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

“…However, using our predefined analytical limits (>5 b.min -1 ), meaningful reductions of 4% (TDHA) and 6% (SDHA) were observed in exercising HR from session 1 to 4 with moderate effect, though CON group did not change (1%). This adaptation is typically concurrent with hypervolemia and is suggested to reduce HR 1 b.min -1 , per ∆P (Convertino, 1991), which was present in HA interventions (4.8% TDHA and 5.2% SDHA), consistent with our analytical limits (>5%) and previous research (Nielsen, et al, 1993;Aoyagi, et al, 1994;Patterson, et al, 2004;Garrett, et al, 2009;Lorenzo, et al, 2010). Another key adaptation is reductions in resting and exercising T re of ~0.6% (Garrett et al, 2011), which significantly altered during TDHA, although this was solely contributed by circadian rhythm variations from morning to afternoon sessions.…”

“…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).…”

Repeated muscle damage blunts the increase in heat strain during subsequent exercise heat stress

“…Further research should investigate whether the observed reduction in thermal strain with repeated EIMD translates into a meaningful performance improvement. In this regard, its noteworthy that a similar reduction in final exercising Tre (-0.3 °C) during exercise-heat-stress after 5 d heat acclimation brought about a 14% improvement in endurance performance during exercise in the heat (Garrett et al 2009). We can have added confidence in our findings, since a particular strength of the current study is that a comparable control group was used in order to discount confounding heat acclimation or passage of time effects.…”