Mechanisms of fatigue.

Fatigue is a common experience of individuals suffering from chronic illnesses. Mechanisms responsible for fatigue may be central or peripheral. Central fatigue may result from proximal events and include the neuromuscular junction and are subdivided into spinal and supraspinal. Peripheral fatigue arises from the muscle and predominately involves muscle bioenergetics or excitation contraction. Fatigue from chronic illnesses can be caused by several mechanisms. Electromyelography, transcranial magnetic stimulation, magnetic resonance imaging, and spectroscopy are helpful tools to clarify the physiologic correlates of fatigue. This state-of-the-art review discusses these physiologic correlates of fatigue and the mechanisms described in a variety of chronic diseases.