Muscle oxygenation kinetics at the onset of exercise do not depend on exercise intensity

The purpose of this study was to determine whether the onset kinetics of muscle oxygenation in localized working muscle (mOxy) was affected by differences in exercise intensity. Five healthy male subjects exercised for 6 min at 125 W, 150 W, and 175 W, and 1 min at 300 W on a cycle ergometer. mOxy was estimated by near-infrared spectroscopy (NIRS) with a continuous wave photometer. The NIRS probe was positioned on the vastus lateralis muscle of the right leg. The relative change in mOxy was calculated from the relative change of the oxygenated hemoglobin (OxyHb) and deoxygenated hemoglobin (DeoxyHb) concentration from their resting values ([mOxy]=Delta[OxyHb]-Delta[DeoxyHb]). Assuming an exponential time course with time delay, the time constants of the mOxy were 5.7 (SD 2.2) s at 125 W, 5.6 (SD 1.9) s at 150 W, 6.0 (SD 2.2) s at 175 W, and 5.6 (SD 2.1) s at 300 W. The time delays of the mOxy were 6.7 (SD 4.2) s at 125 W, 8.6 (SD 1.6) s at 150 W, 6.4 (SD 3.0) s at 175 W, and 5.4 (SD 2.9) s at 300 W. The mean response times of the mOxy were 12.5 (SD 2.7) s at 125 W, 14.2 (SD 2.4) s at 150 W, 12.4 (SD 4.4) s at 175 W, and 11.0 (SD 3.1) s at 300 W. These results indicate that the kinetics of mOxy were not affected by differences in exercise intensity.