Dealing with Heat Stress

Excerpt from Cycling Science by Stephen Cheung, PhD and Mikel Zabala, PhD (Human Kinetics, 2017) with permission from the publisher. Available at www.HumanKinetics.com


Part of the beauty of cycling is that it is an outdoor sport in which training and competition take place across a wide range of weather over the course of a season. Conditions can range from the cold weather associated with the spring Classics in northern Europe through to the heat typically found during the Tour de France and Vuelta a Éspana during the summer. The globalization of cycling also means that major markets for cycling are developing in many hot-weather or tropical locations such as Australia, southeast Asia, and the Middle East.

At the Olympics in Atlanta in 1996 and Athens in 2004, sport scientists, coaches, and athletes alike knew that heat, humidity, and even air pollution would be environmental factors that could seriously affect the performance of the athletes. The positive aspect was that, just as the 1968 Mexico City Games kick-started scientific and applied interest in altitude training that remains strong, these recent Games spurred a lot of fundamental and applied research into ways to minimize the potentially debilitating effects of heat.

The environment, therefore, becomes a major determinant of how a ride or race plays out, and it can greatly affect performance. At the same time, by understanding the effects of environmental stress on how the body responds, cyclists can minimize the negative effects and even use challenging environmental conditions to maximize their capacity.

Photo credit for image of Stephen Cheung, PhD above: Takashi Nakata

Heat Balance

The basic heat balance equation incorporates the four major heat loss pathways (radiation, conduction, convection, and evaporation; see figure 11.1) and metabolic heat production to model the rate of heat storage (Ṡ in W·m^-2):


Figure 11.1 The body exchanges heat with the environment through many pathways.
Reprinted, by permission, from J.H. Wilmore, D.L. Costill, and W.L. Kenney, 2008, Physiology of sport and exercise, 4th ed. (Champaign, IL: Human Kinetics), 257. Adapted from C.V. Gisolfi and C.B. Wenger, 1984, "Temperature regulation during exercise: Old concepts, new ideas," Exercise and Sports Sciences Reviews 12(1): 339-372.

Ṡ = (Ṁ – Ẇ) – (Ṙ + Ċ + K̇ + Ė + Ċ_res + Ė_res)

where

Ṡ = heat storage; a positive value represents a heat gain that could eventually lead to hyperthermia, whereas a negative value represents heat loss that could eventually lead to hypothermia.

Ṁ = metabolic heat production.

Ẇ = mechanical work.

Ṙ = radiation; this component combines direct radiation of the sun and reflected radiation from the ground. It is positive when the surrounding temperature is greater than the skin temperature.

Ċ = conduction; this component involves direct heat transfer from one molecule to another. Its rate depends on the temperature gradient between the skin and surrounding surfaces and the thermal qualities of those surfaces. Conduction is generally not relevant for most cycling situations, because of the small contact points (saddle, handlebars, pedals) between the rider and the bike.

K̇ = convection; this component is often discussed in combination with conduction because the effectiveness of conduction depends on the rate of heat exchange between the skin and the surroundings. As long as ambient temperature is lower than body temperature of 37 to 39 degrees Celsius, the high rate of convective flow over a cyclist moving at speed can still contribute to significant heat dissipation. But when climbing or when cycling in slower disciplines such as mountain biking, convective heat loss may be minimal, accelerating heat storage.

Ė = evaporation from the respiratory tract or the skin; evaporation is the main defense against hyperthermia because, as body temperature increases, the effectiveness of radiation, conduction, and convection as heat-loss mechanisms decreases. Unlike what happens with the other methods of heat exchange, the potential for evaporative heat loss is determined primarily by the water vapor pressure gradient between the body surface and the environment. A warm but humid environment can lead to much higher heat storage than a hotter but dry environment, because of the impairment of evaporative heat loss.

Effect of Heat on Performance

A consensus of scientific evidence—analyzing performance across various climatic conditions in both laboratory and field settings—has clearly demonstrated that human exercise capacity decreases in the heat. In the laboratory, Galloway and Maughan (1997) had subjects exercise to voluntary exhaustion at 70 percent of V̇O_2peak in 4, 10, 20, and 30 degrees Celsius and reported that peak tolerance time occurred at 10 degrees Celsius. Similar and shorter durations occurred at both 4 and 20 degrees Celsius, and a further decrease occurred at 30 degrees Celsius. Although the relatively limited airflow in such laboratory studies may have overamplified the negative effects of temperature in cycling studies (Morrison, Cheung, and Cotter 2014), such decrements are supported by retrospective field studies. For example, Ely et al. (2007) analyzed marathon running times across a range of finishing placings and race-day temperatures and reported the fastest running times in cool temperatures and an exponentially greater impairment in slower runners in warm temperatures. But the decrement in physiological performance is often weighed up in a cost–benefit analysis because of the potential for decreased air density—and thus lower air resistance—with increasing temperature. To take advantage of this reduced air resistance, a recent trend has been to increase velodrome temperatures to nearly 30 degrees Celsius during World Hour Record attempts.

Heat-related exercise impairment likely encompasses a wide range of physiological and psychological mechanisms (Cheung and Sleivert 2004; figure 11.2). These mechanisms include potential reductions in cerebral blood flow, decrements in neuromuscular activation in the brain, decreases in mental arousal, reduced gut blood flow leading to bacterial leakage from the gut and inflammatory response, and reduced cardiovascular efficiency while coping with increased metabolic and thermoregulatory demands. Besides causing physiological effects, higher ambient temperature may alter voluntary effort; the extra heat stress may cause the brain to reduce the willingness to work as hard as in a cooler environment to decrease the risk of heat illness and catastrophic collapse. When tasked to adjust power output to maintain a constant perceived effort similar to that needed for a 20K to 40K time-trial pace, the rate of decline in voluntary power output was higher in hot (35 degrees Celsius) compared with cooler (15 and 25 degrees Celsius) environments (Tucker et al. 2006).


Figure 11.2 Elevated core, brain, and skin temperature during exercise-heat stress can cause fatigue through a variety of interrelated physiological and psychological factors. CNS = central nervous system; RPE = rating of perceived exertion.


Besides reducing exercise capacity, the major clinical danger with exercise in hot environments is an elevated risk of exertional heat illness such as heat exhaustion and heatstroke. An excellent book on this topic specifically for coaches and athletes was written by Dr. Lawrence Armstrong (2007) of the University of Connecticut. An important finding is that exertional heat illness can occur even in cool weather. Dr. Armstrong led the American College of Sports Medicine Position Stand on Exertional Heat Illness (Armstrong et al. 2007), emphasizing that heat exhaustion and exertional heatstroke "occur worldwide with prolonged intense activity in almost every venue (e.g., cycling, running races, American football, soccer)" (556).

How Hot Is Too Hot?

Given that hyperthermia clearly impairs physiological function and exercise capacity, the main question for athletes is just how hot is too hot, whether that be ambient temperature or actual body temperature. This question is difficult to answer, because a slight or even significant rise in body temperature is not always an indication of problems during exercise, especially in fit or elite athletes. Ultramarathoners can exercise at maximal capacity for 4 hours in moderate ambient temperatures with only minor elevations in core temperature, suggesting strong ability to thermoregulate even under conditions of high metabolic heat production. At the same time, some endurance athletes can sustain elevated core temperatures of greater than 40 degrees Celsius throughout a marathon or triathlon without major issues (Kenefick, Cheuvront, and Sawka 2007). Especially in cycling outdoors, in which the cycling itself often generates high wind speed and thus convective cooling, high temperatures may not be as dangerous as expected. As noted previously, in certain situations a higher ambient temperature may benefit cycling performance because the lower air density reduces air resistance.

In contrast, although exertional heat illnesses occur most frequently in hot and humid conditions, such problems can occur even in cool conditions with intense or prolonged exercise. In one case study a well-trained male experienced collapse near the finish of a marathon in 6 degrees Celsius ambient conditions and presented a rectal temperature of 40.7 degrees Celsius approximately 30 minutes postcollapse (Roberts 2006). Such a wide range in responses highlights the high degree of individual variability in response to heat stress, making it difficult to develop predictive models for race organizers who are trying to protect participant safety and for coaches and sport scientists who are trying to individualize training programs or predict performance outcomes.

Protective Effects of Aerobic Fitness

Aerobic fitness provides protective physiological responses to exercise similar to those from acclimatization to hot environments (Cheung, McLellan, and Tenaglia 2000). These benefits include greater evaporative heat dissipation through improved sweating response, resulting from both lower core temperature thresholds for the initiation of sweating and greater sensitivity of sweating response to increasing core temperatures. Improved aerobic capacity also leads to elevated plasma volume and cardiac output, minimizing the competition for blood distribution between skeletal muscle and skin (heat dissipation and sweating output) during exercise and heat stress (Sawka et al. 1992). Other important benefits of aerobic fitness are a lowered resting core temperature coupled with an elevated endpoint core temperature that can be tolerated before voluntary exhaustion (Cheung and McLellan 1998). This latter effect results directly from aerobic fitness rather than body composition because differences across fitness groups remained evident when highly and moderately fit subjects were normalized for differences in body fatness (Selkirk and McLellan 2001). Therefore, fitness appears to benefit heat tolerance because of both a greater capacity for tolerating high temperatures and a slower rate of heat storage. These benefits, however, appear unique to long-term changes in aerobic fitness rather than transient changes brought about by short-term training interventions (Cheung and McLellan 1998).

Heat Adaptation

When discussing long-term responses to heat exposure, definitions of some basic terminology can help to clarify discussion and avoid confusion.

Adaptation is the overall response by humans to a different environment such as heat. Responses can be both physiological (e.g., increased sweat rate) and psychological (less perceived distress).

Acclimation is the process of adaptation through exposure to an "artificial" stimulus. An example is the training that Bradley Wiggins did for the 2011 Vuelta in his garden shed with the heaters turned up. Acclimatization, on the other hand, is the process of adaptation through natural exposure to the stimulus. This method would include moving to a hot environment to train and live there for several weeks before competition. Moving to a hot environment and spending the majority of the day in air conditioning, however, reduces total natural heat exposure, thus qualifying more as acclimation than acclimatization and slowing the rate of overall adaptation.

Habituation generally refers to the psychological desensitization to a stimulus. Put your hand into cold water. The first minutes feel painful and horrible, but that feeling of strong discomfort fades after a while, even though your hand itself may actually be at a lower temperature.

Scientific research on heat stress, and specifically our physiological responses and adaptation to prolonged heat exposure, is well studied. In general, humans have a high ceiling for physiological adaptation to prolonged heat exposure. Several major changes can occur:

• Resting core temperature decreases slightly, theoretically permitting more heat storage before hyperthermia-induced exercise impairments occur, along with decreasing the risk of catastrophic exertional heat illness.
  
  
• Plasma volume—the liquid component of blood—increases, resulting in greater total blood volume. Note that hemoglobin and red blood cells typically do not increase, so hematocrit generally decreases with heat adaptation.
  
  
• One consequence of higher plasma and blood volume is decreased cardiovascular strain from pumping blood to both the muscles and skin,resulting in a lowered resting and exercise heart rate.
  
  
• The beneficial sweat response adaptations include an earlier onset of sweating, a redistribution of the sweat response, and a conservation of sodium. Adapted individuals are able to dissipate heat more readily because they will begin to sweat at a lower core temperature and willsweat more from the limbs (the trunk is the main area of sweat loss for an unacclimatized person), thus making better use of the skin surface area for evaporative heat loss. Of course, the flip side of enhanced sweating is that the rate of fluid loss from the body increases.
  
  
• Sweat becomes more dilute as the sweat glands become better at reabsorption of electrolytes, which helps to minimize overall electrolyte loss with the higher sweat rate.
  
  
• Both separately and in conjunction with these physiological changes,perceptual sensitivity to heat decreases. Thermal discomfort and ratings of perceived exertion decrease while resting or exercising in the heat.
  
  

Adaptation Timeline

Generally, the rate of heat adaptation follows an exponential path, but the physiological adaptations to heat occur at differing rates (Périard, Racinais, and Sawka 2015; table 11.1). The immediate adaptation is a reduction in heart rate (HR). Most of the observed reduction occurs within 4 to 5 days, and full acclimatization appears to occur after about 7 days of exposure, coinciding with most of the beneficial core and skin temperature reductions. Thermoregulatory adaptations take a little longer than cardiovascular ones, but when following a structured acclimatization protocol, full adaptation often occurs after 10 to 14 days of exposure. Maximal sweat response improvements may take a month, and resistance to exertional heat illness may take twice that long.

Besides the occurrence of these physiological changes, important questions are whether heat adaptation is capable of eliciting improved performance to exercise in these hot conditions and how rapidly these performance changes may occur. Conversely, an interesting question is whether the adaptations caused by chronic heat exposure lead to improved response and performance when returning to a more temperate environment (Lorenzo et al. 2010). If this is the case, then heat adaptation protocols may serve a double benefit, enhancing performance across both temperate and hot environments.


Table 11.1 Magnitude of Response to Heat Adaptation Along With Comparative Timeline

Adaptation	Change from baseline
Resting heart rate	–5 bpm STHA, MTHA –12 bpm LTHA
Mean exercise heart rate	–12 bpm
Resting Tskin	No effect
Exercising Tskin	–0.57° C
Core temperature	–0.17° C STHA, MTHA –0.32° C LTHA (1 study)
Sweat rate	+5% STHA +29% MTHA, +33% LTHA
Thirst sensation	Moderate decrease
Rating of perceived exertion (RPE)	Moderate decrease
Thermal sensation	Small decrease
Exercise capacity/performance	+7% STHA +22% LTHA, +21% MTHA

 
STHA = short-term (< 1 week), MTHA = medium-term (8–14 days), LTHA = long-term (> 14 days) heat adaptation. Comparisons on different rows indicate significant time differences between adaptation durations.

Data from C.J. Tyler, T. Reeve, G.J. Hodges, and S.S. Cheung, 2016, "The effects of heat adaptation on physiology, perception, and exercise performance in the heat: A meta-analysis," Sports Medicine 46(11): 1699-1724.

A logistically challenging field experiment performed by a Qatari-Danish collaboration suggests that heat acclimatization can eventually restore performance to levels found in temperate conditions, but not exceed them (Karlsen et al. 2015a; Karlsen et al. 2015b). Highly trained Danish cyclists performed a 43.4-kilometer outdoor time trial in cool (5 degree Celsius) conditions in Denmark and then moved to Qatar (30 to 36 degrees Celsius ambient temperature) for a 2-week heat training camp. Physiological testing and a time trial of identical distance were performed at 1, 6, and 13 days into the camp. Impairment was marked upon acute exposure to heat (about 85 percent power), clearly supporting the classical consensus that heat stress severely impairs performance. Performance steadily improved over the course of the heat camp; progressive increases occurred in power output at day 6 (+14 percent) and again at day 13 (+5 percent) compared with day 1. The big caveat is that, although heat acclimatization helped performance in hot ambient temperatures, it only ever got the subjects back to square one. That is, even after 13 days, their power output only returned to nearly 100 percent of what they could do in a quite cold (5 degrees Celsius) temperature back in Denmark. Overall, then, full heat acclimatization for performance outcomes appears to require close to 2 weeks of exposure, and this timeline needs to be integrated into training and tapering programs.

The second half of the field study tested whether the increased blood volume, higher sweat rate, and other heat adaptation responses helped the cyclists when they went back to Denmark and competed in a cool environment. The interesting answer is no; no improvements occurred in time-trial performance compared with either the initial precamp test in Denmark or any of the TTs done in Qatar. This result suggests that heat acclimatization is highly specific to a hot environment and that it is not effective as an ergogenic tool for competitions in temperate environments.

Heat Acclimation Protocols

Nobody really disputes that heat adaptation occurs, but the devil is in the details for sport scientists. Specifically, what are the best and most efficient heat exposure protocols? How much daily exposure (time? intensity?) is required for optimal adaptation? How do other individual factors (e.g., fitness, hydration) affect the rate of heat adaptation?

For maximum acclimatization, exercise should be performed in elevated ambient temperatures; the primary stimulus is a sustained rise in core temperature. But no standard heat acclimatization protocol exists for frequency, duration, or intensity. A gradual increase in thermal stress is required to minimize the risks of prematurely causing exertional heat illness.

Rather than dehydration being perceived as a negative health or performance risk, recent research suggests that permitting a slight state of dehydration in conjunction with exercising in the heat may accelerate the rate of heat adaptation (Garrett et al. 2012, 2014). This effect appears to occur through either magnification of the stimulus from exercise-heat stress by dehydration or through parallel interrelated response pathways between dehydration and heat exposure.

People with high levels of aerobic fitness tend to have greater levels of heat tolerance and acclimatize much more readily than people at lower fitness levels (Pandolf, Burse, and Goldman 1977). The reasons for this differential response are likely multifactorial and linked to training-induced adaptations and increased ability to tolerate higher levels of thermal stress.

Factors such as alcohol consumption, sleep loss, and illness have all been linked to reductions in acclimatization-related benefits. Such factors should be carefully considered, especially when any acclimatization protocol causes significant disruption to the normal routine of the athlete (e.g., excessive travel and relocation associated with training camps).

The environment is also a significant component of a heat acclimation program. Wearing heavy sweat clothing in a cool environment may elicit the same increased sweating response as standard heat exposure does (Dawson 1994). Therefore, training indoors with a lower fan speed or wearing an extra clothing layer may be a useful method if training in a cold environment and traveling to a race at which hot conditions are expected. Overall, the thing to focus on with heat adaptation appears to be getting the core temperature to rise to the point at which sweating response is strongly stimulated and to maintain that high temperature and sweating for about 60 to 90 minutes a session for four to eight sessions, depending on fitness. Passive heat exposure, such as sitting in a sauna, can possibly provide the same stimulus but is not practical because staying in the sauna for long periods may be dangerous and time spent in the sauna is time not spent on training or recovering. With altitude training, maximal adaptation requires maintaining hypoxic exposure for as much of the day as possible. Currently, no research exists on whether nontraining heat exposure—such as during rest and sleep—can accelerate or optimize heat adaptation. But any benefits from nontraining exposure would likely be offset by negative consequences to recovery.

Limited data exist regarding the decay of acclimatization, but physiological adaptations may begin to subside after 3 days and may last no longer than 3 weeks. The beneficial adaptations observed in physiological parameters are lost earlier for HR than for rectal temperatures and sweat rate, so it appears that HR monitoring can be a good indicator of acclimatization decay. Differing advice can be found regarding when a "top up" of acclimatization is required. Some data suggest that one additional bout of exposure should be considered for every 5 days away from significant thermal stress (Pandolf, Burse, and Goldman 1977). Thermoregulatory research tends to separate trials by 7 days to minimize the effect of any heat acclimatization (Barnett and Maughan 1993), so a prudent recommendation is that if the adaptations are required for a sustained period following heat exposure, reexposure should take place at least once during the subsequent 7 days.

Cooling Strategies

The use of cooling protocols to improve performance or to counteract the risks of heat stress and hyperthermia has gained increasing popularity in cycling. The consistent core temperatures at the point of voluntary fatigue with a constant workload in the heat would certainly suggest that the removal of heat before exercise could increase exercise tolerance by increasing possible heat storage beforehand. Early athletic proponents include the Australian rowing teams, who used vests containing ice packs during warm-ups before competing in the 1996 Atlanta Olympics. Since then, many professional teams have used various ice vests while warming up on stationary bikes for a time trial. During riding, cyclists have used lightweight cooling collars, doused their heads with cool water, or used ice socks down their necks and backs to keep cooler.

Precooling

Although the rate of heat storage may remain unchanged with precooling, the lowered baseline temperature may enable both core temperature and heart rate to remain lower with prolonged exercise over time. Precooling also can decrease perceptions of heat stress and thereby possibly promote an upregulation of work intensity. This phenomenon has been reported in both elite runners and rowers with precooling of about 0.5 degree Celsius using cool water of about 20 degrees Celsius or ice vests (Arngrimsson et al. 2004).

Precooling is typically adopted to enhance athletic performance in hot environments rather than to increase the safety of exercising in such conditions. Decreasing core temperature before exercise or slowing the rate at which it rises during exercise can increase the time it takes to reach the high internal temperatures associated with the termination of exercise, whereas reductions in skin temperature can improve perceptions of thermal comfort. Precooling has regularly been shown to enhance endurance, exercise performance, and capacity in hot environmental conditions for moderate-duration exercise such as a 5K run (Tyler, Sunderland, and Cheung 2015; table 11.2). The improvement, however, does not always require a reduction in core temperature, which suggests that the benefit may also arise from reductions in skin temperature and the magnitude of perceived heat stress (Tyler and Sunderland 2011).

The potential benefits of precooling vary greatly across cycling disciplines, related to both their duration and their level of neuromuscular demand. Precooling does not appear to improve short-duration or sprinting exercise. Such activity is unlikely to be compromised because of thermoregulatory strain, and precooling may in fact negatively affect performance because of lowered muscle temperature and contractile capacity (Thornley, Maxwell, and Cheung 2003). Indeed, precooling before intermittent cycling sprints did not result in any improved power (Cheung and Robinson 2004; Duffield et al. 2003), and a higher ambient temperature may actually facilitate higher pedaling cadences and power outputs during maximal cycling sprints (Ball, Burrows, and Sargeant 1999). Precooling benefits may also be minimal with events such as a sportif ride or Ironman triathlon, because of the prolonged nature of the event and because the high metabolic heat production rapidly overwhelms any precooling-induced heat storage reduction.


Table 11.2 Relative Effectiveness of Precooling Overall, Broken Down by Exercise Modality, With Relative Effectiveness of Cooling During Exercise

Exercise type	Effect
PRECOOLING (23 STUDIES) ↑↑
Sprint	↓
Intermittent	↑
Prolonged performance (e.g., TT)	↑↑
Prolonged steady rides to exhaustion	↑↑↑
COOLING DURING EXERCISE (5 STUDIES) ↑↑

 
Data from C.J. Tyler, C. Sunderland, and S.S. Cheung, 2015, "The effect of cooling prior to and during exercise on exercise performance and capacity in the heat: A meta-analysis," British Journal of Sports Medicine 49(1): 7-13.

Ice vests are popular for precooling because they cover the important torso region and are relatively nonconstricting during cycling compared with cooling pants. Vests also permit the legs to exercise and get the usual benefits of warming up before competition while keeping the torso as cool as possible. Research has shown that precooling the legs eliminates most of the benefits of warming up (Sleivert et al. 2001). Cooling hoods are also often used because they theoretically keep the brain cool. Furthermore, the head and face play a major role in the overall perception of thermal stress and comfort (Cotter and Taylor 2005), and head cooling by itself can be beneficial in improving high-intensity running in the heat.

Cooling During Exercise

Cooling during exercise has been studied less than precooling, but the effects and mechanisms of action (reduced core and skin temperature, heart rate, thermal sensation, and perceived exertion) appear to be similar. Although cooling during exercise can offer benefits to exercise performance, cooling jackets and vests can also increase the energy demands of exercise because of their extra mass, can create a microenvironment around the body that impairs evaporative heat dissipation, and can cause discomfort and skin irritation.

Cooling the head may achieve greater thermoregulatory advantage by directly targeting the thermoregulatory centers in the brain and the high perceptual thermosensitivity associated with the head and neck. This cooling effect may be achieved by using a cooling collar, wearing an ice sock underneath a jersey, or pouring cool or cold water over the head and neck. Cooling the head and neck region during exercise and physical activity in hot conditions has been shown to improve exercise capacity and performance (Tyler and Sunderland 2011). Physiological improvements (e.g., core temperature and HR) are equivocal, but perceptual thermal discomfort and ratings of perceived exertion are often improved. Head cooling may also protect some aspects of mental functioning when thermal strain is high, potentially a huge benefit in a sport that requires as many instantaneous decisions and carries as much risk as cycling does.

Although head and neck cooling may make athletes feel better, it is unlikely to reduce the level of thermal or cardiovascular strain experienced, so this technique should not be used as a method to lower body temperature. An important caveat and point of education with athletes is that with head and neck cooling, the improved perceptual strain in the absence of physiological improvements may provide false signals regarding the thermal state of the body and may pose a danger to the individual’s health.

Applying the Science

Although some increase in body temperature during exercise is acceptable and even desirable, extreme temperatures impose extra stress on the body that diverts its focus from generating power at the muscles and delivering it to the pedals. How can you apply some of the research on heat stress and cooling strategy to cycling?

Exercise capacity is clearly reduced in both warm and hot environments. Therefore, anticipating this reduction is important in planning training and racing. For example, a cyclist who travels to a hot environment to train or race will need to adjust power or performance targets until full adaptation occurs.

Although full adaptation in physiology and performance can occur within about 2 weeks, individual responses to heat and adaptation rate can differ greatly. Therefore, cyclists need to monitor their own responses to heat. Many can do this by carefully logging training and responses during the initial heat wave of the summer.

Cyclists should keep as cool as possible before an event! They should do everything possible to stay out of the sun and heat before an event. Every bit of heat exposure is unnecessary, additional stress that detracts from training or recovery.

Precooling can be useful when competing in hot environments, but it is not universally positive as an ergogenic aid. It appears to be most effective for moderate-duration events lasting 10 to 60 minutes. Precooling appears to be counterproductive for sprint and power sports (e.g., downhill MTB, BMX, track sprinting) because of negative effects from cooling the muscles. Cooling may be effective, however, between competition heats. Precooling is also likely ineffective for long-duration events such as sportifs and many road races.

If precooling or cooling during exercise, cooling the head and torso may be the preferred strategy. The torso may strike the ideal balance between cooling the largest surface area without impeding the legs. Cooling the head and neck may improve perceptual sensations and performance despite having only minimal effect on actual heat storage.