The effects of high-intensity interval training and moderate-intensity continuous training for heat acclimation on...

This study investigated the effects of high-intensity interval training (HIIT) and moderate-intensity continuous training (MICT) during a 5-day heat acclimation (HA) protocol on resting thermoregulation and heart rate variability (HRV). Forty-eight male college students were randomly assigned to four groups: MICT in normal ambient temperatures (N-MICT), MICT in hot ambient temperatures (H-MICT), HIIT in normal ambient temperatures (N-HIIT), and HIIT in hot ambient temperatures (H-HIIT). Participants completed 5 consecutive days of either MICT (50 % maximal power output) or HIIT (3 min at 80 % maximal power output with 2 min of passive rest), all performed on a cycle ergometer in either normal (23 °C) or hot ambient (32 °C) conditions. Core temperature (T_core), mean skin temperature (T_skin), heart rate (HR), and HRV were measured at rest in hot conditions before and after the intervention. The HIIT HA protocol led to significant reductions in T_core, T_skin, and HR, along with improvements in HRV. In contrast, the MICT HA protocol significantly reduced HR but did not alter T_core, T_skin, or HRV. Post-intervention, the H-HIIT group showed greater reductions in T_core (H-HIIT: 36.96 °C ± 0.27 °C; H-MICT: 37.19 °C ± 0.15 °C, p = 0.02), HR (H-HIIT: 66.62 ± 9.24 bpm; H-MICT: 74.59 ± 7.37 bpm, p = 0.048), as well as more pronounced elevations in root mean square of successive differences and the percentage of successive Normal-Normal intervals that are greater than 50 ms (pNN50) compared to the H-MICT group. Additionally, T_skin was lower in the H-HIIT group than in the N-HIIT group post-intervention. These results indicate that HIIT based HA induced a decrease in body temperature, along with improved HRV, suggest that a 5-day HIIT HA protocol could enhance thermoregulatory and cardiovascular functions.