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Keeping your cool: precautions to take while exercising in the heat

American Fitness, July-August, 1996 by Deborah Riebe

During the summer, physical activities are often complicated by environmental conditions. High temperatures and humidity add to the demands put on the body by exercise. Complications due to excess body temperature, such as heat exhaustion and heat stroke, can be serious. Between 1968 and 1987, 50 high school and college football players died as a direct result of heat-related disorders. Although the number of deaths per year has declined, fitness professionals still need to educate clients how to safely exercise in the heat.

Body Temperature Regulation

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When air temperature is greater than body temperature, the body absorbs heat from the environment. On warm days, radiation from the sun also contributes. Heat is gained from the reactions of energy metabolism. When muscles become active, they generate a tremendous amount of heat. During a vigorous aerobic workout, the metabolic rate can increase 20 to 25 times above basal levels. If cooling mechanisms didn't exist, body temperature would increase 1.5 to 2 degrees F every five minutes.

Luckily, our bodies have several cooling mechanisms. Evaporation provides the major defense against overheating. Our bodies have approximately 2 to 3 million sweat glands that secrete a weak saline solution. The skin is cooled when sweat absorbs heat from the body and evaporates. It's the evaporation of sweat that has cooling power. When it eventually falls from the body, it is wasted and doesn't help reduce heat.

The total sweat vaporized depends on the amount of skin exposed, air currents, temperature and humidity. Humidity is the most important factor because evaporation is slowed when water vapor pressure in the air increases. Although you may be sweating more on a humid day, less sweat is evaporated. The body is then unable to effectively cool itself.

When exercising on hot, humid days with little wind, reduce your intensity and duration. Be sure to also drink fluids before, during and after exercise, and wear light-colored clothing. If exercising indoors, use a fan to assist evaporation.

Physiological Responses

The combination of exercise and heat stress is one of the greatest challenges faced by the cardiovascular system, according to Loring Rowell, M.D., a leading heat physiologist. The, ,kin and exercising muscles compete for blood flow, and their combined needs can exceed the pumping capacity of the heart. Muscles need an adequate supply of blood so enough oxygen is delivered to sustain aerobic energy metabolism. However, during exercise in the heat, blood vessels near the skin expand and blood is shunted away from the center of the body. This serves the important purpose of transporting heat produced by the working muscles to the skin, where it can be dissipated. However, blood shunted to the skin cannot deliver oxygen to the working muscles.

When blood is shunted to the skin, a decline in stroke volume results because less is available to fill the heart, says Rowell. During low-intensity exercise, heart rate increases to compensate for the lower stroke volume and cardiac output is maintained. As exercise gets more intense, cardiac output declines.

The lack of adequate blood flow to the muscles and a lower cardiac output results in performance decrements. Research shows individuals have a lower maximal oxygen consumption in the heat compared to a neutral environment.

Dehydration

During prolonged exercise on a hot day, excess water loss is another consideration. An individual doing high-intensity exercise can lose up to three liters of water per hour due to sweating. Dehydration results in a decrease in plasma volume (the liquid portion of the blood), exaggerating other effects of dehydration. Less blood is available to simultaneously perfuse the skin and the active skeletal muscle and fill the heart. Greater decreases in stroke volume and cardiac output result.

Additionally, dehydration leads to higher body temperatures during exercise, and sweating may be reduced. This puts individuals at a higher risk of heat injury. Research has also found a water deficit as little as 2% of body weight affects performance by increasing heart rate, while decreasing maximal oxygen uptake, muscle strength and endurance.

Drink before, during and after workouts to prevent dehydration. Don't rely on your sensation of thirst because it is not a sensitive indicator of the need for fluids. In fact, most individuals will incur a 2% weight loss before feeling thirsty. In other words, an individual weighing 150 pounds will lose three pounds of fluid before sensing it is time to take a drink.

Adapting to the Heat

The good news is that after spending approximately 14 days in a hot Climate, humans become acclimatized. A number of physiological adaptations improve the body's thermoregulatory function and alleviate circulatory strain. These adaptations include increases in sweat capacity, and rate, plasma and stroke volume, and decreases in heart rate. Additionally' sweating begins at a lower core temperature and hormonal changes occur that help the body retain more water.

Keeping your cool: precautions to take while exercising in the heat

American Fitness, July-August, 1996 by Deborah Riebe

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