Heat Acclimation

Original Editor - Kapil Narale

Top Contributors - Kapil Narale, Uchechukwu Chukwuemeka and Vidya Acharya  

Introduction[edit | edit source]

Heat Acclimation is the body's process of adapting to the heat. Heat acclimation is a process that can occur to individuals at any age. Individuals who desire to achieve maximal heat acclimation must exercise in the heat, despite them being able to achieve heat acclimation in a cool environment. Exercising in the heat poses a demand in maintaining normal body temperature and fluid balance. [1]

Heat acclimation occurs when the body physiologically adapts to the hot environment and can maintain homeostasis. The adaptations improve exercise tolerance in hot environments and reduce the risk of heat injury. [1]

Heat acclimation can occur within 1-2 weeks. [1]

The body temperature needs to be regulated within a homeostasis value of roughly 37 degrees Celsius since cellular structures and metabolic pathways are affected by body temperature. An increase in body temperature to 45 degrees Celsius can be quite dangerous to the body, as it can lead to an inability to produce cellular energy, and lead to cellular death, and death of the organism. Therefore heat loss in the body needs to equal heat gain in the body. [1]

The homeostasis of the body can be referred to as the human 'furnace' or 'thermostat'. The body's temperature is relatively high and is regulated with gradual heat loss. [1]

Here is a video discussing training in the heat. It explains exercise physiology, thermic balance (body temperature regulation), cardiovascular adaptation, endocrine adaptation, hydration, and how the body adapts to heat training.


Exercise in a Hot Environment[edit | edit source]

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Hot environmental conditions decrease performance in both submaximal (marathon or triathlon) and high-intensity exercise (rugby or soccer). [1]

There are three major contributors to impaired performance in the heat: [1]

  1. Quicker muscle fatigue
  2. Impaired cardiovascular function
  3. Impaired central nervous system (CNS) function

These factors can separately contribute to impaired exercise performance, though they are more likely to work together to result in impaired exercise performance. These contributions depend on the exercise duration and intensity. [1]

Quicker muscle fatigue[edit | edit source]

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There are three major changes that occur in muscle metabolism that interact to cause muscle fatigue: [1]

  1. Muscle glycogen breakdown is accelerated - this contributes to muscle fatigue during prolonged exercise
  2. Muscle lactate production is increased with exercise in a hot environment, thus decreasing pH and contributing to muscle fatigue
  3. Free radical production in skeletal muscles is increased with exercise in a hot environment, which can also contribute to muscle fatigue

Muscle fatigue can contribute to CNS dysfunction due to messages to the brain from the fatiguing muscles, which leads to a reduced central motor drive. [1]

Impaired Cardiovascular Function[edit | edit source]

Exercise in a hot environment can produce an increased heart rate, decreased stroke volume, and decreased muscle blood flow. Without dehydration, in light or moderate exercise, muscle blood flow is regulated, thus doe not lead to impaired exercise performance. However, there is a gradual decrease in muscle blood flow with high-intensity exercise, due to a demand for blood between the working muscles and the skin. Since more of the blood flow is going towards the skin, to cool the body, and not the working muscles, this can lead to impaired exercise performance. Due to the reduced muscle blood flow in the heat, there is reduced oxygen delivered to the muscles, which would lead to quicker muscle fatigue. [1]

Prolonged exercise in the heat also causes a reduced VO2 max. This may be caused by the inability to reach maximal cardiac output relative to a VO2max test at normal conditions. This is caused by a reduced cardiovascular reserve in response to an increased skin and core temperature. With an elevated skin blood flow with prolonged exercise in the heat, the reduced VO2max would cause a higher relative metabolic intensity at an absolute work level. [3]

An increased physiological load in the heat will definitely increase the rate of perceived exertion, and discomfort from the heat, therefore decreasing exercise performance. [3]

Impaired Central Nervous System[edit | edit source]

An impaired central nervous system can cause dehydration and hyperthermia, thus impairing exercise performance. Hyperthermia reduces the neural drive for motor unit recruitment. [1]

See the Heat Illness in Sports page for an explanation of hyperthermia and other sports related heat illnesses.

Physiology[edit | edit source]

If heat loss is less than heat gained, then there is an increase in body temperature, thus body temperature rises. This is the case where homeostasis needs to be maintained. Contracting skeletal muscle produces a large amount of heat, thus prolonged exercise in hot environments poses a challenge to maintaining homeostasis. [1]

An important function of the circulatory system is to transport heat. Blood is a facilitator in transporting heat, since it has a high capacity to store heat. When the body wants to lose heat, blood flow is increased to the skin to facilitate heat loss to the environment. [1]

Individuals who are heat acclimated have a lower body temperature and a lower heart rate with submaximal exercise. It is recommended that individuals should partake in strenuous interval training or continuous exercise at an intensity higher than 50% VO2max to facilitate the higher core temperature since a higher core temperature is what drives heat acclimation. [1]

Initial physiological changes that take place with heat acclimation are: [1]

  • Increased plasma volume
  • Earlier onset of sweating
  • Higher sweat rate
  • Reduced salt loss in sweat
  • Reduced skin blood flow
  • Increased synthesis of heat shock proteins

As mentioned heat acclimation can occur quickly, in as fast as 1-2 weeks.

Some of these physiological changes will be explained:

There is a 10-12% increase in plasma volume. This maintains central blood volume, stroke volume, and sweat capacity, and allows the body to store more heat with a smaller temperature gain. [1]

There is an earlier onset of sweat production and an increased sweat rate. This indicates that sweating begins right after the exercise starts, with less heat storage at the beginning of exercise, and decreased core temperature. In fact, heat acclimation can increase the sweat volume to almost 3x the amount without being acclimatized to the heat. [1]

Higher amounts of evaporative cooling in the body are possible, which is a benefit in minimizing the storage of heat during prolonged work. [1]

Environments with high heat and humidity make the body less able to lose heat by radiation/convection and evaporation. The inability to lose heat produces a greater core temperature and a higher rate of sweat loss, compared to a more comfortable environment. With fluid loss and a high core temperature, there is a risk of hyperthermia and heat injury. [1] More information on heat injuries and illnesses is discussed on this page, Heat Illness in Sports.

As mentioned on the Physiology of Sweat page, the only way to lose body heat, when exercising in hot environments, is through the evaporation of sweat.

See the page Physical Activity and Perspiration for a section on sweat rates and exercise.

Effects[edit | edit source]

Heat gain of the body, and thus effects of exercising in the heat, would occur when environmental temperatures are greater than skin temperatures. [4]

Exercise in the heat, for 10-14 days at,

  • low-intensity, <50% VO2 max, and long duration, 60-100 mins, or
  • moderate to high intensity, 75% VO2 max, and a short duration, 30-35 mins,

results in, [4]

  • increased plasma volume, blood flow, perspiration
  • increased VO2 max, maximal cardiac output, and power output at the lactate threshold
  • reduces body temperature and heart rate responses with submaximal exercise
  • reduces salt loss in sweat and the occurrence of sodium depletion
  • increases aerobic fitness capacity and performance

This is the process of acclimation and helps with safeguarding against heat illnesses.

As noted the effects of heat acclimation take about 7-14 days of consecutive exercise to occur. Individually, the timeline to acclimation of these effects are: [1]

  • Decreased heart rate, within 3-7 days
  • Increased plasma volume, within 3-6 days
  • Decreased perceived exertion, with 5-9 days
  • Increased sweat rate, within 8-14 days

Benefits[edit | edit source]

As mentioned above, with the higher sweat production and rate from heat acclimation, the storage of heat is minimized with prolonged activity, and there is a higher rate of evaporative cooling in the body. [1]

With heat acclimation, there is a lower loss of sodium and chloride, from an increased secretion of aldosterone. Despite this adaptation resulting in a lower loss of electrolytes during exercise, there is still definitely a need to replace the water loss during exercise, especially prolonged exercise in the heat. [1]

With becoming heat acclimated, not only does one have a lower body core temperature and heart rate throughout their exercise duration, there is a decreased rating of perceived exertion and an improved exercise performance in the heat. [1]

Heat Shock Proteins[edit | edit source]

With heat acclimation, there is cellular production of heat shock proteins in skeletal muscle fibers, the heart, and all other cells in the body. These 'stress' proteins are produced as an adaptation to heat stress, and they protect from cellular damage from stresses such as heat. They protect cells from thermal injury by stabilizing and remodelling damaged proteins. [1]

Risks of prolonged exposure to the Heat[edit | edit source]

As mentioned on the Heat Illness in Sports page, prolonged sporting activities in the heat can cause adverse reactions, which could even result in death. It is important to recognize signs and symptoms, and prevent the onset of these illnesses rather than treat or manage the conditions. [4]

Loss of Acclimation[edit | edit source]

Similar to the use it or lose it principle, there is a rapid loss of heat acclimation with the cessation of exercise. [1]

There is a decrease in heat tolerance within a few days of no activity in the heat. Heat tolerance is significantly reduced within 7 days of no heat exposure. Heat tolerance is completely removed after 28 days without heat exposure. [1]

To maintain heat acclimation one must have continuous or repeated exposure to the heat. [1]

Application to Sports - Running[edit | edit source]

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With running in the heat, the exertional stress on the body will depend on the distance/intensity and duration of the run. [3]

Middle distance runs, from 800m to 5k are at a relatively high intensity than a longer distance, producing a higher metabolic rate of heat production. Since the run times of these distances are relatively short, the risks of exertional heat illnesses are quite low. Performance in these types of events can be related to dehydration, fatigue, or illness, rather than environmental conditions. [3]

Longer distances, from 10k-42k, would be classified as lower intensity, with a lower rate of metabolic heat storage and an increased heat storage time. Crucial factors with the longer distance and lower intensity include physical fitness, heat acclimation, and fluid imbalances. Since the skin and the contracting muscles are both in demand for blood, as already mentioned, gradual dehydration is common during long-duration running in the heat. However, if a runner is over-hydrated, and their body fluid is in excess, they can experience hyponatremia, low sodium concentration in the blood (also explained on the Physical Activity and Perspiration page). [3]

With ultra-distance races, above 26.2 miles (42km) up to 100 miles, the intensity is even lower than in a marathon, and these runners are well trained, heat acclimatized, and well hydrated with a race hydration practice. Ultramarathoners can experience different challenges which can encompass muscle damage, changing environmental conditions, or loss of sleep, significantly if such races exceed 24 hours. [3]

As mentioned, duration and intensity are important to consider, since inappropriate pacing for a certain individual may be hazardous in a hot environment. Proper precautions and training need to be undertaken to prevent any heat-related illnesses. [3]

Working in the Heat[edit | edit source]

Here is a video describing heat acclimation, when it comes to working in the heat:


References[edit | edit source]

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.22 1.23 1.24 1.25 1.26 1.27 Powers, Scott K. Howley, Edward T. editors. Exercise and the Environment. Exercise Physiology - Theory and Application to Exercise and Performance. 10th Ed. New York: McGraw-Hill Education. 2018. p269-292.
  2. The Movement System. Training in the Heat | Hydration, Cardiovascular Adaptation, and Heat Acclimatization. Available from: http://www.youtube.com/watch?v=jzEbj3w8bDY [last accessed 21/7/2023]
  3. 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Casa Douglas J. Sport and Physical Activity in the Heat - Maximizing Performance and Safety. Storrs: Springer International Publishing, 2018.
  4. 4.0 4.1 4.2 Powers, Scott K. Howley, Edward T. editors. Exercise and the Environment. Exercise Physiology - Theory and Application to Exercise and Performance. 10th Ed. New York: McGraw-Hill Education. 2018. p548-572
  5. Centre for Research in Occupational Health and Safety (CROSH). Heat Acclimatization Overview. Available from: http://www.youtube.com/watch?v=RR9n-ca1Kj4 [last accessed 21/7/2023]