Regional tissue oxygenation during high-intensity exercise following voluntary isocapnic hyperpnea versus inspiratory threshold loading in endurance–trained individuals: a randomized controlled trial

Abstract This study contrasted the effects of five weeks of voluntary isocapnic hyperpnea (VIH) versus inspiratory threshold loading (ITL) on tissue oxygenation at the prefrontal cortex (PFC), respiratory muscles (m.Intercostales), and locomotor muscles (m.Vastus Lateralis) during high intensity constant load cycling (CLT) in endurance trained individuals. Twenty participants (14 men, 6 women) were randomly assigned to VIH (n = 10) or ITL (n = 10) training. Before and after intervention, participants completed a CLT at 80% of peak power output until exhaustion. Tissue oxygenation was continuously monitored using near–infrared spectroscopy (NIRS) at three sites. Changes in oxygenated (ΔO2Hb), deoxygenated (ΔHHb), and total hemoglobin (ΔtHb), along with tissue saturation index (TSI), were analyzed using a three–way ANOVA. VIH significantly improved peak oxygen–uptake (p = 0.016), maximal lung ventilation (p = 0.004), respiratory rate (p = 0.030), and tidal volume (p = 0.022), whereas ITL significantly increased maximal inspiratory pressure (p = 0.003). A significant main effect of time (%CLT) was observed for all NIRS variables at the three measurement sites (p < 0.05), except TSI at the PFC and ΔO2Hb at the m.Vastus Lateralis. A main effect of training was detected only for TSI at the m.Vastus Lateralis (p = 0.036, η²p = 0.22; mean difference 3.2%, 95% CI: 0.3 to 6.1%), though direct physiological interpretation requires caution given the modest effect magnitude. No significant group effects or interactions were observed. VIH and ITL elicit distinct adaptations in respiratory function and aerobic capacity without modifying regional tissue oxygenation dynamics during high intensity exercise. The observed locomotor muscle TSI trend warrants confirmation in future studies.