Unfortunately, heat is not something you can "beat," but you can optimize your performance by preparing for and acclimating to it.
It is important to understand your body's individual physiological response to a hot and/or humid environment and how much energy it must expend to cool itself. You may be surprised that most of the chemical energy used for muscular contraction is lost as heat—up to 90 percent at high intensities!
And we must maintain core temperature in a range of 35 to 42 degrees C during exercise or our bodies will shut down. The worst conditions an athlete can face are hot, humid, sunny, and windless, in which exercise is occurring at a high rate of intensity.
Lance Armstrong brought an interesting device into the media spotlight during his last Tour. The CoreControl glove is a relatively simple tool used to help cool the body down. The athlete places their hand in the glove and a slight vacuum is applied while the hand is cooled.
Why the hand? The body loses heat rapidly from certain areas of the body where there is more blood flow, such as the face, feet, and hands. Why would this make a difference? As noted, our bodies expend enormous amounts of energy to maintain core temperatures, and in an event such as a time trial, in which the podium is gained by fractions of a second, even a slight cooling effect will make a difference. Obviously, cooling the body is of great importance for optimal athletic performance.
How Your Body Stays Cool
The body cools itself through a variety of means. Heat is conducted from the muscles to the skin surface and transferred to the surrounding air by convection. Heat is also drawn to the surface of the body via sweat, which then evaporates on the skin surface causing heat loss. The respiration process dispels heat as water vapor, as does excretion of body wastes. If the environment is colder than the skin temperature, it will absorb body heat through radiation.
However, if exercise is undertaken in an environment warmer than the skin temperature, the body absorbs heat from the environment and can not lose heat through convection/radiation. The sweat/evaporation process becomes the primary means of heat loss during exercise in hot and humid conditions.
Not everyone will react to heat and humidity the same. In fact, there is a wide variance in efficiency between athletes exercising in the same conditions, and individual heat/humidity tolerance should absolutely factor into race planning strategy. Smaller athletes have a distinct advantage and may be able to maintain rates of adequate heat loss as larger athletes shut down. Smaller athletes have greater surface area to body mass, and comparatively, larger athletes will have to sweat more and utilize more energy in order to produce the same heat loss at the same running speed.
Body fat percentage is another factor that affects heat dissipation. Body fat is an insulator, and while it is advantageous for cold weather training, it retains heat in warmer conditions. Beginner or de-conditioned athletes will have less heat tolerance and greater sweat rates compared to highly conditioned athletes, and are at greater risk for heat illness/stroke. Slower athletes will have less air flow over the body, further reducing heat loss.
Finally, it requires one to two weeks to acclimate fully to a new environment, and once acclimated, an athlete will have a considerable advantage over another that is not. An acclimated athlete will have a lower heart rate, salt content of sweat, core temperature, and sweat rate; however, athletes that race in their normal training environment may still have a home court advantage over those recently acclimated.
Temperature is but one factor that influences cooling effect. Wind speed, humidity, and the radiant heat absorbed from the environment also affect heat dissipation. For instance, an athlete will absorb more radiant heat on an asphalt road in the sun versus a shady trail in the same heat/humidity/wind.
Under very humid conditions, sweat rolls off the body at certain points with less cooling effect, whereas wind speed increases the convective heat loss. It is important to recognize and attempt to prepare for all of the environmental factors you will face.