To provide best-practice recommendations for developing and implementing heat-acclimatization strategies in secondary school athletics.
An extensive literature review on topics related to heat acclimatization and heat acclimation was conducted by a group of content experts. Using the Delphi method, action-oriented recommendations were developed.
A period of heat acclimatization consisting of ≥14 consecutive days should be implemented at the start of fall preseason training or practices for all secondary school athletes to mitigate the risk of exertional heat illness. The heat-acclimatization guidelines should outline specific actions for secondary school athletics personnel to use, including the duration of training, the number of training sessions permitted per day, and adequate rest periods in a cool environment. Further, these guidelines should include sport-specific and athlete-specific recommendations, such as phasing in protective equipment and reintroducing heat acclimatization after periods of inactivity. Heat-acclimatization guidelines should be clearly detailed in the secondary school's policy and procedures manual and disseminated to all stakeholders. Heat-acclimatization guidelines, when used in conjunction with current best practices surrounding the prevention, management, and care of secondary school student-athletes with exertional heat stroke, will optimize their health and safety.
Exertional heat stroke (EHS) is consistently ranked among the top 3 causes of sudden death in sport and physical activity.1,2 Previous position,3–6 consensus,7 and interassociation task force8 statements have provided recommendations for heat acclimatization, the process whereby repeated exposure to heat stress elicits physiological adaptations to improve thermal tolerance, as a method of attenuating the EHS risk. Although the implementation of heat-acclimatization guidelines in secondary school athletics has been effective in reducing the EHS risk,9,10 the current recommendations provide limited guidance regarding implementation across all sports.
A multidisciplinary group of content experts was formed and a roundtable meeting convened on May 28, 2019, in Orlando, Florida, to discuss preseason heat safety for secondary school student-athletes. The multidisciplinary group comprised 33 practitioner and scientist content experts who met the following criteria: (1) expertise in the management and care of patients with exertional heat illness; (2) active involvement in providing clinical care to secondary school student-athletes; and (3) expertise in biometeorology, thermal physiology, or epidemiology. Specifically, the roundtable participants consisted of scientists with expertise in the management and care of EHS (n = 12), clinically practicing secondary school athletic trainers (n = 7), sports medicine physicians (n = 5), scientists with expertise in thermal physiology (n = 5), scientists with expertise in biometeorology (n = 2), an emergency department physician (n = 1), and a sports injury epidemiologist (n = 1).
From the roundtable attendees, a working group (n = 14) was formed to draft this document. Their primary purpose was to develop evidence-based recommendations and implementation strategies for heat acclimatization in secondary school athletics. The working group responsible for drafting this document comprised scientists with expertise in the management and care of exertional heat stroke (n = 6), clinically practicing secondary school athletic trainers (n = 3), scientists with expertise in thermal physiology (n = 3), a sports medicine physician (n = 1), and a sports injury epidemiologist (n = 1).
The Delphi method, a framework for establishing consensus through the unbiased aggregation of expert opinion on a topic,11 was used to develop consensus among the roundtable attendees. Aligning with current best practices for the use of the Delphi method, we followed a 2-stage process: an exploration phase and an evaluation phase. A full description of this process is available elsewhere12,13 ; however, we provide a brief overview in the sections that follow.
The working-group members conducted a review of the relevant literature, which served as the foundation for developing action-oriented recommendations. Specifically, the review focused on the induction and decay of heat acclimatization; physiological adaptations obtained from heat acclimatization; time course of heat acclimatization; sport-specific heat-acclimatization considerations; age, sex, and environmental heat-acclimatization considerations; and heat-acclimatization implementation strategies.
After the exploration phase, the working group prepared action-oriented recommendations for use in secondary school athletics. Once the recommendations were finalized, the roundtable participants were emailed a link to an anonymous survey (Qualtrics) for scoring. Roundtable attendees assessed each recommendation for validity, feasibility, and clarity using a Likert-type scale (range = 1–9), in which a higher score indicated that a recommendation was more valid, feasible, or clear, respectively. If they wished, attendees could provide comments to support their score for each recommendation. For scoring purposes, validity was operationally defined as whether the recommendation was substantiated based on current data, theory, literature, or other scientific evidence. Feasibility was operationally defined as whether it was realistic to expect individuals or campuses to implement the recommendation, keeping in mind the widely varying resources and competing demands that individuals and campuses may face. Clarity was operationally defined as whether the recommendation was clear and easily understood.
We compiled the scores from each attendee and calculated the mean scores for each construct (ie, validity, feasibility, and clarity) and each recommendation. Any recommendation with a score <4 was discarded, and any recommendation with a score ≥7 was retained in the final version. All mean scores of 4 to 6 were revisited for further discussion; the working group revised these recommendations using input from the survey open comments. All roundtable attendees then performed a second round of scoring for the revised recommendations using the same methods as in the first round. Recommendations with mean scores ≥4 but <7 after the second round of scoring were assessed by 3 roundtable members (W.M.A, Y.H., D.J.C.) and modified by consensus.
Physiological Adaptations to Heat Acclimatization
Heat acclimation or acclimatization involves repeated exposure to environmental conditions that elicit profuse sweating, increase skin blood flow, and elevate skin and core body temperature, leading to physiological adaptations that reduce the negative effects of heat stress on health and performance. Heat acclimation refers to the adaptations that occur after a period of heat exposure undertaken in artificial or laboratory settings, whereas heat acclimatization occurs in natural outdoor environments. Although the physiological adaptations elicited by artificial and natural environments are similar,14,15 exercise heat acclimatization has been suggested to provide more specific adaptations due to the regular exposure to particular training environments (ie, ambient temperature, humidity, solar radiation).16
The adaptations that develop during heat acclimation or acclimatization include plasma volume expansion and better maintenance of fluid balance, lower exercising core temperature, enhanced sweating and skin blood-flow responses, improved cardiovascular stability, and less reliance on carbohydrate metabolism during exercise in the heat at a given workload.16–18 A complete list of physiological adaptations afforded by heat acclimatization is available in the Figure. These adaptations allow for improved submaximal exercise performance and increased maximal aerobic uptake (o2max),19,20 attenuation of perceived fatigue within training sessions,21 and enhanced thermal comfort in the heat.22–24 However, support for enhanced thermal comfort is lacking in the contemporary literature. As we explain later, the magnitude and time frame of induction vary among individuals and depend on intensity, duration, frequency, and number of heat exposures, as well as the environmental conditions (eg, dry or humid heat).
Time Course of Heat Acclimatization and Decay
Generally speaking, 75% to 80% of heat-acclimation adaptations are achieved during the first 7 days of heat exposure,16,25 yet large interindividual variations exist.26,27 Although a few days of heat exposure provide some level of heat acclimation or acclimatization, athletes generally require >10 days for full expression of cardiovascular and sudomotor (ie, sweating and skin blood-flow responses) adaptations20,28,29 and at least 2 weeks to optimize their physical performance in hot ambient conditions.30 Repetitive heat exposures within a day (ie, multiple practices in a single day) do not appear to expedite the process; thus, single daily exposures (ie, 1 practice per day) during 2 full weeks of heat exposure is the safest effective strategy.31 Further, multiple heat exposures in a given day (eg, 2 practice sessions per day) may be detrimental to an athlete's safety based on evidence32 of greater thermal strain during a second exercise session compared with the first in a given day.
Once acclimation occurs, adaptations decay in the absence of heat exposure. The authors33 of a recent meta-analysis determined that approximately one-third of adaptations were lost in the 2 weeks after heat acclimation in the absence of heat exposure. The rate of decay in the adaptations of heart rate and body core temperature was approximately 2.5% per day without heat exposure. Importantly, training in cool conditions and regular heat exposure (even passive heat exposure) delay the loss of adaptation. Moreover, a second heat acclimation or acclimatization phase during the decay period (within 1 month) appears to provide a faster rate of adaptation than the first phase.33 For example, 2 and 4 days of reacclimation after 12 and 26 days without heat exposures were sufficient to regain an acclimation level similar to that obtained during the initial 10 days of acclimation.34 Therefore, for athletes in individual sports (and possibly those in team sports), coaches can plan for an initial comprehensive heat-acclimatization period of at least 2 weeks, followed by a tapering period and then reacclimation in the few days before competition.35
Induction of Heat Acclimatization
The induction of heat acclimation or acclimatization involves daily heat-exposure sessions of 60 to 90 minutes16,36 and can be achieved through a variety of passive, active, or combination protocols.33 Although passive heat acclimation (eg, posttraining saunas or hot baths) can be used after training in a cool environment,37–40 the lack of appropriate monitoring and supervision in the secondary school setting renders this method inappropriate, so it should not be attempted. Self-paced exercise heat acclimatization (ie, the individual self-selection of exercise intensity) was developed by the military to ensure that recruits could adapt at their own pace.41,42 This approach is still used by large athletic teams (eg, American football and soccer teams)26,43,44 and endurance athletes.28 Other approaches that are more laboratory based (ie, requiring greater control) include performing exercise at a constant work rate20 or attaining and sustaining a given core body temperature (eg, 38.5°C).45
The logistical constraints associated with these approaches make them more difficult to implement, particularly with large groups of athletes. An alternative may be to maintain a predetermined heart rate when exercising in the heat, as it provides a good overall indicator of the cardiovascular strain elicited by exercise in a given environment.16 This approach requires a heart-rate monitor and strict maintenance of a given heart rate, which may not be practical or feasible in many secondary school settings. A more practical approach uses a rating of perceived exertion to ensure that a particular exercise intensity is maintained. However, the use of perceived exertion as a tool for monitoring intensity may be fraught with concerns such as misreporting by the athletes. Regardless of how intensity is gauged, high-intensity efforts during the heat-acclimatization process should be completed at the start of a given practice or training session so as to reduce the risk of heat illness and preserve performance. The choice of approach may therefore be determined by the athlete population (eg, individual versus team sport), availability of equipment for monitoring physiological responses (eg, core body temperature, heart rate), available facilities (eg, heat chamber, sauna), and prevailing ambient conditions.46 Within the context of secondary school athletics (ie, large athletics teams with limited physiological monitoring capabilities), the most feasible approach is likely to be the self-paced method.
Sex-, Age-, Sport-, and Fitness-Specific Considerations for Heat Acclimatization
Sex-Specific Considerations for Heat Acclimatization
Before the 1960s, investigators questioned whether women had the physiological capacity to acclimate to heat.47 However, this notion was discounted in later studies48,49 that demonstrated women indeed acclimated to heat. Early work also led to the misconception that women thermoregulated less effectively than men before acclimatization but more effectively after acclimatization,49–51 perhaps because of greater sweating efficiency in women.52 More recent data50,53,54 suggests that these past findings were confounded by sex differences in biophysical factors (body mass and body surface area) that explained most of the variability between sexes in thermoregulatory responses during heat stress. That said, women have reduced sweating sensitivity as a result of less sweat output per gland at high exercise intensities (500 W or 300 W/m2),55 which ultimately results in less evaporative heat loss compared with men of comparable body size.56,57
Regardless of sex differences in sweating, if matched for aerobic fitness and the ratio of surface area to mass, men and women have similar responses to heat acclimatization.52,58 Furthermore, the process by which men and women acclimatize and the outcomes of acclimatization do not seem to differ.59,60 The time course of heat acclimatization has also typically been considered similar in men and women48,49,51 ; historically, 7–10 days are necessary for near-full expression. Shorter regimens (eg, 4–5 days) have also been shown to be effective in both men and women.59,61,62 However, cardiovascular and core body temperature responses in men appear to stabilize after 5 days, whereas women require >5 days to achieve same.59,61,62 Therefore, allowing >10 days of heat acclimatization provides near-optimal benefits for both males and females.
Age-Specific Considerations for Heat Acclimatization
In younger populations and in the context of secondary school athletics, age itself does not affect thermoregulation. Nevertheless, pubertal status has some influence. The idea that prepubertal children are more vulnerable to heat stress was purported for many years, but this notion has been challenged by experts and perceptions have altered.63–65 The larger ratio of body surface area to mass in children and adolescents affords the possibility of greater convective and radiative heat exchange than in adults, depending on the external environment. Thus, prepubertal children are more dependent on convective and radiative heat-loss mechanisms. In addition, even though sweat rates in prepubertal children are lower than those in adults, their sweat droplet dispersion results in equally efficient evaporation.63,64,66 Significant differences in cardiovascular responses between prepubertal and postpubertal children exercising in the heat have not been found.65 Based on these examinations of thermoregulatory and cardiovascular responses, prepubertal and postpubertal children appear to adapt similarly during heat acclimatization. In the United States, puberty has begun in the average female by age 13 and in the average male by age 14.67 As a result, the majority of high school sport athletic teams will be composed of pubertal or postpubertal children. The same set of heat-acclimatization guidelines can be used for athletes at all competitive levels in the secondary school system (ie, freshman, junior varsity, and varsity). If a secondary school includes younger ages (ie, middle school athletes), differences in heat-loss mechanisms should be considered, particularly in terms of break frequency and length. For middle school–aged athletes, in whom reliance on convective and radiative heat loss is greater, modifying work-to-rest ratios and providing access to a shaded or cool environment during activity may help mitigate risk.
Sport-Specific Considerations for Heat Acclimatization
As long as the aforementioned principles are followed (ie, increasing sweating, skin blood flow, and core body and skin temperatures), no evidence indicates that different types of physical activity (eg, soccer versus American football) produce different or delayed adaptations.33,68 Metabolic heat generation is the biggest driver of core body temperature. Regardless of the specific sport, appropriate work-to-rest ratios balance active periods of high metabolic heat production with rest periods of lower metabolic heat production that facilitate heat dissipation.3,5 This balance is imperative in reducing the risk of exertional heat illness, particularly EHS.69–71 Therefore, training in sports that involve continuous endurance activity with limited to no breaks (eg, cross-country) should be based on relative (rather than fixed) exercise intensity (eg, perceived exertion) and exercise length restricted during the heat-acclimatization time frame to ensure student-athlete safety.
Sport protective equipment can cover large portions of the body.72–75 Depending on the padding material and its proximity to the body, heat transfer may be affected. With protective equipment, extra clothing, or both, evaporative heat loss will also be limited due to garment or equipment saturation with sweat. Further, convective heat loss is attenuated due to the microenvironment between the protective equipment or clothing, which will remain constant and approach skin temperature.72–75 Gradually adding protective equipment over time during the heat-acclimatization period (eg, 1 new piece of equipment per day for the first few days) allows the body to adapt to the added stressor; however, when protective equipment, extra clothing, or both are worn, the rate of rise in core body temperature will be greater than in conditions without these added garments, regardless of heat-acclimatization status. Player positions of concern in secondary school sports because of equipment or clothing include all American football positions; all men's lacrosse positions; goalies in field hockey, soccer, and women's lacrosse; and catchers in baseball or softball.
Training-Status–Specific Considerations for Heat Acclimatization
Training status (ie, overall fitness) should also be considered when deciding how to develop heat-acclimatization guidelines. Evidence suggests that individuals who are more physically fit demonstrate partial heat acclimatization, as supported by their habitual training, which elevates exercising core body temperature and skin temperature and elicits an increased sweat rate, all of which improve thermoregulatory capacity.46 Even though individuals with greater physical fitness may exhibit partial heat acclimatization, the magnitudes of physiological adaptations to heat acclimation are similar when individuals of various aerobic fitness levels are compared.39 Thus, heat acclimatization in the context of secondary school athletics may not need to be altered based on overall athlete fitness levels. However, based on the evidence supporting fitness-mediated physiological adaptations for improving thermoregulatory capacity,46 a period of training aimed at improving cardiovascular fitness before heat exposure may be of benefit in reducing the EHS risk.76
Implementing Heat Acclimatization in Sport and Training
Recently, Kerr et al77 identified a lack of compliance with the guidelines provided in the 2009 National Athletic Trainers' Association consensus statement on heat acclimatization (https://meridian.allenpress.com/jat/article/44/3/332/110945/Preseason-Heat-Acclimatization-Guidelines-for). Specifically, 3.9% of athletic trainers working in high schools with football reported full compliance with the guidelines.77 When state high school athletics associations or legislative bodies required local high schools to develop policies, the risk of exertional heat illness appeared to decrease by approximately 55%.9 Despite this, implementation of heat-acclimatization guidelines in secondary school athletics at the state level was disparate.78,79 Within the United States, only 8 of the 51 state and District of Columbia high school athletics associations required their member schools to meet all of the current best practices for heat acclimatization.79 This is an important point to consider in the context of local adoption of such policies: prior researchers77 found that secondary schools were more likely to adopt a comprehensive policy when it was required at the state level. Despite the influence of state-level policy on local adoption, barriers preventing the adoption and implementation of health and safety policies in secondary school athletics persist.
Perceived barriers related to the adoption and implementation of these guidelines in secondary school settings have been identified (Table 1).80 Strategies to overcome such barriers are recommended to help facilitate proper implementation.81 It is important to acknowledge that every secondary school has unique characteristics and that tailored approaches, which adapt but do not drastically alter the guidelines, will help to meet the needs of individual schools. Further, tailored approaches should consider stakeholder feedback to help foster “buy in.” Proactive planning will aid in proper implementation as the sport season begins.
From the narrative review, the working group developed an initial list of 27 recommendations for review. After the first round of review, 4 recommendations received scores ≥4 but <7 for validity, 5 recommendations received scores ≥4 but <7 for feasibility, and 4 recommendations received scores ≥4 but <7 for clarity. In total, consensus occurred on 20 recommendations (scores ≥7 in all 3 constructs), which were carried forward to the final document. The working group used the open-ended comments obtained during the initial round of scoring to address and revise the 7 recommendations that scored ≥4 but <7 on any 1 construct. Based on these comments, the working group removed 2 recommendations, and the 5 revised recommendations were disseminated to the meeting attendees for a second round of scoring.
After the second round of scoring, 2 recommendations received scores ≥4 but <7 for clarity, and 1 recommendation received a score of 6.77 for feasibility. After reviewing the relevant open-ended comments, the working group revised the recommendations and included them in the final list of 25 recommendations, which addressed (1) heat-acclimatization induction and time course, (2) practice-structure considerations, (3) sport-specific considerations, (4) athlete-specific considerations, and (5) implementation strategies.
Heat-Acclimatization Induction and Time Course
Exercise heat acclimatization can be achieved via repeated heat exposure, coupled with exercise, that elicits sweating, increases skin blood flow, and elevates internal body temperature.
At least 10 training sessions over 14 consecutive days of heat acclimatization are recommended.
A total of 60 to 90 minutes of training per day in the heat is advised to induce and optimize the physiological adaptations associated with heat acclimatization.
During the heat acclimatization period, training may take place when the environmental heat is less (eg, early morning or late afternoon or evening) to reduce the risk of heat-related illness but still initiate the adaptation process.
Regardless of the environmental conditions, the length of any single training session during days 1 through 7 of the heat-acclimatization period should last ≤120 minutes.
The length of any single training session during days 8 through 14 of sport activity should be ≤150 minutes.
For days 1 through 6 of the heat-acclimatization period, only 1 training session per day is permitted. An additional 60-minute walkthrough session is permitted on days 1 through 6 for instructional or strategy purposes only and should be separated from the training session by at least 3 hours of continuous rest in a cool (eg, indoor, air-conditioned) environment.
Two training sessions per day are not permitted before day 7 of the heat-acclimatization period; once initiated, double practices may not occur on consecutive days. The time between training sessions must be at least 3 hours, with student-athletes allowed to recover in a cool environment.
Break stations (ie, shaded areas where athletes can rest and have unlimited access to fluids) should be available at the training venue. If possible, cooling devices (eg, fans, cold towels) should be used during breaks.
If a conditioning session that includes high-intensity work is planned during the heat-acclimatization period (days 1–14), it should either (a) be planned as a separate session and constitute the only training on that day or (b) occur at the beginning of practice to reduce the risk of exertional heat illness.
During the heat-acclimatization period, practice sessions that are solely dedicated to conditioning (rather than sport-specific skills) should occur indoors in an air-conditioned environment or during times of the day (early morning or evening) when the environmental heat is low and in conjunction with environment-based activity modifications.13 Conditioning that is outside of the normal sport practice (eg, punishment conditioning sessions) should be prohibited.
Secondary school athletics associations should ensure that ≥14 days of heat acclimatization occur before the first sanctioned competition for sports whose seasons begin when the environmental heat is high.
For athletes in equipment-laden sports (eg, American football, lacrosse, field hockey), the following guidelines regarding protective equipment should be followed: (a) during days 1 and 2, only helmets or headgear are permitted; (b) during days 3 through 5, helmets and shoulder pads are permitted; and (c) full protective equipment can be worn beginning on day 6.
For athletes in endurance sports (eg, cross-country), training in the heat should be based on relative intensity (ie, the same perceived exertion or percentage of maximum heart rate as during training in temperate conditions) rather than absolute intensity (ie, the same pace as during training in temperate conditions).
The guidelines used during the heat-acclimatization period should not differ by (1) level of sport (eg, freshman, junior varsity, or varsity), (2) sex, or (3) conditioning history (eg, an individual participating versus not participating in summer conditioning activities). The guidelines for an institution should apply to all athletes independent of previous individual heat exposure.
Given the variability across sports with respect to mode, intensity, duration, and protective equipment worn during exercise, modifications to the heat-acclimatization guidelines should be considered (Table 2).
Before the start of the heat-acclimatization protocol, approximately 2 weeks of aerobic exercise training should be conducted in a temperate (eg, climate-controlled, indoor, or cool) environment to improve the athlete's cardiorespiratory fitness and reduce the magnitude of stress imposed on the body. If a temperate environment is not available, these workouts should take place during the times of lowest environmental heat (eg, early morning or late afternoon or evening).
Athletes with a recent illness or injury that required significant time loss (>5 days) may experience the loss of heat-acclimatization adaptations. Once physical fitness and function have been restored during the return-to-play process after illness or injury, these individuals may need to restart the heat-acclimatization process.
Recovery between training sessions should occur in a comfortable (ie, air conditioned) environment to ensure that body temperature returns to baseline before the next training session. Adequate sleep (7–9 hours) will also help optimize recovery between training sessions.
Heat-acclimatization guidelines should be used in conjunction with regional (geographic) environment-based activity modifications to dictate work-to-rest ratios, hydration breaks, removal of protective equipment, and rescheduling training or competition to optimize student-athlete safety.
Given the risks to players and the potentially catastrophic outcomes associated with EHS, a credentialed health care provider trained in the proper management and treatment of exertional heat illness (eg, athletic trainer) should be accessible or onsite during all training sessions.
In case of suspected EHS, immediate ice-water immersion or other water-mediated cooling should be provided, following the principle of “cool first, transport second.” If a credentialed health care provider is not onsite to initiate cooling, the coaching staff should be educated to initiate cooling while waiting on the arrival of emergency health care services.
Each secondary school should conduct regular training and education sessions with all stakeholders (ie, athletic trainer, team physician, coaches, administrators, emergency medical services) to review and practice the procedures regarding exertional heat illness prevention, management, and care. Each secondary school sports medicine health care team should regularly review and update the school's policies and procedures regarding student-athlete health and safety.
Each secondary school should have a written policies and procedures manual that contains the current standards for the prevention, management, and care of exertional heat illness.
The written policies and procedures manual should be disseminated to all stakeholders involved in secondary school athletics. Stakeholders should provide written acknowledgment that the manual was received and read.
Given the physiological benefits afforded by heat acclimatization in optimizing human health and performance during exercise in hot environmental conditions, it is prudent that proper guidelines and protocols be in place in secondary school athletics that address equipment-laden, intermittently aerobic (eg, basketball, soccer), and endurance or aerobic (eg, cross-country) sports. The consensus recommendations from this Delphi process offer clinicians, administrators, and coaches a data-driven approach for developing and implementing heat-acclimatization procedures across all secondary school sports to mitigate the risk of exertional heat illness. Implementation of these consensus recommendations (Table 3) in conjunction with current evidence-based recommendations on the recognition, management, and care of patients with EHS permits a comprehensive approach to heat safety for secondary school athletes.
We acknowledge the National Athletic Trainers' Association, Korey Stringer Institute, and American College of Sports Medicine for providing support for the roundtable meeting. We also thank the meeting attendees for their contributions at the roundtable meeting and to the Delphi voting process.