Exercise modality but not hypoxia alters heart rate variability after a single repeated-sprint session.

PURPOSE: In exercise physiology, the concept of overload states that training load should be sufficient to challenge body homeostasis and thus trigger positive adaptations. Repeated-sprint training in hypoxia has been proposed as a potent modality to enhance performance without altering mechanical work or subjective difficulty compared to normoxia, while physiological effects of such training remain poorly understood. We proposed here to use heart rate variability (HRV) as a proxy for the level of physiological stress induced by different modalities of repeated-sprint exercises in normoxia and hypoxia. METHODS: Twelve participants performed 6:24 (sprint: recovery duration in s) or 10:20 repeated-sprints in normoxia and hypoxia in a balanced randomized single-blinded crossover design. Time, frequency and non-linear domains of HRV were analysed within the first 15 min of recovery after each exercise, as well as in the morning following the session. RESULTS: During immediate recovery, modality (6:24 vs. 10:20), but not hypoxia had a significant effect on HRV indices, with the 10:20 condition delaying parasympathetic reactivation. Variables measured during the morning after exercise displayed less pronounced changes, with the 10:20 condition that tended to reduce HRV. Hypoxia non significantly increased resting heart rate and had a negative effect on parasympathetic activity only in the 6:24 condition. CONCLUSION: Overall, the 10:20 modality in normoxia appears as a potent stimulus to disrupt body homeostasis in recreationally trained participants. Our results suggest that addition of hypoxia during supramaximal exercises would not represent a significant stimulus to trigger additional alteration of the autonomic nervous system balance.