It has been hypothesized that the signals of near infrared spectroscopy (NIRS) would reflect muscle O
2 uptake (mVO
2). Although it is not definite that NIRS signals accurately reflect mVO
2, there is every possibility that NIRS signals at least reflect regional O
2 uptake (rVO
2). The phase II kinetics of pulmonary oxygen uptake (pVO
2) is regarded as reflecting mVO
2 at the onset of exercise. To examine whether the rVO
2 on-kinetics measured by NIRS reflects the mVO
2 on-kinetics at the onset of exercise, we compared the rVO
2 as measured by NIRS with the phase II kinetics of pVO
2 at the onset of exercise. Twelve healthy male subjects cycled a Monark ergometer at three different intensities: below the ventilatory threshold (VT) level (below-VT), on the VT level (on-VT), and above the VT level (above-VT), for 6 minutes on three separate occasions. The rVO
2 was calculated from the concentration of oxyhemoglobin and deoxyhemoglobin, as measured by NIRS every 3 seconds. The pVO
2 was determined by the breath-by-breath method. A significant relationship between the amount of increases of pVO
2 and rVO
2 from rest to the end of exercise among all levels of exercise intensity was found (r=0.935, P<0.001). The time constants of rVO
2 (rVO
2-Tc: below-VT: 6.514±2.159 s, on-VT: 7.760±2.035 s, above-VT: 9.532±2.342 s) were significantly faster than the time constants of pVO
2 (pVO
2-Tc: below-VT: 23.8±4.4 s, on-VT: 25.9±5.1 s, above-VT: 26.3±5.7 s) (P<0.001). There was no significant relationship between rVO
2-Tc and pVO
2-Tc for each intensity (P>0.05). We conclude that the rVO
2 on-kinetics measured by NIRS does not necessarily reflect the mVO
2 kinetics at the onset of exercise.
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