Investigating the Acute Effect of Different Training Protocols on Heart Rate Variability

This study examined the acute effects of high-intensity interval training (HIIT) and prolonged endurance training (ET) on heart rate variability (HRV) in elite Greco-Roman wrestlers and assessed the usefulness of HRV for monitoring post-exercise recovery. Using a longitudinal, within-subject, repeated-measures design, 13 elite male wrestlers completed two training protocols-differing in intensity, duration, modality, and work-rest structure-in a fixed, non-randomized order (ET on Day 7, HIIT on Day 22), separated by a 15-day washout period. HRV was recorded at baseline, pre-exercise, during training, and 24 h post-exercise, and analyzed with a linear mixed model and Bonferroni-adjusted post hoc comparisons. A significant main effect of Timepoint was found for all HRV parameters (SDNN, RMSSD, LF/HF ratio, and the lnRMSSD-derived HRV Score), reflecting marked reductions during exercise and partial recovery by 24 h (e.g., SDNN decreased by approximately 73% in ET and 82% in HIIT). SDNN also differed significantly between training types overall, but heart rate did not differ between protocols during exercise (p = 0.127). No between-protocol difference for any HRV parameter survived Bonferroni correction at any timepoint, despite a large effect size for the HRV Score during exercise (d = 0.79); the corresponding effect for raw RMSSD, the primary time-domain outcome, was small (d = 0.36) and non-significant. Both protocols showed comparable recovery at 24 h. Because protocol order was fixed rather than randomized, these findings should be interpreted as a comparison between two specific field-based conditioning sessions rather than a controlled test of exercise intensity alone.