Your body is a constantly at work heat pump. Aerobic exercises like running can produce body heat at a rate ten to twenty times faster than while resting.
Your body regulates its core temperature by attempting to transfer excess heat to the surrounding air. The human body is not always efficient at accomplishing heat transfer so some awareness and management are necessary to prevent heat-related injury.
Researchers have learned that aerobic exercise in air temperatures as low as 65 degrees F can increase your body’s core temperature. Your heart rate increases as your body’s core temperature increases. Blood moves away from muscle toward the surface of the skin. The depletion rate of body water from sweating increases. This combination decreases your ability to perform and increases your probability of heat injury.
Air temperatures of 88 degrees F and higher stress the body’s heat transfer mechanism to the limit, and if left unmanaged, exercise in these conditions can raise your core temperature to a dangerous level. Heat cramps are the mildest and most common form of heat related injury.
Core body temperatures at 106 degrees F are a life-threatening medical emergency. At excessive core temperatures you can suffer heatstroke, when the sweating mechanism shuts down and the skin gets hot and dry. Fever thermometers are not standard exercise gear but it’s not a bad idea to occasionally bring one along and check your core temperature immediately after completing an exercise session. You can learn if you have to be more aggressive about managing your body temperature.
Sweating is the body’s most important heat transfer mechanism. Any condition that slows or blocks the transfer of heat from the body by evaporation causes heat storage, which results in an increase in body temperature. Light weight, loose fitting clothing is the best way to handle this. Performance fabrics like Coolmax wick moisture away from the skin while allowing air to pass through.
Relative humidity also directly affects evaporative cooling. At 100% relative humidity the air is completely saturated with moisture. Sweat does not evaporate and no cooling effect takes place. Consequently, the body’s core temperature rises, which triggers even more sweating. The net effect is an even more rapid loss of body water… the sure recipe for dehydration and potentially serious problems.
According to U.S. Army physical training manuals plain water is the best replacement fluid to use to prevent dehydration. Fluids with high sugar content slow the absorption of water from the stomach and should be avoided.
To prevent heat injuries, the following hydration guidelines should be used:
Drink 13 to 20 ounces of cool water at least 30 minutes before exercise.
Drink 3 to 6 ounces at 15 to 30 minute intervals during exercise.
Drink to satisfy your thirst, and then some, after exercise.
Cool water between 45 to 55 degrees F is absorbed more quickly.
You do not have to work as hard in high temperatures to achieve a training effect. Increased temperatures and humidity cause increased heart rates. Consequently, it takes much less effort to elevate the heart rate into the training zone, but the training effect is the same. These facts underscore the need to monitor both your body temperature and your heart rate when running or performing other aerobic exercises during the summer and in hot climates.