A CONTINUOUS MEASUREMENT OF OXIDATIVE RATE BY NEAR INFRARED SPECTROSCOPY IN HUMAN MUSCLES

Abstract

1174 Measurements of muscle oxidative rate by near infrared spectroscopy (NIRS) require repeated brief arterial occlusions. Therefore, disadvantages such as a poor time resolution, occur. In this study, we tried to continuously measure muscle oxidative rate by NIRS. In order to achieve this objective, we used an intermittent isometric exercise at high intensity, which elicits a transient occlusion of the blood flow to the muscle due to an increase in intramuscular pressure. Muscle oxygenation and energetics in the finger flexors (N=5) were measured by NIRS and magnetic resonance spectroscopy (MRS), respectively. The sampling time of the data for NIRS and MRS was 0.1 sec. and 2.0 sec., respectively. Changes in muscle oxygenation and phosphocrealine (PCr) concentration were monitored during an exercise at varying frequencies (3-5 sec. contraction/3-5 sec. relaxation) at 50% of maximum voluntary contraction for 3 minutes each. The oxidative rate was measured from the deoxygenation rate during the last 2 sec. of each contraction. The time resolution for the measurement of oxidative rate and PCr was 6 sec. and 10 sec., respectively. Oxidative rates at the onset of exercise followed an exponential time course with a time constant of 42 sec. on average. This value agreed with the time constant of the decrease in PCr (48 sec.). The oxidative rate during exercise increased 15 to 25 fold of the resting value on average, while the decrease in PCr was 58% on average. We observed a significant correlation between the change in oxidative rate and PCr concentration (r=0.95, p<0.05). This result suggests that the oxidative rate was successfully monitored by NIRS during exercise without arterial occlusion. Supported in part by a Grant-in-Aid for Encouragement of Young Scientists, the Ministry of Education, Science, Sports and Culture in Japan (#09780087).