Short-Term Autonomic Responses to Repeated Sprint Exercise Under Normobaric Hypoxia: A Randomized Controlled Trial

Objective: Exercising in hypoxia intensifies physiological stress and alters autonomic nervous system activity, requiring an adequate recovery period for maintaining high-intensity performance. This study examined the short-term autonomic responses to a single session of repeated sprint training in normobaric hypoxia (RSH), via heart rate variability (HRV) measurements. Material and Methods: Sixteen moderately trained males (age: 19.35±1 y) were assigned in a randomized parallel-group design. Participants performed a cyclingbased repeated sprint protocol under normoxic or hypoxic (FiO2=14.5- 14.7%) conditions. The protocol included 4 sets of 5 x 5-second sprints, separated by 25 seconds of rest and 5 minutes of active recovery between sets. HRV was measured at 4 time points: baseline, intervention day, 48h, and 72h post-intervention. Results: Hypoxia exposure significantly reduced standard deviation of normal-to-normal intervals (SDNN) during both exercise (p=0.017) and recovery (p=0.023), and was associated with a higher post-exercise heart rate (p=0.041). Resting HRV showed a significant time effect for root mean square of successive differences (RMSSD) (p=0.032), with both groups displaying increased parasympathetic activity at 72h post-exercise. At the end of the 72h period following the intervention, RMSSD increased by 138% in the hypoxia group and by 76% in the control group. No statistically significant differences or interactions were observed in frequency-domain parameters at any time point between the interventions. Conclusion: RSH elicited a greater increase in sympathetic activation compared to the same training performed in normoxia. These findings highlight the importance of carefully planning hypoxic training sessions with adequate recovery intervals to balance performance and recovery processes.