Physiological Adaptations to Exercise in the Heat

Part I: Physiological Adaptations to Exercise in the Heat

Exercising and physical exertion in hot weather puts greater demands on the body. It is more difficult for the body to produce the same force or work required for an activity when it also has to keep its own core temperature from getting too high. Normal body temperature is regulated to 97.7 to 99.5 deg F as it strives to maintain a heat balance. The hypothalamus, located in the brain, can be thought of as the body’s thermostat, or control center. It senses when body temperature is too high or low and triggers numerous physiological processes to return to normal temperature. If the body cannot return its core temperature to the normal range, serious damage or even death may occur. For this reason, it is imperative that we understand some of the science behind exercising in the heat, as well as how to recognize it in ourselves and others, and what to do when it happens.

 

Physiological adaptations to exercise in the heat can be thought of as short-term and long-term. Short-term physiological changes occur in the first few days of exercising in higher temperatures as an effort to return body temperature to normal as quickly as possible. These short-term adaptations are ones that most people have experienced first hand when exerting themselves in hot environments. Individuals who are unacclimatized to hot weather, meaning their body is not prepared to efficiently handle the heat, immediately experience excessive sweating to cool skin temperature and an associated increased heart rate. Internally, their veins constrict (vasoconstriction) to protect blood pressure which negatively affects cardiac output (how much oxygenated blood the heart is putting out). These short-term adaptations come at a cost, performance suffers as there is less energy being produced to perform work.

 

As more demands are placed on the circulatory system to help decrease skin temperature, the ability to transport oxygen to muscles is degraded. Blood plasma volume decreases and when this is coupled with increased fluid loss from sweating, can compromise total blood volume. To truly adapt to exercising in the heat, we need to exercise in the heat; simply spending time in hot weather may not be sufficient to being about necessary long-term adaptations.

 

Long-term adaptations to exercise in the heat can take up to a month or more. These adaptations include increased blood plasma volume, primarily plasma protein, which supports normal stroke volume and central blood volume.  Blood flow to the skin also decreases, sweating begins at a lower skin temperature, and sweating capacity increases. These adaptations help improve the heart’s ability to deliver oxygenated blood to the muscles while the body efficiently regulates its core temperature.

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There are several ways to help prepare for the demands of hot environments, even if you do not live in an area that gets hot or humid. After your normal workouts, commit to 20-40 minutes in a sauna or hot bath as your “cool-down”. This should be performed as much as possible for one to two weeks. MAKE SURE TO DRINK PLENTY OF WATER to ensure that you stay properly hydrated. To properly train for an event in a hot and/or humid environment, it is recommended to spend one to two weeks (or more) exercising in similar conditions, for 60-90 minutes at a time. This will give your body a similar stimulus that it will experience during the event and will help you practice your hydration, gear, and pacing strategy; think dress rehearsal.

 

The most important point about exercising in the heat is to pay attention to how much water you are losing and replacing. It is crucial to drink more water when in hot environments, and to ensure that you are consuming electrolytes as well. There is such a thing as drinking too much water, which can lead to a harmful condition called hyponatremia, which occurs when sodium levels are dangerously low. Good sources of electrolytes include Nuun tablets, Gatorade, SaltStick capsules, or Pedialyte.

 

Part II of this blog post will focus more on nutrition, hydration, and fueling strategies during exercise in hot and humid environments.

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