Carl A James, Alan J Richardson, Peter W Watt, Ashley G B Willmott, Oliver R Gibson, Neil S Maxwell

Following heat acclimation (HA), endurance running performance remains impaired in hot vs temperate conditions. Combining HA with precooling demonstrates no additive benefit in intermittent sprint, or continuous cycling exercise protocols, during which heat strain may be less severe compared to endurance running. This study investigated the effect of short-term heat acclimation (STHA) combined with mixed-methods precooling, on endurance running performance and directly compared precooling and HA. Nine amateur trained runners completed 5 km treadmill time trials in the heat (32°C, 60% RH) under four conditions; no intervention (CON), precooling (PC), short-term heat acclimation (5 days - HA) and short-term heat acclimation with precooling (HA+PC). Mean (±SD) performance times were; CON 1476 (173) s, PC 1421 (146) s, HA 1378 (116) s and HA+PC 1373 (121) s. This equated to the following improvements versus CON; PC -3.7%, HA -6.6% and HA+PC -7.0%. Statistical differences were only observed between HA and CON (p=0.004, d=0.68, 95% CI [-0.27, 1.63]) however, similar effect sizes were observed for HA+PC vs CON (d=0.70, 95% CI [-0.25, 1.65]), with smaller effects between PC vs CON (d=0.34, 95% CI [-0.59, 1.27]), HA vs PC (d=0.33, 95% CI [-0.60, 1.26]) and HA+PC vs PC (d=0.36, 95% CI [-0.57, 1.29]). Pilot testing revealed a time trial typical error of 16 s (1.2%). Precooling offered no further benefit to performance in the acclimated individual, despite modest alleviation of physiological strain. Maintenance of running speed in HA+PC, despite reduced physiological strain, may indicate an inappropriate pacing strategy therefore, further familiarisation is recommended to optimise a combined strategy. Finally, these data indicate HA, achieved through cycle training, yields a larger ergogenic effect than precooling on 5km running performance in the heat, although precooling remains beneficial when HA is not possible.