The purpose of this study was to confirm both the reproducibility of indices (NIRS slope, NT2 %NIRS fall) and the specificity obtained by analyzing the muscle oxygenation curve measured by near-infrared spectroscopy (NIRS) during ramp exercise. Ten healthy men participated in this study. The NIRS probe was placed on the vastus lateralis muscle. An increase in oxygenation was observed from rest to warm-up at 0 watts (Delta NIRS). Oxygenation began to decrease lineally as the workload increased (NIRS slope). In the latter phase of exercise, the oxygenation curve flattened out despite an increasing workload, and as a result, an inflection point was formed (NT2). The minimum value of oxygenation during ramp exercise was indicated as "%NIRS fall."
Protocol 1. After a warm-up period of 3 min at 0 watts, a ramp exercise (20 watt/min) test was performed until volitional fatigue. The test was performed for each subject twice (test-1, test-2) with a 1-week interval. Protocol 2. A test was performed with three consecutive ramp exercises (10 watt/min . 20 watt/min . 30 watt/min) up to 120 watt each with sufficient rest between the exercises.
NT2 was observed in 7 of 10 subjects. Test-1 and test-2 mean values of Delta NIRS, NIPS slope, watts at NT2 (NT2) and %NIRS fall were not significantly different, and the correlations between test-1 and test-2 were highly significant (r = 0.94, P < 0.0001 : Delta NIRS, r = 0.99, P < 0.0001 : NIRS slope, r = 0.91, P < 0.002 : NT2 and r = 0.78, P < 0.005: %NIRS fall). The regression lines obtained for correlations of results of test-1 and test-2 were y = -5.89 + 1.38X (Delta NIRS), y = 0.02 + 1.03X (NIRS slope), y = 31.52 + 0.83X (NT2), and y = 19.91 + 0.61X (%NIRS fall). No significant differences in both intercept and coefficient between the regression line and identity line were found in the NIPS slope and NT2. The rate of decrease in the oxygenation curve became steeper with an increase in workload from 10 watts/min to 20 watts/min and to 30 watts/min. However, the mean values of the NIRS slope, modified by watts, were 0.29+/-0.06%/watt, 0.29+/-0.07%/watt and 0.29+/-0.07%/watt, respectively. There were no significant differences of the NIRS slopes among these exercises. The results indicate constancy of the rate of decrease in oxygenation per workload.
In conclusion, these findings demonstrate the reproducibility of the NIRS slope and the appearance of NT2 during ramp exercise, and the specific way in which the decrease in muscle oxygenation reflects workload. They suggest that analysis of the muscle oxygenation curve can be used to estimate muscular metabolism and indices of training effects.