IMPACT OF HIGH-INTENSITY INTERVAL TRAINING (HIIT) ON HEART RATE VARIABILITY IN RUNNERS

Introduction: Heart rate variability (HRV) measures have been increasingly used to assess autonomic regulation changes induced by training. HRV analysis is a valuable tool for monitoring training load responses, physical conditioning, and overtraining in athletes. Objective: To analyze heart rate variability and performance in runners undergoing a high-intensity interval training (HIIT) protocol. Methods: Eleven well-trained runners (age: 31±5.78 years; running experience: 10.23±6.11 years; best 5K time: 16.94±1.82 minutes) participated in the study. A maximal incremental treadmill test was conducted pre- and post-training to determine maximal aerobic speed (MAS). The test began at 10 km/h, increasing by 1 km/h every 2 minutes without rest. The HIIT program lasted four weeks, with two sessions per week. Training intensity was progressively adjusted. Each session included 1-minute intervals at a percentage of MAS, alternated with 1-minute active recovery at 50% of the corresponding load, until exhaustion or a 90-minute ceiling. R-R intervals were recorded at rest using a POLAR® RS800cx monitor at four time points: before the initial test (PRE HIIT), before the first session in week 3 (110% MAS), before the first session in week 4 (Taper), and before the final test (POST HIIT). HRV was analyzed using Kubios software in time domain, nonlinear parameters, and overall HRV metrics. Statistical analyses included Student’s t-test (VO₂ max and MAS) and One-Way ANOVA with Tukey and Dunn post hoc tests (resting HRV). Results: Positive adaptations were observed with maintained cardiorespiratory fitness and resting HRV. Autonomic modulation adapted to training load, with HRV reduction during intense phases (110% MAS and Taper) and recovery post-training. Significant differences (p<0.05) were found in SDNN, RMSSD, and SD2 between phases. Conclusion: HRV is sensitive to HIIT effects, proving effective for monitoring cardiac autonomic regulation and guiding training load. HIIT also supports performance maintenance in runners.