The plateau in the NIRS-derived [HHb] signal near the end of a ramp incremental test does not indicate the upper limit of O2 extraction in the vastus lateralis

This study aimed to examine, at the level of the active muscles, whether the plateau in oxygen (O 2 ) extraction normally observed near the end of a ramp incremental (RI) exercise test to exhaustion is caused by the achievement of an upper limit in O 2 extraction. Eleven healthy men (27.3 ± 3.0 yr, 81.6 ± 8.1 kg, 183.9 ± 6.3 cm) performed a RI cycling test to exhaustion. O 2 extraction of the vastus lateralis (VL) was measured continuously throughout the test using the near-infrared spectroscopy (NIRS)-derived deoxygenated hemoglobin [HHb] signal. A leg blood flow occlusion was performed at rest (LBF OCC1 ) and immediately after the RI test (LBF OCC2 ). The [HHb] values during the resting occlusion (108.1 ± 21.7%; LBF OCC1 ) and the peak values during exercise (100 ± 0%; [HHb] plateau ) were significantly greater than those observed at baseline (0.84 ± 10.6% at baseline 1 and 0 ± 0% at baseline 2 ) ( P < 0.05). No significant difference was found between LBF OCC1 and [HHb] plateau ( P > 0.05) or between the baseline measurements ( P > 0.05). [HHb] values at LBF OCC2 (130.5 ± 19.7%) were significantly greater than all other time points ( P < 0.05). These results support the existence of an O 2 extraction reserve in the VL muscle at the end of a RI cycling test and suggest that the observed plateau in the [HHb] signal toward the end of a RI test is not representative of an upper limit in O 2 extraction.