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Posted: July 26, 2005

Athletics: Beat The Heat

By Kathryn Dempsey

You have been through it before, and it is no fun – having to cut a nice jog or even a race short on a hot sunny day because you’ve overdone yourself in the heat. You may feel the need to stop, sit down in the shade, and sip some cool water for quite some time before you feel yourself again. In extreme cases, runners may need medical attention if they require intravenous fluids or ice-packing to cool their overheated bodies. Why does the sun have such an ability to zap us of our strength and energy? Though you may have the will power to muscle through the discomfort, exercising too hard in hot weather can be dangerous. Fortunately, you can protect yourself against various types of heat illness with some basic scientific knowledge. In this article, you’ll discover the physiological effects of the sun on your body and the hyperthermia (a.k.a. overheating) warning signs to look out for. But first, you’ll get a crash course on the basics of thermoregulation – that is, maintenance and control of body temperature – that occur during exercise.

How Your Body Keeps Cool

Your miraculous body has an impressive ability to maintain a relatively constant internal body temperature. For most people, body temperatures do not fluctuate more than 1.8º F under normal circumstances. Throughout life, your body stays within the range of 97.0º F to 100.0º F except in the case of extremely hot or cold conditions, illness, or prolonged intense exercise. Lucky for us runners, humans are able to tolerate a reasonable amount of exercise in heat because of our well-adapted thermoregulatory power, a power that keeps our body temperature in check.

In order to stay cool, your body keeps a balance between the heat you gain (that which your body produces and that which you absorb from the environment) and the heat you lose. You can lose heat from your body in any of four ways: Conduction, Convection, Radiation, and Evaporation.

Conduction is the transfer of heat through direct contact. That is, body heat travels from your core, molecule-to-molecule, cell-to-cell, tissue-to-tissue, until it reaches your skin, where it is passed to clothing or air molecules that touch your skin.

Convection is the movement of heat with the help of a gas or liquid. When air moves across your skin (wind, for example), it picks up heat from your skin and carries it away from our bodies. Similarly, when you swim in water that is cooler than your skin, heat is swept from the skin as the water moves across you.

Radiation is the transfer of heat in the form of electromagnetic waves, in this case infrared rays. If you have a higher temperature than your surroundings, you will radiate heat and your temperature will decrease.

Evaporation is the most important method by which your body sheds excess heat during exercise! When water on your body is in contact with air (in your lungs, at the lining of your mouth, and on your skin when you are sweating), excess body heat is used to vaporize the water, and you are cooled in the process.

How Circulation Plays A Part

Apparent in these descriptions, heat needs to be at the skin’s surface in order to be shed from the body; yet, most heat is produced within your muscles and tissues. Although conduction helps with this predicament, the most efficient way to transport heat from deep in your body to your skin is with your blood. This happens naturally with your circulation. The greater the quantities of blood near your skin, the more heat is transported to your skin. Thus, controlling the blood supply to the skin is one part of how your body regulates temperature while you exercise in the heat. The other method by which heat is exerted is through the activation of your sweat glands – this evaporation is the main avenue of heat loss when you run in the sun. Table 1 illustrates the relative significance of the four methods of ridding your body of excess heat:

Mechanism of Heat Loss: (1)
 % Heat Loss At Rest % Heat Loss During Exercise
Conduction and Convection 20 15
Radiation 60 5
Evaporation 20 80

This table calculates averages, as the numbers will vary with body size, fitness level, and weather conditions. But it still displays that conduction, convection, and radiation play much less of a role during exercise than at rest. This is because in warmer weather conditions, the temperature of your skin and core is not much more than that of the outside environment, so heat transfer with these methods becomes much less efficient. You may even gain heat, for example, due to radiation from the sun. So when you go running in the heat, you rely primarily on the evaporation of your sweat to protect you from overheating!

What Heat Can Do To Your Running

Now lets take a look at what happens when you run in the sun. Say it’s a clear, 75ºF day with 20% humidity and no wind. As you jog along, getting a nice sock tan, your body heats up simply from the physical exertion – you gain metabolic heat. Also, since there is no cloud cover to diminish the sun’s intensity, you gain even more heat from the sun’s radiation, and perhaps even more from hot pavement and other objects. Since there’s no wind, you are losing very little heat through conduction and convection. It is inevitable that your body temperature begins to rise.

Next comes the crucial role of the hypothalamus. The hypothalamus, located in your brain, receives impulses from sensory receptors in your skin relaying information on the environmental temperature. It also keeps track of the temperature of your blood as it passes through your brain. With this information, your hypothalamus tells your body what to do to keep the temperature consistent. In our example, it would tell the blood vessels near your skin to dilate (vasodilation), consequently allowing more heat to flow to the skin. It would also activate your sweat glands, so that you could begin to bring your body heat back down by way of evaporation.

Unfortunately, even with your fine-tuned natural thermostats working as they should, you can’t compensate 100% when you’re running in the aforementioned hot weather condition. This is due to a competition for blood supply between your skin and your muscles. Once the hypothalamus has kicked in, a large amount of blood is directed out to your skin. This leaves less blood for the rest of your body. As you know, running requires more blood to be directed to the working muscles – and so you’ve set up a bit of a pickle. At any given time, there is a lot of blood both in your working muscles, and near your skin; therefore the volume of blood returning to your heart is less than it would be without vasodilation near the skin. Thus, your stroke volume is reduced. Stroke volume is the amount of blood that is pumped out of your heart’s left ventricle when it contracts, sending blood to your body.

Don’t worry…yet; the human body is an amazing opus. Even with less blood being pumped by your heart with each contraction, cardiac output (which is the volume of blood pumped out by the heart per minute) will generally remain constant for up to 30 minutes of exercise in warm weather. This is due to a gradual increase in heart rate throughout your exercise bout. However, your body will reach a point when the cardiovascular system becomes overburdened and you have to stop running, or at least slow down. The heart can’t comfortably pump sufficient blood to fuel your muscles’ efforts while still diverting the proper amount of blood to your skin (to keep the body from overheating). All the while, your blood volume is decreasing as you lose more and more fluid to sweat, making it even harder to supply blood where it is needed.

Exercising beyond this point is unwise! It may be difficult to know when you have reached this point, but there are some general symptoms to keep in mind on hot days when heat stress and dehydration can occur.

The Dangers Of Heat

Heat Cramps – this is the least severe of the three main heat-related disorders. It usually affects muscles that are used the most, so in running perhaps your gastrocnemius, hamstring, calves, or quadriceps. The muscle becomes dehydrated and depleted of minerals, and it cramps painfully.

Heat Exhaustion – this disorder generally occurs when the total blood volume decreases to a point where the cardiovascular system is overwhelmed. The body has lost so much fluid and minerals through sweating that it can not supply both your working muscles and your skin with enough blood to both fuel and cool. Your body temperature rises, and headache, goose bumps, pale and cool skin, dizziness and weakness may set in.

Heat Stroke – this degree of heat disorder requires immediate medical attention! The body temperature has risen to more than 104ºF; your body stops sweating and your skin is hot, breathing and heart rate become rapid, and you become confused or even unconscious. If untreated, this condition can lead to nerve damage or even death.2

Clearly, the repercussions of pushing yourself too hard for too long in the heat can be grim. Be sure that if you feel any of the symptoms coming on, stop, get to a shady area, rest and drink some cool fluids. Always seek medical help if you’re seriously concerned; better safe than sorry.

Knowledge Is Safety

In extreme heat conditions, performance is hindered primarily by cardiovascular limitations rather than glycogen depletion in the muscles. Thus, proper fluid intake should be your priority. The amount of water you need is unique to the individual and there are many expert opinions on the precise number of ounces per body weight. If you have found a formula that is comfortable and safe for you, stick with it. If not, a good rule to go by is simply listening to your stomach. Drink plenty of water before the run, but if your stomach feels full or if it sloshes when you move around, stop until your body has absorbed some of the water that has been consumed. Throughout the workout, you should intake fluid every 10-15 minutes. In order to replenish the electrolytes that are lost through sweating, and also to increase palatability, use a sports drink. A good sports drink will have 15-20 grams of carbohydrate, or 60-80 calories, per 8 ounces. A drink that is more concentrated is likely to upset your stomach, especially while running. Food is also important in that you do not want a large, heavy, warm meal right before you work out.

Workout time in the heat should be limited to one hour; additional time may subject you to the “danger zone.” If it is exceptionally hot, try to take a few short cooling breaks during which you may rest for a minute under a shady tree and drink water. What you wear is also important; you want to give yourself as much of a shield against the sun as you can. Light colored, loose fitting clothes that breathe well are best. Cool Max or other such wicking materials are optimal for running in the heat as they increase the evaporation process and help shield your skin from the sun’s rays. A hat and sunscreen is also beneficial as sunburn impedes the skin's ability to cool itself through sweating.

One way to determine your hydration status is the before-and-after weight test. Weigh yourself before your workout. Then, workout and drink as you normally would during the run. When you are finished running, weigh yourself again before you start drinking your post-workout water. A loss of 4% of your water weight can mean up to a 30% decrease in performance, and a 5% loss is dangerous. Make sure to always replace the amount of weight you have lost with the proper amount of fluid (1 lb weight loss = 16 ounces of sweat loss), preferably with a 6% carbohydrate-electrolyte sports drink rather than plain water to avoid hyperhydration.

Now you have a basic working knowledge of what occurs in your body when you run in the heat. An additional action you can take to protect yourself and get the most of your training sessions is to slowly acclimate yourself. This takes some time and you should build up gradually; don’t jump into hard intervals on the year’s second day of summer weather. However, there is evidence that heat acclimatization can be achieved with repeated exercise in warm weather. This means that your body will begin to sweat earlier in an exercise bout, and your sweat ducts will adjust so that they retain more minerals. This makes the process of evaporation more efficient. Also, over time, your heart rate will not have to increase as much to maintain cardiac output, as the retained minerals will help keep your blood volume from falling. All in all, you can run longer and with less discomfort. Just be careful of running in the heat for too many days in a row; muscle glycogen stores are depleted much more quickly with heat stress than in mild weather.

Remember, “warm” or “hot” weather is more complex than just the thermometer reading. Of course air temperature is important, but also take note of the humidity, wind, and cloud cover. In humidity, sweat will not evaporate as it should because the air is already saturated with water. Since evaporation is our primary means for cooling, you won’t be cooling off as quickly and are more susceptible to Heat Cramps, Exhaustion, and Stroke. When it comes to heat, wind is our friend. In windy weather, heat will be carried away from your body much more quickly than when the air is still. And cloud cover keeps you cool by blocking a portion of the sun’s radiation. Check with the weather channel as to the UV index for the day.

Be smart and you’ll be safe. Good luck on the roads!


1 Costill, David, and Jack Wilmore. Physiology of Sport and Exercise, Human Kinetics. Champaign, IL., p.315, 1999.

2 The Nicholas Institute of Sports Medicine and Athletic Trauma., "Sports Nutrition Corner: Fluid." May 16, 2005. 1996-2002.

Kathryn Dempsey lives in San Francisco and loves to run! She has extensive knowledge of physiology (Biology degree) and fitness training from experience on the cross country ski team at Williams College. Currently she works for Equinox Fitness and can be reached at

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