Effects of normobaric hypoxia and hyperthermia on ventilatory responses to high-intensity interval training bouts

Purpose

Respiratory frequency (fR) and tidal volume (VT) show distinct responses during high-intensity interval training (HIIT) bouts, yet the underlying mechanisms remain unclear. We investigated the effects of hypoxia and hyperthermia on ventilatory responses to HIIT bouts.

Methods

Ten recreationally trained males (mean ± SD: peak oxygen uptake: 3.98 ± 0.62 L/min, age: 28 ± 6 years) performed the same HIIT protocol to exhaustion in three randomized conditions: normobaric hypoxia (HYP, 15% O2), hyperthermia (HOT, 35 °C, 40% humidity), and control (CON, 18 °C and 40% humidity). Work–recovery phases (30–30 s) were performed at 109% of peak power output from a prior incremental test and 50 W, respectively.

Results

Time-to-exhaustion (TTE) differed (P < 0.05) across CON (18.0 ± 4.4 min), HYP (9.4 ± 2.8 min), and HOT (12.6 ± 3.1 min) conditions. Iso-time fR was associated with changes in TTE and positively correlated with perceived exertion (P < 0.001; r = 0.85) and aural temperature (P < 0.001; r = 0.58). On average, fR increased during the work phase and decreased during recovery, primarily driving pulmonary ventilation (

) during work–recovery alternations. Conversely, VT showed a smaller and opposite response, with higher values during recovery. VT plateaued in all conditions, but higher values were observed in HYP versus CON and HOT, accompanied by higher capillary blood lactate levels and lower blood oxygen saturation.

Conclusions

fR is associated with changes in exercise tolerance irrespective of the environmental conditions tested and is more closely related to perceived exertion than to aural temperature. fR primarily drives the

response to high-intensity work–recovery alternations, while VT appears fine-tuned on fR levels and the magnitude of metabolic inputs.