Contribution of intramuscular oxidative metabolism to total ATP production during forearm isometric exercise at varying intensities.

It is not fully clear whether intramuscular oxidative metabolism contributes to total adenosine triphosphate (ATP) production during forearm isometric exercise at varying intensities. We tested hypothesis that oxidative metabolism with intramuscular O2 contributes to lessen the dependence on anaerobic metabolism, in particular phosphocreatine (PCr) breakdown. Seven male subjects were tested for changes in muscle oxygenation (MO2) and high-energy phosphates in forearm flexor muscles at rest and during exercise under arterial occlusion by 31-phosphorus magnetic resonance spectroscopy (31P-MRS) and near infrared spectroscopy (NIRS). Isometric wrist flexion exercise was performed for 1 min or until exhaustion at intensities corresponding to 30%, 50% and 70% of maximal voluntary contraction (MVC) under intramuscular O2 (Intramuscular O2-Ex) and anaerobic (Anaero-Ex) conditions. Oxidative ATP production in Intramuscular O2-Ex was calculated as 0.05 +/- 0.01 mM/s for 30%MVC, 0.08 +/- 0.01 mM/s for 50%MVC and 0.11 +/- 0.01 mM/s for 70%MVC. At a lower intensity (30%MVC), PCr breakdown rate (0.17 +/- 0.02 mM/s) of Anaero-Ex was significantly higher than the rate (0.13 +/- 0.01 mM/s) of Intramuscular O2-Ex (p < 0.05). There was no significant difference in ATP production rates through PCr breakdown and glycolysis between Intramuscular O2-Ex and Anaero-Ex at the higher intensities (50% and 70%MVC). In conclusion, intramuscular oxidative metabolism plays a significant role in reducing the dependence on PCr breakdown during isometric exercise at a lower intensity (30%MVC).