Application of evidence-based recommendations for heat acclimation: Individual and team sport perspectives

ABSTRACT

Heat acclimation or acclimatization (HA) occurs with repeated exposure to heat inducing adaptations that enhance thermoregulatory mechanisms and heat tolerance leading to improved exercise performance in warm-to-hot conditions. HA is an essential heat safety and performance enhancement strategy in preparation for competitions in warm-to-hot conditions for both individual and team sports. Yet, some data indicate HA is an underutilized pre-competition intervention in athletes despite the well-known benefits; possibly due to a lack of practical information provided to athletes and coaches. Therefore, the aim of this review is to provide actionable evidence-based implementation strategies and protocols to induce and sustain HA. We propose the following suggestions to circumvent potential implementation barriers: 1) incorporate multiple induction methods during the initial acclimation period, 2) complete HA 1–3 weeks before competition in the heat to avoid training and logistical conflicts during the taper period, and 3) minimize adaptation decay through intermittent exercise-heat exposure or re-acclimating immediately prior to competition with 2–4 consecutive days of exercise-heat training. Use of these strategies may be desirable or necessary to optimize HA induction and retention around existing training or logistical requirements.

Introduction

Individual and team sport endurance performance in warm-to-hot environmental conditions (≥25°C) is impaired compared to cooler conditions [1–5]. This performance decrement most likely occurs because of the combination of elevated perceptual heat stress and compulsory increases in skin blood flow and sweat rate to transfer metabolic heat from the muscle to the body surface and into the environment. These thermoregulatory responses occur at the expense of elevated physiological and cardiovascular strain [6]. Shunting blood to the skin reduces skeletal muscle blood flow, oxygen and nutrient delivery to tissues and organs, and metabolic waste removal from working muscle [7]. Sweating during exercise contributes to dehydration, further impairing cardiac output and heat loss mechanisms exacerbating thermal, physiological, and perceived strain at performance intensities [7–9]. When ambient temperature exceeds skin temperature or evaporation of sweat decreases, from high environmental water vapor pressure, core body temperature (T_c) rises more rapidly and performance is further reduced.

Ambient temperatures ≥25°C impair performance at running distances ≥800 m [4,10], with greater decrements observed when athletes are dehydrated [9,11,12]. High-level sporting events are often scheduled in geographic locations where warm-hot conditions are typical making it critical for athletes to prepare for environmental stress (e.g., 2020 Tokyo Olympics) [13–15]. For example, prior to the 2015 IAAF World Athletics Championships in Beijing, China; meteorologists forecasted air temperatures ranging 26–33°C with ~73% relative humidity (RH), yet <15% of athletes arrived at the competition heat acclimated [16].

Repeated exposure to exercise in the heat induces heat acclimation (HA), which improves thermoregulatory mechanisms, skeletal muscle metabolism, cardiovascular stability, and whole body thermotolerance [17–19]. Nonetheless, integrating HA into training to enhance endurance performance and reduce exertional heat illness risk [20] in warm-to-hot competitions may be an underutilized strategy in athletes [16]. While HA guidelines and consensusopinions [13,14] are available, they provide limited practicalinformation for athletes and coaches to integrate HA into trainingand periodization models [15]. This review aims to discuss actionable implementation strategies for individual and team sports in three sections: 1) expected performance enhancements, 2) important induction variables, and 3) theoretical case reports illustrating induction tactics. HA can be induced in both outdoor (acclimatization) and artificial indoor (acclimation) environments. These terms are used interchangeably in this review.

Section 1: The ergogenic potential of heat acclimation

The benefits of HA are clearly demonstrated in athletes competing in prolonged hot weather events [10]. A meta-analysis of 96 studies showed time-to-exhaustion and time trial performance improves up to ~23% and ~7%, respectively, following HA [17]. These improvements may be due to enhancements in maximal oxygen consumption (V̇O₂max; ~6%), lactate threshold (~1.0 mmol kg⁻¹ lower), movement economy (~2.5%), and thermotolerance among other adaptations (Figure 1). Enhanced thermal comfort may be a vital element associated with performance optimization in the heat by removing reservations of high intensity exercise enabling proper pacing during competition [21,22]. Behavioral strategies to reduce sweat-induced dehydration further augment endurance performance [23,24].

Figure 1.

Heat acclimation adaptations that support exercise performance and safety in the heat.

HA may also improve aerobic performance in cooler conditions up to 6% [1]. However, other investigations showed no effect [25,26]. Regardless, no evidence indicates HA impairs aerobic performance in cooler conditions, so HA integrated into annual training plans may improve performance in both hot and cooler conditions without compromising higher priority training objectives. HA may also lessen physiological strain at altitude [27,28] through epigenetic [29], cellular [30], and system level adjustments [31]. The potential of improved performance via cross-tolerance among environmental conditions is an exciting new area of discovery.

Section 2: Developing a heat acclimation program

There is no single HA protocol that would elicit “optimal” adaptations in all athletes. Generally, athletes can fully heat acclimate with 10 consecutive days of exercise for 90 minutes in 30°C wet-bulb globe temperature [32]. Customization of this recommendation should be based on the athlete’s training status, recent environmental exposure, available training time (both per day and number of days), tolerance for additional training stress, available equipment, access to hot environments, competition environment, and optimal training phase timing. While guidelines for adjusting HA program variables based on training phase are available [14], current evidence for HA customization that manipulates exercise intensity, duration, protocol length, frequency, environment, and mode remain under examined.

Daily exercise intensity and duration

The driving impulse for HA is the combination of elevated T_c and skin temperature producing increased skin blood flow and sweat production during exercise-heat exposures [33]. Several exercise prescription permutations can achieve this goal. Gibson and colleagues suggest than an exercise-heat induced T_c elevation to 38.5°C for ~60 minutes is the minimal thermal impulse for effective HA [34]. Table 1 outlines intensities and time intervals to achieve this stimulus [35]. Well-trained athletes running for 30–35 min day⁻¹ at 75% V̇O₂max or walking 60 min day⁻¹ at 50% V̇O₂max in the heat over nine days develop similar magnitudes of HA [36]. Although HA may be induced with shorter duration/higher intensity protocols in highly trained athletes, prudent recommendations are that athletes should aim for 60–90 min of exercise-heat exposure [19], and when possible build toward longer durations up to 2 hours to enhance HA effectiveness and delay adaptation decay [37]. This duration may be comprised of exercise-heat exposure alone or a combination of exercise- and passive-heat exposure (see Practical Question #2). HA exercise duration and intensity should replicate sport competition as performance adaptations are condition specific [17].

Table 1.

Exercise intensity to raise T_c within various time limits in 40°C and 39% relative humidity.

Tc rise	+1.0°C			+1.5°C			+2.0°C
Time (min)	20	30	40	20	30	40	20	30	40
Relative power (W kg−1)*	2.7	1.9	1.5	4.0	2.7	2.1	5.2	3.6	2.7
Absolute power (% max)	64	48	40	88	64	52	112	80	64
RPE (AU)	17	15	12	>20	17	14	>20	>20	17
V̇O2 utilization (% V̇O2peak)	68	55	48	89	68	58	>100	82	68
HR (% HRpeak)	95	79	70	>100	95	83	>100	>100	95

* Strongest predictor of T_c change during exercise in the heat. Resting T_c ranges 36.5–37.5°C. RPE = ratings of perceived exertion on Borg’s 6–20 point scale. HR = heart rate. Table adapted from Gibson et al. [35]. Once target T_c is reached, exercise intensity should decrease so T_c does not continue to rise.

The physiological stress of a hot environment must be incorporated into training load calculations [15], often requiring a reduction in exercise intensity that sacrifices sport specificity. This paradox may be partly resolved by progressively increasing exercise intensity after the 3^rd day of heat exposure when the initial cardiovascular and thermoregulatory adaptations have been established. By the 4^th day of exposure, the risk of exertional heat illness risk is lower [38] and exercise-heat tolerance is increased [39], allowing for higher intensity training sessions nearer performance intensities. High exercise intensities toward the end of HA may maintain plasma volume expansion [40]. Common metrics to monitor training load and progression are reviewed in previous publications [41,42] and include session-RPE [43], training impulse (TRIMP) [44], blood biomarkers [45] and the Training Stress Score® (TrainingPeaks, Boulder, CO). Most of these metrics are rare in HA research but may be integral in properly managing overall training load when adding HA.

The five general induction methodologies to manipulate exercise intensity or T_c are controlled heart rate (HR), controlled hyperthermia, self-paced exercise, passive-heat exposure, and/or constant work rate (Table 2). Most HA guidelines are based upon a single induction method. However, using more than one HA method can offer greater flexibility while simultaneously training and inducing HA [46].

Table 2.

Various heat acclimation induction methods with advantages and disadvantages.

Protocol	Duration	Intensity	Length	Advantages	Disadvantages
Variable workload
Controlled hyperthermia	60–90 min	Initially high to achieve Tc = 38.5°C within ~30 min, then lower intensity to sustain Tc thereafter	5–14 days	Sustained thermal adaptive impulse	Requires valid Tc measurement [60,61]
				Potential optimization of adaptations [18]*	Ideal (usually high) exercise intensity to achieve Tc ≥ 38.5°C within 30 min unknown and changes as adaptations occur; may be challenging for novice athletes
				Plasma volume, HR, and the emergence of thermoregulatory and metabolic adaptations improve thermal tolerance and performance after only 5 d in trained athletes [75]	Overall mean relative work rate and HR ↓ over protocol [75]
Self-paced	60–90 min or duration of practice	Varies	7–14 days	Very practical for team-based sports	Less control of exercise variables and adaptive stimuli, day-to-day improvements not easily measured
				Induce HAwhile training sports skills during practices or pre-season activities [91–93]	Requires motivated athletes
				Trained athletes can sustain intensities within narrow range [94]	May not induce all HA adaptations
Controlled heart rate	60–90 min	~70% heart rate reserve or heart rate at lactate threshold	5–14 days	Sustained relative exercise intensity and cardiovascular strain [32,78]	Thermal adaptive impulse difficult to quantify
				May use sport-relevant intensities	Cardiac drift may lower absolute exercise intensity
Constant workload
Average fitness athletes	100 min	40–50% V̇O2max	10–14 days	Easy to implement	Thermal strain/adaptation stimuli ↓ as HA progresses
				Day-to-day improvements easily measured	May ↓ magnitude of adaptation*
High fitness athletes	30–45 min	75% V̇O2max	5–9 days	Easy to implement	Thermal strain/adaptation stimuli ↓ as HA progresses
				Day-to-day improvements easily measured	May ↓ magnitude of adaptation
				May supplement with passive-heat exposures
Passive-heat exposure
Sauna bathing	20–30 min	50–100°C, 10–20% RH	5–21 days	↓ exercise-heat training volume	Requires sauna room
				May use every other day	Time constraint due to exercise immediately before or after
Hot water immersion	30–60 min	40–44°C	14–21 days	↓exercise-heat training volume	Time constraint due to exercise immediately before or after
				May use every other day

T_c = core temperature. V̇O₂max = maximal oxygen consumption. Table modified from [95]. * One studied showed controlled hyperthermia and constant workload protocols exhibited similar physiological and cellular adaptations [76].

Protocol length

Physical training reduces the number of days required to achieve full HA [38,47–49] but physical training without HA does not protect athletes from heat stress. Trained athletes adapt well to heat within 5–7 days while continuing to improve aerobic performance [38,47–49]. However, more effective [10,17,36,39,40] and longer lasting [37] adaptations occur with protocols lasting 8–14 days versus ≤7 days leading many to suggest optimal HA requires at least 10–14 days [14,32]. Longer protocols are recommended in less trained athletes, athletes competing in prolonged or multi-day events, and in team settings given the large inter-individual differences in fitness and HA status.

Frequency

HA is most rapidly induced by consecutive days of exercise-heat exposure. Multiple exercise-heat exposures per day does not hasten HA nor enhance adaptations compared to one training bout per day [50,51]. Missing 1–3 days of exercise-heat exposures during a 10–14 day acclimation period will not likely impede HA. When heat exposure occurs every 2–3 days, it can take a full month to induce HA [52–55]. While daily exercise-heat exposures are preferable, alternating days of “heat training” and “normal training” may be an acceptable alternative, especially when athletes train twice per day.

Environment

The HA induction environment should match or be slightly more oppressive than the worst anticipated competition environment, allowing athletes to experience the exact nature of the environmental stress [56] and induce adaptations specific to that setting [32]. Generally, a wet-bulb globe temperature ≥30°C provides sufficient environmental stress to induce HA and higher wet-bulb globe temperatures enhance sweat responses [37]. HA may occur in milder conditions, but adaptations will require higher exercise intensities or layering of additional clothing to create added heat stress (see Practical Question #2).

Athletes sustain higher work rates at higher temperatures during outdoor exercise as wind flow enhances convective cooling [5,57]. Therefore, artificially generating wind with a fan matched to performance velocity during indoor HA may aid in competition environment emulation and increasing work rate. Purposefully engaging in physical activity in the heat elevates exertional heat illness risk, but with mindful applications of HA principles, long-term risk is reduced.

Exercise mode

Walking, jogging, running, cycling, stepping, rowing, sport training (e.g., rugby, American football, soccer) and manual labor tasks have all been used to induce HA in military personnel, workers, and athletes. Incorporating competition activities into the acclimation stressors helps preserve sport-specific skills and adaptations. Athletes may also fine-tune pacing strategy and other approaches to optimize performance in the heat (e.g., hydration, pre-cooling) toward the end of the HA protocol [2,58,59].

Section 3: Practical questions

• 1. How do I know if I am heat acclimatized?

Several metrics can be used to quantify HA status (Figure 1) including resting and exercising T_c using validated tools [60,61], but reductions in exercising HR, sweat rate, and perception of exercise effort or thermal stress are inexpensive, practical, and efficient assessment strategies. Adaptations can be referenced against a recent meta-analysis [17] for confirmation although inter-individual variability is noteworthy [15,17,62]. Optimally, these metrics are collected during standardized training activities completed periodically throughout HA; for example, before HA and every 4–5 days thereafter to track changes. A reduction in environmental symptoms measured with a 12-question survey mirrors T_c and HR reductions following HA [63]. Ultimately, for many coaches and athletes improved performance during standardized sport relevant activities is sufficient evidence of HA. A performance plateau (no further change) during a standardized training activity (e.g., yo-yo intermittent test, time trial, time-to-exhaustion, etc.) in the heat may indicate adequate HA.

• 2. How can sufficient heat stress for HA be implemented into training for athletes who do not live or train in a warm-to-hot climates?

Athletes can HA by traveling to “heat camps,” training in an environmental chamber or artificially heated indoor facility, or arriving early at the competition locale. Arriving 5–14 days prior to competition allows athletes to heat acclimate in the most likely environment for the contest and is preferred but not always feasible. Matching the expected warm-to-hot conditions in an indoor facility is an effective alternative to early arrival for athletes who do not live and train in a climate similar to the competition site. Few athletes have access to commercially built environmental chambers, so mimicking the competition site can be challenging. Adding space heater(s) to raise ambient temperature in an enclosed area can create a sufficient thermal stress, especially when a misting spray is added to increase the RH. In a small room, RH increases quickly once sweating begins and reducing RH requires a dehumidifier. Training outdoors in the hottest time of the day [64] or indoors [65] while layering clothing aids in raising T_c in cooler areas and is practical for teams and individuals without access to temperature controlled chambers. It is important to monitor both the environmental conditions and the athlete for signs and/or symptoms of exertional heat illness (every ~5–10 min) as both can change rapidly during exercise-heat stress [20].

Concomitant exercise and heat exposure may not be necessary to induce HA. HA with hot water immersion [66–68] and dry heat or sauna exposure [69–71] are time and training load efficient alternatives that use equipment that are typically accessible to athletes. Hot water immersion studies of untrained men show successful HA by alternating days of 30–60 minutes of hot water immersion (40–44°C) with training in the heat over 2–3 weeks [66,67], hot water immersion before exercise in the heat [72], or hot water immersion after exercise in temperate conditions [68]. These studies indicate that the volume of exercise in the heat can be reduced and still produce HA adaptations on par with ~5 days of HA [17], regardless of time of day [73].

Sauna bathing shows promise for inducing HA in both trained men and women. In trained men, daily post-exercise sauna bathing for 10–15 thirty minute sessions over 2–3 weeks (80–100°C, 10–20% RH) increased plasma volume 7–18% [69,70] and time to exhaustion by 32% [69]. In trained women, a sauna suit worn for 20 minutes in 50°C and 30% RH each day before 5 days of controlled hyperthermia HA showed plasma volume expansion, sweat rate increases, and lower peak exercising HR, skin temperature, and T_c [71]. Passive sauna bathing without exercise induces minimal thermoregulatory adaptations [74].

A limitation to passive-heat exposure approaches is that they may not fully prepare the athlete for the sensations and physiological responses to exercise in the heat, particularly at competition intensities. Manipulating passive and exercise HA approaches may alleviate these concerns and those of detraining due to reduced exercise training intensity and volume to accommodate HA [10,15]. For example, 30 minutes of exercise in the heat at or near an intensity sufficient to raise T_c to ~38.5°C followed by 30–60 minutes of hot water immersion or sauna exposure may provide an acceptable mix of exercise and thermal adaptive stimuli. Separating heat and exercise stressors may also allow high exercise intensity in “normal” conditions while supporting the specific adaptations needed for the target environment. This approach may be particularly valuable for HA or re-acclimating during the taper prior to competition.

• 3. How quickly are heat adaptations lost and how can athletes sustain HA?

In the absence of repeated exercise-heat stress, full HA is lost within a month. After two weeks without heat exposure end-exercise HR, sweat rate, and T_c can be expected to decay ~35%, ~30%, and ~6%, respectively [37]. The shorter the HA induction protocol, the faster the decay rate. This rapid decay prompted a suggestion that elite athletes should heat acclimate prior to competition (e.g., during the taper) using ~5 days of controlled hyperthermia [75]. Acclimating to heat during the taper is possible for some but also presents many training challenges potentially counterintuitive to tapering [15,76]. Alternatively, completing HA 1–3 weeks prior to a competition in the heat does not interfere with the taper or the long-term training plan [14,37,46]. Heat re-acclimation and intermittent exercise-heat exposure are two evidence-based approaches to regain or prolong adaptations to heat, respectively.

The exact explanation for this phenomenonis unclear but may be because initial adaptations have not fullydecayed at the time of reinduction or select molecular adaptationsare retained for months yielding a “heat acclimation memory” [77]. Re-acclimation studies show HR, sweat rate, and T_c adaptations require 2–4 consecutive days of exercise-heat exposures within 2–4 weeks after initial HA to regain heat tolerance levels [37]. Rapid re-induction may be a more feasible strategy than initial induction during a training taper and can potentially be completed at the competition site. Re-induction using the controlled HR method is a practical approach to manage exercise volume and recovery during tapers. For example, five consecutive days of cycling in the heat resulted in cardiovascular and thermoregulatory adaptive stimuli with 30% less mechanical stress [78].

HA can be prolonged by using intermittent exercise-heat exposure. One exercise-heat exposure (40°C, 40% RH, 45% V̇O₂max) for 120 minutes every 5 days sustained HR and T_c adaptations for 1 month after initial HA [46]. In two elite sailors, an intermittent exercise-heat exposure (walking 5.5 km hr⁻¹, 10% incline, 35°C, 60% RH) for 30 minutes every four days retained many heat adaptations 3 weeks after initial HA [79]. In both cohorts, athletes continued their regular out-of-heat training after HA, which appears vital to sustaining adaptations [46,80]. Both lower intensity/longer duration and higher intensity/shorter duration exercise-heat exposures appear effective at retaining heat adaptations, although the latter is more time efficient. More research is needed to determine if intermittent exercise-heat exposure and heat re-acclimation are effective beyond one month. Regardless, heat training should be completed at least two days before major competitions to avoid residual fatigue effects [78].

• 4. Should an athlete alter food and fluid intake during heat acclimation?

The rate of muscle glycogen depletion is accelerated during exercise in the heat [81]. Consuming carbohydrates before and/or during exercise prolongs exercise while high carbohydrate diets effectively replenish muscle glycogen stores [82] and extends exercise in the heat [83]. Athletes should consume 6–10 g kg⁻¹ day⁻¹ of carbohydrates with 30–60 g hr⁻¹ (in a fructose-maltodextrin blend) during and immediately after exercise lasting 1–2.5 hours [84].

Consuming 1.5–3.2 g of sodium daily appears sufficient to sustain fluid-electrolyte balance [85] except in “salty sweaters” who may require greater daily sodium intakes. Lightly salting foods and adding 0.5–0.7 g L⁻¹ of sodium to fluids during exercise can drive thirst sensation and increase fluid intake [86], attenuating fluid-electrolyte imbalances during the initial days of HA. Athletes prone to cramping may benefit from increased sodium supplementation to 1.5 g L⁻¹ [87] or preferably matching sodium intake with sweat sodium losses. Athletes wishing to maximize plasma volume expansion or that consume <1.5 g sodium per day (e.g., vegan diets or diets low in prepackaged, highly processed, and preserved foods) may benefit from increasing sodium intake.

Athletes should maintain euhydration throughout HA by drinking ad libitum or preferably equal to sweat rate during exercise and replace exercise related body weight losses up to 150% before the next training session [23]. As HA proceeds, sweat sodium concentration lowers, sweat volume increases, and behavioral fluid consumption increases as does gut acceptance leading to enhanced intake and fluid-electrolyte balance [88]. Permissive dehydrationduring consecutive days of exercise-heat stress induces HA butthere have not been cohort comparisons with respect to outcomesor safety [89].

Challenges of heat acclimation in sport

Integrating HA into training periodization without disrupting other training objectives can be complex and a potential barrier to implementation. This section illustrates the theoretical integration of HA into an individual and team sport training plan.

Individual sport athlete

An elite level triathlete lives in San Francisco, CA, USA, where temperatures range 13–22°C [90] and has a series of races >2 h in duration over a 6 month (May-October) racing season mainly in warm conditions. The first nine weeks of the training season (including HA) are outlined in Figure 2. The first major race (Race 2, Figure 2) occurs the last week of May for which the athlete will taper for one week. The final race is the Ironman World Championships in October where temperatures in Kona, HI, USA average 29–30°C. Competitions of lower importance (e.g., Race 1) are also scheduled throughout the season. This athlete is unable to travel to warm race locations more than four days prior to any race.

Figure 2.

Integration of heat acclimation into a periodization scheme for an elite triathlete.

The athlete starts the training season in January with traditional training free from added environmental heat stress to enhance cardiorespiratory fitness. About four weeks prior to the first race of the season in heat (Race 1), HA should begin. The objective of each exercise-heat exposure is to 1) sweat profusely and 2) increase T_c to at least 38.5°C for 60 minutes (see Table 1 for intensity prescriptions). Preferably, this training would occur in a heated room matching Race 1’s anticipated environmental conditions. If this is not feasible, training can be done in the hottest part of the day with extra-layered clothing to force a rise in T_c and sweating. In the absence of a valid T_c measurement [60,61], intensity during exercise in the heat can be prescribed using the controlled HR method (e.g., 70% HR reserve [78] or lactate threshold HR [32]).

The initial exposures to HA training may require a reduction in duration and/or intensity from the usual training load. The training duration and/or intensity should progressively increase after three days of HA to replicate race pace. During the initial two weeks of HA, there should be no more than one rest day between exercise-heat exposures. This triathlete completes daily bike or run HA training, with concurrent regular (no heat) training on most days. For example, swim workouts will not incur significant heat stress and continue as normal. Additionally, longer duration and race pace specific bike and run workouts should continue during the cooler hours of the day, but with sufficient rest time from the HA training. Exercise-heat exposures can be replaced with passive-heat exposures (i.e., sauna or hot tub) following intense training bouts in cooler temperatures to sustain training quality. Training load metrics can be used to monitor load when combining HA with normal training to avoid excessive training stress.

Once heat acclimated, this athlete should engage in 30–120 min of exercise-heat exposures every 4–5 days through the final race of the season. These exercise-heat exposures can be either continuous or interval training using controlled HR or self-paced methods to meet training needs. Passive-heat stress immediately following exercise in temperate conditions may maintain HA when an exercise-heat exposure would compromise the training schedule. If this athlete requires a significant break from exercise heat-exposure, a 2–4 day re-acclimation protocol can be used [37]. Prior to Race 1 and 2, this triathlete will optimize HA using 2–3 days of re-acclimation, which should also be repeated prior to the championship race in Kona, HI.

Team-based sports

Heat acclimation for team-based sports is complex for a variety of factors including individual athlete differences, positional requirements, and team schedules. Consider the HA need of a NCAA Division III women’s soccer team residing in Philadelphia, PA, USA that will compete in San Antonio, TX, USA for the NCAA Championships December 1–4. San Antonio is a tournament location where high temperatures for these dates in 2017 ranged 25.5–29.4°C [90]. Average high temperatures in Philadelphia leading into the championships were cooler, averaging 9.0–17.7°C [90]; therefore, athletes will not be heat acclimated. This team heat acclimated during fall pre-season training but adaptations decayed over the season as no major matches occurred in the heat for the past two months.

These athletes require fewer HA days as they would be well trained and likely partially heat acclimated [80] due to training throughout the competitive season. Therefore, in preparation for the championships, this team should complete a 6-day self-paced HA protocol (November 19–26). This timing allows a one-week taper prior to the championship with travel November 28^th. The team will complete one day of training in the competition environment and one rest day before matches begin.

To emulate the competition environment, HA can occur in a gymnasium with the thermostat set to increase the air temperature to 26.6–27.7°C. To aid in T_c elevation, athletes can layer clothing and complete 20 minutes of high intensity interval running followed by skill-based training for 50–70 minutes. A sport specific performance test in the heat (e.g., yo-yo intermittent test) will be implemented November 19^th and 26^th to help determine HA status and will replace interval running on these days. If the duration of training in the heat is shortened to spare time for higher intensity practice(s) in temperate conditions, post-exercise passive-heat exposure can be used to extend the thermal adaptive impulse with either hot water immersion (30 min; 40°C water) or sauna exposure (10–15 min; 90°C, 10% RH). If only one modality is available limiting accessibility, prioritize athletes who run greater distances and engage in more frequent high intensity sprints (mid-fielders and forwards) or athletes with a history of exertional heat illnesses. Hydration, body cooling, an on-site athletic trainer, and other safety precautions should be implemented during HA (Table 3) [20]. No other practices should occur during the first two days of HA. Beginning the 3^rd HA day, outside practices can resume but consist of lower durations due to the added heat stress of HA. Within the abilities of the athletes, heat training duration should increase to 90 minutes then begin to escalate work intensity.

Table 3.

Pre-heat acclimation coaches checklist.

Training Facility	Is there a cold-water immersion treatment area closely available to all athletes participating in the exercise-heat exposure?
	Are the signs and symptoms of exertional heat illness posted for all athletes so they are aware when they should immediately stop training?
	Is there a person besides the athlete designated to monitor the temperature and humidity of the training facility?
Individual Athlete	Has the athlete’s bodyweight changed by <1% from the previous day?
	Has the athlete had an adequate night of sleep >6 hours in a cool environment?
	Has the athlete avoided supplements that may affect thermoregulation within the past 6 hours (e.g., “pre-workout”)?
	Is the athlete free of any fever, flu, cold-like, or digestive problem symptoms in the previous 24 hours?

In both individual and team sports, coaches should review the following checklist regarding their heat acclimation environment and each individual athlete prior to every exercise-heat exposure session. If the answer to any question is “no”, exercise heat acclimation for the day should be suspended for the team or individual. The next exercise-heat bout should occur when all responses to these questions are “yes”, beginning with the missed training bout. See Casa et al. [20] for additional information on exertional heat illness prevention strategies.

Conclusions

There is no single “optimal” HA induction protocol that applies to all athletes. Ten days of consecutive exercise-heat exposure for 90 minutes in 30°C wet-bulb globe temperature is a good starting point for customization. HA should reflect the expected competition environment and be sport-specific whenever possible. Incorporating several induction methodologies may allow high training quality while acclimating to the heat. While the ideal timing of HA prior to competition is debated, current evidence indicates HA should be completed 1–3 weeks prior to competition to allow sufficient recovery from heavy training loads (including HA) and enable a training taper. Intermittent exercise-heat exposure, heat re-acclimation, sauna bathing, or hot water immersion can be used to sustain or rapidly regain HA throughout the season. Despite substantial progress over the last several decades, gaps in knowledge remain requiring future work regarding exercise and heat including the female response/adaptation, the immune and microbiome response/adaptation, and the genetic and epigenetic connection with HA induction and decay.

Acknowledgments

The authors thank Jim Vance and Jacobo Morales for their careful review and thoughtful suggestions.

Disclosure statement

No potential conflict of interest was reported by the authors.

Abbreviations

T_c core body temperature HR heart rate RH relative humidity HA heat acclimation V̇O₂ maximal oxygen consumption