Active muscle oxygenation dynamics measured during high-intensity exercise by using two near-infrared spectroscopy methods.

Near-infrared spectroscopy is a noninvasive optical technique used to monitor tissue oxygenation. Generally, the modified Beer-Lambert's law (MBL) using continuous-wave light has been used to measure active muscle oxygenation during exercise; however, it cannot measure absolute changes in the oxy- (oxy-[Hb + Mb]), deoxy- (deoxy-[Hb + Mb]), and total hemoglobin/myoglobin concentrations (total-[Hb + Mb]) because the pathlength and scattering coefficient are not measured. In contrast, the time-resolved spectroscopy (TRS) using a ultra short pulsed laser can be used to determine absolute changes in the concentration, although the temporal resolution is inferior to that in MBL. This study evaluated the absolute changes in active muscle oxygenation and the optical mean pathlength and scattering and absorption coefficient during high-intensity exercise by using the TRS system. In addition, the difference between the changes determined using TRS and MBL measurements was assessed. When the TRS and MBL measurements obtained during high-intensity exercise were compared, the total-[Hb + Mb] and oxy-[Hb + Mb] dynamics differed markedly during high-intensity exercise, while the deoxy-[Hb + Mb] dynamics and kinetics did not differ.