FATIGUE. A term common both to physi ology and to psychology. In the former, fatigue may provisionally be defined as a cer tain change in function of muscle, nerve or gland resulting from continued stimulation. In psychology two usages of the term are to be distinguished; qualitatively, fatigue is that perceptual experience which we have when we "feel tired)); functionally, it is an explanatory term to account for the decrement in amount and quality of work done by an organism. Whether these three points of view are really three aspects of one and the same thing is a question which cannot now be satisfactorily answered. We shall do well, therefore, to re gard them separately and then correlate where, in the light of present knowledge, correlation is possible.
We begin with physiological fatigue. If an extirpated muscle is successively stimu lated at regular intervals by electric shocks, and the resulting contractions recorded graphi cally, then it appears that the contractions at first increase and then decrease more or less regularly in extent until ultimately they may cease entirely. A similar curve may be obtained from living muscle by means of the ergograph, an instrument in which the flexor muscle of the middle finger is made to lift successively a small weight. In both cases the gradual re duction in contractility is known as fatigue, and the complete loss as exhaustion. Both fatigue and exhaustion are, however, relative terms; if, for example, the interval between shocks is long enough then fatigue does not appear, and if after exhaustion stimulation ceases and the muscle is allowed to rest, then it regains, to some extent at least, its con tractility. The cause of muscular fatigue is twofold: In active muscle, certain "fatigue lactic acid, carbon dioxide and acid potassium phosphate, are produced which tend to inhibit contraction. Secondly, processes of dissimilation and assimilation are constantly going on, the fats and even the substance of the muscle itself are being consumed and re placed. When the processes of dissimilation are more rapid than those of assimilation, then the muscle must ultimately cease to function and exhaustion appears. An interval of rest, on the other hand, gives the blood time to re move the fatigue substances and the processes of assimilation to make repairs.
Nervous tissue seems for the most part to resist fatigue. It is true that the appearance of some nerve cells changes with continued activity; there is first an in.crease and then a decrease in size and color of cell and nucleus, but this change seems to have no functional significance. Nerve fibres, apparently, also resist fatigue, i.e., impulses will continue to pass over a nerve, that has been stimulated, for several hours. There is, however, a point in the neuromuscular chain known as the motor end-plate, the point where the motor nerve is attached to the muscle, that according to some investigators is more easily fatigued than the muscle itself. Structurally, this end-plate is a bed of granular protoplasm through which are distributed a number of small nerve fibrils, and it is supposed that the fatigue substances cause these fibrils to contract and thereby to block the conduction of the nervous impulse. It is also held by some physiologists that the synapse, the point of junction between two neurones, is fatigable; the matter is still de bated, however, and awaits further experi mentation.
We turn to the first of the two psychological points of view. Ordinarily, fatigue as a sensory experience or a is not distinguished from weariness, tiredness, jadedness, ennui and the like. There is, however, a typical fatigue which is not the same as the weariness of ennui, that is the mere disinclination to exert one's self, and which is simpler in character than the gen eral lassitude of physical weakness or of long continued exertion. This simple fatigue may easily be induced by the ergograph experiment or by alternately extending and flexing the weighted arm. Under these conditions fatigue appears first as a dragging pressure localized in the muscle and qualitatively identical with muscular sensation. Later it becomes a sense feeling usually but not always unpleasant, and at high intensities the sensory component is an achy pain. There can be little doubt of the correlation of this typical fatigue with the fatigue of physiology, but the specific nature of the correlation is, at present, unknown.