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Abstract
Exercising in the heat induces thermoregulatory and other physiological strain that can lead to impairments in endurance exercise capacity. The purpose of this consensus statement is to provide up-to-date recommendations to optimise performance during sporting activities undertaken in hot ambient conditions. The most important intervention one can adopt to reduce physiological strain and optimise performance is to heat acclimatise. Heat acclimatisation should comprise repeated exercise-heat exposures over 1–2 weeks. In addition, athletes should initiate competition and training in a euhydrated state and minimise dehydration during exercise. Following the development of commercial cooling systems (eg, cooling-vest), athletes can implement cooling strategies to facilitate heat loss or increase heat storage capacity before training or competing in the heat. Moreover, event organisers should plan for large shaded areas, along with cooling and rehydration facilities, and schedule events in accordance with minimising the health risks of athletes, especially in mass participation events and during the first hot days of the year. Following the recent examples of the 2008 Olympics and the 2014 FIFA World Cup, sport governing bodies should consider allowing additional (or longer) recovery periods between and during events, for hydration and body cooling opportunities, when competitions are held in the heat.
- Thermoregulation
- Hypohydration
- Heat acclimatisation
- Heat stress
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Aim and scope
Most of the major international sporting events such as the Summer Olympics, the FIFA World Cup and the Tour de France—that is, the three most popular events in terms of television audience worldwide—take place during the summer months of the northern hemisphere, and often in hot ambient conditions. On the 23rd and 24th of March 2014, a panel of experts reviewed and discussed the specificities of Training and Competing in the Heat during a topical conference held at Aspetar Orthopaedic and Sports Medicine Hospital in Doha, Qatar. The conference ended with a round-table discussion, which has resulted in this consensus statement.
This document is intended to provide up-to-date recommendations regarding the optimisation of exercise capacity during sporting activities in hot ambient conditions. Given that the performance of short duration activities (eg, jumping and sprinting) is at most marginally influenced, or can even be improved, in hot ambient conditions,1 but that prolonged exercise capacity is significantly impaired,2 the recommendations provided in this consensus statement focus mainly on prolonged sporting events. For additional information on Training and Competing in the Heat, the reader is referred to the supplement issue published in the Scandinavian Journal of Medicine and Science in Sports, which includes targeted reviews and original manuscripts.3
Introduction
When exercising in the heat, skin blood flow and sweat rate increase to allow for heat dissipation to the surrounding environment. These thermoregulatory adjustments, however, increase physiological strain and may lead to dehydration during prolonged exercise. Heat stress alone will impair aerobic performance when hyperthermia occurs.2 ,4–6 Consequently, athletes perform endurance, racket or team-sports events in the heat at a lower work rate than in temperate environments.7–12 In addition, dehydration during exercise in the heat exacerbates thermal and cardiovascular strain,13–18 and further impairs aerobic performance.2 ,17 ,19 This document contains recommendations and strategies to adopt in order to sustain/enhance performance during training and competition in the heat, as well as to minimise the risk of exertional heat illness. As presented in the first section, the most important intervention one can adopt to reduce physiological strain and optimise performance is to heat acclimatise. Given that dehydration can impair physical performance and exacerbate exercise-induced heat strain, the second section of the consensus statement provides recommendations regarding hydration. The third section highlights the avenues through which it is possible to decrease core and skin temperatures, before and during exercise, via the application of cold garments to the skin such as ice packs, cold towels and cooling vests, as well as through cold water immersion (CWI) or ice slurry ingestion.
Given the lack of data from real competitions, the International Olympic Committee (IOC) recently highlighted the necessity for sports federations, team doctors and researchers to collaborate in obtaining data on the specific population of elite athletes exercising in challenging environments.20 Several international sporting federations such as FIFA, FINA, FIVB, IAAF and ITF have responded to this challenge by initiating a surveillance system to assess environmental conditions during competition, along with their adverse outcomes.12 ,21–23 A number of sporting federations have also edited their guidelines to further reduce the risks of exertional heat illness. These guidelines are reviewed in the fourth section of this consensus statement. Recommendations are offered to event organisers and sporting bodies on how to best protect the health of the athlete and sustain/enhance performance during events in the heat.
Section 1: Heat acclimatisation
Although regular exercise in temperate conditions elicits partial heat acclimatisation,24 it cannot replace the benefits induced by consecutive days of training in the heat.24–27 Heat acclimatisation improves thermal comfort and submaximal, as well as maximal, aerobic exercise performance in warm-hot conditions.11 ,28 ,29 The benefits of heat acclimatisation are achieved via increased sweating and skin blood flow responses, plasma volume expansion and hence improved cardiovascular stability (ie, better ability to sustain blood pressure and cardiac output), and fluid-electrolyte balance.19 ,30 ,31 Exercise-heat acclimatisation is therefore essential for athletes preparing competitions in warm-hot environments.30 This section describes how to practically implement heat acclimatisation protocols and optimise the benefits in athletes.
Induction of acclimatisation
Duration
Most adaptations (ie, decreases in heart rate, skin and rectal temperature, increases in sweat rate and work capacity) develop within the first week of heat acclimatisation and more slowly in the subsequent 2 weeks.32–34 Adaptations develop more quickly in highly trained athletes (up to half the time) compared with untrained individuals.24 ,35 Consequently, athletes benefit from only a few days of heat acclimatisation,36–38 but may require 6–10 days to achieve near complete cardiovascular and sudomotor adaptations,28 ,29 ,39 and as such 2 weeks to optimise aerobic performance (ie, cycling time trial) in hot ambient conditions.11
Training
The principle underlying any heat acclimatisation protocol is an increase in body (core and skin) temperature to induce profuse sweating and increase skin blood flow.19 ,30 Repeated heat-exercise training for 100 min was originally shown to be efficient at inducing such responses.40 Reportedly, exercising daily to exhaustion at 60% VO2max in hot ambient conditions (40°C, 10% RH) for 9–12 consecutive days increases exercise capacity from 48 to 80 min.28 Ultimately, the magnitude of adaptation depends on the intensity, duration, frequency and number of heat exposures.30 ,31 For example, Houmard et al41 reported similar physiological adaptations following moderate-intensity short-duration (30–35 min, 75% VO2max) and low-intensity long-duration (60 min, 50% VO2max) exercise.
As acclimatisation develops, constant workload exercise protocols may result in a progressively lower training stimulus (ie, decreases in relative exercise intensity). In turn, this may limit the magnitude of adaptation if the duration and/or the intensity of the heat-exercise training sessions are not increased accordingly.42 When possible, an isothermic protocol (eg, controlled hyperthermia to a core temperature of at least 38.5°C) can be implemented to optimise the adaptations.43 ,44 However, isothermic protocols may require greater control and the use of artificial laboratory conditions, which could limit their practicality in the field. Alternatively, it has recently been proposed to utilise a controlled intensity regimen based on heart rate to account for the need to increase absolute intensity and maintain a similar relative intensity throughout the acclimatisation process.31 Lastly, athletes can adapt by training outdoors in the heat (ie, acclimatisation) using self-paced exercise, or maintaining their regular training regimen. The efficacy of this practice has been demonstrated with team-sport athletes,45 ,46 without interfering with their training regimen.
Environment
Heat acclimatisation in dry heat improves exercise in humid heat47 ,48 and vice versa.49 However, acclimatisation in humid heat evokes higher skin temperatures and circulatory adaptations than in dry heat, potentially increasing maximum skin wettedness and therefore the maximum rate of evaporative heat loss from the skin.30 ,31 ,50 Although scientific support for this practice is still lacking, it may potentially be beneficial for athletes to train in humid heat at the end of their acclimatisation sessions to dry heat to further stress the cardiovascular and thermoregulatory systems. Nevertheless, despite some transfer between environments, other adaptations might be specific to the climate (desert or tropic) and physical activity level.51 Consequently, it is recommended that athletes predominantly acclimatise to the environment in which they will compete.
Athletes who do not have the possibility to travel to naturally hot ambient conditions (so-called ‘acclimatisation’) can train in an artificially hot indoor environment (so-called ‘acclimation’). However, while acclimation and acclimatisation share similar physiological adaptations, training outdoors is more specific to the competition setting as it allows athletes to experience the exact nature of the heat stress.52–54
Decay and periodisation of short-term acclimatisation
Heat adaptations decay at different rates with the fastest adaptations also decaying more rapidly.35 However, the rate of decay of heat acclimatisation is generally slower than its induction, allowing maintenance of the majority of benefits (eg, heart rate, core temperature) for 2–4 weeks.34 ,55–58 Moreover, during this period, individuals re-acclimatise faster than during the first acclimatisation period57 (table 1). These studies are, however, mainly based on physiological markers of heat acclimatisation and the decay in competitive sporting performance remains to be clarified.
Examples of heat-acclimatisation strategies
Individualised heat acclimatisation
Heat acclimatisation clearly attenuates physiological strain.59 ,60 However, individual acclimatisation responses may differ and should be monitored using simple indices, such as the lessened heart rate increase during a standard submaximal exercise bout.33 ,61–63 Other more difficult and likely less sensitive markers for monitoring heat acclimatisation include sweat rate and sodium content,64 core temperature33 and plasma volume.65 The role of plasma volume expansion in heat acclimatisation remains debated, as an artificial increase in plasma volume does not appear to improve thermoregulatory function,66 ,67 but the changes in haematocrit during a heat-response test following short-term acclimatisation correlate to individual physical performance.45 ,46 This suggests that plasma volume changes might represent a valuable indicator, even if it is probably not the physiological mechanism improving exercise capacity in the heat. Importantly, measures in a temperate environment cannot be used as a substitute to a test in hot ambient temperatures.45 ,46 ,68
As with its induction, heat acclimatisation decay also varies between individuals.32 It is therefore recommended that athletes undergo an acclimatisation procedure months before an important event in the heat to determine their individual rate of adaptation and decay20 ,45 (table 1).
Heat-acclimatisation as a training stimulus
Several recent laboratory or uncontrolled-field studies have reported physical performance improvement in temperate environments following training in the heat.29 ,46 ,62 ,69 ,70 Athletes might therefore consider using training camps in hot ambient conditions to improve physical performance in-season62 and preseason46 (table 1). Bearing in mind that training quality should not be compromised, the athletes benefiting the most from this might be experienced athletes requiring a novel training stimulus,46 whereas the benefit for highly-trained athletes with limited thermoregulatory requirement (eg, cycling in cold environments) might be more circumstantial.71
Summary of the main recommendations for heat acclimatisation
Athletes planning to compete in hot ambient conditions should heat acclimatise (ie, repeated training in the heat) to obtain biological adaptations lowering physiological strain and improving exercise capacity in the heat.
Heat acclimatisation sessions should last at least 60 min/day, and induce an increase in body core and skin temperatures, as well as stimulate sweating.
Athletes should train in the same environment as the competition venue, or if not possible, train indoors in a hot room.
Early adaptations are obtained within the first few days, but the main physiological adaptations are not complete until ∼1 week. Ideally, the heat acclimatisation period should pass 2 weeks in order to maximise all benefits.
Section 2: Hydration
The development of hyperthermia during exercise in hot ambient conditions is associated with a rise in sweat rate, which can lead to progressive dehydration if fluid losses are not minimised by increasing fluid consumption. Exercise-induced dehydration, leading to a hypohydrated state, is associated with a decrease in plasma volume and an increase in plasma osmolality that are proportional to the reduction in total body water.19 The increase in the core temperature threshold for vasodilation and sweating at the onset of exercise is closely linked to the ensuing hyperosmolality and hypovolaemia.72 ,73 During exercise, plasma hyperosmolality reduces the sweat rate for any given core temperature and decreases evaporative heat loss.74 In addition, dehydration decreases cardiac filling and challenges blood pressure regulation.75–77 The rate of heat storage and cardiovascular strain is therefore exacerbated, and the capacity to tolerate exercise in the heat is reduced.78–80
Despite decades of studies in this area,81 the notion that dehydration impairs aerobic performance in sport settings is not universally accepted and there seems to be a two-sided polarised debate.82–84 Numerous studies report that dehydration impairs aerobic performance in the condition that if exercise is performed in warm-hot environments and that body water deficits exceed at least ∼2% of body mass.13 ,49 ,81 ,85–90 On the other hand, some recent studies suggest that dehydration up to 4% body mass does not alter cycling performance under an ecologically valid conditions.82 ,83 ,91 However, these results must be interpreted in context; that is, in well-trained male cyclists typically exercising for 60 min in ambient conditions up to 33°C and 60% relative humidity, and starting exercise in a euhydrated state. Nonetheless, some have advanced the idea that the detrimental consequences of dehydration have been overemphasised by sports beverage companies.92 As such, it has been argued that athletes should drink to thirst.82 ,83 ,91 However, many studies (often conducted prior to the creation and marketing of ‘sport-drinks’) have repeatedly observed that drinking to thirst often results in body water deficits that may exceed 2–3% body mass when sweat rates are high and exercise is performed in warm-hot environments.13 ,47 ,49 ,93–98 Ultimately, drinking to thirst may be appropriate in many settings, but not in circumstances where severe dehydration is expected (eg, Ironman triathlon).84
In competition settings, hydration is dependent on several factors, including fluid availability and the specificities of the events. For example, while tennis players have regular access to fluids due to the frequency of breaks in a match, other athletes such as marathon runners have less opportunity to rehydrate. There are also differences among competitors. Whereas the fastest marathon runners do not consume large volume of fluids and become dehydrated during the race, some slower runners may conversely overhydrate,99 with an associated risk of ‘water intoxication’ (ie, hyponatraemia).100 The predisposing factors related to developing hyponatraemia during a marathon include substantial weight gain, a racing time above 4 h, female sex and low body-mass index.101 ,102 Consequently, although the recommendations below for competitive athletes explain how to minimise the impairment in performance associated with significant dehydration and body mass loss (ie, ≥2%), recreational athletes involved in prolonged exercise should be cautious not to overhydrate during the exercise.
Pre-exercise hydration
Resting and well-fed humans are generally well hydrated,103 and the typical variance in day-to-day total body water fluctuates from 0.2% to 0.7% of body mass.93 ,104 When exposed to heat stress in the days preceding competition, it may, however, be advisable to remind athletes to drink sufficiently and replace electrolyte losses to ensure that euhydration is maintained. Generally, drinking 6 mL of water per kg of body mass during this period every 2–3 h, as well as 2–3 h before training or competition in the heat, is advisable.
There are several methods available to evaluate hydration status, each one having limitations depending on how and when the fluids are lost.105 ,106 The most widely accepted and recommended methods include monitoring body mass changes, measuring plasma osmolality and urine specific gravity. Based on these methods, one is considered euhydrated if daily body mass changes remain <1%, plasma osmolality is <290 mmol/kg and urine specific gravity is <1.020. These techniques can be implemented during intermittent competitions lasting for several days (eg, cycling stage race, tennis/team sports tournament) to monitor hydration status. Establishing baseline body mass is important, as daily variations may occur. It is best achieved by measuring post-void nude body mass in the morning on consecutive days after consuming 1–2 L of fluid the prior evening.81 Moreover, since exercise, diet and prior drinking influence urine concentration measurements, first morning urine is the preferred assessment time point to evaluate hydration status.81 If first morning urine cannot be obtained, urine collection should be preceded by several hours of minimal physical activity, fluid consumption and eating.
Exercise hydration
Sweat rates during exercise in the heat vary dramatically depending on the metabolic rate, environmental conditions and heat acclimatisation status.107 While values ranging from 1.0 to 1.5 L/h are common for athletes performing vigorous exercise in hot environments, certain individuals can exceed 2.5 L/h.108–111 Over the last several decades, mathematical models have been developed to provide sweat loss predictions over a broad range of conditions.112–117 While these have proven useful in public health, military, occupational and sports medicine settings, these models require further refinement and individualisation to athletic populations, especially elite athletes.
The main electrolyte lost in sweat is sodium (20–70 mEq/L),118 ,119 and supplementation during exercise is often required for heavy and ‘salty’ sweaters to maintain plasma sodium balance. Heavy sweaters may also deliberately increase sodium (ie, salt) intake prior to and following hot weather training and competition to maintain sodium balance (eg, 3.0 g of salt added to 0.5 L of a carbohydrate-electrolyte drink). To this effect, the Institute of Medicine103 has highlighted that public health recommendations regarding sodium ingestion do not apply to individuals who lose large volumes of sodium in sweat, such as athletes training or competing in the heat. A salt intake that would not compensate sweat sodium losses would result in a sodium deficit that might prompt muscle cramping when reaching 20–30% of the exchangeable sodium pool.120 During exercise lasting longer than 1 h, athletes should therefore aim to consume a solution containing 0.5–0.7 g/L of sodium.121–123 In athletes experiencing muscle cramping, it is recommended to increase the sodium supplementation to 1.5 g/L of fluid.124 Athletes should also aim to include 30–60 g/h of carbohydrates in their hydration regimen for exercise lasting longer than 1 h,122 and up to 90 g/h for events lasting over 2.5 h.125 This can be achieved through a combination of fluids and solid foods.
Post-exercise rehydration
Following training or competing in the heat, rehydration is particularly important to optimise recovery. If fluid deficit needs to be urgently replenished, it is suggested to replace 150% of body mass losses within 1 h following the cessation of exercise,123 ,126 including electrolytes to maintain total body water. From a practical perspective, this may not be achievable for all athletes for various reasons (eg, time, gastrointestinal discomfort). Thus, it is more realistic to replace 100–120% of body mass losses. The preferred method of rehydration is through the consumption of fluids with foods (eg, including salty food).
Given that exercise in the heat increases carbohydrate metabolism,127 ,128 endurance athletes should ensure that not only water and sodium losses are replenished, but carbohydrate stores as well.129 To ensure the highest rates of muscle glycogen resynthesis, carbohydrates should be consumed during the first hour after exercise.130 Moreover, a drink containing protein (eg, milk) might allow better restoration of fluid balance after exercise than a standard carbohydrate-electrolyte sport drink.131 Combining protein (0.2–0.4 g/kg/h) to carbohydrate (0.8 g/kg/h) has also been reported to maximise protein synthesis rates.132 Therefore, athletes should consider consuming drinks such as chocolate milk, which has a carbohydrate-to-protein ratio of 4:1, as well as sodium, following exercise.133
Summary of the main recommendations for hydration
Before training and competition in the heat, athletes should drink 6 mL of fluid per kg of body mass every 2–3 h, in order to start exercise euhydrated.
During intense prolonged exercise in the heat, body water mass losses should be minimised (without increasing body weight) to reduce physiological strain and help to preserve optimal performance.
Athletes training in the heat have higher daily sodium (ie, salt) requirements than the general population. Sodium supplementation might also be required during exercise.
For competitions lasting several days (eg, cycling stage race, tennis/team sports tournament), simple monitoring techniques such as daily morning body mass and urine specific gravity can provide useful insights into the hydration state of the athlete.
Adequately rehydrating after exercise-heat stress by providing plenty of fluids with meals is essential. If aggressive and rapid replenishment is needed, then consuming fluids and electrolytes to offset 100–150% of body mass losses will allow for adequate rehydration.
Recovery hydration regimens should include sodium, carbohydrates and protein.
Section 3: Cooling strategies
Skin cooling will reduce cardiovascular strain during exercise in the heat, while whole-body cooling can reduce organ and skeletal muscle temperatures. Several studies carried out in controlled laboratory conditions (eg, uncompensable heat-stress), in many cases with or without reduced fanning during exercise, have reported that precooling can improve endurance,134–140 and high-intensity141 and intermittent-sprint or repeated-sprint exercise performance.142–145 However, several other studies reported no performance benefits of precooling on intermittent-sprints or repeated-sprints exercise performance in the heat.142 ,146–148 Whole body cooling (including cooling of the exercising muscles) may even be detrimental to performance during a single sprint or the first few repetitions of an effort involving multiple sprints.149 ,150
Therefore, whereas several reviews concluded that cooling interventions can increase prolonged exercise capacity in hot conditions,151–158 it has to be acknowledged that most laboratory based precooling studies might have overestimated the effect of precooling as compared to an outdoor situation with airflow,159 or do not account for the need to warm-up before competing. As a consequence, the effectiveness of cooling in a competitive settings remains equivocal and the recommendations below are limited to prolonged exercise in hot ambient conditions with no or limited air movement.
Cold-water immersion
A range of CWI protocols are available (for reviews see156 ,160–162), but the most common techniques are whole body CWI for ∼30 min at a water temperature of 22–30°C, or body segment (eg, legs) immersion at lower temperatures (10–18°C).156 However, cooling of the legs/muscles will decrease nerve conduction and muscle contraction velocities,1 and athletes might therefore need to re-warm-up before competition. Consequently, other techniques involving cooling garments have been developed to selectively cool the torso, which may prevent the excessive cooling of active muscles while reducing overall thermal and cardiovascular strain.
Cooling garments
Building on the early practice of using iced towels for cooling purposes, several manufacturers have designed ice-cooling jackets to cool athletes before or during exercise.137 ,142 ,163 ,164 The decrease in core temperature is smaller with a cooling vest than with CWI or mixed-cooling methods,158 but cooling garments present the advantage of lowering skin temperature, and thus reducing cardiovascular strain and, eventually, heat storage.165 Cooling garments are practical in reducing skin temperature without reducing muscle temperature, and athletes can wear them during warm-up or recovery breaks.
Cold fluid ingestion
Cold fluids can potentially enhance endurance performance when ingested before,166 ,167 but not during,168 ,169 exercise. Indeed, it is suggested that a downside of ingesting cold fluids during exercise might be a reduction in sweating and therefore skin surface evaporation,170 due to the activation of thermoreceptors probably located in the abdominal area.171
Ice-slurry beverages
Based on the theory of enthalpy, ice requires substantially more heat energy (334 J/g) to cause a phase change from solid to liquid (at 0°C) compared with the energy required to increase the temperature of water (4 J/g/°C). As such, ice slurry may be more efficient than cold-water ingestion in cooling athletes. However, it is not yet clear if the proportional reduction in sweating observed with the ingestion of cold water during exercise170 occurs with ice slurry ingestion. Several recent reports support the consumption of an ice-slurry beverage since performance during endurance or intermittent-sprint exercise is improved following the ingestion of an ice-slurry beverage (∼1 L crushed ice at ≤4°C) either prior to140 ,172 ,173 or during exercise,174 but no benefit was evident when consumed during the recovery period between two exercise bouts in another study.175 Consequently, ingestion of ice-slurry may be a practical complement or alternative to external cooling methods,155 but more studies are still required during actual outdoor competitions.
Mixed methods cooling strategies
Combining techniques (ie, using both external and internal cooling strategies) has a higher cooling capacity than the same techniques used in isolation, allowing for greater benefit on exercise performance.158 Indeed, mixed methods have proven beneficial when applied to professional football players during competition in the tropics,176 lacrosse players training in hot environments177 and cyclists simulating a competition in a laboratory.139 In a sporting context, this can be achieved by combining simple strategies, such as the ingestion of ice-slurry, wearing cooling vests and providing fanning.
Cooling to improve performance between subsequent bouts of exercise
There is evidence supporting the use of CWI (5 to 12 min in 14°C water) during the recovery period (eg, 15 min) separating intense exercise bouts in the heat to improve subsequent performance.178 ,179 The benefits of this practice would relate to a redistribution of the blood flow, probably from the skin to the central circulation,180 as well as a psychological (ie, placebo) effect.181 In terms of internal cooling, the ingestion of cold water182 or ice-slurry175 during the recovery period might attenuate heat strain in the second bout of work, but not necessarily significantly improve performance.175 Together, these studies suggest that cooling might help recovery from intense exercise in uncompensable laboratory heat-stress and, in some cases, might improve performance in subsequent intense exercise bouts. The effects of aggressive cooling versus simply resting in the prevailing hot ambient conditions, or in cooler conditions, remains to be validated in a competition setting (eg, half time in team-sports).
Summary of the main recommendations for cooling
Cooling methods include external (eg, application of iced garments, towels, water immersion or fanning) and internal (eg, ingestion of cold fluids or ice-slurry) methods.
Precooling may benefit sporting activities involving sustained exercise (eg, middle and long distance running, cycling, tennis and team sports) in warm-hot environments. Internal methods (ie, ice slurry) can be used during exercise, whereas tennis and team sport athletes can also implement mixed cooling methods during breaks.
Such practice may not be viable for explosive or shorter duration events (eg, sprinting, jumping, throwing) conducted in similar conditions.
A practical approach in hot-humid environments might be the use of fans and commercially available ice cooling vests, which can provide effective cooling without impairing muscle temperature. In any case, cooling methods should be tested and individualised during training to minimise disruption to the athlete.
Section 4: Recommendations for event organisers
The most common set of recommendations followed by event organisers to reschedule or cancel an event is based on the wet bulb globe temperature (WBGT) index empirically developed by the US military, popularised in sports medicine by the American College of Sports Medicine183 and adopted by various sporting federations (table 2). However, WBGT might underestimate heat stress risk when sweat evaporation is restricted (ie, high humidity and/or low air movement).184 Thus, corrected recommendations have been proposed185 (table 3). Moreover, the WBGT is a climatic index and does not account for metabolic heat production or clothing and therefore cannot predict heat dissipation.19 Therefore, the recommendations below provide guidelines for various sporting activities rather than fixed cut-offs based on the WBGT index.
Examples of recommended actions by various sporting governing bodies based on the WBGT
Corrected estimation of the risk of exertional heat illness based on the wet bulb globe temperature (WBGT) taking into account that WBGT underestimates heat stress under high humidity
Cancelling an event or implementing countermeasures?
Further to appropriate scheduling of any event with regard to expected environmental conditions, protecting athlete health might require stopping competition when combined exogenous and endogenous heat loads cannot be physiologically compensated. The environmental conditions in which the limit of compensation is exceeded depends on several factors, such as metabolic heat production (depending on workload and efficiency/economy), athlete morphology (eg, body surface area to mass ratio), acclimatisation state (eg, sweat rate) and clothing. It is therefore problematic to establish universal cut-off values across different sporting disciplines. Environmental indices should be viewed as recommendations for event organisers to implement preventive countermeasures to offset the potential risk of heat illness. The recommended countermeasures include adapting the rules and regulations with regard to cooling breaks and the availability of fluids (time and locations), as well as providing active cooling during rest periods. It is also recommended that medical response protocols and facilities to deal with cases of exertional heat illnesses be in place.
Specificity of the recommendations
Differences among sports
Hot ambient conditions impair endurance exercise such as marathon running,7 but potentially improve short duration events such as jumping or sprinting.1 In many sports, athletes adapt their activity according to the environmental conditions. For example, compared to cooler conditions, football players decrease the total distance covered or the distance covered at high intensity during a game, but maintain their sprinting activity/ability,9 ,12 ,186 while tennis players reduce point duration8 or increase the time between points10 when competing in the heat (WBGT ∼34°C). Event organisers and international federations should therefore acknowledge and support such behavioural thermoregulatory strategies by adapting the rules and refereeing accordingly.
Differences among individuals within a given sport
When comparing two triathlon races held in Melbourne, in similar environmental conditions (ie, WBGT raising from 22 to 27°C during each race), 2 months apart, Gosling et al187 observed 15 cases of exertional heat illness (including 3 heat strokes) in the first race that was held in unseasonably hot weather at the start of summer, but no cases in the second race. This suggests that the risk of heat illness was increased in competitors who were presumably not seasonally heat acclimatised187 and supports many earlier studies regarding increased risk of heat illness in early summer, or with hot weather spikes.188 Nevertheless, exertional heat stroke can occur in individuals who are well acclimatised and have performed similar activities several times before, as they may suffer from prior viral infection or similar ailment.19 In one of the very few epidemiological studies linking WBGT to illness in athletes, Bahr et al22 investigated 48 beach volleyball matches (World Tour and World Championships), over 3 years. They reported only one case of a heat-related medical forfeit, which was related to an athlete with compromised fluid balance due to a 3-day period of acute gastroenteritis.22 Moreover, while healthy runners can also finish a half-marathon in warm and humid environments without developing heat illness,189 exertional heat stroke has been shown to occur during a cool weather marathon in a runner recovering from a viral infection.190
In fact, prior viral infection is emerging as a potentially important risk factor for heat injury/stroke.19 ,191 Event organisers should therefore pay particular medical attention to all populations potentially at a greater risk, including participants currently sick or recovering from a recent infection, those with diarrhoea, recently vaccinated, with limited heat dissipation capacity due to medical conditions (eg, Paralympic athletes), or individuals involved in sports with rules restricting heat dissipation capacity (eg, protective clothing/equipment). Unacclimatised participants are also to be considered at risk. Although it is impractical to screen every athlete during large events, organisers are encouraged to provide information, possibly in registration kits, advising all athletes of the risk associated with participation under various potential compromised states and suggesting countermeasures.
Summary of the main recommendations for event organisers
The WBGT is an environmental heat stress index and not a representation of human heat strain. It is therefore difficult to establish absolute participation cut-off values across sports for different athletes and we rather recommend implementing preventive countermeasures, or evaluating the specific demands of the sport when preparing extreme heat policies.
Countermeasures include scheduling the start time of events based on weather patterns, adapting the rules and refereeing to allow extra breaks or longer recovery periods, and developing a medical response protocol and cooling facilities.
Event organisers should pay particular attention to all ‘at risk’ populations. Given that unacclimatised participants (mainly in mass participation events) are at a higher risk for heat-illness, organisers should properly advise participants of the risk associated with participation, or consider cancelling an event in the case of unexpected or unseasonably hot weather.
Overall conclusion
Our current knowledge on heat stress is mainly derived from military and occupational research fields, while the input from sport sciences is more recent. Based on this literature, athletes should train for at least 1 week and ideally 2 weeks to acclimatise using a comparable degree of heat stress as the target competition. They should also be cautious to undertake exercise in a euhydrated state and minimise body water deficits (as monitored by body mass losses) through proper rehydration during exercise. They can also implement specific countermeasures (eg, cooling methods) to reduce heat storage and physiological strain during competition and training, especially when the environmental conditions are uncompensable. Event organisers and sports governing bodies can support athletes by allowing additional (or longer) recovery periods for enhanced hydration and cooling opportunities during competitions in the heat.
Acknowledgments
The authors thank the following conference attendees for their participation in the two days of discussion: Carl Bradford, Martin Buchheit, Geoff Coombs, Simon Cooper, Kevin De Pauw, Sheila Dervis, Abdulaziz Farooq, Oliver Gibson, Mark Hayes, Carl James, Stefanie Keiser, Luis Lima, Alex Lloyd, Erin McLeave, Jessica Mee, Nicholas Ravanelli, Jovana Smoljanic, Steve Trangmar, James Tuttle, Jeroen Van Cutsem and Matthijs Veltmeijer. Bart Roelands is a postdoctoral fellow of the Fund for Scientific Research Flanders (FWO).
References
Footnotes
Twitter Follow Dr Sébastien Racinais at @SebRacinais
Competing interests None declared.
Funding José González-Alonso has received research funding from the Gatorade Sports Science Institute, Pepsico. Michael N Sawka was a member of the Gatorade Sports Science Institute Expert Panel in 2014.
Provenance and peer review Not commissioned; internally peer reviewed.