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fatigue, in physiology, inability to perform reasonable and necessary physical or mental activity. When the metabolic reserves of the body are exhausted and the waste products increased, as for example after prolonged exertion, the body finds it difficult to continue its function and activity. The accumulation of lactic acid in muscle tissue and the depletion of glycogen (stored glucose) results in muscle fatigue. The contractile properties of muscle are reduced, and continued exertion is impossible unless the muscle is allowed to rest. In the normal body a period of rest permits redistribution of nutritive elements to the muscles and tissues and elimination of accumulated waste products; the body is then ready to resume activity. There are some persons in whom fatigue is a chronic state that does not necessarily result from activity or exertion. In some instances this abnormal fatigue may be associated with systemic disorders such as anemia, a deficiency of protein or oxygen in the blood, addiction to drugs, increased or decreased function of the endocrine glands, or kidney disease in which there is a large accumulation of waste products. If excessive fatigue occurs over a prolonged period, exhaustion (marked loss of vital and nervous power) may result. In most persons with chronic fatigue, however, the condition seems to be associated with manic-depressive disorder. Thorough medical and psychiatric examination may be required.
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