FATIGUE, ffi-tg' (from Lat. fatigare, to fat igue; probably connected with of - fat MI, enough). Fatigue usually follows long-sustained appliontion, whether of mind or hotly. The dis tinction is often drawn between physical or bod ily fatigue and mental fatigue. There is, how ever, a common element in the two. The mental experience whieh we call exhaustion or weari ness is of one and the same kind, whether caused by prolonged muscular work or by sustained thinking. But it is customary to include in physical fatigue llie all ered condition of the muscles which renders them, for the time heinc. unlit for use. 'Finis we say that our arm or body is tired, or that we are 'tired all over.' Sim ilarly there is sometimes included in mental fa tigue the laxness of 'mental endeavor' and the increased inattention which follow upon lmrd study or the contention of diverse motives. These concomitants should, however, be kept distinct from the real experience of fatigue.
There is some disagreement among psycholo gists in regard to the ultimate analysis of fatigue. it is argued, on the one haul, that fa tigue is a complex of more or less intensive or ganic sensations (q.v.), usually toned with pleas antness or unpleasantness, and even passing over into pain where the limit of endurance is approxi mated; while. on the other hand, it is urged that fatigue is as simple and unanalyzable as is the sensation of red or the tone of a tuning-fork. The truth seems to lie between the two views. As a rule many processes cooperate in the pro duction of fatigue; but the experience itself is a fusion (q.v.), so close that it is impossible by introspection to wrench the constituent elements out of their union.
The experimental study of fatigue and of its effects, mental and physical, has been carried on with some success during the last twenty years by the continued efforts of physiology and psy chology. The first comprehensive work was done by H. Kronecker of Bern and A. Mosso of Turin. The starting-point was the observation of the course of exhaustion in the muscle taken from the frog's leg. When the nerve supplying the muscle was stimulated at successive intervals by an electric current it was found that the contract ility of the muscle suffered a constant decre ment each time that the current was passed; until, finally, no movement at all was produced.
If, however, the muscle were now thoroughly lathed with a weak saline solution, contract ility was restored. This circumstance has led to thus conclusion that at least part of the effect of fatigue lies in the accumulation of toxic ma terials, which prevent the contraction of the muscle. 'These materials are usually carried off in the blood; as is shown by the fact that the injection of blood from a fatigued into a normal animal gives rise in the latter to all the symp toms of exhaustion. An objective measurement of the effect of fatigue was made possible by Alosso's invention of the ergograph. an instru ment which records the amount of work that a muscle, or rather a restricted group of muscles, performs in lifting a known weight, or pulling against a coiled spring. 'Many modifications of the instrument are now in use.
Over and above the action of poisonous materials, muscular fatigue is undoubtedly con ditioned upon the exhaustion of the energy which is supplied by the blood in the form of nutriment. If the consumption of energy does not exceed the supply, and if the waste products are speedily re newed by the blood, fatigue of a muscle does not ensue. The heart, e.g. beats throughout the individual's lifetime without causing fatigue. Moreover, each muscle seems to have optimal eon ditions for work, which include an appropriate loading and a sufficient interval for rest between contractions. Whether the feeling of fatigue is aroused by the lack of nutriment in the muscle, or only by the poisonous waste products. is not definitely known. In any event. it is necessary to pass beyond the muscle, and to inquire into the immediate nervous conditions of fatigue. It is clear that a muscle could not feel fatigue, apart from a nervous system. The conducting nerve fibres do not appear to he exhausted by fatigue; for they will continue to functionate for hours after the muscles have refused to contract. But the ease is different with the nerve-eells of the brain. It has been found that these cells undergo serious alterations. both during artificial stimn lation by electricity and during muscular exer cise. l'nder exhaustion the cell-body becomes shriveled and the nucleus takes on a different appearance. It is fair to suppose that these changes are directly connected with the mental processes of fatigue.