ESTIMATION OF ENERGY METABOLISM DURING THE INITIAL PHASE OF SUSTAINED EXERCISE AT VARYING INTENSITIES ARYING INTENSITIES

Abstract

The purpose of this study was to estimate the energy metabolism in muscle during the initial phase of sustained wrist flexion exercise under aerobic and anaerobic conditions. Seven male subjects were tested for the metabolic rate at rest (resting metabolic rate) prior to the principal experiment and during the sustained exercise. Sustained wrist flexion exercise at 30, 50 and 70% of maximum voluntary contraction (MVC) was performed until exhaustion under aerobic and anaerobic conditions. Exercise under anaerobic conditions started immediately after muscle oxygen content (MO2) was completely depleted by 6 min cuff occlusion at rest. Changes in high energy phosphates and MO2 in the forearm flexor muscle were continuously measured by 31-phosphorus magnetic resonance spectroscopy (31P-MRS) and near infrared spectroscopy (NIRS), respectively. The MO2 level was calculated by defining the resting MO2 level as 100%, while MO2 depleted level as 0%. Assuming that b-ATP is 8.2mM, the resting metabolic rate was 0.01 ± 0.001 (mean ± S.E.) ATP mM/s. From the changes of these parameters during exercise in each intensity, the metabolic rates during initial 30 s were calculated. Each rate of ATP production during exercise under two conditions were elevated with an increase in exercise intensity. In lower intensity (30%MVC), PCr breakdown rate under anaerobic conditions was higher than the rate under aerobic conditions. However, in the highest intensity (70%MVC), no significant difference were found in ATP production rates through PCr breakdown and glycolysis between aerobic and anaerobic conditions. The contribution of oxidative ATP production was lower in higher intensities (50% and 70% MVC) under aerobic conditions. These results suggest that, under both conditions, the ATP production through glycolysis contributed to an increase in exercise intensity to a large extent.