The word fatigue is used in everyday living to describe a range of afflictions, varying from a general state of lethargy to a specific work-induced burning sensation within one's muscles. Physiologically, "fatigue" describes the inability to continue functioning at the level of one's normal abilities due to an increased perception of effort. Fatigue is ubiquitous in everyday life, but usually becomes particularly noticeable during heavy exercise.
Fatigue has two known forms; one manifests as a local, muscle-specific incapacity to do work, and the other manifests as an overall, bodily or systemic, sense of energy deprivation. Due to these two divergent facets of fatigue symptoms, it has been proposed to look at the causes of fatigue from "central" and "peripheral" perspectives.
Fatigue can be dangerous when performing tasks that require constant concentration, such as driving a vehicle. When a person is sufficiently fatigued, he or she may experience microsleeps (loss of concentration). However, objective cognitive testing should be done to differentiate the neurocognitive deficits of brain disease from those attributable to tiredness.
The sense of fatigue is believed to originate in the reticular activating system of the lower brain. Musculoskeletal structures may have co-evolved with appropriate brain structures so that the complete unit functions together in a constructive and adaptive fashion. The entire systems of muscles, joints, and proprioceptive and kinesthetic functions plus parts of the brain evolve and function together in a unitary way).