Adaptation to Hot Environmental Conditions: An Exploration of the Performance Basis, Procedures and Future Directions to Optimise Opportunities for Elite Athletes

Abstract

Extreme environmental conditions present athletes with diverse challenges; however, not all sporting events are limited by thermoregulatory parameters. The purpose of this leading article is to identify specific instances where hot environmental conditions either compromise or augment performance and, where heat acclimation appears justified, evaluate the effectiveness of pre-event acclimation processes. To identify events likely to be receptive to pre-competition heat adaptation protocols, we clustered and quantified the magnitude of difference in performance of elite athletes competing in International Association of Athletics Federations (IAAF) World Championships (1999-2011) in hot environments (>25 °C) with those in cooler temperate conditions (<25 °C). Athletes in endurance events performed worse in hot conditions (~3 % reduction in performance, Cohen's d > 0.8; large impairment), while in contrast, performance in short-duration sprint events was augmented in the heat compared with temperate conditions (~1 % improvement, Cohen's d > 0.8; large performance gain). As endurance events were identified as compromised by the heat, we evaluated common short-term heat acclimation (≤7 days, STHA) and medium-term heat acclimation (8-14 days, MTHA) protocols. This process identified beneficial effects of heat acclimation on performance using both STHA (2.4 ± 3.5 %) and MTHA protocols (10.2 ± 14.0 %). These effects were differentially greater for MTHA, which also demonstrated larger reductions in both endpoint exercise heart rate (STHA: -3.5 ± 1.8 % vs MTHA: -7.0 ± 1.9 %) and endpoint core temperature (STHA: -0.7 ± 0.7 % vs -0.8 ± 0.3 %). It appears that worthwhile acclimation is achievable for endurance athletes via both short-and medium-length protocols but more is gained using MTHA. Conversely, it is also conceivable that heat acclimation may be counterproductive for sprinters. As high-performance athletes are often time-poor, shorter duration protocols may be of practical preference for endurance athletes where satisfactory outcomes can be achieved.

Effects of hydration status during heat acclimation on plasma volume and performance

“…Prolonged (>60 minutes) and repeated exercise in the heat (5‐14 days), known as heat acclimation or acclimatization (HA), elicits favorable physiological adaptations such as increased PV, increased sweat rate, and earlier onset of sweating. All of these adaptations facilitate cooling and attenuate the rise in core temperature ( T c ) and heart rate (HR) during exercise in the heat, with most studies demonstrating improved endurance performance (for recent reviews see: Refs ). The associated hypervolemia has also been shown to result in an elevated stroke volume, leading to higher maximal cardiac outputs and …”

“…6 The physiological stressors required to optimize HA remain to be fully elucidated, but several key variables are potentially implicated: (a) total duration and intensity of heat stress; (b) subject hydration status throughout HA; and (c) baseline PV and/or fitness status. 4,9,10 Several research groups have hypothesized that acutely restricting fluid intake during HA protocols, causing permissive dehydration, may further augment physiological strain and fluid regulatory hormonal responses resulting in even greater adaptations compared to euhydration protocols. [11][12][13] Indeed, a couple of studies have examined the impact of permissive dehydration (~2% decrease in body weight [BW]) during short-term HA protocols but have had equivocal results.…”

“…Group averages (SD) over 5 days of heat acclimation (HA) training for heart rate (%HR max ) (A); B, average daily training power outputs relative to V ⋅ O 2max wattage maximum. (%W max ; [B]); accumulated thermal load relative to baseline T c (AUC;°C •min; [C]) and psychothermal ratings (T sen [0-9], T com[1][2][3][4][5], RPE(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20); [D]) for DEH, EUH and CON. # CON different than DEH or EUH (P < 0.001); $ EUH different than DEH or CON (P < 0.05); * DEH and EUH different than CON (P < 0.05).…”

Ischaemic preconditioning does not alter the determinants of endurance running performance in the heat

“…These changes are significant given that the lactate turnpoint (LTP) remains a strong predictor of time trial performance in hot environments (Lorenzo et al 2011) and appears sensitive to the effects of IP (Bailey, Jones, et al 2012), whilst V" O 2max is considered the best individual predictor of performance, explaining 90.2% of the total variance in a 16-km run (McLaughlin et al 2010). Furthermore, evidence exists that occlusion may modify thermoregulatory responses, through the muscle metaboreflex, independently of body temperature (Kondo et al 1999 (James et al 2015;Randall et al 2015) and heat acclimation (Gibson et al 2015;Mee et al 2015;Willmott et al 2016), that seek to alleviate the significant impairment to endurance performance afforded by heat stress (Guy et al 2015).…”

The effect of ice slurry ingestion on body temperature and cycling performance in competitive athletes

“…The failure to find any effects of the cooling measures on performance may be due to the exercise protocol not being of sufficient length to detect changes. In fact, thermal strain is not likely to be key determinant in shorter events like running (Guy et al, 2015). The present study examined 45 minutes of SS cycling at 70% VO 2max followed by a 10km TT.…”