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1 Department of Medicine, University of California San Diego, La Jolla, California 92093; and 2 Department of Radiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6021
The relationship between skeletal muscle intracellular PO2 (iPO2) and progressive muscular work has important implications for the understanding of O2 transport and utilization. Presently there is debate as to whether iPO2 falls progressively with increasing O2 demand or reaches a plateau from moderate to maximal metabolic demand. Thus, using 1H magnetic resonance spectroscopy of myoglobin (Mb), we studied cellular oxygenation during progressive single-leg knee extensor exercise from unweighted to 100% of maximal work rate in six active human subjects. In all subjects, the Mb peak at 73 ppm was not visible at rest, whereas the peak was small or indistinguishable from the noise in the majority of subjects during progressive exercise from unweighted to 50-60% of maximum work rate. In contrast, beyond this exercise 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 skeletal muscle PO2 was variable before 60% of maximum work rate but in general was relatively high (>18 Torr, the measurable PO2 with the poorest signal-to-noise ratio, in the majority of cases), whereas beyond this exercise intensity iPO2 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 iPO2 across increasing work rates. Instead, this study documents variable but relatively high iPO2 from rest to moderate exercise and again confirms that from 50-60% of maximum work rate iPO2 reaches a plateau that is then invariant with increasing work rate.
oxygen; work rate; diffusion; oxygen transport
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