Magnetic resonance spectroscopy studies of human muscle.

The use of MRS has become more widespread as cost and availability have improved. It has been demonstrated that MRS of human skeletal muscle can play a significant role in (1) understanding healthy muscle metabolism and the mechanisms of muscle fatigue, (2) understanding the effects of disease on muscle metabolism and function, (3) monitoring the efficacy of therapeutic intervention, and (4) the confirmation of disease diagnoses. The results of the 31P MRS studies of disease are summarized in the Table 1. A few conditions (McArdle's, PFK deficiency) are associated with failure to develop acidosis during exercise. This response appears to be relatively specific to these metabolic myopathies. For most of the conditions reviewed here, however, the metabolic findings of reduced PCr/Pi and greater acidosis during exercise with impaired recovery of PCr/Pi and pH are very similar. The nonspecificity of the MRS results suggests the possibility that a common mechanism may be at work in all of these diseases. A major question to arise from clinical studies using MRS concerns the extent to which deconditioning may have played a role in some of these findings. This is because conditions associated with muscle weakness, rapid fatiguability, and muscle pain during or following vigorous physical activity may also be those that lead to deconditioning. In virtually all studies reviewed here, healthy, active subjects were used as controls. There are no examples in which controls were appropriately matched to the subjects for their level of conditioning. Conditioning could be assessed by questionnaire, activity logs, activity monitoring devices, or measurements of conditioning effects such as maximal oxygen consumption. The role of deconditioning in the decreased quality of life of persons with chronic diseases has not been fully explored. Future studies of chronic disease using MRS should combine MRS with other techniques to further probe the mechanisms of muscle metabolism under various conditions, and the extent to which these mechanisms are sensitive to the level of physical conditioning.