Hypoxia equally reduces the respiratory compensation point and the NIRS‐derived [HHb] breakpoint during a ramp‐incremental test in young active males

This study investigated the effect of reduced inspired fraction of O(2) (FiO(2)) in the correspondence between the respiratory compensation point (RCP) and the breakpoint in the near‐infrared spectroscopy‐derived deoxygenated hemoglobin signal ([HHb](bp)) during a ramp‐incremental (RI) test to exhaustion. Eleven young males performed, on two separated occasions, a RI test either in normoxia (NORM, FiO(2) = 20.9%) or hypoxia (HYPO, FiO(2) = 16%). Oxygen uptake ( [Formula: see text] O(2)), and [HHb] signal from the vastus lateralis muscle were continuously measured. Peak [Formula: see text] O(2) (2.98 ± 0.36 vs. 3.39 ± 0.26 L min(−1)) and PO (282 ± 29 vs. 310 ± 19 W) were lower in HYPO compared to NORM condition, respectively. The [Formula: see text] O(2) and PO associated with RCP and [HHb](bp) were lower in HYPO (2.35 ± 0.24 and 2.34 ± 0.26 L min(−1); 198 ± 37 and 197 ± 30 W, respectively) when compared to NORM (2.75 ± 0.26 and 2.75 ± 0.28 L min(−1); 244 ± 29 and 241 ± 28 W, respectively) (p < .05). Within the same condition, the [Formula: see text] O(2) and PO associated with RCP and [HHb](bp) were not different (p > .05). Bland–Altman plots mean average errors between RCP and [HHb](bp) were not different from zero in HYPO (0.01 L min(−1) and 1.1 W) and NORM (0.00 L min(−1) and 3.6 W) conditions. The intra‐individual changes between thresholds associated with [Formula: see text] O(2) and PO in HYPO from NORM were strongly correlated (r = .626 and 0.752, p < .05). Therefore, breathing a lower FiO(2) during a RI test resulted in proportional reduction in the RCP and the [HHb](bp) in terms of [Formula: see text] O(2) and PO, which further supports the notion that these physiological responses may arise from similar metabolic changes reflecting a common phenomenon.