thought, change of occupation is likely to augment fatigue. Intellectual activity, to be worth while, should be intense. Nerve-centres are then exhausted, and for this reason the muscles themselves are weakened. Maggiora tested the endurance of the middle finger of his left hand in Mosso's laboratory. In the morning tests his finger contracted fifty-three times before temporary exhaustion. In the afternoon he examined twelve students—candidates for the doctor's degree—during three and one-half hours. Then he again tested the endurance of his finger and found that it was exhausted by twelve contractions. Even after two hours of complete repose it had not regained its strength of the morning. A tired brain evidently means a tired body.
This relation between brain and body in the matter of fatigue is especially important in connection with school work. First of all, it should be said that children rarely study hard enough to become fatigued. With them what is called fatigue is more commonly languor arising from lack of interest—or it is the outgrowth of nervous excitement resulting from the confusion of commands and prohibitions. At home and in school, at different times, children are forbidden and permitted to do the same thing, according to the momentary mood of those over them, until rules of action become chaotic. Again, "dangerous fatigue is the result of unhealthy confinement within doors, or is owing to unwholesome shocks, and puzzlings, and confusions, and conflicts of impulses resulting from the imposition of scatter-brain notions of teaching and discipline, imposed much too fast for the child to grow to, or even to comprehend. . . . Other sources of dangerous fatigue are overstimulated ambitions or disproportionate pressure and rivalries, instigated by home or class coin panonis.
The "dangerous fatigue" from unhealthy confinement within do ors, to which reference has just been made, is worth a moment's digression. One of the problems of fatigue is how to escape it without unnecessary reduction of work, and for children, at least, the open-air schools seem to have given the solution. They remove the fatigue-limit to the evening and night, where it belongs. "Observations have shown that the pupils in outdoor and open-window schools are not only kept more healthy, but learn more quickly than those in the ordinary school"; and their work is also more accurate. Recent investigation indicates that the harmfulness of confined and respired air is not due so much to its chemical components as to its physical features. Confined air is too warm, too dry, and too still. But whether the effect of confined air is due to its physical or chemical condition, there is no doubt of its influence in producing the feelings of fatigue, if not fatigue itself; and the greater wear and tear of work done under unfavorable atmospheric conditions promotes derangement of nervous functions and deterioration of tissues.
Let us turn now to one or two of the latest investigations of fatigue in school children. The most recent extensive test was made by Heck. This investigation is of unusual significance because of the large number and variety of children included, and because of the natural conditions under which the tests were given. Over ',too children in four New York City schools, "representing different nationalities and different grades of social. and hygienic opportunities" were tested under the usual school-room organization. The work was given at different times of the day—shortly after nine and eleven o'clock in the morning, and at one o'clock and half after two in the afternoon. The material was a modified form of one of Courtis' Standard Tests in arithmetic. The results showed that "mental fatigue in relation to the daily school program is far less than generally believed.... The small amount of fatigue noticeable during the school-day was more probably caused by improper conditions of ventilation, lighting, etc., than by the school work itself. Unhygienic conditions in the school and physical defects, how-ever slight, in the children are undoubtedly the great causes of fatigue in most schools. The decrease in quality of work of children as the day advances, supposed to be more or less general in schools, is due less to exhaustion of the energy-producing material of the nerve-cells of the body, and to autopoisoning of the nervous system by waste products from this process, than to a loss of interest in school work, with its lack of vital and varied appeal, and its monotony of instruction and environment. The bored child, unconsciously or consciously, rebels and does a less correct amount of work. Continued work produces boredom and continued boredom decreases efficiency, on account of the close mutual relation between physiological and mental attitudes. With sound bodies, a hygienic school, proper classification, frequent relaxation, a vital and varied curriculum, and live teachers, most children will show no problem of fatigue in relation to the daily school programme."
Heck also tested 573 children in the Lynchburg, Virginia, schools. As before, two periods were taken in the morning and two in the afternoon. The length of the test was increased from ten to twenty-five minutes to determine whether a longer period would reveal fatigue. The results strengthened the former conclusions.
Still another test was made by Heck, this time with 467 boys and girls in the Roanoke, Virginia, schools. "The final conclusion to be drawn from this experiment in Roanoke with reasoning-tests in arithmetic, as well as from those in Lynchburg and New York' with the fundamental operations, is that normal, healthy children in the grammar grades, in a hygienic school environment, can Meet the requirements of the usual daily school programme without injury to themselves or their work."
Short tests of ten minutes and also those involving continuous effort extending through an hour or more have been made recently in the grades of the Winthrop Training School. The short tests consisted of examples in addition and subtraction, while the longer ones were drawn from algebra, history, and Latin. The results indicate that "in general there is more weariness than fatigue in the children; and mental activities are more necessary than complete idleness. Here, then, is the teacher's problem: How to provide the proper mental stimulations for keeping up the interest in school. . . . If a boy sees no purpose in learning he will not learn."
"A fair claim to make on the basis of the results obtained," says Thorndike, in summing up his investigation of fatigue in school children, "is that a regular day's work in the grammar school does not decrease the ability of the child to do mental work. . . The chief responsibility for mental exhaustion in scholars falls, I should be inclined to think, not on a Creator who made our minds so that work hurts them, nor on the public opinion which demands that children shall do a given amount of work, but upon the unwise choice of material for study, the unwise direction of effort, the unwise inhibition of pleasurable activities, the unwise abuse of sense-organs, and unattractiveness of teachers and teaching."
We have quoted from these experiments in some detail because the information is needed as an antidote to the sentimentality regarding fatigue. Maudlin emotions threaten to deprive children of the advantages of a busy, thoughtful life during a small part of the day. As a matter of fact, if children are as fresh after two hours in school as when they entered, they have missed something worth while. Moderate fatigue, if caused by work and not by foul air or nervous irritation, is not bad, provided school is forgotten on the playground and sleep is abundant and undisturbed. Probably one of the chief causes of injurious fatigue in children is the constant nagging to which they are so commonly subjected. A shrill, penetrating voice, exploding with "don'ts," has little efficiency beyond nervous irritation.
Experiments on fatigue presuppose that it will always reveal itself in the quantity or quality of work accomplished. Yet there are two physiological facts which seem to deny this assumption. And this contribution from physiology has not received sufficient attention from investigators of mental fatigue. The first of these two facts is the stimulating effect of small quantities of fatigue-substances.
"If present in small quantity, or moderate quantity, for a brief time, each substance," says Lee, "causes an augmentation of activity of the muscle, which is characterized by an increase in irritability and working power, an increase in the height to which the load is lifted, and an increase in the total amount of work performed." ' And again, as Bayliss says: "It appears that the presence of a small quantity of products of activity is favorable." This has been demonstrated only for muscle, but the assumption that it is also true of mental activity is a possibility which cannot, at any rate, be denied. If this stimulating effect of a moderate amount of fatigue-substances shall be found true also of mental activity, we may then expect, for a brief time, an improvement in the quantity and quality of work; and one investigator' thinks that there is such a period when fatigue acts as a stimulant in mental as well as in muscular work.
Let us now turn to the second physiological fact to which reference has been made. Gruber fatigued a muscle with an hour's work and then allowed it to rest for an hour and a half. The result of this rest was a gradual but steady recovery of vigor. Then adrenalin was injected, and the effect, five minutes after the injection, was a further, but at the same time abrupt, recovery of 61 per cent. Subsequent rest for an hour and a half produced no further recovery. In another experiment the effect of adrenalin, after an hour's work, was substantially the same, showing a recovery of 62 per cent. Gruber proved that the effect of adrenalin is a counteraction of fatigue by determining the threshold stimulus for muscle and nerve-muscle in non-fatigued animals before and after injecting adrenalin. He found that in the case of non-fatigued muscle there was no lowering of the threshold, "a result in marked contrast with the pronounced and prompt lowering in fatigued muscle by this agent."
"It is quite conclusive," says Gruber, "that adrenalin, in some way, causes a rapid recovery of normal irritability of muscle after fatigue. The question whether this is done by neutralizing, transforming, or destroying the fatigue toxins is still obscure. That the action may be on the muscle itself has been definitely shown in this" [Gruber's] "paper; its effect, however, upon the nervous elements or on the region of the neuromuscular union cannot be denied. . . . Adrenalin acts quickly, requiring five minutes or less to produce its effect on the threshold" [of fatigue]. "In that length of time, in some cases, it reduces the threshold to normal, whereas rest would require fifteen minutes to two hours." '
The experiments just quoted show that adrenalin injected into fatigued muscle has a remarkable recuperative effect. The inquiry arises at once then, Is there any arrangement in the body by which adrenalin is supplied and made to serve the same purpose during life? Experiments have answered this question in the affirmative. The suprarenal capsules are glands, situated above the kidneys, which secrete a substance, to which the name adrenalin has been given. This secretion passes into the blood. investigations have demonstrated that artificial stimulation of the splanchnic nerves increases this secretory activity, and as a result the adrenalin in the blood is increased. Here, then, is a mechanism by which the adrenal glands can be made to discharge their secretion into the blood.
Several years ago, it was observed' that, with artificial stimulation of the splanchnic nerve, a muscle did "for a short period 8o per cent more work than before splanchnic stimulation, and for a considerably longer period exhibited an intermediate betterment of its efficiency." At that time a considerable part of this improvement was ascribed to the increased blood-flow resulting from splanchnic stimulation. The investigators raised the question, however, as to whether this explanation was sufficient. Later investigation verified this effect of increased blood-flow through excitation of the splanchnic nerve, but it proved, in addition, that the recovery of muscle and its strengthened action was due in part to a specific action of adrenalin itself. Splanchnic stimulation is thus seen to promote recovery of muscle from fatigue and increase its action in two ways, first, by increasing the arterial blood-pressure and so cleansing the working muscles with fresh blood, and second, by liberating adrenalin, which acts specifically upon muscles, restoring their working power.
How, then, does this work out in life? It is clear, as Cannon has indicated, that increased arterial pressure would be highly serviceable to animals in times of stress. It would clear away the waste and fatigue products. Now adrenalin, secreted by the adrenal glands and passed into the blood, does this and more. "The heart, the lungs, and the brain, as well as the skeletal muscles, are in times of excitement abundantly supplied with blood taken from organs of less importance in critical moments." It also tones up the muscles that have become fatigued through continued activity. Emotional excitement, of course, always accompanies stress and danger. Consequently, the relation of excitement to adrenal secretion is important.
Adrenal secretion is known to be increased during strong emotions. This has clearly been demonstrated in a cat placed near a barking dog, and Elliott' observed that no greater excitement is needed in animals than the strangeness of new quarters to induce a greater discharge of adrenalin into the blood.
Have we not here, then, a possible explanation for the failure to observe fatigue in school children? The youngsters know that they are to be tested for something. The conditions of class routine are changed. Perhaps a stranger comes into the school to give the tests. The situation is surely as "exciting" as the new quarters for the animals that Elliott tested. One who has given unusual tests of any sort knows well how alert and animated the children become. It is also a matter of common experience that the fatigue of adults often vanishes in even mild excitement. We know that under excitement the adrenal glands secrete and as a result physical fatigue disappears. To be sure, this has been proven only for muscular fatigue, but as Gruber has said: "Its effect upon nervous elements . . . cannot be denied." If the blood circulation of the brain is controlled by the autonomic system, and there is some evidence for this, then the tonic effect of adrenalin, already demonstrated in muscular fatigue, is also operative in mental activity. Further, there is no doubt that the increased arterial pressure flushes the nerve-cells of the higher centres as well as the muscular tissue, removing the fatigue products, and this is itself invigorating.
There are, then, two physiological reasons why fatigue in its early stages may not be discovered by the tests usually given; the first is that fatigue substances in small quantities are stimulating, and the second is that adrenalin secreted under excitement and passed into the blood dissipates fatigue by increasing arterial pressure and by acting directly upon muscles, and perhaps upon nervous elements. Thus the condition that is sought disappears by reason of its very presence or because of the animation aroused by the attempts to find it. One may seriously doubt, however, whether fatigue is present in many cases. Children fortunately are endowed with an indifference to the demands of their teachers. If they did all that they are told to do and did it as well as they are told they should, every school must needs be equipped with an adrenalin laboratory—like Weichardt's antifatigue-toxin factory. Happily, native indolence comes to the aid of children, and they refuse to be overworked. Undoubtedly, they could do much more without fatigue than they actually accomplish if the things at which they are set appealed to them as worth while. Subjects of study should not be made easy, but their value and significance should be evident to those working in them. And, after all, this is a very human demand.
As one looks through the voluminous literature on fatigue' one is impressed by the fact that "fatigue," as ordinarily investigated and measured, is exceedingly complex and that many times it is not fatigue that is tested but inattention and the inability to ignore sensations, feelings, and thoughts of one sort or another which have no definite relation to fatigue. Sleepiness, discomfort from the hard, straight-backed chair, temporary ennui for the task in hand, and staleness on account of the ennui, as well as thoughts of pleasanter activities, are a few illustrations from the many that might be mentioned. " I was constantly surprised," says Thorndike, in his analysis of this condition, "to find myself when feeling, as I would certainly have said, `mentally tired,' unable to demonstrate in the feeling anything more than emotional repugnance to the idea of doing mental work. On at least half the occasions this seemed to be all there was."
The feeling of lassitude, again, is quite commonly the result of lack of physical exercise. It has been observed that adults who engage in vigorous out-of-door work or sports, without overdoing, require less sleep and accomplish more mental work without fatigue. The sensations of strain and the feeling of effort may be due, also, to the disagreeable monotony of the task. As a relief from ennui, mere change is recuperative, and this is probably another reason why experiments and tests so frequently reveal little fatigue. The student may be weary from his previous work, but not fatigued.
Curves showing the progress of work, aside from the practice effect, are curves of a good many more things than fatigue. Pleasure and displeasure are important factors in postponing or hastening fatigue. Wright noticed in his investigations that "the fatigue accompanying work is not so great when the person is working under the direct stimulus of a definite aim, notwithstanding the fact that he has at the same time produced an increase in the amount of work." The aim gives point and zest to what would otherwise be a disagreeable task, and it prevents the sensations, feelings, and thoughts mentioned above from arising in the mind. This is always the effect of a purpose in which one is interested. Aimless work is soon reduced to drudgery, and few activities are more fatiguing. The more immediate and direct the aim and the more it concerns the present interests of the worker, the less likely is the mental condition commonly regarded as fatigue to appear. Since we may suppose that toxic products are always produced by continuous physical or mental activity, the difference in the effect of pleasant and dreary work would seem to lie in the rapidity with which they are disposed of or eliminated. It is not unlikely that the freer blood-circulation and the buoyant feeling attending the exhilaration of plea-sure carries away these toxic products more rapidly than in the more sluggish condition of ennui. At all events, it is clear that disagreeable, monotonous work fatigues and wears one out more quickly than pleasant occupations. Perhaps this is one reason for the prodigious and at the same time unimpairing work of von Humboldt, Mommsen, and Edison. Such men are fortunate enough to have found work in which they could engage with unmitigated joy.