The relationship between muscle pain and fatigue.

Pain and fatigue may occur together during sustained exhausting muscle contractions, particularly as the limit of endurance is approached, and both can restrict muscle performance. Patients with neuromuscular disorders may have chronic myofascial pain (e.g. fibromyalgia) or contraction-induced pain (e.g. in metabolic myopathies). In some patients these two types of pain may coexist and both may inhibit central motor drive during exercise. Little is known about the central motor adaptations that occur in patients with neuromuscular disorders and how the effects of pain are mediated. Transcranial magnetic brain stimulation has made it possible to investigate the changes in excitability of the central motor pathway during fatiguing muscle activity and have thrown light on the mechanisms of fatigue in normal subjects and individuals with chronic fatigue syndrome and multiple sclerosis, but there have been few studies in patients with neuromuscular disorders. Repetitive magnetic brain stimulation protocols can now be used to modulate the excitability of the motor system during exercise to delay the onset of peripheral fatigue, and to reduce chronic pain. The possible application of these techniques in patients with neuromuscular disorders warrants further investigation.