A perception of tiredness, often a physiological state in which muscles become fatigued by the LACTIC ACID accumulating in them as a result of their activity. Oxygen is required to remove lactic acid in the recovery phase of muscular contraction and, if the supply of oxygen is not plentiful enough, or cannot keep pace with the work the muscle is doing, then lactic acid accumulates and fatigue results. There is also a nervous element in muscular fatigue in that it is diminished by stimulation of the sympathetic nervous system. (See also MUSCLE.)

Chronic fatigue, unrelated to the above mechanism, is a symptom of some illnesses such as ANAEMIA, CHRONIC FATIGUE SYNDROME, HYPOTHYROIDISM, MONONUCLEOSIS, MOTOR NEURONE DISEASE, MYASTHENIA GRAVIS, MYALGIC ENCEPHALOMYELITIS (ME) and others. Some drugs may also induce a feeling of fatigue.