By Carl V. Gisolfi, Ph.D. Department of Exercise Science, University of Iowa
Specific steps must be taken to prepare athletes for training and competing in hot weather. Proper preparation can improve athletic performance and reduce the potential for thermal injury.
Under adverse climatic conditions, including high temperature and humidity, heat gained from the combination of physical exertion and the hot environment can exceed the body's capacity to remove heat through perspiration. In such instances, heat cramps, heat exhaustion and heat stroke may occur as a result of dehydration and elevated body temperature. Such thermal injuries can occur regardless of the athlete's physical condition or ability to adapt to hot weather.
How The Body Handles Excess Heat During Exercise
During exercise, heat produced by working muscles exceeds heat released by the body, and body temperature rises. The rise in body temperature causes an increase in sweating and blood flow to the skin. As a result heat is removed by the evaporation of sweat from the skin, radiated form the body to the cooler surroundings, and is lost by convection to moving air (e.g. wind chill).
When the rate at which heat is produced during exercise equals the rate at which heat is lost from the body, the body temperature will plateau at an elevated level. However, when more heat is produced during exercise then the body can lose, body temperature will rise to a potentially dangerous level. High environmental temperatures and humidity contribute to the risks of thermal injury because they reduce the body's ability to remove heat. Athletes who are untrained and unacclimatized (not accustomed to the heat) can maintain an elevated, but safe, body temperature during moderate exercise in temperatures ranging from 60 to 85 degrees Fahrenheit. With proper training and heat acclimatization, athletes can safely increase the intensity and duration of exercise in even hotter environments.
There is little chance that under hot, humid weather conditions thermal injury will occur in competitive events lasting up to 10 minutes (the vast majority of track events). But, body temperature could rise to dangerous levels (e.g., 106 degrees Fahrenheit) under these conditions during exercise lasting 12 - 16 minutes or longer, especially if the competition is preceded by a vigorous warm-up that elevates the body temperature.
How To Acclimate Your Athletes
Athletes can acclimate their bodies to excessive heat and humidity by performing mild to moderate exercise in a hot environment. After one and one-half to four hours of exercise per day for five-to-fifteen days, the body will adjust (acclimate) to hot and humid weather conditions.
Successful heat acclimatization results in (1) a lower resting body temperature, (2) lower skin and core temperature during exercise, (3) decreased exercise heart rate and metabolism, and (4) increased sweating and evaporative cooling. All of these changes help athletes safely improve their performance in hot weather. The acclimatization process is similar in men and women, is not influenced by the menstrual cycle, and does not seem to be affected by age.
Endurance training, even in a cool environment, produces physiological adjustments similar to those caused by heat acclimatization. Training and acclimatization enable athletes to exercise at considerably greater exercise intensities while maintaining safe body temperatures. Both training and heat acclimatization are required for an optimal ability to exercise in the heat.
How To Prevent Thermal Injuries
In preparing for outdoor competition under hot, humid conditions, warm up in the shade to avoid raising body temperature too high, too soon.
Rest in the shade between competitive events. Exposure to the sun can cause blood to accumulate in the skin. This makes less blood available to muscles during subsequent exercise.
Wear minimal, loose-fitting clothing to help promote heat loss.
During prolonged exercise in the heat, body fluids lost as sweat must be replaced as frequently as possible to avoid dehydration and subsequent thermal injury. Drinking 12 to 20 ounces (1 ½ to 2 ½ cups) of fluid 10-20 minutes prior to competition is a good idea, but can not substitute for ingesting fluids during exercise. Running through a shower or being hosed with water also will not prevent the rise in body temperature during exercise.
Fluids (e.g., sports drinks) ingested during exercise should contain 5-8 percent carbohydrate and a small amount of electrolytes. Such beverages will provide a source of fuel for the working muscles, encourage drinking by enhancing taste, promote fluid retention, and will facilitate the absorption of water and glucose from the intestines.
Throughout heavy work or prolonged exercise, at least eight ounces (1 cup) of fluid should be consumed every 15-to-20 minutes.
The pre-season regimen for competitive football, long distance running and other sports under hot and/or humid conditions should be preceded by one-to-two weeks of conditioning. This means working one-to-two hours per day in the heat, while wearing minimal clothing and drinking liberally. This will help athletes gradually achieve heat acclimatization.
Practice sessions under hot, humid conditions should be limited to very moderate workouts or be cancelled.
Athletes who typically train and compete in cool weather but are scheduled to compete in the heat can markedly improve their heat tolerance by training in excess clothing. Carefully supervised use of this practice will simulate a warm environment and improve the heat-acclimatization process. Similar to exercise in warm weather, frequent fluid consumption is a must during this type of exercise.
Applying proper precautionary steps prior to and during exercise can help athletes avoid thermal injuries. Techniques, such as warming-up in the shade, ingesting adequate fluids, and wearing loose-fitting clothing can safeguard athletes health and maximize performance.
Dr. Gisolfi is a professor of Exercise Science, Physiology and Biophysics at the University of Iowa, Iowa City. He is past president of the American College of Sports Medicine and a member of the Gatorade Sports Science Institute.