Induction and decay of short-term heat acclimation

Abstract

Abstract:Most advice for heat adaptation is to use long-term (>10 d) regimes, in which hydration status is maintained. We tested the hypothesis that short-term (5-day) heat acclimation would confer substantial improvements in physiological strain and exercise tolerance for exercise in the heat, and fluid regulatory strain provides a thermally-independent stimulus for such adaptations. Ten moderately-fit males were heat acclimated using controlled hyperthermia (rectal temperature 38.5°C) for 90 min on five consecutive days (T a = 40°C, 60% RH), on two occasions separated by a five-week washout, in a randomly assigned, cross-over design; one with euhydration (EUH) and one with dehydration (DEH) during acclimation bouts. One week before, then on the 2 nd day after each acclimation regime, a heat stress test (HST) was completed, comprising cycling at 40% peak power output for 90 min (T a = 35°C, 60% RH), before incrementing to exhaustion. Plasma volume (PV) at rest was measured using CO rebreathing. Acclimation exercise-induced response of[aldo] p became more pronounced across DEH (  178 pg . mL -1 ; 95%CI: 33 to 324) but not EUH (  -47 pg . mL -1 : -209 to 115) and this difference was significant (P=0.02).Compared to EUH, permissive DEH during acclimation bouts conferred larger acclimation-induced increases in resting PV (4.1%: -1.5 to 9.8%; P=0.06), F Q  (4.2: 0.7 to 7.8 ml . min -1. 100 ml -1 ; P=0.009), FVC (0.06: 0.02 to 0.10 ml . 100ml Tissue -1. min -1. mmHg -1 ; P=0.006) and decreased end-exercise c f by 17% (19: -29 to 9 b·min-1; P=0.05). In conclusion, short-term (5-day) heat acclimation was effective with several adaptations more pronounced after fluid-regulatory strain from a dehydration acclimation regime.

The impact of submaximal exercise during heat and/or hypoxia on the cardiovascular and monocyte HSP72 responses to subsequent (post 24 h) exercise in hypoxia

“…These differences become more apparent when participants were separated into trained (>50 mL · kg · min -1 , 55.8 ± 5.5 mL · kg · min -1 ; n  = 6) and untrained (<40 mL · kg · min -1 , 38 ± 2.4 mL · kg · min -1 ; n  = 6) groups. It is well established that endurance-trained athletes behave physiologically as though already adapted to heat stress [44] via an increased heat loss capacity and decreased rectal temperature [45]. This is illustrated by the slower adaptation to heat seen in those with lower levels of aerobic fitness, compared to their trained counterparts [24].…”

Consensus Recommendations on Training and Competing in the Heat

“…In turn, this may limit the magnitude of adaptation if the duration and/or the intensity of the heat–exercise training sessions are not increased accordingly [42]. When possible, an isothermic protocol (e.g., controlled hyperthermia to a core temperature of at least 38.5 °C) can be implemented to optimize the adaptations [43, 44]. However, isothermic protocols may require greater control and the use of artificial laboratory conditions, which could limit their practicality in the field.…”

Isothermic and fixed intensity heat acclimation methods induce similar heat adaptation following short and long-term timescales

“…STHA utilises r5 daily exposures, facilitating rapid, but, incomplete adaptation ( $ 75% compared to LTHA (Pandolf, 1979)). Notwithstanding, STHA still remains an effective tool used by practitioners for augmenting adaptation before exposure to hot environments, improving tolerance to exercise or work (Garrett et al, 2012(Garrett et al, , 2009.…”

“…Isothermic heat acclimation (also known as controlled hyperthermia) is imposed based upon endogenous (internal) criteria (Castle et al, 2012;Garrett et al, 2014Garrett et al, , 2012Garrett et al, , 2009Hom et al, 2012;Machado-Moreira et al, 2006;Magalhães et al, 2010aMagalhães et al, , 2010bPatterson et al, 2014Patterson et al, , 2004, and might provide sustained targeting and attainment of specific and individualised internal temperatures through a combination of active and passive heat acclimation (Fox et al, 1963). The balance between work and rest to target and maintain specific core temperatures ensures a consistency, or a progression of endogenous heat strain to induce adaptation, albeit requiring alterations in administration throughout each session.…”