Exercise in the Heat

Temperatures exceeding 90 degrees Fahrenheit and relative humidities exceeding 90% are common during the summer months in many climates. Such conditions are likely to affect athletes and spectators alike.

What generates heat during exercise?

No engine is 100% efficient. This means that some of the energy that is utilized goes not into work of the engine, but heat production. Skeletal muscle is an engine that is only 20% efficient when it converts ATP (adenosine tri-phosphate) into work. A great deal of the ‘combustion’ of ATP generates heat within the muscle. The higher the work rate, the greater the heat production. Consequently, the core temperature of the body must increase during exercise. Core temperature increases of 5 to 6 degrees are common with intense, prolonged exercise.

Heat balance during exercise

How does the body dissipate heat?

There are three mechanisms by which objects either gain or loose heat:

Radiation is heat transfer by infrared rays. If the body is hotter than the environment it will loose heat to the environment by radiation. In direct sunlight, the body absorbs radiant heat from the sun.

Conduction is the process of heat transfer by direct contact with another surface. If the skin touches a frozen piece of metal, heat is lost from the skin by conduction.

Convection occurs when air or fluid passes over a heated service. When standing in front of a fan which is blowing air at the same temperature as the environment, you loose heat by convection. If you are a swimmer, you don’t typically notice that you are hot until you stop. A great deal of heat is lost to the water by fluid movement across the body surface when swimming.

When water evaporates from any surface it changes from a liquid to a gas. The so called latent heat of vaporization is relatively enormous compared to the other forms of heat dissipation noted above. Consequently, when sweat evaporates from the skin it carries away a great deal of heat at the same time. Sweat that drips of the body has not accomplished its role of cooling the body effectively.

How does a hot, humid environment affect cooling?

Radiant energy from the environment is enormous in the hot sunlight and from flat surfaces which reflect heat. Later in the day, when pavement and track surfaces have absorbed a great deal of heat, radiation from these surfaces is worse. Any event which lasts longer than 20 minutes, done late in the day with the sun still high in the sky, does not have the athletes’ health in mind.

High humidity severely restricts the utility of evaporative heat loss since the air is already close to saturated with water vapor. As a result, though athletes sweat profusely, they do not cooling effectively. Core temperatures either continue to climb or the athlete has to decrease the intensity of the performance.

What is the importance of fluid intake?

The NISMAT article on fluid contains information about fluid replacement.