Intermittent normobaric hypoxia at rest is associated with divergent cardiac autonomic trajectories in master cyclists

PURPOSE: This randomized, participant-blinded, sham-controlled 2 × 2 crossover trial examined whether a 4-week intermittent normobaric hypoxia intervention performed at rest (IHNT; 12 sessions; FiO₂ ≈ 0.12) is associated with changes in indices of cardiac parasympathetic modulation in master cyclists. METHODS: Twenty-three master cyclists aged 50-60 years completed both IHNT and normoxic sham conditions across two periods with a 4-week washout. HRV was assessed from standardized supine morning recordings summarized as 7-day means at Baseline, Post-intervention, and Follow-up. Primary outcomes were lnRMSSD and lnHF, analyzed using linear mixed-effects models with autoregressive residual structure, condition-specific heterogeneous variances, and Holm-adjusted contrasts. RESULTS: Significant Condition × Time interactions were observed for lnRMSSD (χ²(2) = 16.27, p < 0.001) and lnHF (χ²(2) = 18.18, p < 0.001), reflecting divergent autonomic trajectories across conditions. Within IHNT, lnRMSSD increased by + 13.6% (p < 0.001) and lnHF by + 17.9% (p < 0.001) Post-intervention, with partial retention at Follow-up (+ 4.5% and + 7.0%, respectively). Sham showed significant lnHF decreases at Post (- 20.5%) and Follow-up (- 21.3%; both p = 0.030), while lnRMSSD remained stable. Pairwise between-condition contrasts did not reach significance after Holm-Bonferroni adjustment. A baseline-adjusted sensitivity analysis confirmed the lnHF interaction (χ²(1) = 12.04, p < 0.001). Blinding was maintained (52.4% accuracy, p = 0.287). CONCLUSION: IHNT at rest was associated with divergent cardiac autonomic trajectories versus sham in master cyclists, with consistent within-IHNT increases in parasympathetic indices and concurrent vagal withdrawal under sham. These proof-of-concept findings support a physiological signal warranting confirmation in adequately powered trials.