fatigue, in engineering, microscopic cracking of materials, especially metals, after repeated applications of stress. Fissures may be formed within pieces of metal during their manufacture when, while cooling from the molten state, they shrink and tensile stresses arise. Once a crack has started it spreads under repeated stress until the metal ruptures. Examples of fatigue are found in steel rails, beams, and girders. Metallic fatigue resulted in the catastrophes encountered by many of the Liberty ships built during World Wars I and II and the crashes of a number of the earliest jet aircraft constructed. Materials used in construction are tested for fatigue strength, or endurance limit, by being subjected mechanically to cyclic applications of stress. Steel parts are sometimes treated by shot blasting to increase their fatigue resistance.

fatigue, in physiology, inability to perform reasonable and necessary physical or mental activity. Muscle fatigue, for example, results when the contractile properties of muscle are reduced, and continued exertion is impossible unless the muscle is allowed to rest. In muscle tissue, the depletion of glycogen (stored glucose), a source of energy for muscle cells, and the accumulation of lactic acid, which is produced through the breakdown of glucose, was long thought to the cause of muscle fatigue, but it is now known that the lactic acid produced is used as an energy source as well. A new explanation of muscle fatigue suggests that it is related to the control of the flow of the calcium ions in muscle. The release of those ions causes muscle contraction, while their storage causes relaxation. After prolonged exercise, the channels that control calcium flow become leaky, diminishing the muscle cells ability to contract. In the normal body the damaged channels are repaired after a period of rest. 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 bipolar disorder. Thorough medical and psychiatric examination may be required.

chronic fatigue syndrome (CFS), collection of persistent, debilitating symptoms, the most notable of which is severe, lasting fatigue. In other countries it is known variously as myalgic encephalomyelitis, chronic fatigue and immune dysfunction syndrome, and postviral fatigue syndrome. It was first recognized as a syndrome in the 1860s by Dr. George Beard, who called it neurasthenia. He believed it to be a neurosis with a fatigue component. Definitions and theories of its cause have changed over the years; many cases have been misdiagnosed as imaginary because doctors could find no cause. In the mid-1980s it came to the public's attention, as affluent women in their thirties began to seek treatment. (For unknown reasons, more women than men seek treatment for the disease.)

Symptoms and Identification

As currently defined, chronic fatigue syndrome is the presence of severe, disabling fatigue lasting for six or more consecutive months. The fatigue is persistent or relapsing, and is new (i.e., not lifelong), not relieved by rest, not the result of ongoing exertion, and interferes with normal work, social, educational, or personal activities. Diagnosis also requires at least four of the following symptoms, each persistent or recurring and not present before the fatigue: impairment of short-term memory or concentration, sore throat, tender lymph nodes in the neck or axillary region, muscle pain, joint pain, headaches peculiar to the syndrome, unrefreshing sleep, and malaise of more than one day's duration following exertion. Chronic fatigue that does not meet all these criteria is termed "idiopathic fatigue."

The course of the disease varies. Many people first experience symptoms after a cold or bout of the flu. Most people reach a plateau early on; the symptoms come and go with varying severity afterward. Some experience complete remission; others have their symptoms indefinitely.

There are no specific diagnostic tests. Diagnosis must first rule out known causes of fatigue such as hypothyroidism, cancer, multiple sclerosis, and major depression with psychotic or melancholic features (e.g., schizophrenia, bipolar disorder). Chronic fatigue syndrome and nonpsychotic, nonmelancholic depression, however, are not mutually exclusive. Substance abuse and side effects of prescribed medications must also be eliminated as possible causes.

Cause and Treatment

There is no known single cause of CFS. Some authorities believe it is a condition shared by many different underlying diseases rather than an entity unto itself; others believe it is caused by a defect of the immune system. Hormonal deficits, low blood pressure, and viral infections have been studied as possible causes or contributors. The postulated causal link with Epstein-Barr virus hypothesized in the mid-1980s has been discounted.

There is no definitive treatment for CFS, although success has been reported anecdotally with antidepressants, antianxiety medications, antivirals, and immune boosters. Symptomatic treatment for the muscle and joint pains is helpful in some cases. Counseling and peer support groups help some patients cope with the frustrating nature of the disease.

Sudden debilitating fatigue of unknown cause. It may follow a nonspecific illness with mild fever, tender lymph nodes, sore throat, headaches, weakness, muscle and joint pain, and confusion or difficulty in concentrating. To meet the criteria of CFS, the syndrome must be new, with a definite point of onset, and must persist more than six months. Once dismissed as imaginary, CFS remains controversial. Many authorities question whether it is a distinct disorder, since there is considerable overlap with other conditions such as fibromyalgia and Gulf War syndrome. No diagnostic test for CFS exists. Although a number of theories about the cause of CFS have been advanced, none has been proved. No cure has been found, but most patients improve gradually.

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Weakened condition of metal parts of machines, vehicles, or structures caused by repeated stresses or loadings, ultimately resulting in fracture under a stress much weaker than that necessary to cause fracture in a single application. Fatigue-resistant metals have been developed and their performance improved by surface treatments, and fatigue stresses have been significantly reduced in aircraft and other applications by designing to avoid stress concentrations.

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In engineering, manifestation of progressive fracture in a solid under cyclic loading, as in the case of a metal strip that ruptures after repeated bending back and forth (see metal fatigue). Fatigue fracture begins with one or several cracks that spread in the course of repeated application of forces until complete rupture suddenly occurs when the small unaffected portion is too weak to sustain the load. Seealso ductility, testing machine.

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