Training methods for indoor heat acclimation training and outdoor heat acclimatization in extreme sports

LIU Lei; GU Liqun; KONG Yalin; HAN Yuming; WU Feifei; LI Yuan; GUAN Lin; LI Ning; ZHANG Xiangyang; NING Bo

doi:10.3969/j.issn.2097-1753.2025.04.019

Abstract：Objective: To explore scientific and effective heat acclimatization training methods for dealing with extreme sports. Methods: Fourteen male soldiers from a coastal military unit in Hainan were selected from January 2022 to February 2024 and randomly divided into an indoor training group (7 people, indoor heat acclimatization training) and an outdoor training group (7 people, outdoor heat acclimatization training). The indoor training group trained in an environment with a temperature of (35.00±0.20)°C and relative humidity of (40.00±2.00)%. They used a treadmill without power, with a slope of 5% and resistance of 7, to complete a 5 km run as hard as possible. The outdoor training group adjusted their clothing according to the real-time outdoor temperature and completed a 6 km run daily (divided into three sessions with 10 minutes of rest between each session). Both groups had a training period of 7 days, and heat tolerance tests were conducted before and after the training. The physiological strain index (PSI) was used to evaluate the improvement of heat tolerance after heat acclimatization training, with a PSI ≤ 3 set as the standard for excellent heat tolerance. The indoor training group continuously monitored rectal temperature and heart rate during daily training, calculated sweat volume after training, and calculated PSI. The outdoor training group continuously monitored ear canal temperature and heart rate during daily training, calculated sweat volume after training, and calculated PSI. Results: After training, the indoor training group showed lower maximum heart rate (P=0.002) and highest core temperature (P=0.003) compared to before training, with significant improvement in PSI (P<0.001). The heat tolerance test after training showed a PSI of 2.33±0.35 (≤3), meeting the excellent standard. The outdoor training group also showed lower maximum core temperature (P<0.001) and maximum heart rate (P<0.001) after training compared to before training, with a heat tolerance test PSI of 2.54±0.33 (≤3), also meeting the excellent standard. Between-group comparison: The indoor training group had a higher average maximum core temperature but lower average sweat volume than the outdoor training group (P < 0.001 for both). Conclusion: Whether in simulated environments or natural outdoor environments, the heat acclimatization training method used in this study enabled participants to achieve the excellent standard of PSI ≤ 3, thereby enhancing their ability to adapt to heat stress during extreme sports.