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.