The effects of single versus twice daily short term heat acclimation on heat strain and 3000m running performance in hot, humid conditions

Ashley Willmott, Oliver Gibson, Mark Hayes, Neil Maxwell

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Endurance performances are impaired under conditions of elevated heat stress. Short term heat acclimation (STHA) over 4-6 days can evoke rapid adaptation, which mitigate decrements in performance and alleviate heat strain. This study investigated the efficacy of twice daily heat acclimation (TDHA) compared to single session per day heat acclimation (SDHA) and normothermic training, at inducing heat acclimation phenotype and its impact upon running performance in hot, humid conditions. Twenty one, moderately trained males were matched and assigned to three groups; SDHA (mean ± SD) (peak oxygen consumption [V̇O2peak] 45.8±6.1, body mass 81.3±16.0 kg, stature 182±3 cm), TDHA (46.1±7.0, 80.1±11.9 kg, 178±4 cm) or control (CON) (47.1±3.5, 78.6±16.7 kg, 178±4 cm). Interventions consisted of 45 min cycling at 50% V̇O2peak, once daily for 4d (SDHA) and twice daily for 2d (TDHA), in 35°C, 60% relative humidity (RH), and once daily for 4d (CON) in 21°C, 40% RH. Participants completed a pre- and post-intervention 5 km treadmill run trial in 30°C, 60% RH, where the first 2 km were fixed at 40% V̇O2peak and the final 3 km was self-paced. No statistically significant interaction effects occurred within- or between-groups over the 2-4d intervention. While within-group differences were found in physiological and perceptual measures during the fixed intensity trial post-intervention, they did not statistically differ between-groups. Similarly, TDHA (-36±34s [+3.5%]) and SDHA (-26±28s [+2.8%]) groups improved 3 km performances (p=0.35), but did not differ from CON (-6±44s [+0.6%]). This is the first study to investigate the effects of HA twice daily and compare it with traditional single session per day STHA. These STHA protocols may have the ability to induce partial adaptive responses to heat stress and possibly enhance performance in environmentally challenging conditions, however, future development is warranted to optimise the administration to provide a potent stimuli for heat adaptation in athletic and military personnel within a rapid regime.
Original languageEnglish
Pages (from-to)59-67
Number of pages9
JournalJournal of Thermal Biology
Publication statusPublished - 1 Feb 2016

Bibliographical note

© 2016. This manuscript version is made available under the CC-BY-NC-ND 4.0 license


  • Heat Acclimation
  • Short Term Heat Acclimation
  • Heat stress
  • Twice daily
  • Thermoregulation
  • Athletics
  • Military


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