Background  Sustained aerobic exercise in high temperature environments can increase the risk of heat stroke and injury, and precooling has become a hot topic of research.

  Objective  To systematically evaluate the effects of precooling on the physiological indexes and exercise performance of physical trainers in a hyperthermic environment and provide references for the development of precooling strategies for military physical training.

  Methods  A combination of subject-word and free-word search was used to search the literature in Chinese and English databases such as PubMed, The Cochrane Library, Elsevier, Embase, Web of science, Scopus, China Knowledge Network (CNKI), China Biomedical Literature Database (CBM), and Wanfang database. The literature was screened according to the inclusion and exclusion criteria, then the data were extracted, and the PEDro scale was used to complete the literature quality evaluation. The data were meta-analyzed using Stata 16.0 software.

  Results  Totally 15 English articles and 2 Chinese articles with a quality evaluation score of 6.235 ± 0.437 were finally included, with 333 study subjects, 238 males and 95 females, 15 studies showing a mean age of 20-32.6 years, 1 study with age of 18-35 years, and 1 not mentioned, all of whom were long-term exercisers without heat-access clothing. The test of heterogeneity I² was 0, with P value of 0.794 for the comparison of heart rate values after exercise with precooling or without cooling, and the results of meta-analysis showed a combined utility measure of MD was -0.25 (95% CI: -0.44 to -0.07) (Z=-2.662, P=0.008), suggesting that precooling slowed the increase in heart rate after exercise; the comparison of core body temperature change values between precooling and without cooling by heterogeneity test was I²=75.2% (P<0.001), and meta-analysis showed combined utility of MD=-1.27 (95% CI: -1.85 to -0.69) (Z=-4.278, P<0.001), suggesting that precooling was effective in slowing the rate of increase in core body temperature; Heterogeneity test for comparison of precooling and without cooling thermal sensation showed I²=52.3% (P=0.040), and the results of meta-analysis showed a combined utility of MD=-0.31 (95% CI: -0.80 to 0.18) (Z=-1.233, P=0.217), indicating that the difference between precooling and post-exercise thermal sensation was not statistically significant; the results of the heterogeneity test comparing precooling and no-cooling sweating rate showed I²=71.0% (P<0.001), and meta-analysis results showed a combined utility measure MD= -0.48 (95% CI: -0.95 to 0) (Z=-1.971, P=0.049), indicating a significant effect of precooling on reducing exercise sweat rate. The raw data including in this studies showed that precooling measures contributed to exercise performance except for three studies that did not assess exercise performance and one study that suggested a non-significant difference in precooling.

  Conclusion  Pre-cooling is effective in improving athletic performance and effectiveness, slowing the rate of body temperature increase, and reducing the incidence and hazards of exercise heat stroke.