Introduction: This comparative analysis aimed to determine whether repeated sprint training in hypoxia (RSH) or heat (RSHT) more effectively improves repeated sprint ability (RSA) in normoxic, temperate conditions, while also examining potential sex differences. Methods: Males and females participated in two randomized controlled studies consisting of a 7-week repeated sprint training 2×/week in hypoxia (HYP; FiO2 = 0.146, 18-20°C, 55% relative humidity (RH); males: n= 12, females: n= 10), heat (HEAT; FiO2 = 0.209, 30°C, 60% RH; males: n= 12, females: n= 14), or control conditions (CON; FiO2 = 0.209, 18-20°C, 55% RH, males: n= 23, females: n= 21). RSA and aerobic and anaerobic capacities were evaluated in normoxic, temperate conditions before and after training. Results: The delta (delta) from Pre- to Post-test was compared between training environmental conditions and sexes for the variables showing improvements after training. The delta in sprint number during the RSA test was higher in the intervention than the control group (HYP vs. CON, delta = + 6 ± 1 sprints, p < 0.001; HEAT vs. CON, delta = + 4 ± 1 sprints, p = 0.001) with no difference between HYP and HEAT. A tendency for a main sex effect (p = 0.071), and for a sex*ENV interaction (p = 0.070) was observed. The ? in power output during the RSA test, mean power during the Wingate test, and VO2 peak showed no differences between environmental conditions or sexes, while a tendency for a main sex effect in RSA total work was detected (p = 0.055). Discussion: Seven weeks of RSH and RSHT similarly enhanced RSA in males and females under normoxic, temperate conditions, with a trend towards further improvements with hypoxia in males.
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