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.

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

“…During this study, T re rose linearly with time during HA, however, lower T repeak were evident in all groups across the four sessions, suggesting the magnitude of physiological strain was not sufficiently elevated compared to other studies (Patterson, et al, 2004;Garrett, et al, 2009Garrett, et al, , 2011Gibson et al, 2015). This may have led to reduced and partial adaptive responses, whereas, if controlled hyperthermia was administrated, greater adaptation may have been achieved (Fox, et al, 1973).…”

“…However, despite total phenotypic adaptations occurring within 14 to 21 days during LTHA, ≤ 5 daily exposures during short term HA (STHA) facilitates practically significant thermoregulatory and cardiovascular (~75%) adaptations (Pandolf, 1998), over a more applicable duration for athletes and military personnel. Previous STHA studies (Cotter et al, 1997;Patterson et al, 2004;Sunderland et al, 2008;Garrett et al, 2009Garrett et al, , 2011Costa et al, 2014;Mee et al, 2015;Gibson et al, 2015) have reported physiological and athletic performance improvements within hot conditions. During training, prior to competition or in the lead up to military deployment, STHA may appear more feasible for inclusion in established schedules due to lesser disruption and costs, particularly when tapering and will avoid unnecessary reductions in the quality of training or heat-related illnesses.…”

“…Predefined analytical limits to highlight meaningful adaptations among typical STHA adaptations, assist in determining efficacy and robustness of the protocol (Atkinson & Nevill 1998). The a priori meaningful limits for adaptation in resting and exercising parameters for this study have been calculated from numerous STHA research studies (Cotter et al, 1997;Patterson et al, 2004;Sunderland et al, 2008;Garrett et al, 2009Garrett et al, , 2011Garrett et al, , 2014Petersen et al, 2010;Brade et al, 2012;Mee et al, 2015;Gibson et al, 2015;Neal et al, 2015) and are; ∆T re >0.20ºC, ∆HR >5 b.min -1 and ∆PV >5%.…”

“…Single session per day and twice daily heat acclimation STHA studies typically consist of ≤7 consecutive daily exposures within hot (30-40ºC) and humid (20-60% RH) conditions, for 90 min fixed intensity or controlled hyperthermia (Cotter et al, 1997;Patterson et al, 2004;Garrett et al, 2009Garrett et al, , 2011Gibson et al, 2015, Neal et al, 2015. Further comparable studies which have investigated a shorter 4 to 5 day protocol, sport specific or high intensity exercise, either over consecutive or non-consecutive days, have primarily reported "partial" heat acclimation adaptions and performance improvements (Sunderland et al, 2008;Petersen et al, 2010;Brade et al, 2012).…”

Physiological and psychological responses in Fire Instructors to heat exposures

“…However, repeated 8 exposure to heat and exercise may be beneficial in some circumstances. Given suitable 9 preparation and safeguard exercise in the heat on repeated occasions may lead to 10 improved performance, a process termed acclimation (Gibson et al 2015). Berkshire) and Woodway (ELG2, GmbH) treadmills set at 1% incline (Jones and 5 Doust, 1996).…”