The effects of high-intensity interval training and moderate-intensity continuous training for heat acclimation on resting thermoregulation and heart rate variability.

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 (Tcore), mean skin temperature (Tskin), 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 Tcore, Tskin, and HR, along with improvements in HRV. In contrast, the MICT HA protocol significantly reduced HR but did not alter Tcore, Tskin, or HRV. Post-intervention, the H-HIIT group showed greater reductions in Tcore (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, Tskin 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.