title: Isothermic and fixed intensity heat acclimation methods induce similar heat adaptation following short and long-term timescales.
abstract: Heat acclimation requires the interaction between hot environments and exercise to elicit thermoregulatory adaptations. Optimal synergism between these parameters is unknown. Common practise involves utilising a fixed workload model where exercise prescription is controlled and core temperature is uncontrolled, or an isothermic model where core temperature is controlled and work rate is manipulated to control core temperature. Following a baseline heat stress test; 24 males performed a between groups experimental design performing short term heat acclimation (STHA; five 90 min sessions) and long term heat acclimation (LTHA; STHA plus further five 90 min sessions) utilising either fixed intensity (50% VO2peak), continuous isothermic (target rectal temperature 38.5 °C for STHA and LTHA), or progressive isothermic heat acclimation (target rectal temperature 38.5 °C for STHA, and 39.0 °C for LTHA). Identical heat stress tests followed STHA and LTHA to determine the magnitude of adaptation. All methods induced equal adaptation from baseline however isothermic methods induced adaptation and reduced exercise durations (STHA = -66% and LTHA = -72%) and mean session intensity (STHA = -13% VO2peak and LTHA = -9% VO2peak) in comparison to fixed (p < 0.05). STHA decreased exercising heart rate (-10 b min(-1)), core (-0.2 °C) and skin temperature (-0.51 °C), with sweat losses increasing (+0.36 Lh(-1)) (p<0.05). No difference between heat acclimation methods, and no further benefit of LTHA was observed (p > 0.05). Only thermal sensation improved from baseline to STHA (-0.2), and then between STHA and LTHA (-0.5) (p<0.05). Both the continuous and progressive isothermic methods elicited exercise duration, mean session intensity, and mean T(rec) analogous to more efficient administration for maximising adaptation. Short term isothermic methods are therefore optimal for individuals aiming to achieve heat adaptation most economically, i.e. when integrating heat acclimation into a pre-competition taper. Fixed methods may be optimal for military and occupational applications due to lower exercise intensity and simplified administration.
authors.affiliation: Centre for Sport and Exercise Science and Medicine (SESAME), Exercise in Extreme Environments Laboratory, University of Brighton, Welkin Human Performance Laboratories, Denton Road, Eastbourne, UK. Electronic address: o.r.gibson@brighton.ac.uk.
authors.forename: Oliver R
authors.initials: OR
authors.lastname: Gibson
authors.affiliation: Centre for Sport and Exercise Science and Medicine (SESAME), Exercise in Extreme Environments Laboratory, University of Brighton, Welkin Human Performance Laboratories, Denton Road, Eastbourne, UK.
authors.forename: Jessica A
authors.initials: JA
authors.lastname: Mee
authors.affiliation: Muscle Cellular and Molecular Physiology (MCMP) and Applied Sport and Exercise Science (ASEP) Research Groups, Department of Sport Science and Physical Activity, Institute of Sport and Physical Activity Research (ISPAR), University of Bedfordshire, Bedford Campus, Polhill Avenue, Bedfordshire, UK.
authors.forename: James A
authors.initials: JA
authors.lastname: Tuttle
authors.affiliation: Muscle Cellular and Molecular Physiology (MCMP) and Applied Sport and Exercise Science (ASEP) Research Groups, Department of Sport Science and Physical Activity, Institute of Sport and Physical Activity Research (ISPAR), University of Bedfordshire, Bedford Campus, Polhill Avenue, Bedfordshire, UK.
authors.forename: Lee
authors.initials: L
authors.lastname: Taylor
authors.affiliation: Centre for Sport and Exercise Science and Medicine (SESAME), Exercise in Extreme Environments Laboratory, University of Brighton, Welkin Human Performance Laboratories, Denton Road, Eastbourne, UK.
authors.forename: Peter W
authors.initials: PW
authors.lastname: Watt
authors.affiliation: Centre for Sport and Exercise Science and Medicine (SESAME), Exercise in Extreme Environments Laboratory, University of Brighton, Welkin Human Performance Laboratories, Denton Road, Eastbourne, UK.
authors.forename: Neil S
authors.initials: NS
authors.lastname: Maxwell
mesh_terms: D000064:Acclimatization; D000328:Adult; D001833:Body Temperature Regulation; D006358:Hot Temperature; D006801:Humans; D008297:Male; D013312:Stress, Physiological; D013997:Time Factors; D055815:Young Adult
keywords: Heat illness; Heat stress; Hyperthermia; Taper; Temperature; Thermoregulation