This article is part of a series:
→ Part 1 — What heat does to you.
→ Part 2 — How to stay cool on race day.
→ Part 3 — How to heat acclimate before race day.
Also check out my related series on:
Training & racing in the cold.
→ Part 1 — What heat does to you.
→ Part 2 — How to stay cool on race day.
→ Part 3 — How to heat acclimate before race day.
Also check out my related series on:
Training & racing in the cold.
Training and racing in the heat. Part 1 of 3:
What does heat do to your body during exercise?
Thomas Solomon PhD.
16th Oct 2021.
It is always hot somewhere in the world. If you are in that part of the world and are too hot, you need to cool it. Why? Because being too hot is bad for you. This 3-part series on training and racing in the heat will delve into what heat does to you (Part 1) and, most importantly, what you can do about it on race day (Part 2) and before race day (Part 3) so you can keep on rocking...
Reading time ~20-mins.
or listen to “audiobook” Podcast version here.
or listen to “audiobook” Podcast version here.
Your body never sleeps. Its biochemical reactions are constantly raging — “burning” fuel and producing chemical energy (ATP). This is your basal metabolic rate. When you move, you “burn” fuel at a faster rate to produce more ATP to help power your muscle contractions (mechanical work). But, there is a downside… During your sessions and races, your body generates a lot of heat, aka thermogenesis — the more chemical energy you produce, the more heat energy you generate. But, your body also “absorbs” (gains) heat from its surroundings, via radiation (from the sun), convection (from hot air or sitting in a sauna), or conduction (from sitting in a hot bath).
The heat generated by your never-ending internal activity is useful because it helps you maintain your core body temperature — you are endothermic — meaning that, as a human, you do not rely on the energy of sunlight to stay warm. But, because your biochemical reactions and nerve conductance operate optimally at ~37°C, your body also needs to maintain core body temperature within a very narrow range despite the large daily fluctuations in the temperature of your surroundings. Therefore, for all the heat you “generate” (from metabolism) or “absorb” (from your surroundings), you need to lose some of it into the surrounding air to maintain thermoneutrality at ~37°C.
Some animals, like the Turkey Vulture, pee on themselves to cool down as temperatures soar. Lucky for you, neither of your parents is a Turkey Vulture so you skipped the “piss on thyself” gene. Although, it is possible that you do know what it feels like to piss your pants — warmth followed by cooling. That aside, and (hopefully) more likely, you will probably know what it feels like to walk past a big block of ice, have cool air blasted in your face, touch a bunch of frozen peas, or hug a cold relative. Hence, you are aware that your body can also lose heat in the same ways it gains heat: radiation, convection, or conduction to cooler “things”.
Image Copyright © Julien Périard, Thijs Eijsvogels, and Hein Daanen (2021) Physiol Rev. All rights reserved.
Licensed under Open-access Creative Commons Attribution CC-BY 4.0.
If the air is cooler than your body, some heat might radiate away from your body. If your running speed and/or the wind speed is high, some heat might convect away from your body. If you run through water that is cooler than your body, some heat might conduct away from your body. But, there are a lot of “mights”. Fortunately, evolution has educated your brain in that sentiment and, to maximise the likelihood of heat leaving your body during exercise when your furnace is roaring, it tells your skin cells to actively secrete water onto your skin so you can harness the full power of evaporation to stay cool (read all about sweat gland physiology here).
Hello, sweat.
A large amount of heat energy — about 580 kcal per litre— is required to turn water from liquid to gas so it can evaporate from your skin (or your breath). So, whenever water leaves your body via evaporation, a corresponding amount of heat also leaves your body into the surrounding air, helping you cool down.
Pretty freaking “cool”.
The main factors governing your body heat production and, therefore, your sweat rate during exercise are:
Your body weight — greater body weight and greater lean body mass = a higher rate of baseline kcal per min energy expenditure (aka resting metabolic rate, your RMR) = more heat production (or thermogenesis).
Your exercise intensity — greater running speed = a higher rate of kcal/min energy expenditure = more heat production. And,
The ambient conditions — higher air temperature = less conductive heat loss and greater heat gain; higher relative humidity = slower evaporative cooling and less heat loss; and lower wind speed = lower convective cooling and less heat loss.
Consequently, sweat rates are highly variable within and between different people. And, to complicate matters further, there is no single body temperature at which sweating begins — the onset of sweating occurs at different core body temperatures in different people. Therefore, the fact that you are sweating does not necessarily mean you are “too” hot nor does it indicate that your performance is declining. Sweat is necessary. Sweat is your friend. But exercise in the heat is more than simply becoming a sweaty, smelly mess. It is important to fully understand what happens to your hot body so you can identify strategies to help mitigate the performance-harming effects of heat. So...
As you already know, the more fuel you “burn”, the more chemical energy (ATP) you produce, and the more heat energy you generate. During exercise, you inhale and use (aka “consume”) oxygen to “burn” your stored fuel (glucose and fatty acids), and the rate of oxygen consumption (VO2) is relative to the intensity of exercise. For every 1 litre of oxygen consumed, you produce about 5 kcal of energy. But because the human body is only about 20-25% efficient, only about 1 kcal of that energy is used for mechanical work and the remaining 4 kcals are released as heat energy.
Let’s conduct a quick thought experiment: Imagine you weigh 65 kg and during a 30-minute race you work at 90% of your VO2max, which is 60 mL/kg/min. This means you will consume a total of ~105 L of O2 during the race (calculation = [60mL × 65kg × 30min × 90%] ÷ 1000) so your muscles can “burn” stored fuel to produce ~525 kcal of energy, of which ~⅕ (~105 kcal) will be used for mechanical work (moving forward) and the remaining ~⅘ (~420 kcal) will be released as heat.
But, then what?
“Central” thermoreceptors in your body constantly detect the change in your core body temperature caused by the heat produced by metabolism and send signals to your brain. Similarly, “peripheral” thermoreceptors in your skin detect changes in air temperature, telling your brain how hot (or cold) it is around you, making you “feel” a certain way. These signals are processed by your hypothalamus, which gets to work to help cool you down by sending signals to your sweat glands (to initiate sweating), your skin’s arterioles (to dilate capillaries and increase skin blood flow), your muscles (to make them slow the eff down), and your adrenal medulla (to stimulate epinephrine/adrenaline secretion, which increases heart rate to maintain skin blood flow while also increasing glycogen breakdown and carbohydrate metabolism).
So, one simple thing — a rise in temperature — has a butterfly effect:
Thermal responses = ↑ core temperature & ↑ skin temperature.
Cardiovascular responses = ↑ heart rate, ↑ breathing rate, ↓ stroke volume & cardiac output, ↓ VO2max.
Cerebral responses = ↓ brain blood flow.
Perceptual responses = ↓ thermal comfort, ↑ thirst, ↑ RPE, ↓ motivation.
Metabolic responses = ↑ glycogen breakdown, ↑ carbohydrate oxidation, & ↑ hydrogen ion (H+) accumulation.
As you sweat more to cool down, you lose more water from your total body water “tank” (aka dehydration). If dehydration persists for a long time, plasma volume will drop making it harder for your blood to perfuse the skin to release heat and allow cooling. Therefore, you can probably surmise why starting exercise in the heat in a hypohydrated (or “dehydrated”) state and/or not adequately maintaining hydration status during exercise in the heat, makes it more difficult to cool down (see my series on Hydration for more info). Plus, a higher sweat rate is correlated with a greater sweat sodium concentration (see here and here). So, during a long session or race on a hot day, you might be at a higher risk of a dehydration-related performance impairment and you might need to supplement with sodium (again, please see my series on Hydration for more info).
The ultimate consequence of all the reflexes and responses to exercise in the heat is impaired endurance performance.
When core body temperature rises to 39-40°C most folks start to dwindle during exercise and most people stop exercise due to fatigue at ~40°C. A 2020 systematic review from Kusumoto and colleagues found that core body temperature is the most important contributor of all factors that influence the variation in athletic performance. And, of course, the rise in your core temperature during exercise is influenced by the external air temperature, which provides a “feeling” of a high skin temperature. For example, randomised controlled trials have shown that peak core body temperature, peak heart rate, and peak RPE during a 30-km time trial are incrementally higher when trained cyclists ride in air temperature of 20˚C, 28˚C, and 36˚C, while average power output and time-to-completion deteriorate. Epidemiological observations over ~40 to 50-years at the Berlin and New York City marathons support these findings — generally, runners are slower on hotter days (see here, here, here, and here, ). And, such findings have been confirmed by observations of ~8000 athletes racing in ~1200 races from 3 km up to 50 km — the likelihood for peak performance was greatest when air temperature was 10 to 17.5°C (or 7.5-15 °C WBGT, something I will delve into later) with a 0.3-0.4% performance decrease for every degree increment in temperature.
For example, over the 45-year history of the Berlin marathon, the fastest marathon runners achieved the fastest race times on race days with higher maximum temperatures and elite athletes run faster on days with higher temperatures and achieve World Records and world-class performances on hotter days.
These observations might indicate that the best world-class endurance athletes are well adapted to the heat. But this is not a rule...
Ahead of the 20201 Olympics, a group of Dutch scientists studied ~100 elite athletes across several sports in the Thermo Tokyo Project to understand how their Olympians would be affected in the heat of Japan. Athletes’ exercise capacity and thermoregulatory, cardiovascular, and perceptual responses to exercise were studied on two occasions under control conditions (~16°C, ~55% relative humidity) and “Tokyo” conditions (~32°C, ~74% RH). The heat and humidity caused a greater exercise-induced increase in core body temperature and peak core temperature, a higher heart rate, greater sweat rate, higher thermal sensation, lower thermal comfort, and, importantly, a 16±7% reduction in power output with a 26±11% (16±8 min) reduction in time-to-exhaustion. These effects are alarming — the subjects are world-class athletes! But, there was huge between-person and between-sport variability in athletes’ thermoregulatory responses and exercise capacity in the simulated “Tokyo” heat and humidity, emphasising that no two athletes respond the same to exercise in the heat.
Sweating is your bodys’ only physiological method of cooling down. As you know, sweating is awesome. But, sometimes, it is not… If the relative humidity is high, the air around you is already holding a lot of water and you will notice that sweat “drips” off your skin more than usual. If sweat is dripping off, it is still liquid and is having a hard time evaporating into gas to float away into the air carrying your body heat. Similarly, if the air temperature is higher than your core temperature, heat cannot move (radiate/convect) away from your body and, instead, you will gain heat from your surroundings. Even convective heat loss due to the wind has a negligible effect on a super hot day because you are simply being shrouded in hot air. Simply put, hot and humid conditions make it difficult to keep cool during exercise.
For these reasons, your body’s natural heat dissipating defence — sweating — is insufficient to cool you down and then your body temperature may keep rising and rising if you keep soldiering on. So, an objective measure of “hot” can help inform whether training/racing today is a good idea or not and/or whether you will need to take any special precautions.
The air temperature can, of course, provide clues about how “hot” it is but there is no single temperature that says “don't go out”. The wind speed is also useful — higher wind speed = greater cooling. Relative humidity also helps — higher humidity = harder for sweat to evaporate = less cooling. The dew point is similarly useful since it is the temperature to which air must be cooled to become saturated with water vapour — lower dew point = harder for sweat to evaporate = less cooling. For a general outlook of how conditions will be, I typically use Weather Underground combined with “current condition” updates from my local Mountain Association if I plan to go off-piste. Some such sites provide a “feels like” temperature, but I rarely find this of use since the calculations are hidden in a proprietary black box.
There are also “heat stress indices” like the physiological equivalent temperature and the wet-bulb globe temperature index, which are used to classify risk of heat stress. A recent meta-analysis identified ~300 heat stress indices but the most popular is the wet-bulb globe temperature (aka WBGT), which is a single metric that combines measures of air temperature, evaporative cooling (derived from relative humidity and air flow), and heat intensity (from sunlight & reflections). For this reason, there are several ways to get a “it is too hot” WBGT — the air temperature can be disgustingly high; or the air temperature can be moderately high but with intense sunshine; or the air temperature can be lower but with very high humidity.
A recent analysis of ~8000 athletes racing in distances of 3 to 50km, found that higher WBGTs are associated with poorer race day running performance. During the the Tokyo 2020 Olympic marathons, it was hot and humid — the WBGT was 29°C in the womens’ race (29°C air temp; 67% relative humidity) and 27°C in the mens’ race (28°C air temp; 72% RH). The conditions were not nice. And, it showed — the DNF beast captured 17% (15 of 88) of women and 28% (30 of 106) of men.
WBGT levels for modification or cancellation of workouts or athletic competition for healthy adults.
Image Copyright © ACSM Position Stand on Exertional Heat Illness during Training and Competition (2007) Med Sci Sports Exerc.
Don’t rely on someone else to tell you how you feel and that it is fine for you to continue.
There is considerable debate about how to quantify “heat stress” objectively. Given the limitations of existing heat stress indices (e.g. WBGT), the ifs and maybes of how to respond to them, and the variable heat tolerance between us all, it is probably best to stay calm and follow a simple rule-of-thumb like:
If it “feels” too bloody hot and you feel really bloody rubbish, it is probably too hot for you. So, stop and be cool.
But, what happens if you don’t? What happens when you exercise when it is too hot for you? Or, more accurately... What happens when you exercise and cannot cool down?
If your core body temperature gets too high — hyperthermia — it increases your risk of an exertional heat illness, including exertional heat stroke and exertional heat exhaustion, which can both be life-threatening, and the less serious exercise-associated muscle cramps.
Exertional heatstroke is defined as a rectal temperature greater than 40°C accompanied by symptoms or signs of organ system failure, most frequently central nervous system dysfunction, which manifests as a loss of coordination and stumbling, perhaps collapse. The signs and symptoms are often nonspecific but:
At the time of collapse, folks will have sweat-soaked and pale skin then the following can develop: disorientation, confusion, dizziness, irrational or unusual behaviour, inappropriate comments, irritability, headache, inability to walk, loss of balance and muscle function resulting in collapse, profound fatigue, hyperventilation, vomiting, diarrhoea, and delirium, and in extreme cases, seizures & coma.
A change of personality or performance in hot & humid conditions is a classic sign of exertional heat stroke but, as you can see, things can get worse than that — you need rapid cooling and should seek medical attention immediately. Following heatstroke, a “return to play” is not possible today — it may kill you.
Exertional heat exhaustion is what happens when your cardiac output drops and your body has a hard time sustaining blood pressure and cerebral blood flow, causing fatigue. This occurs because as your core temperature rises, skin and muscle capillaries dilate and sweating rate increases, lowering plasma volume due to ongoing dehydration. Exertional heat exhaustion may lead to a full collapse that can be exemplified by glycogen depletion and low blood glucose as a result of prolonged vigorous exercise. Exertional heat exhaustion is, therefore, your brain basically “putting the brakes on”. Signs and symptoms are nonspecific but include:
appearing sweaty and pale with low blood pressure but can also include headache, weakness, dizziness, heat “sensations” on the head or neck, chills, goose flesh/pimples, nausea, vomiting, diarrhoea, irritability, and decreased muscle coordination
Serious cases may require hospitalisation and IV fluids. And, just like exertional heat stroke, following heat exhaustion, a “return to play” is not possible today — it may kill you. But, wth milder cases, return to play is often possible 24-48 hours later following rest, meals, and rehydration.
Exercise-associated muscle cramps are triggered due to a loss of neuromuscular control and/or due to electrolyte imbalances and sodium depletion (see my separate deep-dive article at veohtu.com/cramp). They typically occur during long-duration and/or high-intensity sessions/races when you are pushing into territory unfamiliar to your fitness, training history, and terrain experience. After cramps, which can be intense and sometimes painful, following some rest and/or sodium-containing foods/fluid, immediate “return to play” is often (but not always) possible.
But, how common are exertional heat illnesses in sport?
Fortunately, they are generally uncommon. For example, observations at the Boston marathon found only 3.7 cases of exertional heat stroke per 10,000 starters, representing 0.5% of all medical encounters. A 2020 systematic review examining the epidemiology of exertional heat illnesses in sport found a huge range in incidence rates across sports — 0.01 (cheerleading) to 4.19 (American football) cases per 1000 athletic-exposures* (* = a session or competition) — although very hot desert ultra-marathons had a very high incidence of ~54 cases per 100 participants. Meanwhile, a 2021 systematic review and meta-analysis of life-threatening events over 10-years in ~1-million runners during marathons and ½-marathons found 1.02 cases of exertional heat stroke per 100,000 runners. This analysis also found a clear effect of race day air temperature: ~4 cases of exertional heat stroke per 100,000 when the temperature was above 20°C, ~1 per 100,000 when the temperature was 10-20°C, and ~0.3 per 100,000 at less than 10°C. To put this in perspective, the London marathon, which has about 30,000 starters, will see ~1 person with a life-threatening event due to exertional heat stroke every 3-years.
But, what puts you at risk of an exertional heat illness?
Remember that a high core temperature leads to heat stress, which can cause a heat illness. A higher rate of energy expenditure (a higher metabolic rate) will generate more heat, which the body needs to remove to keep your core temperature within the healthy range. A larger body weight and a higher exercise intensity are correlated with greater energy expenditure. Therefore, a bigger runner will generate more heat than a smaller runner and a faster runner will generate more heat than a slower runner.
Heat illnesses can occur during short sessions/races, even ones that are too short to cause significant dehydration, which means that dehydration is sufficient, but not a requisite, for exertional heat illness. Heat stroke is more likely in shorter high-intensity sessions/races, up to 30-minutes, because the rate at which your core temperature rises is relative to your metabolic rate, which can be driven very high in short races. Heat stroke is less of a concern in longer bouts since it is very difficult to push your core temp so high because you cannot work so hard. Heat exhaustion is often more likely in longer events, partly because dehydration is also more of a concern during longer races since you have more time to sweat.
But…
The heat “stress” a runner will experience is influenced by the factors that allow (or prevent) heat to be removed from their body. High air temperature, high humidity, low air velocity (the sum of your speed and the wind speed), and intense solar radiation make strenuous exercise riskier because they all prevent heat from leaving your body. So too can dehydration because if your total body water “tank” is low, your skin cells won’t place water on your skin so it can evaporate — sweating — since this would cause further dehydration… Double trouble.
There are many other risk factors, including pregnancy, poor fitness, current illness, underlying disease, alcohol, drugs, supplements/stimulants, sunburn, even race start time (see a 2020 systematic review by Westwood et al. for an overview). But, because most of the factors that increase your core temperature and/or prevent you cooling down are all intertwined, it is often hard to pin down individual risk factors — exertional heat illness is complex!
Risk factors for exertional heat illness.
Image Copyright © Julien Périard, Thijs Eijsvogels, and Hein Daanen (2021) Physiol Rev. All rights reserved.
Licensed under Open-access Creative Commons Attribution CC-BY 4.0.
Be aware of the weather conditions.
Learn to recognise the signs and symptoms of exertional heat illness.
Keep em peeled… Is it hot, and/or is it humid, and/or is the sun intense? If so and you’re involved in a session or race, either as an athlete or a spectator, having this awareness will keep you (or someone else) healthy and could save you (or someone else’s) life.
If you are aware and sensible and adequately hydrated (see my series on Hydration), you will likely avoid the issues associated with exercising in the heat. But, if you have any medical conditions and/or you are pregnant, you should consult your doctor before exercising in hot conditions.
So, what can you do to train safely?
Be aware of your own body and be aware of how well heat adapted you are — if you’ve spent more than the last 2-weeks hiding from the heat by running in the cool, you’re likely not well heat-adapted (as you’ll learn in Part 3, it takes at least 2-weeks for full acclimation).
To stay safe in the heat, maintain adequate daily hydration so you are euhydrated before you leave the house and aim to stay hydrated during your sessions (see my series on Hydration).
Know the signs and symptoms of dehydration and exertional heat stroke and heat exhaustion.
If it's super hot, train earlier (for cooler and shadier times), go slower (for a lower rate of energy expenditure) and go shorter (for less total energy expenditure and less total exposure to the environment), all of which will minimise your risk of overheating and developing a heat illness.
During your sessions, monitor how you feel — if you get dizzy, lightheaded, uncomfortably hot, or develop irrational behaviour, that’s the time to stop.
So, now you know what your body goes through when you smash a session or a race in hot conditions. The next obvious question is “How can you mitigate the effects of heat during exercise?”. Well well, what a great question. But, that is the story for next time...
Thanks for joining me for this “hot” session. Until next time, keep training smart.
The heat generated by your never-ending internal activity is useful because it helps you maintain your core body temperature — you are endothermic — meaning that, as a human, you do not rely on the energy of sunlight to stay warm. But, because your biochemical reactions and nerve conductance operate optimally at ~37°C, your body also needs to maintain core body temperature within a very narrow range despite the large daily fluctuations in the temperature of your surroundings. Therefore, for all the heat you “generate” (from metabolism) or “absorb” (from your surroundings), you need to lose some of it into the surrounding air to maintain thermoneutrality at ~37°C.
Some animals, like the Turkey Vulture, pee on themselves to cool down as temperatures soar. Lucky for you, neither of your parents is a Turkey Vulture so you skipped the “piss on thyself” gene. Although, it is possible that you do know what it feels like to piss your pants — warmth followed by cooling. That aside, and (hopefully) more likely, you will probably know what it feels like to walk past a big block of ice, have cool air blasted in your face, touch a bunch of frozen peas, or hug a cold relative. Hence, you are aware that your body can also lose heat in the same ways it gains heat: radiation, convection, or conduction to cooler “things”.
Licensed under Open-access Creative Commons Attribution CC-BY 4.0.
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But... by far your most specialised mechanism for losing “excess” body heat is via evaporation... a little bit from your breath and a massive amount from your sweat.
Sweating is awesome.
During exercise, your metabolic rate (aka your rate of energy expenditure) increases above your resting level and energy metabolism (“burning” fats and carbs) produces heat. A greater rate of energy expenditure — which increases as you run faster — will generate a higher amount of heat. Heat production must be equalised by heat loss to maintain “thermoneutrality” and prevent your core body temperature from rising above critical levels, which is a signal for fatigue. The faster you move, the more heat you will produce and, therefore, you need to lose more heat.If the air is cooler than your body, some heat might radiate away from your body. If your running speed and/or the wind speed is high, some heat might convect away from your body. If you run through water that is cooler than your body, some heat might conduct away from your body. But, there are a lot of “mights”. Fortunately, evolution has educated your brain in that sentiment and, to maximise the likelihood of heat leaving your body during exercise when your furnace is roaring, it tells your skin cells to actively secrete water onto your skin so you can harness the full power of evaporation to stay cool (read all about sweat gland physiology here).
Hello, sweat.
A large amount of heat energy — about 580 kcal per litre— is required to turn water from liquid to gas so it can evaporate from your skin (or your breath). So, whenever water leaves your body via evaporation, a corresponding amount of heat also leaves your body into the surrounding air, helping you cool down.
Pretty freaking “cool”.
The main factors governing your body heat production and, therefore, your sweat rate during exercise are:
Your body weight — greater body weight and greater lean body mass = a higher rate of baseline kcal per min energy expenditure (aka resting metabolic rate, your RMR) = more heat production (or thermogenesis).Note: contrary to popular belief, a 2021 systematic review and meta-analysis shows that a higher body fat level is not associated with poorer thermoregulation during exercise in the heat. But, existing studies have done a poor job of controlling for confounding factors such as metabolic heat production, bodyweight, heat acclimatization, and body surface area.
Your exercise intensity — greater running speed = a higher rate of kcal/min energy expenditure = more heat production. And,
The ambient conditions — higher air temperature = less conductive heat loss and greater heat gain; higher relative humidity = slower evaporative cooling and less heat loss; and lower wind speed = lower convective cooling and less heat loss.
What does heat do to your physiology and exercise performance?
We humans are certainly among the mammals with the greatest capacity for exercising in extreme heat. Firefighters spend lots of time hauling heavy gear in or near one of the hottest things we know: fire. Soldiers frequently train and march in extreme heat. And, some athletes compete in “silly” things like the multi-day 250 km desert march at Marathon Des Sables. But these are extremes and not everyone wants to be extreme. That said, most of us will do a session/race in hot conditions at some point in our lives and most of us will train during periods of heat and humidity.As you already know, the more fuel you “burn”, the more chemical energy (ATP) you produce, and the more heat energy you generate. During exercise, you inhale and use (aka “consume”) oxygen to “burn” your stored fuel (glucose and fatty acids), and the rate of oxygen consumption (VO2) is relative to the intensity of exercise. For every 1 litre of oxygen consumed, you produce about 5 kcal of energy. But because the human body is only about 20-25% efficient, only about 1 kcal of that energy is used for mechanical work and the remaining 4 kcals are released as heat energy.
Let’s conduct a quick thought experiment: Imagine you weigh 65 kg and during a 30-minute race you work at 90% of your VO2max, which is 60 mL/kg/min. This means you will consume a total of ~105 L of O2 during the race (calculation = [60mL × 65kg × 30min × 90%] ÷ 1000) so your muscles can “burn” stored fuel to produce ~525 kcal of energy, of which ~⅕ (~105 kcal) will be used for mechanical work (moving forward) and the remaining ~⅘ (~420 kcal) will be released as heat.
But, then what?
“Central” thermoreceptors in your body constantly detect the change in your core body temperature caused by the heat produced by metabolism and send signals to your brain. Similarly, “peripheral” thermoreceptors in your skin detect changes in air temperature, telling your brain how hot (or cold) it is around you, making you “feel” a certain way. These signals are processed by your hypothalamus, which gets to work to help cool you down by sending signals to your sweat glands (to initiate sweating), your skin’s arterioles (to dilate capillaries and increase skin blood flow), your muscles (to make them slow the eff down), and your adrenal medulla (to stimulate epinephrine/adrenaline secretion, which increases heart rate to maintain skin blood flow while also increasing glycogen breakdown and carbohydrate metabolism).
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You might have noticed some of these things happening to you but, no doubt, you are also aware of how you “feel” when you exercise on a hot day. Yes, some folks feel comfy; many do not. If your “thermal comfort” decreases, you will have one more distraction on race day. Exercise in the heat can also “feel” harder — your RPE (rating of perceived exertion) goes up because of the thermal discomfort you experience and this can be coupled with a decrease in motivation to keep on pushing. And, because of your in-built “cooling” mechanism — sweating — more water leaves your body (more dehydration occurs) during exercise on a hot day, which will make you “feel” thirsty.
So, one simple thing — a rise in temperature — has a butterfly effect:
Thermal responses = ↑ core temperature & ↑ skin temperature.
Cardiovascular responses = ↑ heart rate, ↑ breathing rate, ↓ stroke volume & cardiac output, ↓ VO2max.
Cerebral responses = ↓ brain blood flow.
Perceptual responses = ↓ thermal comfort, ↑ thirst, ↑ RPE, ↓ motivation.
Metabolic responses = ↑ glycogen breakdown, ↑ carbohydrate oxidation, & ↑ hydrogen ion (H+) accumulation.
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So, you feel hotter and the effort feels harder, and...
The ultimate consequence of all the reflexes and responses to exercise in the heat is impaired endurance performance.
Note: for a phenomenal dive into the literature on all things related to exercise in the heat, including performance implications, and mitigation strategies, I thoroughly recommend reading a 2021 narrative review from Julien Périard, Thijs Eijsvogels, and Hein Daanen (2021) Physiol Rev.
Heat slows us down but there are always exceptions to the rule.
It is clear that a high core body temperature impairs endurance performance but heat tolerance is massively variable between people and some endurance athletes have been reported with core temperatures as high as 41.5°C at the end of a race without reaching fatigue or showing signs or symptoms of exertional heat illness. Furthermore, the best endurance athletes can perform rather well when it is warm.For example, over the 45-year history of the Berlin marathon, the fastest marathon runners achieved the fastest race times on race days with higher maximum temperatures and elite athletes run faster on days with higher temperatures and achieve World Records and world-class performances on hotter days.
These observations might indicate that the best world-class endurance athletes are well adapted to the heat. But this is not a rule...
Ahead of the 202
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All of the aforementioned factors — your sweat rate (which is influenced by your bodyweight and exercise intensity), the ambient conditions, your feelings/sensations, etc — are part of the mechanism that allows some folks to tolerate heat on race day more than others. However, no matter how “tolerant” you are, if you’re giving it large and it is super hot and humid, things can get bad for anyone. So, you might be asking yourself...
When is “hot” too hot?
No doubt you are aware of how you feel when you’re in hot conditions. But, your perception of “hot” is highly-subjective. If you live in the UK, you might consider an air temperature of 20°C as a “hot” day and be in shorts and a t-shirt. On the other hand, if you live in Dubai, you might consider 20°C as “deep winter”. No matter who you are or where you live, your body works optimally within a narrow core temperature range around ~37°C. And, if you are gaining heat faster than you are losing it, your core temperature will continue rising until it reaches a level that either forces you to voluntarily stop or, worse, involuntarily stop, at which point you are probably risking your health.Sweating is your bodys’ only physiological method of cooling down. As you know, sweating is awesome. But, sometimes, it is not… If the relative humidity is high, the air around you is already holding a lot of water and you will notice that sweat “drips” off your skin more than usual. If sweat is dripping off, it is still liquid and is having a hard time evaporating into gas to float away into the air carrying your body heat. Similarly, if the air temperature is higher than your core temperature, heat cannot move (radiate/convect) away from your body and, instead, you will gain heat from your surroundings. Even convective heat loss due to the wind has a negligible effect on a super hot day because you are simply being shrouded in hot air. Simply put, hot and humid conditions make it difficult to keep cool during exercise.
For these reasons, your body’s natural heat dissipating defence — sweating — is insufficient to cool you down and then your body temperature may keep rising and rising if you keep soldiering on. So, an objective measure of “hot” can help inform whether training/racing today is a good idea or not and/or whether you will need to take any special precautions.
The air temperature can, of course, provide clues about how “hot” it is but there is no single temperature that says “don't go out”. The wind speed is also useful — higher wind speed = greater cooling. Relative humidity also helps — higher humidity = harder for sweat to evaporate = less cooling. The dew point is similarly useful since it is the temperature to which air must be cooled to become saturated with water vapour — lower dew point = harder for sweat to evaporate = less cooling. For a general outlook of how conditions will be, I typically use Weather Underground combined with “current condition” updates from my local Mountain Association if I plan to go off-piste. Some such sites provide a “feels like” temperature, but I rarely find this of use since the calculations are hidden in a proprietary black box.
There are also “heat stress indices” like the physiological equivalent temperature and the wet-bulb globe temperature index, which are used to classify risk of heat stress. A recent meta-analysis identified ~300 heat stress indices but the most popular is the wet-bulb globe temperature (aka WBGT), which is a single metric that combines measures of air temperature, evaporative cooling (derived from relative humidity and air flow), and heat intensity (from sunlight & reflections). For this reason, there are several ways to get a “it is too hot” WBGT — the air temperature can be disgustingly high; or the air temperature can be moderately high but with intense sunshine; or the air temperature can be lower but with very high humidity.
A recent analysis of ~8000 athletes racing in distances of 3 to 50km, found that higher WBGTs are associated with poorer race day running performance. During the the Tokyo 2020 Olympic marathons, it was hot and humid — the WBGT was 29°C in the womens’ race (29°C air temp; 67% relative humidity) and 27°C in the mens’ race (28°C air temp; 72% RH). The conditions were not nice. And, it showed — the DNF beast captured 17% (15 of 88) of women and 28% (30 of 106) of men.
Image Copyright © ACSM Position Stand on Exertional Heat Illness during Training and Competition (2007) Med Sci Sports Exerc.
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The WBGT was developed by the US military, popularised by the American College of Sports Medicine, and is used by several sporting bodies (including FIFA, WTA, World Athletics, World Triathlon, etcs) to inform decisions about canceling events (see this Table for more info ). Because of the variability in “heat tolerance” shown between different people, it is difficult to establish a single WBGT “threshold” across all sports beyond which events should be cancelled. It is the race organisers’ responsibility to keep you safe and they will make an informed decision for your safety. So, you do not need to own a device that measures WBGT. Besides, they are very expensive. It is also important to note that the WBGT is an environmental heat stress index and not a measure of your heat strain. Furthermore, when evaporative cooling (sweating) is restricted due to high humidity and/or low wind speed, WBGT can underestimate your risk of heat stress. And, because WBGT does not account for clothing choices or metabolic heat production, it cannot predict heat dissipation. My point being that:
Don’t rely on someone else to tell you how you feel and that it is fine for you to continue.
If it “feels” too bloody hot and you feel really bloody rubbish, it is probably too hot for you. So, stop and be cool.
Exertional heat illness.
If you’ve followed some major marathons in the last couple of years, you will have seen examples of heat illness. British runner, Callum Hawkins, became a complete mess as he led the Commonwealth Games marathon while the Brazilian, Daniel do Nascimento, had multiple dramatic collapses with delirium at the Tokyo Olympic marathon.If your core body temperature gets too high — hyperthermia — it increases your risk of an exertional heat illness, including exertional heat stroke and exertional heat exhaustion, which can both be life-threatening, and the less serious exercise-associated muscle cramps.
Exertional heatstroke is defined as a rectal temperature greater than 40°C accompanied by symptoms or signs of organ system failure, most frequently central nervous system dysfunction, which manifests as a loss of coordination and stumbling, perhaps collapse. The signs and symptoms are often nonspecific but:
At the time of collapse, folks will have sweat-soaked and pale skin then the following can develop: disorientation, confusion, dizziness, irrational or unusual behaviour, inappropriate comments, irritability, headache, inability to walk, loss of balance and muscle function resulting in collapse, profound fatigue, hyperventilation, vomiting, diarrhoea, and delirium, and in extreme cases, seizures & coma.
Exertional heat exhaustion is what happens when your cardiac output drops and your body has a hard time sustaining blood pressure and cerebral blood flow, causing fatigue. This occurs because as your core temperature rises, skin and muscle capillaries dilate and sweating rate increases, lowering plasma volume due to ongoing dehydration. Exertional heat exhaustion may lead to a full collapse that can be exemplified by glycogen depletion and low blood glucose as a result of prolonged vigorous exercise. Exertional heat exhaustion is, therefore, your brain basically “putting the brakes on”. Signs and symptoms are nonspecific but include:
appearing sweaty and pale with low blood pressure but can also include headache, weakness, dizziness, heat “sensations” on the head or neck, chills, goose flesh/pimples, nausea, vomiting, diarrhoea, irritability, and decreased muscle coordination
Exercise-associated muscle cramps are triggered due to a loss of neuromuscular control and/or due to electrolyte imbalances and sodium depletion (see my separate deep-dive article at veohtu.com/cramp). They typically occur during long-duration and/or high-intensity sessions/races when you are pushing into territory unfamiliar to your fitness, training history, and terrain experience. After cramps, which can be intense and sometimes painful, following some rest and/or sodium-containing foods/fluid, immediate “return to play” is often (but not always) possible.
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But, how common are exertional heat illnesses in sport?
But, what puts you at risk of an exertional heat illness?
Heat illnesses can occur during short sessions/races, even ones that are too short to cause significant dehydration, which means that dehydration is sufficient, but not a requisite, for exertional heat illness. Heat stroke is more likely in shorter high-intensity sessions/races, up to 30-minutes, because the rate at which your core temperature rises is relative to your metabolic rate, which can be driven very high in short races. Heat stroke is less of a concern in longer bouts since it is very difficult to push your core temp so high because you cannot work so hard. Heat exhaustion is often more likely in longer events, partly because dehydration is also more of a concern during longer races since you have more time to sweat.
But…
The heat “stress” a runner will experience is influenced by the factors that allow (or prevent) heat to be removed from their body. High air temperature, high humidity, low air velocity (the sum of your speed and the wind speed), and intense solar radiation make strenuous exercise riskier because they all prevent heat from leaving your body. So too can dehydration because if your total body water “tank” is low, your skin cells won’t place water on your skin so it can evaporate — sweating — since this would cause further dehydration… Double trouble.
There are many other risk factors, including pregnancy, poor fitness, current illness, underlying disease, alcohol, drugs, supplements/stimulants, sunburn, even race start time (see a 2020 systematic review by Westwood et al. for an overview). But, because most of the factors that increase your core temperature and/or prevent you cooling down are all intertwined, it is often hard to pin down individual risk factors — exertional heat illness is complex!
Image Copyright © Julien Périard, Thijs Eijsvogels, and Hein Daanen (2021) Physiol Rev. All rights reserved.
Licensed under Open-access Creative Commons Attribution CC-BY 4.0.
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So, to exercise safely in the heat…
Be aware of the weather conditions.
Learn to recognise the signs and symptoms of exertional heat illness.
If you are aware and sensible and adequately hydrated (see my series on Hydration), you will likely avoid the issues associated with exercising in the heat. But, if you have any medical conditions and/or you are pregnant, you should consult your doctor before exercising in hot conditions.
Note: for a thorough overview of exertional heat illness, including signs & symptoms, and treatment, I recommend the 2007 ACSM Position Stand on Exertional Heat Illness during Training and Competition and the 2021 ACSM Expert Consensus Statement on Exertional Heat Illness: Recognition, Management, and Return to Activity.
What can you add to your hot training tool box?
You know that heat makes exercise feel harder and slows you down. But, hot and humid conditions also increase your risk of an exertional heat illness because sweating is insufficient to cool you down. While event organisers use weather metrics to inform their race day decisions, most summer races happen in hot conditions and you will most definitely train in the summer. Plus, some races (e.g. Marathon des Sables) often plough on in “too hot” conditions. Consequently, you will at some point or another find yourself training and racing in the heat.So, what can you do to train safely?
Be aware of your own body and be aware of how well heat adapted you are — if you’ve spent more than the last 2-weeks hiding from the heat by running in the cool, you’re likely not well heat-adapted (as you’ll learn in Part 3, it takes at least 2-weeks for full acclimation).
To stay safe in the heat, maintain adequate daily hydration so you are euhydrated before you leave the house and aim to stay hydrated during your sessions (see my series on Hydration).
Know the signs and symptoms of dehydration and exertional heat stroke and heat exhaustion.
If it's super hot, train earlier (for cooler and shadier times), go slower (for a lower rate of energy expenditure) and go shorter (for less total energy expenditure and less total exposure to the environment), all of which will minimise your risk of overheating and developing a heat illness.
During your sessions, monitor how you feel — if you get dizzy, lightheaded, uncomfortably hot, or develop irrational behaviour, that’s the time to stop. Thanks for joining me for this “hot” session. Until next time, keep training smart.
Disclaimer: I occasionally mention brands and products but it is important to know that I am not affiliated with, sponsored by, an ambassador for, or receiving advertisement royalties from any brands. I have conducted biomedical research for which I have received research money from publicly-funded national research councils and medical charities, and also from private companies, including Novo Nordisk Foundation, AstraZeneca, Amylin, A.P. Møller Foundation, and Augustinus Foundation. I’ve also consulted for Boost Treadmills and Gu Energy on their research and innovation grant applications and I’ve provided research and science writing services for Examine — some of my articles contain links to information provided by Examine but I do not receive any royalties or bonuses from those links. These companies had no control over the research design, data analysis, or publication outcomes of my work. Any recommendations I make are, and always will be, based on my own views and opinions shaped by the evidence available. My recommendations have never and will never be influenced by affiliations, sponsorships, advertisement royalties, etc. The information I provide is not medical advice. Before making any changes to your habits of daily living based on any information I provide, always ensure it is safe for you to do so and consult your doctor if you are unsure.
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About the author:
I am Thomas Solomon and I'm passionate about relaying accurate and clear scientific information to the masses to help folks meet their fitness and performance goals. I hold a BSc in Biochemistry and a PhD in Exercise Science and am an ACSM-certified Exercise Physiologist and Personal Trainer, a VDOT-certified Distance running coach, and a Registered Nutritionist. Since 2002, I have conducted biomedical research in exercise and nutrition and have taught and led university courses in exercise physiology, nutrition, biochemistry, and molecular medicine. My work is published in over 80 peer-reviewed medical journal publications and I have delivered more than 50 conference presentations & invited talks at universities and medical societies. I have coached and provided training plans for truck-loads of athletes, have competed at a high level in running, cycling, and obstacle course racing, and continue to run, ride, ski, hike, lift, and climb as much as my ageing body will allow. To stay on top of scientific developments, I consult for scientists, participate in journal clubs, peer-review papers for medical journals, and I invest every Friday in reading what new delights have spawned onto PubMed. In my spare time, I hunt for phenomenal mountain views to capture through the lens, boulder problems to solve, and for new craft beers to drink with the goal of sending my gustatory system into a hullabaloo.
Copyright © Thomas Solomon. All rights reserved.
I am Thomas Solomon and I'm passionate about relaying accurate and clear scientific information to the masses to help folks meet their fitness and performance goals. I hold a BSc in Biochemistry and a PhD in Exercise Science and am an ACSM-certified Exercise Physiologist and Personal Trainer, a VDOT-certified Distance running coach, and a Registered Nutritionist. Since 2002, I have conducted biomedical research in exercise and nutrition and have taught and led university courses in exercise physiology, nutrition, biochemistry, and molecular medicine. My work is published in over 80 peer-reviewed medical journal publications and I have delivered more than 50 conference presentations & invited talks at universities and medical societies. I have coached and provided training plans for truck-loads of athletes, have competed at a high level in running, cycling, and obstacle course racing, and continue to run, ride, ski, hike, lift, and climb as much as my ageing body will allow. To stay on top of scientific developments, I consult for scientists, participate in journal clubs, peer-review papers for medical journals, and I invest every Friday in reading what new delights have spawned onto PubMed. In my spare time, I hunt for phenomenal mountain views to capture through the lens, boulder problems to solve, and for new craft beers to drink with the goal of sending my gustatory system into a hullabaloo.
Copyright © Thomas Solomon. All rights reserved.