Induction and Decay of Short-Term Heat Acclimation in Moderately and Highly Trained Athletes

Abstract

A rethinking of current heat-acclimation strategies is required as most research and advice for improving physiological strain in the heat includes maintaining hydration using long-term acclimation protocols (>10 days). Furthermore, these strategies have tended to use untrained and moderately trained participants. Therefore, the aims of this review were to (i) investigate the effectiveness of short-term heat acclimation (STHA) with moderately and highly trained athletes; (ii) determine the importance of fluid regulatory strain, which has a thermally independent role in heat adaptation; (iii) assess the impact of STHA on a marker of thermotolerance (inducible heat-shock protein 70 [HSP70]); and (iv) provide further information on the decay of acclimation to heat. The review suggests that 5-day STHA is effective, and adaptations may be more pronounced after fluid regulatory strain from a dehydration-acclimation regimen. Furthermore, highly trained athletes may have similar physiological gains to those who are less trained using STHA. However, research has tended to focus on untrained or moderately trained participants and more information is required for highly trained populations. HSP70 response is upregulated across STHA. This indicates increased thermotolerance and protective adaptive change that may indicate HSP70 response as a useful marker of heat acclimation. Physiological adaptations after heat acclimation are relatively short term and may vanish only a few days or weeks after removal from heat exposure. From a practical perspective 5-day STHA may be the preferred acclimation regimen for moderately and highly trained athletes as it has been shown to be effective, less expensive and less likely to disrupt the tapering for competition in elite performers. Furthermore, updated information on the time course of acclimation decay may allow a reliable estimate of how long individuals can be free from heat exposure before reacclimation is required. This is particularly pertinent in present times as many athletes, civilians and military personnel increasingly have to relocate to different climates of the world, often within a short period of time.

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

“…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. No other statistically significant changes were evident.…”

“…Although a key adaptation associated with STHA is indicated by a ~6-7% improvement in cardiovascular stability (Garrett et al, 2011), no statistically significant changes were evident in resting or exercising HR after interventions. 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%).…”

“…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).…”