Heat Acclimation Strategy Research Articles

Implications of Heat Stress-induced Metabolic Alterations for Endurance Training.

Inducing a heat-acclimated phenotype via repeated heat stress improves exercise capacity and reduces athletes̓ risk of hyperthermia and heat illness. Given the increased number of international sporting events hosted in countries with warmer climates, heat acclimation strategies are increasingly popular among endurance athletes to optimize performance in hot environments. At the tissue level, completing endurance exercise under heat stress may augment endurance training adaptation, including mitochondrial and cardiovascular remodeling due to increased perturbations to cellular homeostasis as a consequence of metabolic and cardiovascular load, and this may improve endurance training adaptation and subsequent performance. This review provides an up-to-date overview of the metabolic impact of heat stress during endurance exercise, including proposed underlying mechanisms of altered substrate utilization. Against this metabolic backdrop, the current literature highlighting the role of heat stress in augmenting training adaptation and subsequent endurance performance will be presented with practical implications and opportunities for future research.

Heat preparedness and exertional heat illness in Paralympic athletes: A Tokyo 2020 survey

ABSTRACT Paralympic athletes may be at increased risk for exertional heat illness (EHI) due to reduced thermoregulatory ability as a consequence of their impairment. This study investigated the occurrence of heat-stress related symptoms and EHI, and the use of heat mitigation strategies in Paralympic athletes, both in relation to the Tokyo 2020 Paralympic Games and previous events. Paralympic athletes competing in Tokyo 2020 were invited to complete an online survey five weeks prior to the Paralympics and up to eight weeks after the Games. 107 athletes (30 [24–38] years, 52% female, 20 nationalities, 21 sports) completed the survey. 57% of respondents had previously experienced heat-stress related symptoms, while 9% had been medically diagnosed with EHI. In Tokyo, 21% experienced at least one heat-stress related symptom, while none reported an EHI. The most common symptom and EHI were, respectively, dizziness and dehydration. In preparation for Tokyo, 58% of respondents used a heat acclimation strategy, most commonly heat acclimatization, which was more than in preparation for previous events (45%; P = 0.007). Cooling strategies were used by 77% of athletes in Tokyo, compared to 66% during past events (P = 0.18). Cold towels and packs were used most commonly. Respondents reported no medically-diagnosed EHIs during the Tokyo 2020 Paralympic Games, despite the hot and humid conditions in the first seven days of competition. Heat acclimation and cooling strategies were used by the majority of athletes, with heat acclimation being adopted more often than for previous competitions.

Effect Of Dry And Wet Heat Acclimation Strategies On Exercise Endurance During Warm Water Dives

Diving in warm water imposes greater thermal risk to Navy divers during exercise compared to thermoneutral waters. Heat acclimation strategies to reduce thermal load of warm water diving have rarely been examined. PURPOSE: Evaluate diver exercise endurance in warm and hot water conditions after wet and dry heat acclimation. METHODS: Twenty male divers completed this study at the Navy Experimental Diving Unit. Subject pairs were matched by VO2max and randomly assigned to one of two heat acclimation groups. One group (n = 9) cycled at 125-150 W for one hour in a non-immersed (DRY) condition (34.4 °C, 50% relative humidity) while the second group (n = 11) cycled at 50 W for one hour while immersed in 34.4 °C water (WET). Following ten days of acclimation, diver endurance was tested via exercise time to exhaustion on an underwater cycle ergometer in 35.8 °C (WARM) and 37.2 °C (HOT) water at a fixed intensity of 50 W on separate days. Core temperature (Tc) was continuously recorded and for all dives, subjects wore light “dive skins” used by Special Operations Forces (SOF) divers. Data was analyzed with two-way analysis of variance compared between water temperatures and acclimation groups. RESULTS: Time to exhaustion was reduced in WARM compared to HOT water (p < 0.01) in both DRY (92.7 ± 41.6 min in 35.8 °C vs. 43.4 ± 17.5 min in 37.2 °C) and WET (98.7 ± 38.4 min in 35.8 °C vs. 51.7 ± 26.7 min in 37.2 °C) groups, but did not differ between groups (p = 0.53). Rate of Tc rise was greater with higher water temperature (main effect: p < 0.01) but was not different between DRY and WET groups (p = 0.74). Maximum Tc (p = 0.74 and p = 0.95) and Tc change from baseline (p = 0.95 and p = 0.56) was not different between water temperatures or acclimations group, respectively. CONCLUSIONS: Diver endurance decreased with increased water temperature but was unchanged between WET and DRY heat acclimation. Divers became exhausted at a similar core temperature during WARM and HOT water exercise. Mechanisms and applications of heat acclimation for warm water diving should be further explored. The views expressed in this abstract are those of the authors and do not reflect the official policy of the Department of the Army, Department of Navy, Department of Defense, or the U.S. Government.

Effectiveness of skin-heating using a water-perfused suit as passive and post-exercise heat acclimation strategies.

The purpose of the present study was to evaluate the effectiveness of passive and post-exercise heat acclimation strategies through directly heating the skin with a water-perfused suit. Nineteen young males participated in the heat acclimation (HA) protocols for 10 days, which were conducted at an air temperature of 33oC with 60%RH. The exercise-only condition (N=6) conducted 1-h treadmill walking (6km·h-1) followed by 1-h rest. The post-exercise passive-heating condition (N=6) wore the suit (inflow water temperature 44.2oC) for 1-h after 1-h walking. The passive-heating condition (N=7) donned the suit for 2h. Heat tolerance tests (leg immersion in 42oC water for 60min) were conducted before and after the training to evaluate changes due to the 10-day intervention. Reflecting that suit-wearing for 10 days as both passive and post-exercise HA strategies can effectively induce adaptive changes, significant interaction effects appeared in: increase or decrease in mean skin temperature (P<0.05) and elevation in whole-body sweat rate (P<0.05). Reduction in rectal temperature (P<0.05) and blood pressure (P<0.05) were found most prominently in the passive-heating condition. These results indicate that this new method of heat acclimation training, donning a skin-heating water-perfused suit, can generate thermoregulatory benefits. The passive HA intervention could be applied to individuals for whom doing exercise regularly are not feasible.

Adaptive changes in physiological and perceptual responses during 10-day heat acclimation training using a water-perfused suit

BackgroundWhile active heat acclimation strategies have been robustly explored, not many studies highlighted passive heat acclimation strategies. Particularly, little evidence demonstrated advantages of utilizing a water-perfused suit as a passive heating strategy. This study aimed to explore heat adaptive changes in physiological and perceptual responses during 10-day heat acclimation training using a water-perfused suit.MethodsNineteen young males were divided into three experimental groups: exercise condition (N = 6, HAEXE, 1-h exercise at 6 km h−1 followed by 1-h rest in a sitting position), exercise and passive heating condition (N = 6, HAEXE+SUIT, 1-h exercise at 6 km h−1 followed 1-h passive heating in a sitting position), and passive heating condition (N = 7, HASUIT, 2-h passive heating in a sitting position). All heating programs were conducted for 10 consecutive days in a climatic chamber maintained at 33 °C with 60% relative humidity. The passive heating was conducted using a newly developed water-perfused suit with 44 °C water.ResultsGreater whole-body sweat rate and alleviated perceptual strain were found in HASUIT and HAEXE+SUIT after 5 and/or 10 days (P < 0.05) but not in the exercise-only condition (HAEXE). Lower rectal temperature and heart rate were found in all conditions after the training (P < 0.05). Heat adaptive changes appeared earlier in HASUIT except for sweat responses.ConclusionsFor heat acclimation in hot humid environments, passive and post-exercise heat acclimation training using the suit (water inflow temperature 44 °C) were more effective than the mild exercise (1-h walking at 6 km h−1). This form of passive heating (HASUIT) may be an especially effective strategy for the elderly and the disabled who are not able to exercise in hot environments.

Post-exercise Hot Water Immersion Elicits Heat Acclimation Adaptations That Are Retained for at Least Two Weeks.

Heat acclimation by post-exercise hot water immersion (HWI) on six consecutive days reduces thermal strain and improves exercise performance during heat stress. However, the retention of adaptations by this method remains unknown. Typically, adaptations to short-term, exercise-heat-acclimation (<7 heat exposures) decay rapidly and are lost within 2 weeks. Short-term protocols should therefore be completed within 2 weeks of relocating to the heat; potentially compromising pre-competition/deployment training. To establish whether adaptations from post-exercise HWI are retained for up to 2 weeks, participants completed a 40-min treadmill run at 65% max in the heat (33°C, 40% RH) before (PRE) and 24 h after (POST) the HWI intervention (n = 13) and then at 1 week (WK 1) and 2 weeks (WK 2) after the HWI intervention (n = 9). Heat acclimation involved a 40-min treadmill run (65% max) on six consecutive days in temperate conditions (20°C), followed by ≤40 min HWI (40°C). Post-exercise HWI induced heat acclimation adaptations that were retained for at least 2 weeks, evidenced by reductions from PRE to WK 2 in: resting rectal core temperature (Tre, −0.36 ± 0.25°C), Tre at sweating onset (−0.26 ± 0.24°C), and end-exercise Tre (−0.36 ± 0.37°C). Furthermore, mean skin temperature (Tsk) (−0.77 ± 0.70°C), heart rate (−14 ± 10 beats⋅min–1), rating of perceived exertion (−1 ± 2), and thermal sensation (−1 ± 1) were reduced from PRE to WK 2 (P < 0.05). However, PRE to POST changes in total hemoglobin mass, blood volume, plasma volume, the drive for sweating onset, sweating sensitivity and whole body sweating rate did not reach significance (P > 0.05). As such, the reduction in thermal strain during exercise-heat stress appears likely due to the reduction in resting Tre evident at POST, WK 1, and WK 2. In summary, 6 days of post-exercise HWI is an effective, practical and accessible heat acclimation strategy that induces adaptations, which are retained for at least 2 weeks. Therefore, post-exercise HWI can be completed during an athlete’s pre-taper phase and does not suffer from the same practical limitations as short-term, exercise-heat-acclimation.

Passive Heating: Reviewing Practical Heat Acclimation Strategies for Endurance Athletes.

Heat acclimation protocols—both active and passive—have been employed by athletes in an effort to attenuate the detrimental effects of heat stress on physical capacities and sports performance. Active strategies have been extensively reviewed, but have various practical and economic limitations. The purpose of this review was therefore to provide an overview of the passive strategies that have received less attention, yet may be more practical or economically viable; recommendations for athletes are also provided. With a systematic search of the relevant databases ending in June 2018, 16 articles on passive heat acclimation that met the inclusion criteria were included in the review. The review highlighted that passive heat acclimation strategies can successfully induce heat adaptations, evident by reports of improved exercise performance, thermoregulatory, cardiovascular, and perceptual responses accompanying such interventions. Based on the review it is apparent that the use of sauna, hot-water immersion and environmental chambers may be used to provide heat stress under passive conditions, for the purpose of acclimation. To maximize the thermoregulatory-adaptive responses, exercise bouts should be employed prior to passive heat stress, rather than passive heating alone, with a minimal delay between exercise and the application of heat stress. Heating bouts should have a minimum duration of 30 min per session and be employed on consecutive days, when possible, with a minimum of 6–7 exposures to induce adaptation. This review identified that information regarding the magnitude of performance changes that can occur, as well as the perceptual responses to passive heating protocols is limited. Future research should investigate the use of passive heat exposures before and/or after repeated heat training sessions, to assess if a further boost to heat adaptation can be achieved with this strategy.

Alterations in core temperature during World Rugby Sevens Series tournaments in temperate and warm environments

Purpose: To characterize player core temperature (Tc) across two separate World Rugby Sevens Series (WRSS) tournaments in temperate and warm environments. Methods: Tc was collected in seventeen playing members of one men’s team competing at the Singapore (n = 12) and London (n = 11) WRSS tournaments. Exertional heat illness (EHI) symptoms, cooling strategy use, playing minutes and wet blub globe temperature (WBGT) were also collected. Linear mixed models and magnitude-based inferences assessed differences in Tc between all periods within-and between tournaments and were also used to assess the effect of WBGT and playing minutes on Tc. Results: Several players experienced Tc in excess of 38°C during warm-ups and 39°C during games. The highest mean Tc values were observed in the final game on all days and in Singapore Day Two, there were substantial game-on-game increases in mean Tc. These Tc responses were associated with playing minutes (effect size; ±90% CL = 0.38; ±0.20), although the effect of WBGT was trivial and unclear. Further, there were no differences in Tc between the two tournaments in the different environments. Despite high individual peak Tc values (Singapore 39.9°C; London 39.6°C); no signs/symptoms of EHI were reported, voluntary post-game cooling usage was minimal, and pre- and mid-cooling strategies were not implemented. Conclusions: During WRSS matches, peak Tc values approached thresholds associated with EHI (>40°C) and exceeded those demonstrated to reduce repeated sprint performance (>39°C). Practitioners may consider the use of compatible cooling and heat acclimation strategies to minimize Tc increase and maximize player preparedness and recovery.

Effect of two-weeks endurance training wearing additional clothing in a temperate outdoor environment on performance and physiology in the heat

ABSTRACTThis investigation assessed performance, physiological and perceptual responses to wearing additional clothing during endurance training for two-weeks in temperate environments, to determine if this approach could be used as a practical, alternative, heat acclimation strategy for athletes. Fifteen trained male triathletes assigned to performance-matched groups completed a two-week unsupervised endurance cycling and running program in either (i) shorts and a short sleeve top (CON; n = 8) or (ii) additional clothing of full-length pants, a “winter” jacket and gloves made from nylon, polyurethane and polyester (AC; n = 7). Participants completed three separate (i.e. familiarisation, pre-program and post-program), identical, pre-loaded cycling time-trials (20 min at 180 W followed by a 40 min self-paced time trial) in 32.5 ± 0.1°C and 55 ± 6% RH. Core and skin temperatures, heart rate, sweat rate, perceived exertion, thermal sensation and thermal comfort were measured across the pre-loaded time trials, and heart rate and thermal sensation were measured across the training program. All of the participants recorded in their diaries that they completed all of the programmed training sessions in the required attire. Mean thermal sensation was most likely hotter in AC (5.5 ± 0.4 AU) compared to CON (4.4 ± 0.4 AU; ES = 1.61, ± 0.68) during the training sessions. However, follow up tests revealed no physiological or perceptual signs of heat acclimation, and the change in time-trial performance from pre-post between groups was trivial (CON: −3.5 ± 12.0 W, AC: −4.1 ± 9.6 W; difference = -0.7%, ± 5.4%). Training in additional clothing for two-weeks in a temperate environment was not an effective heat acclimation strategy for triathletes.

Post-exercise hot water immersion induces heat acclimation and improves endurance exercise performance in the heat.

We examined whether daily hot water immersion (HWI) after exercise in temperate conditions induces heat acclimation and improves endurance performance in temperate and hot conditions. Seventeen non-heat-acclimatized males performed a 6-day intervention involving a daily treadmill run for 40min at 65% V̇O2max in temperate conditions (18°C) followed immediately by either HWI (N=10; 40°C) or thermoneutral (CON, N=7; 34°C) immersion for 40min. Before and after the 6-day intervention, participants performed a treadmill run for 40min at 65% V̇O2max followed by a 5-km treadmill time trial (TT) in temperate (18°C, 40% humidity) and hot (33°C, 40% humidity) conditions. HWI induced heat acclimation demonstrated by lower resting rectal temperature (Tre , mean, -0.27°C, P<0.01), and final Tre during submaximal exercise in 18°C (-0.28°C, P<0.01) and 33°C (-0.36°C, P<0.01). Skin temperature, Tre at sweating onset and RPE were lower during submaximal exercise in 18°C and 33°C after 6days in HWI (P<0.05). Physiological strain and thermal sensation were also lower during submaximal exercise in 33°C after 6days in HWI (P<0.05). HWI improved TT performance in 33°C (4.9%, P<0.01) but not in 18°C. Thermoregulatory measures and performance did not change in CON. Hot water immersion after exercise on 6days presents a simple, practical, and effective heat acclimation strategy to improve endurance performance in the heat.

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

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 Effect Of Acute Hypoxia On Heat Shock Protein 72 Expression And Oxidative Stress In Vivo

The inducible human stress protein heat shock protein 72 (HSP72) performs vital roles within the body at rest and during periods of stress. Recently a previously disclosed quadratic trend in basal HSP72 expression was shown to be reliable and repeatable. PURPOSE: The notion of a physiological stressor such as hypoxia disrupting this basal quadratic trend is an interesting one. Research in-vivo exploring HSP72 expression in response to hypoxia and any potential link to oxidative stress is lacking. METHODS: On trial day one basal monocyte expressed HSP72 was determined every 3 h, via Flow Cytometry, over a 12 h period in twelve healthy male subjects (19.8±3.5 years, 175.5±10.8 cm, 73.1±8.0 kg) within a temperature controlled laboratory. Measures of oxidative stress (TBARS) were taken pre and post exposure. RESULTS: A significant quadratic trend was demonstrated in the control condition (F = 23.5; p = 0.002; partial h2 = 0.77), where the basal HSP72 expression decreased between 0800 and 1100 and then increased thereafter. Trial day two involved a repeat of trail day one with the addition of a hypoxic exposure consisting of 75 mins at a simulated altitude of 2,980 m, commencing and ceasing at 0930 and 1045 respectively. The hypoxic condition failed to display any polynomial trend in expression, with HSP72 values significantly different (elevated) at 1100 (p = 0.002), 1400 (p < 0.001), 1700 (p = 0.034) and 2000 (p = 0.041). Additionally, there was a significant increase in plasma TBARS in the hypoxia condition post-intervention (p < 0.001) compared to control, with a significant increase over time for the hypoxic condition (p < 0.001), but no significant change in the control condition (p = 0.82). CONCLUSIONS: The results demonstrate a 75 min bout of normobaric hypoxia (simulated altitude of 2980 m) is sufficient to induce significant increases in HSP72 expression, which disrupts the basal quadratic trend shown by others. This increase may be linked to the observed changes in oxidative stress. These results may provide a tool for manipulating basal monocyte HSP72 expression with possible applications in augmenting human hyperthermic exercise performance and heat acclimation strategies.