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It’s Hot! Safe Exercise with a High Heat Index: Ask the Trainer

Q: I like to run and bike outside during my lunch hour. It has been so hot and humid this summer that the weather has put a crimp in my routine! Do you have tips for getting a great workout despite the high temps?

A:  The key to beating the heat and making summertime fitness progress is to have the facts. Hydration, exercise intensity, current fitness levels, clothing, and the heat stress index are all important factors to consider. Use this information to develop a strategy for healthy exercise:

Hydration and Heat Cramps

Drinking enough fluid, whether water or a sports drink, is imperative for exercising in hot or humid weather. You need to be hydrated in order for your body to produce enough body fluids and maintain proper temperature. Heat cramps can occur when you have lost a large amount of water and sodium chloride through sweat. See below for how to avoid them—acclimatization to the hot, humid weather can also help you avoid heat cramps.

Sweat cools your body by evaporating off your skin. Visible beads of sweat that don’t evaporate only dehydrate you without the beneficial cooling effect. If you let your body become dehydrated, you’ll find it much more difficult to perform even the lightest of workouts.

To avoid dehydration, weigh yourself before and after your workout and drink 20 ounces of fluid for every pound of body weight lost. Water isn’t the only thing your body loses when sweating. Electrolytes such as sodium, potassium, and chloride are also lost in sweat. Rehydration will occur faster in the presence of sodium, which can be gotten through sports drinks or electrolyte replenishment powders that can be added to water. Try Emergen-C or Coconut Water! 

Exercise Intensity

Depending on the heat stress index you may need to reduce the intensity of your workout, particularly the first few times you are exposed to higher temperatures. The heart must work harder to pump oxygen during extreme heat and humidity, meaning that it is recommended that you use a heart rate monitor to make sure you are in a healthy heart-rate range.

Heat Stress Index

High humidity prevents sweat from evaporating. Use the heat stress index to determine the risk of exercising at various combinations of temperature and humidity. While a 90° F outdoor temperature is relatively safe at 10% humidity, the heat stress of 90° F at 50% humidity is the equivalent of 96° F.

When the heat stress index rises above 90° F, consider postponing your exercise session until later in the day or heading inside for a treadmill workout. Or, plan ahead and beat the day’s heat by working out early in the morning. It may not be your first choice, but at least you’ll be safe and still get a workout in.

When you go outside to exercise, refer to the heat stress index and consider the associated risks:

Below 80° F (27° C): Little or no danger under normal circumstances

80–90° F (27–32° C): Fatigue possible with prolonged exposure

90–105° F (32–41° C): Heat cramps and heat exhaustion are possible

105–130° F (41–54° C): Heat cramps and heat exhaustion likely, heatstroke is
possible

Over 130° F (54° C): Heatstroke is imminent

Fitness

Physical training and heat acclimation can increase your blood volume, helping to regulate body temperature more effectively. Interestingly, the acclimatization process can be completed in seven to 14 days of repeated heat exposure.

Clothing

Wear minimal clothing to provide a greater skin surface area for heat dissipation.

Hot weather workout clothing should be:

Lightweight with a “wicking”-effect to help sweat evaporate

Light-colored to reflect the sun’s rays

Wear a hat to protect the head and keep the sun off your face.

Check out this New York Times article reporting that research shows that drinking ice cold liquid before a workout can increase your exercise capacity significantly: http://www.nytimes.com/2010/04/27/health/nutrition/27best.html?_r=1