University of Missouri Extension

GH1900, Reviewed June 2009

Exercising in the Heat

Stephen D. Ball
Nutritional Sciences Department

In recent years, there have been several well-documented cases of highly conditioned athletes dying unexpectedly while exercising in the heat. Understanding what causes the body to overheat can help you prevent a heat-related illness.

What causes the body to overheat when exercising in hot environments?

The harder you are exercising, the higher your body temperature becomesAt rest, the body is able to balance heat production and heat loss so that its temperature remains around 98 degrees Fahrenheit. However, during exercise body temperature begins to rise as it converts the chemical energy from food into the mechanical energy necessary for movement. The degree of increase is directly proportional to the intensity of the exercise bout. In other words, the harder you are exercising, the higher your body temperature becomes. When you exercise in average indoor or outdoor conditions, the body is usually able to adjust to the increase in body temperature. However, during hot conditions the body loses its ability to radiate heat from its surface. In fact, when the air temperature exceeds body temperature (above 98 degrees Fahrenheit), the body actually gains heat from the environment. In this case, evaporation, or the conversion of a liquid (sweat) to a gas, becomes the body's only legitimate defense against overheating. Millions of sweat glands on the surface of the skin secrete large amounts of liquid, which when evaporated, help to cool the skin, which in turn cools the blood and ultimately the body. Serious problems can arise because high air temperatures stimulate excessively large amounts of sweat production, which if not replaced, can lead to a dehydrated state. Dehydration leads to higher body temperature. Severely dehydrated individuals can suffer circulatory collapse and death. Heat illness is divided into three incremental stages (heat cramps, heat exhaustion and heat stroke), which if not dealt with immediately, can lead to medical complications and even death.

What are heat cramps and what should I do to prevent them?

Heat cramps are involuntary muscle spasms that can occur during or following physical exertion. Generally, heat cramps result from an electrolyte imbalance due to excessive loss of salts. Direct pressure, massage and simple stretching of the cramped muscle should help alleviate the cramping. Heat cramps are the first sign of heat illness and are a signal to drink more water and/or decrease the intensity of physical exertion.

What is heat exhaustion and what prevents it?

Heat exhaustion is a more serious state of heat illness that usually occurs when a person has not become accustomed to exercising in the heat. Heat exhaustion especially can occur in someone who is not used to exercising in the heat if he or she has not consumed enough water before or during a workout. Heat exhaustion may or may not include collapse with or without consciousness. Symptoms include excessive sweating, cold clammy skin, normal or slightly elevated body temperature, paleness, dizziness, weak and rapid pulse, shallow breathing, nausea and headache. Anyone experiencing these symptoms should immediately stop exercising and get into the shade, or better yet, get inside an air-conditioned building. An abundant amount of fluid should be consumed. If the situation seems severe enough, elevate the feet and seek medical help.

What is heat stroke and how can I prevent it?

The most advanced stage of heat illness is called heat stroke. In simple terms, heat stroke is a breakdown of the cooling mechanisms of the body, induced by high body temperature. Someone who is experiencing heat stroke may stop sweating altogether and the skin will appear dry and hot. The pulse is usually strong but rapid. The person may have labored and difficult breathing. Body temperature can exceed 106 degrees Fahrenheit. Heat stroke is an emergency that requires immediate medical attention! Transport the person to the hospital as quickly as possible, ideally in wet sheets. In the meantime, cool the person by any means possible (hose down, ice packs, complete body submersion in cold or icy water). Remove as much clothing as possible and treat for shock by elevating the feet slightly.

My friends tell me that humidity causes heat-related illnesses and I say it is the temperature. Which of us is right?

Humidity or temperatureYou are both correct. During hot conditions the body loses its ability to radiate heat from its surface. Air temperature is a key factor in determining your risk for overheating, but the humidity level, or the water content of air, is equally important. Several deaths of football players have occurred with air temperatures below 75 degrees Fahrenheit at 95 percent humidity levels. High humidity levels reduce the evaporation of sweat from the skin. Even though it may seem that you are sweating a lot when it is humid, the sweat is mostly rolling off the skin and is not really contributing much to the cooling process. Sweat alone does not cool the body. It is the evaporation of sweat that cools the body. The combination of high air temperatures and high humidity levels increase the risk for heat illness to occur.


Measuring heart rate is often referred to as palpation. You can master it easily with a little practice.

  1. Measuring heart rateAt rest (sitting or lying down), take your first two fingers and locate the radial pulse. This artery is found on the thumb side of your wrist on the underneath part of the arm (near where you would wear your watch). Make sure you are using only the first two fingers and not the thumb, because the thumb can have its own inherent pulse. Also, don't press too hard or you could actually block the artery and you won't feel any pulse.
  2. After finding the pulse, look at your watch (or use a stop watch) and count the number of beats in one minute. This number represents your minute heart rate or your resting heartbeat. You could also just count the number of beats in 30 seconds and multiply by 2. The best time to get a true resting heartbeat is immediately upon waking up in the morning, before getting out of bed.

During exercise you may have to stop what you are doing to find your pulse. However, do your best to locate it without stopping. Once you locate the pulse, count the number of beats in 10 seconds and multiply that number by 6 to get your exercise heart rate.

For example, if you count 20 beats in 10 seconds then your exercise heart rate is 120 beats per minute

(20 x 6 = 120). Speed up or slow down, based on your target heart zone. Remember that your target heart rate zone is 55 percent to 90 percent of your maximum heart rate.

How can I determine my Target Heart Rate Zone?

First find your maximum heart rate (MHR) by subtracting your age from 220.

Next multiple (MHR) by 0.55 = lower end of the target heart rate zone.

Then multiply (MHR) by 0.90 = upper end of the target heart rate zone.

These two numbers represent your target heart rate zone. You should strive to stay between these numbers during your workout. Here is an example calculation for a 30-year-old woman:

220 - 30 (age) = 190 = maximum heart rate (MHR)

190 x 0.55 = 105 beats per minute

190 x 0.90 = 171 beats per minute

Target heart rate zone = 105 to 171 beats per minute

Avoiding heat illness

The most effective way to manage heat illness is to prevent it. The following guidelines offer simple steps for the prevention of heat-related illness:


GH1900 Exercising in the Heat | University of Missouri Extension


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