Consensus recommendations on training and competing in the heat
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Scand J Med Sci Sports 2015: 25 (Suppl. 1): 6–19 © 2015 John Wiley & Sons A/S.
doi: 10.1111/sms.12467 Published by John Wiley & Sons Ltd
Consensus recommendations on training and competing in
the heat
S. Racinais1, J. M. Alonso2,3, A. J. Coutts4, A. D. Flouris5, O. Girard6, J. González-Alonso7, C. Hausswirth8, O. Jay9,
J. K. W. Lee10,11,12, N. Mitchell13, G. P. Nassis14, L. Nybo15, B. M. Pluim16, B. Roelands17, M. N. Sawka18, J. E. Wingo19,
J. D. Périard1
1
Athlete Health and Performance Research Centre, Aspetar, Qatar Orthopaedic and Sports Medicine Hospital, Doha, Qatar, 2Sports
Medicine Department, Aspetar Orthopaedic and Sports Medicine Hospital, Doha, Qatar, 3Medical and Anti-doping Commission,
International Association of Athletics Federations (IAAF), Montecarlo, Monaco, 4Sport and Exercise Discipline Group, University of
Technology Sydney (UTS), Lindfield, New South Wales, Australia, 5FAME Laboratory, Department of Physical Education and Sport
Science, University of Thessaly, Trikala, Greece, 6ISSUL, Institute of Sport Sciences, Department of Physiology, Faculty of Biology
and Medicine, University of Lausanne, Lausanne, Switzerland, 7Centre for Sports Medicine and Human Performance, Department of
Life Sciences, College of Health and Life Sciences, Brunel University London, Uxbridge, UK, 8French National Institute of Sport
(INSEP), Research Department, Laboratory of Sport, Expertise and Performance, Paris, France, 9Discipline of Exercise and Sport
Science, Faculty of Health Sciences, University of Sydney, Lidcombe, New South Wales, Australia, 10Defence Medical and
Environmental Research Institute, DSO National Laboratories, Singapore, 11Yong Loo Lin School of Medicine, National University of
Singapore, Singapore, 12Lee Kong Chian School of Medicine, Nanyang Technological University, Singapore, 13British Cycling and
“Sky Pro Cycling”, National Cycling Centre, Manchester, UK, 14National Sports Medicine Programme, Excellence in Football
Project, Aspetar, Qatar Orthopaedic and Sports Medicine Hospital, Doha, Qatar, 15Department of Nutrition, Exercise and Sport,
Section of Human Physiology, University of Copenhagen, Copenhagen, Denmark, 16Medical Department, Royal Netherlands Lawn
Tennis Association (KNLTB), Amersfoort, The Netherlands, 17Department of Human Physiology, Vrije Universiteit Brussel, Brussels,
Belgium, 18School of Applied Physiology, College of Science, Georgia Institute of Technology, Atlanta, Georgia, USA, 19Department of
Kinesiology, University of Alabama, Tuscaloosa, Alabama, USA
Corresponding author: Sébastien Racinais, PhD, Athlete Health and Performance Research Centre, Aspetar, Qatar Orthopaedic and
Sports Medicine Hospital, PO Box 29222, Doha, Qatar. Tel: +974 4413 2544, Fax: +974 4413 2020, E-mail:
sebastien.racinais@aspetar.com
Accepted for publication 10 March 2015
Exercising in the heat induces thermoregulatory and vest), athletes can implement cooling strategies to facili-
other physiological strain that can lead to impairments in tate heat loss or increase heat storage capacity before
endurance exercise capacity. The purpose of this consen- training or competing in the heat. Moreover, event orga-
sus statement is to provide up-to-date recommendations nizers should plan for large shaded areas, along with
to optimize performance during sporting activities under- cooling and rehydration facilities, and schedule events in
taken in hot ambient conditions. The most important accordance with minimizing the health risks of athletes,
intervention one can adopt to reduce physiological strain especially in mass participation events and during the
and optimize performance is to heat acclimatize. Heat first hot days of the year. Following the recent examples of
acclimatization should comprise repeated exercise-heat the 2008 Olympics and the 2014 FIFA World Cup, sport
exposures over 1–2 weeks. In addition, athletes should governing bodies should consider allowing additional (or
initiate competition and training in a euhydrated state longer) recovery periods between and during events for
and minimize dehydration during exercise. Following the hydration and body cooling opportunities when competi-
development of commercial cooling systems (e.g., cooling tions are held in the heat.
Aim and scope specificities of Training and Competing in the Heat
during a topical conference held at Aspetar Orthopaedic
Most of the major international sporting events such as and Sports Medicine Hospital in Doha, Qatar. The con-
the Summer Olympics, the FIFA World Cup, and the ference ended with a roundtable discussion, which has
Tour de France – i.e., the three most popular events in resulted in this consensus statement.
terms of television audience worldwide – take place This document is intended to provide up-to-date rec-
during the summer months of the northern hemisphere, ommendations regarding the optimization of exercise
and often in hot ambient conditions. On 23–24 March capacity during sporting activities in hot ambient condi-
2014, a panel of experts reviewed and discussed the tions. Given that the performance of short-duration
6Training and competing in the heat
activities (e.g., jumping and sprinting) is at most mar- A number of sporting federations have also edited their
ginally influenced, or can even be improved, in hot guidelines to further reduce the risks of exertional heat
ambient conditions (Racinais & Oksa, 2010), but that illness. These guidelines are reviewed in the fourth
prolonged exercise capacity is significantly impaired section of this consensus statement. Recommendations
(Nybo et al., 2014), the recommendations provided in are offered to event organizers and sporting bodies on
this consensus statement focus mainly on prolonged how to best protect the health of the athlete and sustain/
sporting events. enhance performance during events in the heat.
Introduction Section 1: Heat acclimatization
When exercising in the heat, skin blood flow and sweat Although regular exercise in temperate conditions elicits
rate increase to allow for heat dissipation to the sur- partial heat acclimatization (Armstrong & Pandolf,
rounding environment. These thermoregulatory adjust- 1988), it cannot replace the benefits induced by consecu-
ments, however, increase physiological strain and may tive days of training in the heat (Gisolfi & Robinson,
lead to dehydration during prolonged exercise. Heat 1969; Nadel et al., 1974; Roberts et al., 1977; Armstrong
stress alone will impair aerobic performance when & Pandolf, 1988). Heat acclimatization improves
hyperthermia occurs (Rowell, 1974; Galloway & thermal comfort and submaximal as well as maximal
Maughan, 1997; Périard et al., 2011; Nybo et al., 2014). aerobic exercise performance in warm-hot conditions
Consequently, athletes perform endurance, racket, or (Nielsen et al., 1993; Lorenzo et al., 2010; Racinais
team sport events in the heat at a lower work rate than in et al., 2015). The benefits of heat acclimatization are
temperate environments (Ely et al., 2007; Morante & achieved via increased sweating and skin blood flow
Brotherhood, 2008; Mohr et al., 2012; Périard et al., responses, plasma volume expansion, and hence
2014; Nassis et al., 2015; Racinais et al., 2015). In addi- improved cardiovascular stability (i.e., better ability to
tion, dehydration during exercise in the heat exacerbates sustain blood pressure and cardiac output) and fluid-
thermal and cardiovascular strain (Adolph, 1947; electrolyte balance (Sawka et al., 1996, 2011; Périard
Strydom & Holdsworth, 1968; Sawka et al., 1985; et al., 2015). Exercise-heat acclimatization is therefore
Montain & Coyle, 1992; González-Alonso et al., 1998; essential for athletes preparing competitions in warm-
Trangmar et al., 2014) and further impairs aerobic per- hot environments (Sawka et al., 1996). This section
formance (González-Alonso et al., 2008; Sawka et al., describes how to practically implement heat acclimati-
2011; Nybo et al., 2014). The document contains recom- zation protocols and optimize the benefits in athletes.
mendations and strategies to adopt in order to sustain/
enhance performance during training and competition in
the heat, as well as minimize the risk of exertional heat Induction of acclimatization
illness. As presented in the first section, the most impor- Duration
tant intervention one can adopt to reduce physiological
strain and optimize performance is to heat acclimatize. Most adaptations (i.e., decreases in heart rate, skin and
Given that dehydration can impair physical performance rectal temperature, increases in sweat rate, and work
and exacerbate exercise-induced heat strain, the second capacity) develop within the first week of heat acclima-
section of the consensus statement provides recommen- tization and more slowly in the subsequent 2 weeks
dations regarding hydration. The third section highlights (Robinson et al., 1943; Ladell, 1951; Flouris et al.,
the avenues through which it is possible to decrease core 2014). Adaptations develop more quickly in highly
and skin temperatures before and during exercise via the trained athletes (up to half the time) compared with
application of cold garments to the skin such as ice untrained individuals (Pandolf et al., 1977; Armstrong &
packs, cold towels, and cooling vests, as well as through Pandolf, 1988). Consequently, athletes benefit from only
cold water immersion (CWI) or ice slurry ingestion. few days of heat acclimatization (Sunderland et al.,
Given the lack of data from real competitions, the 2008; Garrett et al., 2011; Chalmers et al., 2014), but
International Olympic Committee recently highlighted may require 6–10 days to achieve near complete cardio-
the necessity for sports federations, team doctors, and vascular and sudomotor adaptations (Nielsen et al.,
researchers to collaborate in obtaining data on the spe- 1993; Lorenzo et al., 2010; Karlsen et al., 2015b), and as
cific population of elite athletes exercising in challeng- such 2 weeks to optimize aerobic performance (i.e.,
ing environments (Bergeron et al., 2012). Several cycling time trial) in hot ambient conditions (Racinais
international sporting federations such as FIFA, FINA, et al., 2015).
FIVB, IAAF, and ITF have responded to this challenge
by initiating a surveillance system to assess environmen-
Training
tal conditions during competition, along with their
adverse outcomes (e.g., Grantham et al., 2010; Bahr & The principle underlying any heat acclimatization proto-
Reeser, 2012; Mountjoy et al., 2012; Nassis et al., 2015). col is an increase in body (core and skin) temperature to
7Racinais et al.
induce profuse sweating and increase skin blood flow vice versa (Eichna et al., 1945). However, acclimatiza-
(Sawka et al., 1996, 2011). Repeated heat-exercise train- tion in humid heat evokes higher skin temperatures and
ing for 100 min was originally shown to be efficient at circulatory adaptations than in dry heat, potentially
inducing such responses (Lind & Bass, 1963). Report- increasing maximum skin wettedness and therefore the
edly, exercising daily to exhaustion at 60% VO2max in hot maximum rate of evaporative heat loss from the skin
ambient conditions (40 °C, 10% RH) for 9–12 consecu- (Candas et al., 1979; Sawka et al., 1996; Périard et al.,
tive days increases exercise capacity from 48 to 80 min 2015). Although scientific support for this practice is still
(Nielsen et al., 1993). Ultimately, the magnitude of lacking, it may be potentially beneficial for athletes to
adaptation depends on the intensity, duration, frequency, train in humid heat at the end of their acclimatization
and number of heat exposures (Sawka et al., 1996; sessions to dry heat to further stress the cardiovascular
Périard et al., 2015). For example, Houmard et al. (1990) and thermoregulatory systems. Nevertheless, despite
reported similar physiological adaptations following some transfer between environments, other adaptations
moderate-intensity short-duration (30–35 min, 75% might be specific to the climate (desert or tropic) and
VO2max) and low-intensity long-duration (60 min, 50% physical activity level (Sawka et al., 2003). Conse-
VO2max) exercise. quently, it is recommended that athletes predominantly
As acclimatization develops, constant workload exer- acclimatize to the environment in which they will
cise protocols may result in a progressively lower train- compete.
ing stimulus (i.e., decreases in relative exercise Athletes who do not have the possibility to travel to
intensity). In turn, this may limit the magnitude of adap- naturally hot ambient conditions (so-called “acclimati-
tation if the duration and/or the intensity of the heat- zation”) can train in an artificially hot indoor environ-
exercise training sessions are not increased accordingly ment (so-called “acclimation”). However, while
(Taylor, 2014). When possible, an isothermic protocol acclimation and acclimatization share similar physi-
(e.g., controlled hyperthermia to a core temperature of at ological adaptations, training outdoors is more specific
least 38.5 °C) can be implemented to optimize the adap- to the competition setting as it allows athletes to experi-
tations (Patterson et al., 2004; Garrett et al., 2009). ence the exact nature of the heat stress (Hellon et al.,
However, isothermic protocols may require greater 1956; Edholm, 1966; Armstrong & Maresh, 1991).
control and the use of artificial laboratory conditions,
which could limit their practicality in the field. Alterna-
tively, it has recently been proposed to utilize a controlled
Decay and periodization of short-term acclimatization
intensity regimen based on heart rate to account for the
need to increase absolute intensity and maintain a similar Heat adaptations decay at different rates with the fastest
relative intensity throughout the acclimatization process adaptations also decaying more rapidly (Pandolf et al.,
(Périard et al., 2015). Lastly, athletes can adapt by train- 1977). However, the rate of decay of heat acclimatization
ing outdoors in the heat (i.e., acclimatization) using self- is generally slower than its induction, allowing mainte-
paced exercise, or maintaining their regular training nance of the majority of benefits (e.g., heart rate, core
regimen. The efficacy of this practice has been demon- temperature) for 2–4 weeks (Dresoti, 1935; Lind, 1964;
strated with team sport athletes (Racinais et al., 2012, Weller et al., 2007; Daanen et al., 2011; Flouris et al.,
2014), without interfering with their training regimen. 2014). Moreover, during this period, individuals (re)ac-
climatize faster than during the first acclimatization
period (Weller et al., 2007) (Table 1). These studies are
Environment
however mainly based on physiological markers of heat
Heat acclimatization in dry heat improves exercise in acclimatization and the decay in competitive sporting
humid heat (Bean & Eichna, 1943; Fox et al., 1967) and performance remains to be clarified.
Table 1. Examples of heat acclimatization strategies
Objective Duration Period Content Environment
Pre-/in-season Enhance/boost the 1–2 weeks Pre-season or Regular or additional Natural or artificial
training camp training stimulus in-season training (75–90 min/day) to heat stress
increase body temperature
and induce profuse
sweating
Target competition Optimize future 2 weeks 1 month before Regular or additional Equivalent to or more
preparatory camp reacclimatization and competing in the training, simulated stressful than target
evaluate individual heat competition, and heat competition
responses in the heat response test
Target competition Optimize performance in 1–2 weeks – Just before the Pre-competition training Same as competition
final camp the heat depending on results competition
of preparatory camp
8Training and competing in the heat
Individualized heat acclimatization in the heat) to obtain biological adaptations lowering
Heat acclimatization clearly attenuates physiological physiological strain and improving exercise capacity
strain (Eichna et al., 1950; MacDonald & Wyndham, in the heat.
1950). However, individual acclimatization responses ● Heat acclimatization sessions should last at least
may differ and should be monitored using simple 60 min/day and induce an increase in body core and
indices, such as the lessened heart rate increase during a skin temperatures, as well as stimulate sweating.
standard submaximal exercise bout (Lee, 1940; Ladell, ● Athletes should train in the same environment as the
1951; Buchheit et al., 2011, 2013). Other more difficult competition venue, or if not possible, train indoors in
and likely less sensitive markers for monitoring heat a hot room.
acclimatization include sweat rate and sodium content ● Early adaptations are obtained within the first few
(Dill et al., 1938), core temperature (Ladell, 1951), and days, but the main physiological adaptations are not
plasma volume (Glaser, 1950). The role of plasma complete until ∼1 week. Ideally, the heat acclimati-
volume expansion in heat acclimatization remains zation period should last 2 weeks in order to maxi-
debated as an artificial increase in plasma volume does mize all benefits.
not appear to improve thermoregulatory function (Sawka
& Coyle, 1999; Watt et al., 2000), but the changes in Section 2: Hydration
hematocrit during a heat response test following short-
term acclimatization correlate to individual physical per- The development of hyperthermia during exercise in hot
formance (Racinais et al., 2012, 2014). This suggests ambient conditions is associated with a rise in sweat rate,
that plasma volume changes might represent a valuable which can lead to progressive dehydration if fluid losses
indicator, even if it is probably not the physiological are not minimized by increasing fluid consumption.
mechanism improving exercise capacity in the heat. Exercise-induced dehydration, leading to a hypohy-
Importantly, measures in a temperate environment drated state, is associated with a decrease in plasma
cannot be used as a substitute to a test in hot ambient volume and an increase in plasma osmolality that are
temperatures (Armstrong et al., 1987; Racinais et al., proportional to the reduction in total body water (Sawka
2012, 2014). et al., 2011). The increase in the core temperature thresh-
As with its induction, heat acclimatization decay also old for vasodilation and sweating at the onset of exercise
varies between individuals (Robinson et al., 1943). It is is closely linked to the ensuing hyperosmolality and
therefore recommended that athletes undergo an accli- hypovolemia (Nadel et al., 1980; Fortney et al., 1984).
matization procedure months before an important event During exercise, plasma hyperosmolality reduces the
in the heat to determine their individual rate of adapta- sweat rate for any given core temperature and decreases
tion and decay (Bergeron et al., 2012; Racinais et al., evaporative heat loss (Montain et al., 1995). In addition,
2012) (Table 1). dehydration decreases cardiac filling and challenges
blood pressure regulation (González-Alonso et al., 1995,
Heat acclimatization as a training stimulus 1998; Stöhr et al., 2011). The rate of heat storage and
cardiovascular strain is therefore exacerbated and the
Several recent laboratory or uncontrolled-field studies capacity to tolerate exercise in the heat is reduced
have reported physical performance improvement in (Sawka et al., 1983; Sawka, 1992; González-Alonso
temperate environments following training in the heat et al., 2000).
(Hue et al., 2007; Scoon et al., 2007; Lorenzo et al., Despite decades of studies in this area (Cheuvront &
2010; Buchheit et al., 2011; Racinais et al., 2014). Ath- Kenefick, 2014), the notion that dehydration impairs
letes might therefore consider using training camps in aerobic performance in sport settings is not universally
hot ambient conditions to improve physical performance accepted and there seems to be a two-sided polarized
both in-season (Buchheit et al., 2011) and pre-season debate (Goulet, 2011, 2013; Cotter et al., 2014). Numer-
(Racinais et al., 2014) (Table 1). Bearing in mind that ous studies report that dehydration impairs aerobic
training quality should not be compromised, the athletes performance in the condition that exercise is performed in
benefiting the most from this might be experienced ath- warm-hot environments and that body water deficits
letes requiring a novel training stimulus (Racinais et al., exceed at least ∼2% of body mass (Eichna et al., 1945;
2014), whereas the benefit for highly trained athletes Adolph, 1947; Below et al., 1995; Cheung & McLellan,
with limited thermoregulatory requirement (e.g., cycling 1998; Ebert et al., 2007; Kenefick et al., 2010; Merry
in cold environments) might be more circumstantial et al., 2010; Sawka et al., 2012; Cheuvront & Kenefick,
(Karlsen et al., 2015a). 2014). On the other hand, some recent studies suggest that
dehydration up to 4% body mass does not alter cycling
Summary of the main recommendations for
performance under ecologically valid conditions (Goulet,
heat acclimatization
2011, 2013; Wall et al., 2013). However, these results
● Athletes planning to compete in hot ambient condi- must be interpreted in context; i.e., in well-trained
tions should heat acclimatize (i.e., repeated training male cyclists typically exercising for 60 min in ambient
9Racinais et al. conditions up to 33 °C and 60% relative humidity and 2013). The most widely accepted and recommended starting exercise in a euhydrated state. Nonetheless, some methods include monitoring body mass changes, mea- have advanced the idea that the detrimental consequences suring plasma osmolality and urine specific gravity. of dehydration have been overemphasized by sports bev- Based on these methods, one is considered euhydrated if erage companies (Cohen, 2012). As such, it has been daily body mass changes remain
Training and competing in the heat
During exercise lasting longer than 1 h, athletes should ● Athletes training in the heat have higher daily sodium
therefore aim to consume a solution containing 0.5– (i.e., salt) requirements than the general population.
0.7 g/L of sodium (Casa, 1999; Von Duvillard et al., Sodium supplementation might also be required
2004; Sawka et al., 2007). In athletes experiencing during exercise.
muscle cramping, it is recommended to increase the ● For competitions lasting several days (e.g., cycling
sodium supplementation to 1.5 g/L of fluid (Bergeron, stage race, tennis/team sport tournament), simple
2003). Athletes should also aim to include 30–60 g/h of monitoring techniques such as daily morning of body
carbohydrates in their hydration regimen for exercise mass and urine specific gravity can provide useful
lasting longer than 1 h (Von Duvillard et al., 2004), and insights into the hydration state of the athlete.
up to 90 g/h for events lasting over 2.5 h (Burke et al., ● Adequately rehydrating after exercise-heat stress by
2011). This can be achieved through a combination of providing plenty of fluids with meals is essential. If
fluids and solid foods. aggressive and rapid replenishment is needed, then
consuming fluids and electrolytes to offset 100–
150% of body mass losses will allow for adequate
Post-exercise rehydration rehydration.
Following training or competing in the heat, rehydra- ● Recovery hydration regimens should include sodium,
tion is particularly important to optimize recovery. If carbohydrates, and protein.
fluid deficit needs to be urgently replenished, it is sug-
gested to replace 150% of body mass losses within 1 h
following the cessation of exercise (Shirreffs & Section 3: Cooling strategies
Maughan, 1998; Sawka et al., 2007), including electro-
lytes, to maintain total body water. From a practical Skin cooling will reduce cardiovascular strain during
perspective, this may not be achievable for all athletes exercise in the heat whereas whole-body cooling can
for various reasons (e.g., time, gastrointestinal discom- reduce organ and skeletal muscle temperatures. Several
fort). Thus, it is more realistic to replace 100–120% of studies carried out in controlled laboratory conditions
body mass losses. The preferred method of rehydration (e.g., uncompensable heat stress), in many cases with or
is through the consumption of fluids with foods (e.g., without reduced fanning during exercise, have reported
including salty food). that precooling can improve endurance (Booth et al.,
Given that exercise in the heat increases carbohydrate 1997; González-Alonso et al., 1999b; Quod et al., 2008;
metabolism (Febbraio et al., 1994; González-Alonso Duffield et al., 2010; Ihsan et al., 2010; Ross et al., 2011;
et al., 1999a), endurance athletes should ensure that not Siegel et al., 2012), high-intensity (Marsh & Sleivert,
only water and sodium losses are replenished, but car- 1999) and intermittent- or repeated-sprint exercise per-
bohydrate stores as well (Burke, 2001). To ensure the formance (Duffield & Marino, 2007; Castle et al., 2011;
highest rates of muscle glycogen resynthesis, carbohy- Minett et al., 2011; Brade et al., 2014). However, several
drates should be consumed during the first hour after other studies reported no performance benefits of pre-
exercise (Ivy et al., 1988). Moreover, a drink containing cooling on intermittent- or repeated-sprint exercise per-
protein (e.g., milk) might allow to better restore fluid formance in the heat (Duffield et al., 2003; Cheung &
balance after exercise than a standard carbohydrate- Robinson, 2004; Duffield & Marino, 2007; Brade et al.,
electrolyte sport drink (James, 2012). Combining 2013). Whole-body cooling (including cooling of the
protein (0.2–0.4 g/kg/h) and carbohydrate (0.8 g/kg/h) exercising muscles) may even be detrimental to perfor-
has also been reported to maximize protein synthesis mance during a single sprint or the first few repetitions of
rates (Beelen et al., 2010). Therefore, athletes should an effort involving multiple sprints (Sleivert et al., 2001;
consider consuming drinks such as chocolate milk, Castle et al., 2006).
which has a carbohydrate-to-protein ratio of 4:1, as well Therefore, whereas several reviews concluded that
as sodium, following exercise (Pritchett & Pritchett, cooling interventions can increase prolonged exercise
2012). capacity in hot conditions (Marino, 2002; Quod et al.,
2006; Duffield, 2008; Jones et al., 2012; Siegel &
Laursen, 2012; Ross et al., 2013; Tyler et al., 2015;
Bongers et al., 2015), it has to be acknowledged that
Summary of the main recommendations for hydration
most laboratory-based precooling studies might have
● Before training and competition in the heat, athletes overestimated the effect of precooling as compared with
should drink 6 mL of fluid per kg of body mass every an outdoor situation with airflow (Morrison et al., 2014),
2–3 h in order to start exercise euhydrated. or do not account for the need to warm up before com-
● During intense prolonged exercise in the heat, body peting. As a consequence, the effectiveness of cooling in
water mass losses should be minimized (without competitive setting remains equivocal and the recom-
increasing body weight) to reduce physiological mendations below are limited to prolonged exercise in
strain and help preserve optimal performance. hot ambient conditions with no or limited air movement.
11Racinais et al.
Cold water immersion exercise is improved following the ingestion of an ice
A range of CWI protocols are available (for reviews, see slurry beverage (∼1 L crushed ice at ≤4 °C) either prior to
Leeder et al., 2012; DeGroot et al., 2013; Ross et al., (Siegel et al., 2010, 2012; Yeo et al., 2012) or during
2013; Versey et al., 2013), but the most common tech- exercise (Stevens et al., 2013), but no benefit was evident
niques are whole-body CWI for ∼30 min at a water when consumed during the recovery period between two
temperature of 22–30 °C, or body segment (e.g., legs) exercise bouts in another study (Stanley et al., 2010).
immersion at lower temperatures (10–18 °C) (Ross Consequently, ingestion of ice slurry may be a practical
et al., 2013). However, cooling of the legs/muscles will complement or alternative to external cooling methods
decrease nerve conduction and muscle contraction (Siegel & Laursen, 2012) but more studies are still
velocities (Racinais & Oksa, 2010) and athletes might required during actual outdoor competitions.
therefore need to rewarm up before competition. Conse-
quently, other techniques involving cooling garments Mixed methods cooling strategies
have been developed to selectively cool the torso, which
may prevent the excessive cooling of active muscles Combining techniques (i.e., using both external and inter-
while reducing overall thermal and cardiovascular strain. nal cooling strategies) has a higher cooling capacity than
the same techniques used in isolation, allowing for greater
benefit on exercise performance (Bongers et al., 2015).
Cooling garments Indeed, mixed methods have proven beneficial when
Building on the early practice of using iced towels for applied to professional football players during competi-
cooling purposes, several manufacturers have designed tion in the tropics (Duffield et al., 2013), lacrosse players
ice-cooling jackets to cool athletes before or during exer- training in hot environments (Duffield et al., 2009), and
cise (Cotter et al., 2001; Arngrimsson et al., 2004; cyclists simulating a competition in a laboratory (Ross
Duffield & Marino, 2007; Duffield et al., 2010). The et al., 2011). In a sporting context, this can be achieved by
decrease in core temperature is smaller with a cooling combining simple strategies such as the ingestion of ice
vest than with CWI or mixed-cooling methods (Bongers slurry, wearing cooling vests, and providing fanning.
et al., 2015), but cooling garments present the advantage
of lowering skin temperature and thus reducing cardio-
vascular strain and eventually heat storage (Cheuvront Cooling to improve performance between subsequent
et al., 2003). Cooling garments are practical in reducing bouts of exercise
skin temperature without reducing muscle temperature, There is evidence supporting the use of CWI (5–12 min
and athletes can wear them during warm-up or recovery in 14 °C water) during the recovery period (e.g., 15 min)
breaks. separating intense exercise bouts in the heat to improve
subsequent performance (Yeargin et al., 2006; Peiffer
Cold fluid ingestion et al., 2010). The benefits of this practice would relate to
a redistribution of the blood flow, probably from the skin
Cold fluids can potentially enhance endurance perfor-
to the central circulation (Vaile et al., 2011), as well as a
mance when ingested before (Lee et al., 2008a; Byrne
psychological (i.e., placebo) effect (Hornery et al.,
et al., 2011), but not during (Lee & Shirreffs, 2007; Lee
2005). In terms of internal cooling, the ingestion of cold
et al., 2008b) exercise. Indeed, it is suggested that a
water (Lee et al., 2013) or ice slurry (Stanley et al.,
downside of ingesting cold fluids during exercise might
2010) during the recovery period might attenuate heat
be a reduction in sweating and therefore skin surface
strain in the second bout of work, but not necessarily
evaporation (Bain et al., 2012) due to the activation of
significantly improve performance (Stanley et al., 2010).
thermoreceptors probably located in the abdominal area
Together, these studies suggest that cooling might help
(Morris et al., 2014).
recovery from intense exercise in uncompensable labo-
ratory heat stress and, in some cases, might improve
Ice slurry beverages performance in subsequent intense exercise bouts. The
Based on the theory of enthalpy, ice requires substantially effects of aggressive cooling vs simply resting in the
more heat energy (334 J/g) to cause a phase change from prevailing hot ambient conditions, or in cooler condi-
solid to liquid (at 0 °C) compared with the energy tions, remain to be validated in a competition setting
required to increase the temperature of water (4 J/g/°C). (e.g., half time in team sports).
As such, ice slurry may be more efficient than cold water
ingestion in cooling athletes. However, it is not yet clear if
Summary of the main recommendations for cooling
the proportional reduction in sweating observed with the
ingestion of cold water during exercise (Bain et al., 2012) ● Cooling methods include external (e.g., application
occurs with ice slurry ingestion. Several recent reports of iced garments, towels, water immersion, or
support the consumption of an ice slurry beverage since fanning) and internal methods (e.g., ingestion of cold
performance during endurance or intermittent-sprint fluids or ice slurry).
12Training and competing in the heat
Table 2. Examples of recommended actions by various sport governing bodies based on the wet bulb globe temperature (WBGT)
WBGT (°C) Organization Athlete concerned Recommendation
32.3 ACSM Acclimatized, fit, and low-risk individuals Participation cutoff
32.2 ITF Junior and wheelchair tennis players Immediate suspension of play
32.2 WTA Female tennis players Immediate suspension of play
32.0 FIFA Football players Additional cooling break at 30 and 75 min
30.1 ACSM Non-acclimatized, unfit, and high-risk individuals Participation cutoff
30.1 ITF-WTA Junior and female tennis players 10-min break between second and third sets
30.1 ITF Wheelchair tennis players Suspension of play at the end of the set in progress
28.0 ITF Wheelchair tennis players 15-min break between second and third sets
28.0 Australian Open Tennis players 10-min break between second and third sets
21.0 Marathon in northern Runners in mass participation events Cancel marathon
latitudes
ACSM, American College of Sports Medicine; ITF, International Tennis Federation; WTA, Women’s Tennis Association; FIFA, Fédération Internationale de
Football Association.
Data from Armstrong et al. (2007), Roberts (2010), and from the following website: http://www.fifa.com/aboutfifa/footballdevelopment/medical/
playershealth/risks/heat.html, http://www.itftennis.com/media/194281/194281.pdf, http://www.itftennis.com/media/195690/195690.pdf, http://
www.wtatennis.com/SEWTATour-Archive/Archive/AboutTheTour/rules2015.pdf, http://www.ausopen.com/en_AU/event_guide/a_z_guide.html.
● Precooling may benefit sporting activities involving Table 3. Corrected estimation of the risk of exertional heat illness based
sustained exercise (e.g., middle- and long-distance on the wet bulb globe temperature (WBGT) taking into account that WBGT
running, cycling, tennis, and team sports) in warm- underestimates heat stress under high humidity
hot environments. Internal methods (i.e., ice slurry) Estimated risk WBGT (°C) Relative humidity (%)
can be used during exercise whereas tennis and team
sport athletes can also implement mixed-cooling Moderate 24 50
methods during breaks. Moderate 20 75
Moderate 18 100
● Such practice may not be viable for explosive or High 28 50
shorter duration events (e.g., sprinting, jumping, High 26 75
throwing) conducted in similar conditions. High 24 100
● A practical approach in hot-humid environments Excessive 33 50
Excessive 29 75
might be the use of fans and commercially available Excessive 28 100
ice-cooling vests, which can provide effective
cooling without impairing muscle temperature. In Adapted from the categories proposed by Gonzalez (1995) to estimate the
any case, cooling methods should be tested and indi- risk of exertional heat illness during a marathon.
vidualized during training to minimize disruption to
the athlete.
Cancelling an event or implementing countermeasures?
Further to appropriate scheduling of any event with
Section 4: Recommendations for event organizers regard to expected environmental conditions, protecting
athlete health might require stopping competition when
The most common set of recommendations followed by combined exogenous and endogenous heat loads cannot
event organizers to reschedule or cancel an event is be physiologically compensated. The environmental
based on the wet bulb globe temperature (WBGT) index conditions in which the limit of compensation is
empirically developed by the U.S. military, popularized exceeded depend on several factors, such as metabolic
in sports medicine by the American College of Sports heat production (depending on workload and efficiency/
Medicine (Armstrong et al., 2007), and adopted by economy), athlete morphology (e.g., body surface area
various sporting federations (Table 2). However, WBGT to mass ratio), acclimatization state (e.g., sweat rate),
might underestimate heat stress risk when sweat evapo- and clothing. It is therefore problematic to establish uni-
ration is restricted (i.e., high humidity and/or low air versal cutoff values across different sporting disciplines.
movement) (Budd, 2008). Thus, corrected recommenda- Environmental indices should be viewed as recommen-
tions have been proposed (Gonzalez, 1995) (Table 3). dations for event organizers to implement preventive
Moreover, the WBGT is a climatic index and does not countermeasures to offset the potential risk of heat
account for metabolic heat production or clothing and illness. The recommended countermeasures include
therefore cannot predict heat dissipation (Sawka et al., adapting the rules and regulations with regard to cooling
2011). Therefore, the recommendations below provide breaks and the availability of fluids (time and locations),
guidelines for various sporting activities rather than fixed as well as providing active cooling during rest periods. It
cutoffs based on the WBGT index. is also recommended that medical response protocols
13Racinais et al.
and facilities to deal with cases of exertional heat ill- 2004; Sawka et al., 2011). Event organizers should
nesses be in place. therefore pay particular medical attention to all popula-
tions potentially at a greater risk, including participants
Specificity of the recommendations currently sick or recovering from a recent infection,
those with diarrhea, recently vaccinated, with limited
Differences among sports heat dissipation capacity due to medical conditions (e.g.,
Hot ambient conditions impair endurance exercise such Paralympic Athletes), or individuals involved in sports
as marathon running (Ely et al., 2007), but potentially with rules restricting heat dissipation capacity (e.g.,
improve short-duration events such as jumping or sprint- protective clothing/equipment). Unacclimatized partici-
ing (Racinais & Oksa, 2010). In many sports, athletes pants are also to be considered at risk. Although it is
adapt their activity according to the environmental con- impractical to screen every athlete during large events,
ditions. For example, compared with cooler conditions, organizers are encouraged to provide information, pos-
football players decrease the total distance covered or sibly in registration kits, advising all athletes of the risk
the distance covered at high intensity during a game, but associated with participation under various potential
maintain their sprinting activity/ability (Mohr et al., compromised states and suggesting countermeasures.
2012; Aughey et al., 2014; Nassis et al., 2015), while
tennis players reduce point duration (Morante & Summary of the main recommendations for
Brotherhood, 2008) or increase the time between points event organizers
(Périard et al., 2014) when competing in the heat ● The WBGT is an environmental heat stress index and
(WBGT ∼ 34 °C). Event organizers and International not a representation of human heat strain. It is there-
Federations should therefore acknowledge and support fore difficult to establish absolute participation cutoff
such behavioral thermoregulatory strategies by adapting values across sports for different athletes and we
the rules and refereeing accordingly. rather recommend implementing preventive counter-
measures, or evaluating the specific demands of the
Differences among individuals within a given sport sport when preparing extreme heat policies.
● Countermeasures include scheduling the start time of
When comparing two triathlon races held in Melbourne,
events based on weather patterns, adapting the rules
in similar environmental conditions (i.e., WBGT raising
and refereeing to allow extra breaks or longer recov-
from 22 to 27 °C during each race), 2 months apart,
ery periods, and developing a medical response pro-
Gosling et al. (2008) observed 15 cases of exertional
tocol and cooling facilities.
heat illness (including three heat strokes) in the first race
● Event organizers should pay particular attention to all
that was held in unseasonably hot weather at the start of
“at risk” populations. Given that unacclimatized par-
summer, but no cases in the second race. This suggests
ticipants (mainly in mass participation events) are at
that the risk of heat illness was increased in competitors
a higher risk for heat illness, organizers should prop-
that were presumably not seasonally heat acclimatized
erly advise participants of the risk associated with
(Gosling et al., 2008) and supports many earlier studies
participation, or consider canceling an event in the
regarding increased risk of heat illness in early summer,
case of unexpected or unseasonably hot weather.
or with hot weather spikes (Sartor et al., 1995). Never-
theless, exertional heat stroke can occur in individuals Overall conclusion
who are well acclimatized and have performed similar
activities several times before, as they may suffer from Our current knowledge on heat stress is mainly derived
prior viral infection or similar ailment (Sawka et al., from military and occupational research fields, while the
2011). In one of the very few epidemiological studies input from sport sciences is more recent. Based on this
linking WBGT to illness in athletes, Bahr and Reeser literature, athletes should train for at least 1 week and
(2012) investigated 48 beach volleyball matches (World ideally 2 weeks to acclimatize using a comparable degree
Tour and World Championships) over 3 years. They of heat stress as the target competition. They should also
reported only one case of a heat-related medical forfeit, be cautious to undertake exercise in a euhydrated state
which was related to an athlete with compromised fluid and minimized body water deficits (as monitored by body
balance due to a 3-day period of acute gastroenteritis mass losses) through proper rehydration during exercise.
(Bahr & Reeser, 2012). Moreover, while healthy runners They can also implement specific countermeasures (e.g.,
can also finish a half-marathon in warm and humid envi- cooling methods) to reduce heat storage and physiologi-
ronments without developing heat illness (Byrne et al., cal strain during competition and training especially
2006), exertional heat stroke has been shown to occur when the environmental conditions are uncompensable.
during a cool weather marathon in a runner recovering Event organizers and sport governing bodies can support
from a viral infection (Roberts, 2006). athletes by allowing additional (or longer) recovery
In fact, prior viral infection is emerging as potentially periods for enhanced hydration and cooling opportunities
important risk factor for heat injury/stroke (Sonna et al., during competitions in the heat.
14Training and competing in the heat
Key words: Temperature, exercise, thermoregulation, Dervis, Abdulaziz Farooq, Oliver Gibson, Mark Hayes, Carl
hydration, dehydration, cooling, cold water immersion, James, Stefanie Keiser, Luis Lima, Alex Lloyd, Erin McLeave,
Jessica Mee, Nicholas Ravanelli, Jovana Smoljanic, Steve
acclimation, acclimatization, heat exhaustion, wet bulb Trangmar, James Tuttle, Jeroen Van Cutsem, and Matthijs
globe temperature, performance. Veltmeijer. Bart Roelands is a postdoctoral fellow of the Fund for
Scientific Research Flanders (FWO).
Conflicts of interest: José González-Alonso has
Acknowledgements received research funding from the Gatorade Sports
The authors thank the following conference attendees for their Science Institute, Pepsico. Michael N. Sawka was a
participation in the 2 days of discussion: Carl Bradford, Martin member of the Gatorade Sports Science Institute Expert
Buchheit, Geoff Coombs, Simon Cooper, Kevin De Pauw, Sheila Panel in 2014.
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