Performance changes during and post-exercise in combined heat and hypoxia environments: a systematic review and network meta analysis

This study aims to compare exercise performance, as well as physiological, and psychological changes, when individuals exercise both during and after exposure to the heat environment (HEAT), the hypoxia environment (HYP), and the combined heat and hypoxia environment (CHH). Key physiological and psychological indicators such as maximal oxygen consumption, environmental temperature, oxygen uptake, blood oxygen saturation, heart rate, core temperature, skin temperature, ratings of perceived exertion, thermal sensation, plasma volume, and lactate levels were extracted for review. 23 publications were subjected to risk of bias and quality assessments. Paired meta-analysis based on a random effects model was used to examine the effects of HEAT, HYP, and CHH on these indicators. Bayesian Framework model-based network meta-analysis in STATA software was employed for statistical analysis. Post-exposure data were divided into subgroups based on the exposure duration, long-term (≥ 14 days) or short-term (< 14 days), and all data were analyzed with a 95% confidence interval. This study analyzed data from 23 studies with 414 healthy participants. During exposure, hypoxia environment significantly decreased exercise performance and oxygen uptake while increasing ratings of perceived exertion. Similarly, both heat environment and combined heat and hypoxia environment impaired the performance and increased the heart rate, core temperature, and skin temperature. In long term post-exposure, heat environment acclimation significantly improved thermal sensation. In short term post-exposure, both combined heat and hypoxia environment and heat environment significantly improved plasma volume and lactate, compared to the control and hypoxia environment conditions. The Network Meta-Analysis shed light on performance changes under heat environment, hypoxia environment, and combined heat and hypoxia environment, noting reductions in exercise capacity in all conditions, especially combined heat and hypoxia environment. Minor improvements were observed after exposure to the environments, particularly in lactate levels and plasma volume after short term acclimation to the combined heat and hypoxia environment Our study emphasized the importance of physiological adaptations to tackle extreme environments, offering potential insights for sports training and guidelines for high-altitude activities.