UPDATE: With the hottest part of the summer approaching, I thought it would be a good idea to talk about exercise in the heat. There are a few points that should be made about exercise in the heat to help you understand why performance may decrease.

First, exercise itself produces a significant amount of heat as muscles break down food particles to produce energy and the energy is used for the contractile processes. This heat, plus the heat that is absorbed from the hot air around us, must be removed to avoid increasing body temperature to too high a level. Many physiologists think the best temperature for exercise is about 55 degrees; if hotter than that, the body has trouble removing the extra heat.Second, the body cools itself by sending the heated blood to the skin, where the heat in the blood can be released into the air. Since some of the blood supply is involved in the cooling process at the skin, there is less blood for the muscles you are using to exercise. This decreases the amount of oxygen available, which decreases the energy you can produce, and performance suf-fers.

The third problem relates to the loss of water through sweating. We sweat through evaporation, which is a very efficient means of losing excess heat. In fact, approximately 580 kilocalories are lost into the environment for every liter (about 1 quart) of water that evaporates from our body. The problem is that the sweat loss comes from blood plasma and therefore reduces blood volume. Again, a reduction in the amount of blood decreases our ability to perform.

An even greater problem occurs when the humidity is high. The body sweats to get the evaporative cooling effect, but evaporation doesn't occur because of the amount of water already in the air. This can cause heat to increase to dangerous levels and, along with the water loss, can result in a serious heat disorder. Therefore, if humidity is high, be extremely careful if you exercise very long or at a fairly high intensity.

The following symptoms have been identified by people who get into trouble while exercising in the heat: 1. hot and cold flashes, 2. clammy skin, 3. disorientation and/or headaches, 4. extremely high or lack of sweat production, 5. poor coordination and vision and 6. cramping and spasms of the muscles. If you experience any of these symptoms, you should probably stop exercising, drink some cold water and cool off.

How can you avoid heat-related problems? A recent article in "Running and Fit News" (June 1988) mentioned the following guidelines:

1. Avoid overtraining by inserting rest into your program.

2. Stay well-hydrated all day when it is hot or humid. During long workouts, drink plenty of fluids. Afterward, drink a pint of fluid for every pound of weight lost.

3. Work more slowly at the beginning of the workout. Exercise in the morning or evening hours. Gradually acclimate your body to the increased temperature before competing.

4. Wear light-colored clothes that fit loosely. Wear a hat to keep the head cool. Wear sun block to reduce damage caused by the sun.

High temperatures and humidity don't have to put an end to training or participation, but you must take them into consideration.

-- Garth Fisher is director of the Human Performance Research Center at Brigham Young University.