The effect of the fraction of inspired oxygen on the NIRS-derived deoxygenated hemoglobin "breakpoint" during ramp-incremental test

During ramp-incremental (RI) exercise to exhaustion, the near-infrared spectroscopy-derived deoxygenated hemoglobin ([HHb]) signal in the vastus lateralis muscle shows a linear increase up to a point at which a plateau-like response is manifested ([HHb](bp)). This study investigated if 1) the [HHb](bp) is affected by different fractions of inspired O-2 (FIO2) [hypoxia (16%; HYPO); normoxia (21%; NORM); hyperoxia (30%; HYPER)]; and 2) an abrupt change to hyperoxic-inspired gas just before the occurrence of the [HHb](bp) (HYPERSWITCH) would affect the [HHb] plateau-like response. Ten physically active male participants reported to the laboratory on four separate occasions to perform an RI test to exhaustion in NORM, HYPO, and HYPER and an RI test to exhaustion with an abrupt increase in FIO2 (30%; HYPERSWITCH) 15 W before the power output (PO) associated with [HHb](bp) in normoxia. PO, [HHb], tissue O-2 (St(O2)), and pulse O-2 saturation (Sp(O2)) were recorded continuously. Peak PO was significantly lower in HYPO (290 +/- 21 W) and higher in HYPER (321 +/- 22 W) and HYPERSWITCH (320 +/- 19 W) compared with NORM (311 +/- 18 W). The PO associated with [HHb](bp) was not different between NORM and HYPER (246 +/- 23 vs. 247 +/- 24 W), but it was lower in HYPO (198 +/- 31 W) than NORM and HYPER. The PO associated with the [HHb](bp) in HYPERSWITCH (240 +/- 23) was not different compared with NORM. HYPER and HYPERSWITCH resulted in greater St(O2) and Sp(O2) compared with NORM. These results suggest that the [HHb](bp) response is not dependent of O-2 driving pressure and that other physiological mechanisms might determine its occurrence.