FATIGUE This is the weariness resulting from muscular or functional activity. The vegetative activities of the body, such as the nutritive functions, reproductive activity, smooth muscle action, glandular activity, etc., are not subject to will power. These activities set their own pace and therefore are not subject to fatigue. On the other hand, the animal activities, including work and locomotion, are subject to the will and stimu lation and may thus be set to limits beyond the individual's own welfare. The receptors, neurones and muscles are the sites of fatigue.
(a) Receptor fatigue is fatigue of the end organs. We may particularly distinguish fatigue of the olfactory, auditory and optic end organs. Some odors affect the ability to distinguish similar or other odors, which must be considered a manifesta tion of fatigue. Auditory fatigue only affects the ability to hear sounds in higher register. Optic fatigue is manifested by a reduced visional acuity.
(b) Neuron fatigue occurs centrally, the axone being con sidered indefatigueable. Morphologic changes in the nerve cells and nucleus with a chromatolysis are observable, and physiologic changes, as shown by a rise in the threshold value between neurons, are seen in the spinal reflexes.
(c) Muscle fatigue may reside in the end plate or muscle fiber. The end plate may be fatigued while the muscle is still capable of contracting. The muscle contraction varies with constant or variable loads. With a constant load the con tractions will finally cease, while with a variable load they will go on indefinitely. The rate at which they contract is as im portant as the load in the production of fatigue. The fatigued muscle is less irritable to stimulation, but increased stimulation will produce an increase in the height of contraction. The reaction of the muscular protoplasm changes to acid from neu tral or alkaline, due to the formation of sarcolactic acid and monopotassium phosphate. The acid is dissipated by outward diffusion, or oxidation to carbon dioxid. As a consequence
lactates and lactic acid may appear in the blood and urine following fatigue. Muscular fatigue is due to the accumulation of these waste products in either the muscles or the blood. If in the muscle their accumulation may be due to the failure of the blood stream to remove them. The massage of a fatigued muscle assists in the removal of the fatigue products by the circulation. Recovery is also hastened by oxygen. The ac cumulation of fatigue products in the blood affects muscles other than the ones involved in the exercise, thus local fatigue becomes general.
Fatigue is temporarily overcome by excitement. In both laziness and fatigue work is done with a sense of effort, probably due to a rise in the threshold value, and to the accumulated waste products. When the threshold value of the neuro-mus cular junction rises as fatigue progresses, the innervation spreads to other groups of muscles in order that the work may be accomplished. This brings more nerves and muscles into play and increases the amount of metabolites in the circulation and hence produces a greater sense of fatigue. As fatigue appears the movements of the muscles become less exact and their co-ordination is less perfect. This has a bearing upon industrial accidents as shown in the following figures.
Fatigue also reduces the rapidity and accuracy of action and also diminishes the extent of mental control. There is also an increase in irritability and irascibility. Garrulity and uncontrollable laughter may appear to an extent which is almost hysterical. Normally fatigue is a protective sensation indi cating the need of rest. It reduces the general sensibility of the body.
Nervous fatigue, on the other hand, involves the activities of the central nervous system itself. It is of importance be cause the changing racial habits of our times require constant alertness, necessitating quick adjustments rather than much muscular work.