Fatigue in human metabolic myopathy.

The ability of muscle fibres to sustain force can be related to their economy of energy utilization and to their capacity to regenerate energy under the prevailing conditions (aerobic or anaerobic) of contraction. The pathophysiology of muscle fatigue is analysed in patients with thyroid dysfunction and with impaired glycogenolysis, and in a patient with abnormal mitochondrial function. Muscle from hypothyroid patients, like cooled muscle, is slow in relaxing and shows a reduced energy requirement (energy economy) and reduced fatiguability, whereas muscle of hyperthyroid patients may show the opposite features. In myophosphorylase deficiency the energy economy is normal in the fresh state and increases as contraction proceeds; however, fatigue is premature and associated with impaired excitation rather than an overall depletion of energy stores. With abnormal mitochondrial function the muscle tends to be effectively anaerobic and fatigue is associated with impaired excitation-contraction coupling. This appears to result from either muscle ischaemia or the dominant use of anaerobic metabolism for energy regeneration. Fatigue in these disorders of energy metabolism may ultimately be due to a reduced supply of ATP but direct evidence of this is lacking and, if it occurs, its physiological expression is probably variable.