Correlates of exercise hyperemia and muscle energetics in the human upper arm.

We employed interleaved dynamic 1H magnetic resonance imaging (MRI) and 31P MR spectroscopy in combination with arm-cycling to investigate correlations of exercise hyperemia and muscle energetics in the triceps brachii (TB) muscle of the upper arm of healthy individuals. The parameter hyperemic slope (HS) determined from MRI acquisitions immediately after exercise was used as primary index of maximal TB oxygenation level in response to exercise. We found that HS tended to be inversely correlated with TB acidification during exercise (P =0.06) as opposed to findings in leg muscle. The absolute increase in cardiac-output during exercise was found to be uncorrelated with HS (P =0.19) suggesting that the magnitude of the hyperemic response to exercise involving a minor muscle mass is governed by local rather than systemic factors. Post-exercise, the rate of metabolic recovery was fastest in the study subject with highest HS and slowest for the opposite case, although this correlation failed to reach significance in our small study cohort (P =0.14). This finding fits the conclusion of previous musculoskeletal 31P MRS studies that oxygen supply to working skeletal muscle exerts significant metabolic control over oxidative muscular energy balance even if the physical task only involves a minor muscle mass.