Skeletal muscle intracellular Po-2 assessed by myoglobin desaturation: response to graded exercise

The relationship between skeletal muscle intracellular Po-2 (iPo(2)) and pr ogressive muscular work has important implications for the understanding Of O-2 transport and utilization. Presently there is debate as to whether iPo (2) falls progressively with increasing O-2 demand or reaches a plateau fro m moderate to maximal metabolic demand. Thus, using H-1 magnetic resonance spectroscopy of myoglobin (Mb), we studied cellular oxygenation during prog ressive single-leg knee extensor exercise from unweighted to 100% of maxima l work rate in six active human subjects. In all subjects, the Mb peak at 7 3 ppm was not visible at rest, whereas the peak was small or indistinguisha ble from the noise in the majority of subjects during progressive exercise from unweighted to 50-60% of maximum work rate. In contrast, beyond this ex ercise intensity, a Mb peak of consistent magnitude was discernible in all subjects. When a Mb half saturation of 3.2 Torr was used, the calculated sk eletal muscle Po-2 was variable before 60% of maximum work rate but in gene ral was relatively high (> 18 Torr, the measurable Po-2 with the poorest si gnal-to-noise ratio, in the majority of cases), whereas beyond this exercis e intensity iPo(2) fell to a relatively uniform and invariant level of 3.8 +/- 0.5 Torr across all subjects. These results do not support the concept of a progressive linear fall in iPo(2) across increasing work rates. Instea d, this study documents variable but relatively high iPo(2) from rest to mo derate exercise and again confirms that from 50-60% of maximum work rate iP o(2) reaches a plateau that is then invariant with increasing work rate.