Heat and humidity can turn moderate summer workouts into miserable ones. Overheated muscles, profuse sweating, warm temperatures and high humidity can combine to cause serious physiological problems.
The August 2004 issue of Runner's World Magazine features a timely hot-weather article by Scott Douglas headed "Don't Sweat It," which offers several tips on how to handle the summer heat.
"It's the humidity that's the culprit," said Tina Schmidt- McNulty, clinical exercise specialist at Purdue University Calumet's Fitness Center. "Dehydration is the problem. The longer you exercise in the heat and humidity, the more water and electrolytes you lose. After an hour or so of fluid and electrolyte losses, dehydration will significantly impair your motor performance."
Douglas reported in his article that tests conducted by the Gatorade Sports Science Institute, in weather conditions of 85 degrees and 50 percent humidity, indicated the average runner will lose about 2 to 4 pounds of sweat per hour of exercise.
Larry Armstrong, author of Performing in Extreme Environments, has found runners slow by 3 percent for every 1 percent decrease in body weight caused by dehydration. A 150-pound runner clocking eight- minute miles will slow by almost 15 seconds per mile after losing only 1 1/2 pounds of body weight.
Most people are aware dehydration causes performance problems. In addition, most hot-weather exercisers know they should keep hydrated. What most outdoor exercisers don't know is their rate of sweat loss per hour and how much, as well as how often, to drink.
According to Douglas Casa, director of athletic training education at the University of Connecticut, there's a way to determine your sweat rate.
Casa's formula appeared in the Runner's World article written by Douglas. And, although Casa's recommendations are for runners, his fluid-replacement guidelines apply to anyone exerting one's self in hot, humid conditions.
Step 1: Weigh yourself before you walk, bike, jog, run, hike, Rollerblade, play basketball, soccer or work in the garden. For example, let's say you weigh 154 pounds before you start exercising.
Step 2: Exercise for one hour in the weather conditions and at the same workout intensity you expect to face for an upcoming run, game or special event.
Step 3: After your workout, remove your perspiration-filled workout clothes. Dry yourself thoroughly. Weigh yourself again. For example, let's say you weigh 150 pounds after your workout.
Step 4: To calculate your sweat rate per hour of exercise, subtract your ending exercise weight of 150 pounds from your beginning weight of 154 pounds. The difference of 4 pounds represents your fluid loss during exercise.
Since you should drink 16 ounces of fluid for every pound of fluid you sweat out during exercise, multiply the number of pounds lost by 16. In this example, you should drink 64 ounces of fluid to counteract the symptoms of dehydration and return your body weight to normal.
Step 5: If you drank any fluid during your 60 minutes of exercise, you'll need to add that number to the total amount of fluid lost during exercise that you calculated in Step 4.
Here's another way you can use the information. Once you know your personal sweat rate per hour, you can incorporate fluid breaks in your training so that every 15 or 20 minutes you consume 10 to 12 ounces of fluid.
By the end of your 60-minute exercise session, you won't be so dehydrated, your performance won't slow as dramatically, your body temperature won't be as high and you won't have to drink as much fluid the rest of the day.
John Bobalik is an exercise physiologist and coordinator of Purdue University Calumet's Fitness Center. He can be reached at firstname.lastname@example.org.