Effects of training intensities and heat chamber sitting on heat acclimatization under extreme high-temperature and high-humidity environment

Background Acute exposure to extreme high-temperature and high-humidity environments impairs physical and cognitive function. Developing rapid and effective strategies for heat acclimatization (HA) has become an urgent practical priority. This study aimed to compare HA efficacy and physiological safety among different exercise intensities and medium‑intensity training combined with heat chamber sitting under extreme high-temperature and high-humidity conditions. Methods Sixty healthy male military participants were randomly allocated to four groups using computer generated random numbers: low-intensity training group (L, 60%–70% HRmax), medium-intensity training group (M, 70%–80% HRmax), high-intensity training group (H, 80%–90% HRmax), and medium-intensity training combined with heat chamber sitting group (MHC, 70%–80% HRmax). All participants underwent a 7-day adaptive training protocol at Wet‑Bulb Globe Temperature (WBGT) 33.9 °C and relative humidity of 70 ± 0.3%. Physiological indicators (ear temperature, heart rate, sweat volume), serum biochemical parameters, and ratings of perceived exertion (RPE) were measured before and after the intervention. Results The H group showed significantly greater reductions in maximum heart rate, core temperature, and physiological strain index (PSI) compared with the L and M groups (P < 0.01). Serum electrolyte levels (chloride, potassium, calcium and sodium) remained within normal ranges across all groups. However, the H group exhibited significantly elevated muscle damage markers (CK, CK-MB, LDH), higher RPE scores, and positive urinary protein results (P < 0.01). Notably, the MHC group achieved comparable improvements in sweat rate, core temperature, and PSI to the H group, but with lower CK levels, lower RPE scores, and no urinary protein abnormalities. Conclusion High-intensity training rapidly induces effective HA, whereas medium-intensity training combined with heat chamber sitting achieves an optimal balance between HA induction efficiency and training safety. These findings provide a reliable reference for HA training implementation in extreme hot and humid environments.