Evidence of seasonal heat acclimatisation in recreationally active adults during a mild summer

Objectives: To assess the magnitude of seasonal heat acclimatisation in recreationally active adults and contextualise the process by documenting the factors that influence adaptations. Design: Longitudinal, repeated measures design. Methods: Seventeen (7 females) recreationally active adults (28 +/- 8 yr, VO2peak 54 +/- 8 mL center dot kg-1 center dot min-1) exercising outdoors a minimum of 5 h center dot wk-1 completed a 45-min heat response test running at 60 % VO2peak in 40 degrees C and 30 % relative humidity prior to, midway through, and following summer. Self-reported physical activity diaries were completed at the beginning and end of summer. Daytime wet-bulb globe temperature was calculated for each day of summer. Data were analysed using Bayesian ordinal regressions. Results: Daytime wet-bulb globe temperature was 22.0 +/- 4.4 degrees C, with the most common hour for recreational exercise being 17:00 to 18:00. Following summer, the rise in oesophageal temperature and mean skin temperature during the heat response test was lower by 0.12 degrees C [90 % credible interval: -0.30, 0.06; probability of direction = 87 %] and 0.43 degrees C [-0.74, -0.10; 98 %], respectively. Moreover, forearm local sweat rate increased by 0.26 mg center dot cm-2 center dot min-1 [0.15, 0.36; 100 %]. There was minimal evidence of a change in the increase in heart rate (1 beat center dot min-1 [-3, 5; 62 %]), or whole-body sweat rate (0.03 L center dot h-1 [-0.11, 0.15; 68 %]) during the heat response test. Conclusions: Although there was evidence of partial heat adaptation in recreationally active adults following summer, a combination of exercising later in the day and the prevailing environmental conditions (La Niha in SouthEastern Australia) may have blunted the development of further adaptations. (c) 2023 The Author(s). Published by Elsevier Ltd on behalf of Sports Medicine Australia. This is an open access article