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Heat Acclimation Can Improve Your Cardiovascular Performance in Hot Conditions

Research has shown that athletes who include passive “hyperthermic conditioning” and “active thermal fitness” protocols with their workout regimens have significantly improved personal fitness and endurance levels. Hyperthermic conditioning holistically increases biochemical and other physiological outputs and improves elite performance levels.  In order to achieve the benefits of heat acclimation, core body temperature needs to be increased.

The beneficial adaptations which result from heat acclimation programs include decreased heart rate, improved heat transfer from core to skin, more efficient cardiovascular function, skin and body temperature during exercise, increased blood volume, reduced loss of electrolytes via kidney filtration, and many others. Frequent exposure to simulated heat—such as extended (high heat) sauna exposure – should be started as soon as possible. For many athletes, opportunities to train in the actual competitive environment may not be available. Passive acclimation induces the same cardiovascular and sweat changes without the logistics, the recovery implications or discomfort that generally accompany exercise in the heat.

Research provides a number of recommendations that will help athletes prepare for intensive athletic competition in hot conditions:

  1. Passive hyperthermic-exercise: Frequent exposure to simulated heat—should be started as soon as possible. For many athletes, opportunities to train in the actual competitive environment may not be available. Passive acclimation induces the same cardiovascular and sweat changes without the logistics, the recovery implications or discomfort that generally accompany exercise in the heat.
  2. Active acclimation (exercise in the heat): Actual exercise in a hot environment is crucial for experiencing the physiological and psychological responses to performing exercise in extremely hot weather. Active acclimation is more difficult to arrange, as it typically requires treadmill or cycling sessions in a dry heat sauna or in a small, heated room.

Positive adaptations to both passive and active hyperthermic conditioning can occur with as few as 10 days of regular heat exposure.

Heat Acclimation Science

Heat acclimation can have profound benefits on cardiovascular function and concomitantly aerobic performance in the heat. For example, Racinais et al. (2015) demonstrated that three cycling time trials (43 km) undertaken in hot outdoor conditions (~37 °C) were initiated at a similar power output to that of time trials conducted in cool conditions (~8 °C). However, a marked decrease in power output occurred in the heat following the onset of exercise, which was partly recovered after one week of training in the heat and almost fully restored after two weeks of training in the heat. As expected, heart rate was similar during all trials in the heat and slightly higher than in the cool, which supports the contention that a similar relative exercise intensity (i.e. %VO2 max) was maintained (Périard et al., 2011; Wingo et al., 2012; Périard and Racinais, 2015) and that with heat acclimation absolute intensity (i.e. power output) increased.

Heat acclimation has been shown to improve the VO2 max of trained individuals in hot conditions with Sawka et al. (1985) reporting a 4% (49 °C) improvement and Lorenzo et al. (2010) and Keiser et al. (2015) noting increases of 8–10% (38 °C). Despite these improvements, acute heat stress mediates a reduction in VO2 max relative to values recorded in temperate conditions that cannot be compensated for by heat acclimation (i.e. VO 2 max in the heat remains lower than in cool conditions). Notwithstanding, heat acclimation has been shown to increase cycle exercise time trial performance in the heat in conjunction with an increase in cardiac output and lactate threshold, plasma volume expansion, lower skin temperatures, and a larger core-to-skin gradient after heat acclimation (Lorenzo et al., 2010). Moreover, the increase in performance was proportional to the increase in VO 2 max in the heat (Lorenzo et al., 2010), which further reinforces the notion that relative exercise intensity significantly influences self-paced exercise performance in the heat (Périard et al., 2011; Périard and Racinais, 2015).

Observations of enhanced self-paced exercise performance have also been noted in cool conditions in proportion to improvements in VO2 max under the same conditions (Lorenzo, et al., 2010). This supports previous observations of heat acclimation increasing VO2 max in trained (3–5%) (Sawka et al., 1985; Lorenzo et al., 2010), untrained (13%) and unfit (23%) individuals in cool conditions (Shvartzet al.,1977), and reinforces the 32% increase in run time to exhaustion noted in fit individuals after heat acclimation via post-exercise sauna bathing.

Passive vs. Active Hyperthermic Exercise

Although passive heat exposure induces adaptations commensurate with that magnitude of strain (Takamata et al., 2001; Beaudin et al., 2009; Brazaitis and Skurvydas, 2010) and passive hot water immersion after exercise can improve endurance performance in the heat (Zurawlew et al., 2015), the inclusion of exercise with heat exposure provides additional strain that generally elicits more profound adaptations (Armstrong and Pandolf, 1988; Wenger, 1988). Accordingly, the magnitude of physiological adaptation induced by heat acclimation or acclimatization depends largely on the initial heat exposure status (i.e. recent heat exposure, season, fitness level), as well as the exercise intensity, duration, frequency, and number of heat exposures, along with the induction protocol (Sawka et al., 1996; Taylor, 2000; Taylor, 2014; Périard et al., 2015 a).

Repeated exercise-heat exposure at a constant work rate (i.e., traditional occupational and military heat acclimation protocol) is not likely as effective in eliciting adaptation as maintaining hyperthermia at a given core temperature (e.g. 38.5 °C; controlled hyperthermia or isothermic heat acclimation) (Taylor, 2000, 2014). The traditional heat acclimation model offers a constant forcing function (i.e. fixed endogenous and exogenous thermal loads), which as adaptations progressively develop, results in decreased physiological strain and reduced further adaptations (Eichna et al., 1950; Fox et al., 1963a; Rowell et al., 1967). In contrast, with controlled hyperthermia protocols the forcing function is increased in proportion to the adaptations by manipulating the endogenous and/or exogenous thermal loads (Garrett et al., 2011; Taylor, 2014). Therefore, it is suggested that greater physiological adaptations occur during a given period with controlled hyperthermia than traditional approaches. Interestingly, recent studies have not found greater adaptations with controlled hyperthermia and the explanation for those findings are unclear (Gibson et al., 2015a,B).

 APL (American Performance Labs) is a research group dedicated to the collection, analysis, and dissemination of published research and articles on the science of hyperthermia and the various applications, technologies and protocols for the use of hyperthermic conditioning.


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