Abstract
Although emerging as a cost and time efficient way to prepare for competition in the heat, recent evidence indicates that “short-term” heat acclimation (<7 days) may not be sufficient for females to adapt to repeated heat stress. Furthermore, self-paced performance following either short-term, or longer (>7 days) heat acclimation has not been examined in a female cohort. Therefore, the aim of this study was to investigate self-paced endurance performance in hot conditions following 4- and 9-days of a high-intensity isothermic heat acclimation protocol in a female cohort. Eight female endurance athletes (mean ± SD, age 27 ± 5 years, mass 61 ± 5 kg, VO2peak 47 ± 6 ml⋅kg⋅min−1) performed 15-min self-paced cycling time trials in hot conditions (35°C, 30%RH) before (HTT1), and after 4-days (HTT2), and 9-days (HTT3) isothermic heat acclimation (HA, with power output manipulated to increase and maintain rectal temperature (Trec) at ∼38.5°C for 90-min cycling in 40°C, 30%RH) with permissive dehydration. There were no significant changes in distance cycled (p = 0.47), mean power output (p = 0.55) or cycling speed (p = 0.44) following 4-days HA (i.e., from HTT1 to HTT2). Distance cycled (+3.2%, p = 0.01; +1.8%, p = 0.04), mean power output (+8.1%, p = 0.01; +4.8%, p = 0.05) and cycling speed (+3.0%, p = 0.01; +1.6%, p = 0.05) were significantly greater in HTT3 than in HTT1 and HTT2, respectively. There was an increase in the number of active sweat glands per cm2 in HTT3 as compared to HTT1 (+32%; p = 0.02) and HTT2 (+22%; p < 0.01), whereas thermal sensation immediately before HTT3 decreased (“Slightly Warm,” p = 0.03) compared to ratings taken before HTT1 (“Warm”) in 35°C, 30%RH. Four-days HA was insufficient to improve performance in the heat in females as observed following 9-days HA.
Highlights
Hot ambient temperatures and elevated humidity are known to negatively impact endurance exercise performance (Tatterson et al, 2000; Périard et al, 2011)
Two values were obtained for measurements of resting blood lactate and an additional two values were obtained for blood lactate immediately following hot time trials (HTT)
In this study’s female cohort, short-term heat acclimation” (STHA) did not significantly improve time-trial performance in the heat; 9-days HA did. These results were consistent with the study hypothesis, which predicted that STHA would be insufficient to improve self-paced performance in females, and that a longer heat acclimation stimulus would be required to induce the physiological adaptations needed for performance improvements in the heat
Summary
Hot ambient temperatures and elevated humidity are known to negatively impact endurance exercise performance (Tatterson et al, 2000; Périard et al, 2011). Heat acclimation is an effective strategy to drive favorable physiological adaptations, thereby reducing athletic performance impairments caused by these challenging environments (Sawka et al, 2011; Périard et al, 2015; Racinais et al, 2015). Based on this, Garrett et al (2009) first demonstrate meaningful performance improvements following just 5 days of isothermic heat acclimation, termed “short-term heat acclimation” (STHA). Mee et al (2018) demonstrated that a longer daily heat exposure (achieved via 20-min of sitting in sauna suits in 50◦C, 30%RH immediately before 90-min isothermic heat acclimation) successfully induced heat adaptations in a female cohort following STHA (5-days). The authors concluded that females require either a longer daily heat exposure (Mee et al, 2018), or a greater number of heat exposures (Mee et al, 2015) to elicit favorable physiological adaptations
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