The effects of high-intensity interval training and moderate-intensity continuous training for heat acclimation on resting thermoregulation and heart rate variability.
Level 2 - randomized trial
Individual randomized controlled trial
PubMed 41207096 · doi:10.1016/j.jtherbio.2025.104315
What was done
Forty-eight male college students were randomized into four parallel groups for a 5-day cycle ergometer training protocol: moderate-intensity continuous training at 23 °C (N-MICT; 50% maximal power output), MICT in heat at 32 °C (H-MICT), high-intensity interval training at 23 °C (N-HIIT; 3 min at 80% maximal power output with 2 min rest), or HIIT in heat at 32 °C (H-HIIT). Resting core temperature, mean skin temperature, heart rate, and heart rate variability (HRV) were measured under hot conditions before and after the intervention.
What was found
The H-HIIT protocol significantly lowered resting core temperature, skin temperature, and heart rate while increasing HRV metrics. In contrast, H-MICT reduced heart rate but did not alter core temperature, skin temperature, or HRV. Post-intervention, the H-HIIT group showed lower core temperature (36.96 ± 0.27 °C vs 37.19 ± 0.15 °C, p = 0.02) and lower heart rate (66.62 ± 9.24 bpm vs 74.59 ± 7.37 bpm, p = 0.048) compared to H-MICT, as well as greater increases in RMSSD and pNN50 (exact numeric values for HRV parameters were not reported in the abstract). Skin temperature was also lower in H-HIIT than in N-HIIT.
Why it matters
Short-duration heat acclimation protocols are critical for individuals preparing for heat exposure. These findings indicate that 5 days of interval training in the heat confers superior resting thermoregulatory and autonomic cardiovascular adaptations compared to traditional moderate continuous training.
Limits
The sample size is small (n = 48 across 4 groups, ~12 per arm) and restricted entirely to young male college students. The abstract does not report exact numerical values for HRV metrics, session duration/exercise volume, or functional exercise performance outcomes in the heat.
Cited by
- supports Heat acclimation from repeated heat exposure improves heart rate variability.