MRS Evidence of Adequate O2 Supply in Human Skeletal Muscle at the Onset of Exercise

Purpose At exercise onset, intramuscular oxidative energy production responds relatively slowly in comparison with the change in adenosine triphosphate demand. To determine whether the slow kinetics of oxidative adenosine triphosphate production is due to inadequate O-2 supply or metabolic inertia, we studied the kinetics of intramyocellular deoxygenation (deoxy-myoglobin (Mb)) and metabolism (phosphocreatine (PCr)) using proton (H-1) and phosphorus (P-31) magnetic resonance spectroscopy in six healthy subjects (33 5 yr). Methods Specifically, using dynamic plantarflexion exercise, rest to exercise and recovery were assessed at both 60% of maximum work rate (moderate intensity) and 80% of maximum work rate (heavy intensity). Results At exercise onset, [PCr] fell without delay and with a similar time constant () at both exercise intensities (approximately 33 s). In contrast, the increase in deoxy-Mb was delayed at exercise onset by 5-7 s, after which it increased with kinetics (moderate = 37 +/- 9 s; heavy = 29 +/- 6 s) that was not different from PCr (P > 0.05). At cessation, deoxy-Mb recovered without time delay and more rapidly ( = approximate to 20 s) than PCr ( = approximate to 33 s) (P < 0.05). Conclusions Using a unique combination of in vivo magnetic resonance spectroscopy techniques with high time resolution, this study revealed a delay in intramuscular deoxygenation at the onset of exercise and rapid reoxygenation kinetics upon cessation. Together, these data imply that intramuscular substrate-enzyme interactions, and not O-2 availability, determine the exercise onset kinetics of oxidative metabolism in healthy human skeletal muscles.