Regional 31P magnetic resonance spectroscopy of exercising human masseter muscle

Abstract

Changes in fibre structure and function associated with exercise have been quantified ultrastructurally and biochemically in selected limb muscles, but the biochemical events associated with contraction are rarely studied in the human jaw muscles. Here, 31P NMR spectroscopy, or MRS, was used to examine the multipennate masseter in six adult men at rest and while performing isometric clenching exercises. NMR spectra were acquired from three locations within the muscle with a 2 × 3cm, single-turn, copper receiver coil. The spectra, corrected for partial saturation effects, were quantified on the basis of relative peak area and position. The inorganic phosphate (P i) to creatine phosphate (PCr) ratio ( P i PCr ), which has been shown to be indirectly related to the phosphate potential and hence the metabolic activity, as well as pH, were calculated for each site and exercise. The mean resting P i PCr ratio was greater for the deep part of the muscle than for the superficial and intermediate parts; these differences were significant to p < 0.01. The mean pH was similar in all parts of the muscle at rest. During exercise, a significant increase in mean P i/PCr was found in the superficial and intermediate parts of the muscle; both these differences were significant to p < 0.05. An accompanying decrease in mean pH was observed in all parts of the muscle during exercise. In the superficial part, this decrease was significant to the p < 0.05 level, and in the deep part, to the p < 0.001 level. No significant differences were found for these measures between left and right molar clenching. These findings show that metabolic activity may be monitored in the masseter by 31P NMR spectroscopy, and that task-dependent and regional variations may be demonstrated both at rest and during exercise. They are promising enough to encourage future studies of metabolism in subjects with jaw muscle disorders.