The Anatomy of a Runner: Thermoregulation

The body prefers a relatively stable temperature of 97.7– 99.5°F (36.5–37.5°C). Whether shivering from the cold or sweating from the heat, the body is attempting to maintain the core temperature close to 98.6° Fahrenheit. This process is called thermoregulation.

Early studies concluded we have a thermal circuit-breaker (also known as the Central Governor) that trips when we get too hot, and those studies limited sports research for some time. Subsequent studies, however, have shown that trained athletes are able to push their core temperatures higher than sedentary people. In fact, that thermal circuit-breaker seems to be triggered more by a perception of heat rather than the temperature itself, and our perceptions of heat are blunted by the mere presence of competition.

The first event of the 2016 UCI Road World Championships in Qatar was the women’s team time trial, a mid-afternoon 40-kilometer cycling race in temperatures averaging 98.4°Fahrenheit (36.9 C). Three cyclists from one of the teams swallowed ingestible core-temperature-sensing thermometer pills with their breakfast as part of a study to investigate the effects of hot-weather, competitive exercise. The researchers found that the three women had peak temperatures during the race ranging from 105.4° to 106.7°F (40.8 to 41.5 C).

It had long been accepted that if you ask an athlete to exercise for as long as they can in a hot environment, they’d quit when their core temperature reached somewhere around 104°F (40 degrees Celsius). These three cyclists all reached higher temperatures than the perceived threshold, yet they hadn’t collapsed. They won a medal.

CAPO VELO, Cycling Collective

Exercise produces heat that the body must eliminate so that it can maintain a stable core temperature and prevent over-heating. Exercising in hot conditions is even more challenging since the primary source of eliminating heat through sweating is less effective in hot and humid environments. If the body sweats so much that it depletes itself of fluids and salts, there’s nothing left to sustain the evaporation process. And when the process of regulating ceases, body temperature soars causing heat illnesses or even heatstroke.

Studies find that after a period of heat training/acclimatization, however, our bodies are able to produce more sweat and earlier, overall core temperature and blood lactate is reduced, blood plasma volume increases creating better cardiovascular fitness, skeletal muscle force increases, and we get better while training in a wider range of temperatures including cold weather. In fact, purposely training in the heat may be more beneficial than altitude training since we adapt more quickly to heat stress than to hypoxia (oxygen deprivation).

While many of these benefits can be obtained by simply living in the heat, exercising in heat speeds up the process. And there’s ways to mimic heat training, in case your next race is in a hotter climate than where you live.

This post discusses key points regarding the athlete’s response to heat; hydration, dehydration and sweat; heat versus altitude acclimatization; pre-cooling methodologies; and thermotolerance training techniques and guidelines.

Major Players

Brain: an almond-sized portion of the brain (the hypothalamus) is hyper-sensitive to changes in core temperature. If the core increases by even one degree, it reacts by opening blood vessels near the skin and routing blood to the periphery where it can cool. In an environment where the air, humidity, wind and sun feels warmer than 99.5°, the brain will limit contraction of the muscles as a way of telling the body to stop generating so much heat. This forces the athlete to slow down before becoming too hot.

Skin: As warm blood reaches the skin, pores expand and you begin to perspire. The sweat evaporates and cools the blood directly underneath. If the air is warmer than your core temperature, sweat is actually wasted and your condition worsens since the sweat fails to cool but contributes to dehydration instead. Pouring cold water onto the skin will help, but only temporarily.

Heart: When blood is over 98.6°, and more blood is being pumped near the skin for cooling, the heart is working harder, beating faster. Perceived effort will increase and recovery will be longer.

Of Special Note: Data from a multidecade study of 2,300 Finnish men found that those who hit the sauna four or more times a week were only a third as likely to develop dementia or Alzheimer’s compared with those who took just one sauna a week.

In a 2017 study from Qatar, participants showed a 17 percent boost in muscle strength after 11 days of sitting in a heat chamber at roughly 120 degrees for an hour at a time. The technique might be particularly relevant for injured athletes or those recovering from surgery as a way to maintain their muscles when they can’t exercise. (Alex Hutchinson, OutsideOnline).

Dehydration and Sweat

While fluid plays a role in heat, it is actually more minimal than we may realize. When athletes are allowed to pace themselves in trials where they are limited to small volumes of fluid or do not drink at all, they reach the same core temperature as when fluids are consumed, but they take longer to finish. It’s not necessarily the fluid ingested that keeps us cool, but the metabolic rate, or how hard we are exercising that affects core temperature. The guiding principle here is to always drink to thirst.

Sweat rate also has nothing to do with the rate we burn fat or calories. An individual’s perspiration rate is mostly dependent on genetic make-up, training, and how the body responds to heat stress.

Some people lose more fluids than others, and men perspire more than women. Testosterone can enhance the sweating response, as will anti-depressant, anti-anxiety, allergy, decongestants, and weight loss medications. Caffeine has a similar effect.

Urine color is determined to be a simple way to assess hydration. Observe urine over the course of a day and notice changes in flow and color. Volume and frequency should be consistent and the color should be lighter, or close to clear, toward the end of the day.

Why It Hurts

Core Temperature = Heat Production vs Heat Loss

Heat is produced when muscles contract and is directly proportional to how fast you are running. Run two times faster, twice as much heat is produced. Consequently, it’s the shorter, more intense races that produce higher core temperatures.

Heat loss depends on evaporation, convection and radiation with the environment being the crucial factor:

– high humidity prevents evaporation,

– high air temperature prevents both evaporation and convection from cooling the body.

Runners learn to push through the pain, but to successfully push through the pain means we must also understand the warning signs that would spell disaster in any given situation.

If our perceptions of heat are blunted by the mere presence of competition among other things, overheating could become the unintended outcome. Know the symptoms of overheating: headache, dizziness, disorientation, nausea.

The Gender Gap

In 2011, VF Corporation, the parent company of Smartwool and The North Face, commissioned a 1,200-person study examining how women and men respond to exercise in hot and cold temperatures.

Their findings show that women run warmer than they perceive. In winter, women’s coldest zones are the backs of the hands, the glutes, outer arms, and kneecaps. Women’s upper backs, calves, collarbones, and pelvis emit more heat during stop-and-go cold-climate activities, such as skiing. During hot-weather exercise, women’s legs are markedly cooler than their upper bodies, while men are more evenly balanced. Women’s feet are always colder than men’s, regardless of the outside temperature.

Acclimatization and Thermotolerance

Thermotolerance is the end result of a successful program of heat acclimatization, where an athlete trains with the specific purpose of making the body functional in a warmer climate to which the athlete is accustomed. (Encylcopedia.com)

Acclimatization methods consist of two types: heat and altitude.

The body undergoes a natural acclimatization to warmer temperatures or higher altitudes, known as passive acclimatization. It is possible to speed up this process through a gradual buildup in training volume, known as active acclimization. While both heat and altitude alone are stresses to the body that will contribute to the acclimatization process, heat or altitude without exercise will not be as effective.

Altitude Acclimatization develops the ability of the athlete to better utilize oxygen, which makes them more effective at sea level competition. At higher altitudes (≥8,000 ft / 2,500 m), the body compensates for the decrease in available oxygen by increasing its production of red blood cells, which transport oxygen through the body. Altitude training will increase oxygen capacity by between 2% and 3% in a period of about three months. Although this benefit will remain for several weeks in an ever-decreasing amount, it will be completely lost within three months of returning to lower altitude.

Note: Altitude training is broken further into three types: “live high/train high,” where the athlete both lives and trains at altitude; “live high/train low,” a regime where the athlete lives at altitude but trains at sea level; and sea-level training, where the reduced oxygen environment of higher altitudes may be replicated through an artificially configured house or training “tent.” The extensive scientific research regarding altitude training confirms that all three methods will enhance sea level performance.

In the Heat Acclimatized athlete, cardiac function improves resulting in increased plasma blood volume accompanied by a 15-25% decrease in heart rate. This means there’s more water in the blood stream that can be used by the sweat glands to produce more sweat. Thinner blood means it can also transfer heat more effectively to the skin. (Note: the systems of the body adapt to heat exposure at varying rates.)

Heat acclimatization also reduces muscle glycogen utilization and post-exercise muscle lactate concentration. Chris Minson, a professor of human physiology at the University of Oregon who studies heat acclimation responses in athletes, has also found that changes to the heart’s left ventricle specifically helps to increase oxygen delivery to the muscles.

Hot and dry environments are different from hot and humid environments (desert vs jungle) – sweat rates being higher in humid environments (the rate of sweating influences thermoregulation). Acclimation is also dependent on the volume of exercise, intensity, and how long the core temperature remains elevated.

In a nutshell, heat acclimatization causes the body to shed fluids sooner by sweating sooner, lowering the core temperature, and making the athlete more comfortable – perception of effort being key to exercising longer (a decrease in ‘perceived exertion’ occurs during the first five days of exercise-heat exposure). An added benefit is that many of these adaptations will be useful even in cool weather training.

The human body is very adaptable to heat, and to corresponding humidity, with the major benefits achieved within 10 to 14 days of beginning a heat training program; most athletes will reach an acclimatization of approximately 75% (defined as an ability to perform to 75% of their top level) within five days. If the athlete is not exposed to warm weather conditions on a regular basis, however, the body will require another acclimatization period. On the bright side, re-acclimatization occurs more rapidly than the initial acclimatization when re-exposed to heat (Weller et al., 2007).

The chart below compares the benefits achieved during heat training as compared to passive acclimatization (no exercise) and exercising in cool conditions.

The Central Governor Effect (and sometimes lack thereof)

In a 2012 study, the negative effects of cycling in 89-degree heat were partly erased when the thermometer in the room was rigged to read 79°F. In other studies, athletes react to hot conditions when their skin temperature is warmer to the feel even though their core temperatures were actually lower. There’s also research that suggests our perception of effort is lower in competition, partly because our attention is focused on the competitors rather than our own pain.

Exercise physiologist, Jo Corbett, and a team at the University of Portsmouth put cyclists through a series of 20k time trials in cool and hot conditions, with and without competition. The cyclists hit higher temperatures during the competition than when they were soloing in the heat, although their ‘perceived’ measurements were the same. ”Thermal sensation” (how hot they felt) was the same, as was “thermal comfort” (how pleasant or unpleasant the heat felt). Racing against a competitor created a disconnect between how hot they were and how hot they felt even though they cycled faster and generated more power during the head-to-head competition in hot conditions.

When athletes in one study were equipped with a small electric heat pad tucked in the pocket of their shirt, they gave up 9 percent sooner even though none of the physiological measurements – blood lactate, core temperature, skin temperature, heart rate, stroke volume, cardiac output, oxygen uptake, ventilation – were different. They simply quit because they felt hot.

This doesn’t change the fact that our bodies undergo significant added stress while exercising in the heat, but it may be useful to know that a great deal of the pain is purely psychological.

The Pre-Cooling Option

Numerous studies have shown that pre-cooling before prolonged exercise in hot temperatures may help sustain intensity and speed, however, definitive conclusions on its effectiveness have not yet been established.

Methods of pre-cooling include whole-body cold water immersion (17-30°C for 30 minutes); cold air exposure; cooling garments; cryotherapy; and internal cooling methods, such as cold beverages, ice slurries, and ice bars.

Some athletes follow the low-tech protocol of simply applying ice packs – to the back of the neck, chest, underarms or between the thighs – with preference to areas with the highest blood flow. Because thermoregulating the brain is essential (and ‘perception’ is everything), ice on the neck significantly relieves perceived heat stress. One study also found a 20% increase in cycling power during an intermittent sprint when ice was placed between the thighs.

Nike sent ice vests for pre-cooling to select olympic athletes competing in Athens in 2004 and Beijing in 2008. (One of the athletes, runner Meb Keflezighi, won the silver medal at Athens). Here Rafael Nadal is wearing the Nike cooling vest during a practice at Flushing Meadows. Courtesy: rafaelnadalfans.com

Athletes sometimes report feeling heavy or sluggish following whole-body cold water immersion. An alternative is to expose just part of the body to cold water by soaking garments in cold water, or submerging specific active or inactive body parts directly in cold water (such as the hands or legs).

Practicality is a logical consideration when choosing a pre-cooling protocol.

Pre-cooling:

Hampers the performance of sprinters

Better for sports with intermittent sprints

Best for endurance events, triathlons, cycling races or marathons

* Pre-cooling has also been shown to improve performance in lower ambient temperatures.

Pre-Cooling tips:

  • Make a reduction in skin temperature your major goal;
  • Aim to pre-cool for 8-30 minutes;
  • Practice your chosen pre-cooling technique before using it on race day.

Heat Training Protocols

Heat training approaches can be as simple as running outside when it’s hot; using the thermostat to create a hot environment indoors; wearing extra clothing that is certain to make you run hot; or spend time in a sauna, hot bath or hot tub post-workout.

Because heat is an added stress, however, any protocol that separates the stress of heat from the workout allows the quality of the workout to be preserved. When training in heat, training volume and/or intensity would be reduced initially as the body adapts.

A 2015 study shows that using a six-day, 104°F post-run hot tub protocol was effective at triggering heat adaptation, including a 4.9% improvement in 5k time in 91° heat. The advantage of using a sauna or hot tub is that it prolongs the amount of time the core temperature remains elevated (going for a run in normal conditions elevates the core temperature, and the hot tub prolongs this period of time).

Here’s a graph that shows core temperature (38.0 C is 100.4 F; 40 C is 104 F) at the end of a 40-minute hot run before and after the hot tub protocol:

Heat Training Guidelines

The most successful heat training programs will follow a progression:

  • Training volume and training intensity are reduced initially.
    Both volume and intensity are increased as the athlete begins to adapt.
    Exercise extreme care to ensure proper hydration is maintained at all times – before, during and after training sessions. When dehydration or salt deficits exist, cardiovascular and thermoregulatory responses may be negatively affected, and the theoretical risk of heat illness increases.
    Increase the sodium in your diet for the first few days. Sodium helps the body retain necessary fluid for temperature regulation.
    Take breaks to allow the body time to cool down.

Ultra running coach Jason Koop says, “at a certain level, you have to compromise training quality for the heat acclimation. Acclimating to the heat is additional stress [on the body], just like more miles or intervals, so you can’t simply pile it on. Something on the training side has to give.”

If you want to incorporate heat into your workouts, here’s how Koop recommends doing it safely.

1. First, pick a protocol (sauna, hot bath, or exercising in the heat) that minimizes the impact on training, both physically and logistically.

2. Koop most commonly recommends that his athletes use a dry sauna immediately after running. “It doesn’t impact training nearly as much as running in the heat, and the effects are similarly positive,” he says. He often tells his athletes to not drink water during these sessions to enhance the effect. Koop recommends spending 20-to-30-minutes in the sauna, depending on tolerance.

3. Koop says that when he has his athletes exercise in the heat—either naturally or by wearing extra clothing to simulate the experience—it will be on a long, slow day for 60 to 90 minutes. The time completely depends on the athlete’s tolerance and previous experience. But he stresses to not do this on a recovery day, because heat training is an added stress on the body. Koop recommends drinking 30 to 40 ounces of an electrolyte drink per hour during these sessions And for safety, he advises using low-traffic sidewalks and bike paths—not trails.

4. Despite the benefits of heat training, Koop reminds his athletes that running in the heat is extremely difficult and usually replaces a hard day. “You are substituting one potential gain for another one,” he says. In other words, use it carefully.

Know the risk factors of heat illness:

●Strenuous exercise in high ambient temperature and humidity

●Lack of acclimatization

●Poor physical fitness

●Obesity

●Dehydration

●Acute illness

●External load, including clothing, equipment, and protective gear

This post is not intended nor recommended as a substitute for medical advice, diagnosis, or treatment. Always seek the advice of your own physician or other qualified health care professional regarding medical questions, concerns, and before beginning any new training regimen.
References and Additional Reading:
The Effect of Head-to-Head Competition on Behavioural Thermoregulation, Thermophysiological Strain and Performance During Exercise in the Heat; Sports Medicine May 2018
Competitor presence reduces internal attentional focus and improves 16.1km cycling time trial performance. Sports Medicine
The Psychological Side of Heat Exhaustion; OutsideOnline, Alex Hutchinson, Apr 2018
Heat Slows You Down, Even When It’s Not Real. OutsideOnline, Alex Hutchinson Feb 2019
Heat Acclimatization to Improve Athletic Performance in Warm-Hot Environments; Gatorade Sports Science Institute, Michael N. Sawka, Julien D. Périard, Sébastien Racinais Jan 2016
Is Seltzer Water Just as Hydrating as Regular Water? The Food Network
How much am I supposed to sweat during a workout? OutsideOnline, Cassie Shortsleeved Jun 2015
The Central Governor and the Athlete’s Clock: Pacing and performance; The Science of Sport, Mar 2011
7 Shocking Facts About Drinking Water Cold vs Room Temperature; Bustle, Carolyn Steber Feb 2019
The Performance Lab Studying How Women Sweat; OutsideOnline, Kelly Bastone Jan 2019
Dear Sports Scientists: Will drinking fluids keep me cool? The Science of Sport Sep 2010
Thermotolerance induced at a mild temperature of 40 °C alleviates heat shock-induced ER stress and apoptosis in HeLa cells; Science Direct, Jan 2015
A Post-Workout Soak Might Boost Performance; OutsideOnline, Alex Hutchinson Jan 2019
How Elite Athletes Respond to Extreme Heat; OutsideOnline, Alex Hutchinson Dec 2018
Is Sunscreen the New Margarine?
The Benefits of Sweat; Care2.com Jul 2015
Exercise and Thermotolerance; Encyclopedia.com
HEAT ACCLIMATIZATION; Lawrence E. Armstrong, Ph.D. Department of Sport, Leisure, and Exercise Science, University of Connecticut
The Surprising Benefits of Training in the Heat. Is heat better than altitude? The science seems to say so. Meaghen Brown Jul 2016
How Heat Therapy Could Boost Your Performance; OutsideOnline Aug 2018
Does Precooling With Whole-Body Immersion Affect Thermal Sensation or Perceived Exertion? A Critically Appraised Topic; Timothy M. Wohlfert and Kevin C. Miller, School of Rehabilitation and Medical Sciences, Central Michigan University
Pre-cooling for endurance exercise performance in the heat: a systematic review; Paul R Jones, Christian Barton, […], and Stephanie Hemmings
A study done in 2006 found that runners wearing Nike Ice Vests before a cross country race had a lower core temperature after the race (Warming Up With an Ice Vest: Core Body Temperature Before and After Cross-Country Racing, 2006. Ian Hunter, et al.).