Heat stress reduces high-speed running and sprint performances in highly trained young football players during official match play.

This study aimed to analyze the running performance of highly trained young male football players during official matches played under different heat stress risk conditions. Young highly trained male football players (n = 37) from one club in Brazil were monitored during 50 official matches. Relative and absolute metrics were monitored, including total distance, low-to-moderate running, high-speed running and activities, sprinting distance, number of sprints, accelerations, and decelerations. Heat stress risk conditions were calculated using the wet-bulb globe temperature and were categorized as low, moderate, high. Linear mixed models were used to compare performance metrics under different heat stress risk conditions. Overall, relative and absolute performance metrics were reduced under high-risk conditions compared to low-risk: high-speed running distance and number of sprints and decelerations were reduced by 24%, 25%, and 19%, respectively (p < 0.05); in particular, high-speed running distance was markedly reduced in central defenders (-43%), fullbacks (-25%) and forwards (-19%) under high-risk conditions. Under moderate-risk conditions, compared to low-risk, high-speed running distance and sprinting were reduced (-16% and -26%, respectively; p < 0.01), with major differences in central defenders (-32%) and forwards (-16%). When high-risk conditions were compared with moderate-risk conditions, differences were observed in absolute metrics (p < 0.05): central defenders had reduced total distance and low-to-moderate running by 4-5%, whereas high-speed running distance was reduced in fullbacks (9%). These findings indicate that young highly trained football players reduce high-intensity running performance under heat stress, likely as a pacing strategy to cope with increased physiological strain during matches.