Purpose: To determine the effect of high- versus low-intensity training in the heat and ensuing taper period in the heat on endurance performance. Methods: In total, 19 well-trained triathletes undertook 5 days of normal training and a 1-wk taper including either low- (heat acclimation [HA-L], n=10) or high-intensity (HA-H, n=9) training sessions in the heat (30°C, 50% relative humidity). A control group (n=10) reproduced their usual training in thermoneutral conditions. Indoor 20-km cycling time trials (35°C, 50% relative humidity) were performed before (Pre) and after the main heat exposure (Mid) and after the taper (Post). Results: Power output remained stable in the control group from Pre to Mid (effect size: -0.10 [0.26]) and increased from Mid to Post (0.18 [0.22]). The HA-L group demonstrated a progressive increase in performance from Pre to Mid (0.62 [0.33]) and from Mid to Post (0.53 [0.30]), alongside typical physiological signs of HA (reduced core temperature and heart rate and increased body-mass loss). While the HA-H group presented similar adaptations, increased perceived fatigue and decreased performance at Mid (-0.35 [0.26]) were evidenced and reversed at Post (0.50 [0.20]). No difference in power output was reported at Post between the HA-H and control groups. Conclusion: HA-H can quickly induce functional overreaching in nonacclimatized endurance athletes. As it was associated with a weak subsequent performance supercompensation, coaches and athletes should pay particular attention to training monitoring during a final preparation in the heat and reduce training intensity when early signs of functional overreaching are identified.
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