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 O₂ (FiO₂) 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, FiO₂ = 20.9%) or hypoxia (HYPO, FiO₂ = 16%). Oxygen uptake ((Formula presented.) O₂), and [HHb] signal from the vastus lateralis muscle were continuously measured. Peak (Formula presented.) O₂ (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 presented.) O₂ 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 presented.) O₂ 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 presented.) O₂ and PO in HYPO from NORM were strongly correlated (r =.626 and 0.752, p <.05). Therefore, breathing a lower FiO₂ during a RI test resulted in proportional reduction in the RCP and the [HHb]_bp in terms of (Formula presented.) O₂ and PO, which further supports the notion that these physiological responses may arise from similar metabolic changes reflecting a common phenomenon.