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Exercising in the Heat

Exercising in the heat can endanger health and impede exercise performance. Hot and humid days pose a particular risk: when it’s humid, the ability to dissipate heat is minimized, which can ultimately lead to heat exhaustion and heat stroke. The two most important things your clients can do to prepare themselves for their summer outdoor training sessions are hydrate and acclimatize.

Plenty of research has been done on how to overcome, or at least blunt, the effects of dehydration. Beginning the workout fully hydrated or even “hyperhydrating” (hydrating to a greater degree than normal) before a workout can delay dehydration during exercise, maintain exercise performance and decrease the risk for heat-related illnesses.

Pre-exercise fluid intake enhances the ability to control body temperature and increases plasma volume to maintain cardiac output. Your clients should drink enough fluids before exercising in the heat to begin every workout fully hydrated and should continue to drink during workouts longer than 1 hour. Since rehydrating while running or cycling necessitates carrying fluids, clients should plan some way of drinking during prolonged exercise in the heat. Given the growing popularity of fitness boot camps and of portable equipment that takes resistance training outside, making sure your clients rehydrate during these workouts is more important than ever.

A good indicator of clients’ hydration levels is urine color. You can educate your clients on how to monitor their hydration status. The lighter the urine color, the better the level of hydration, so tell your clients their urine should look like lemonade rather than apple juice.

Chronically exposing oneself to a hot and humid environment simulates adaptations that lessen the stress. Cardiovascular adaptations to exercising in the heat (e.g., decreased heart rate, increased plasma volume) are nearly complete within 3-6 days. Full acclimatization becomes complete after 2 weeks as the increased sweating response catches up to the other adaptations. To be fully acclimatized and prepared for prolonged continuous exercise, your clients should take 2 weeks to introduce themselves slowly to the heat.

Recommendations for Heat Acclimatization

  1. Attain adequate fitness in cool environments before attempting to acclimatize to the heat.
  2. Exercise at intensities > 50% VO2max, and gradually increase the duration (up to 90–100 minutes per day) and intensity of the workouts during the first 2 weeks.
  3. Perform highest-intensity workouts during the cooler morning or evening hours and other training during the hotter times of the day.
  4. Monitor body weight to ensure that proper hydration is maintained as sweat rate increases.

Other Strategies for Exercising in the Heat
If you’re training clients or holding a boot camp outdoors in the summer, the best time to choose is the morning, when the temperature is lower. Not only is it cooler and thus safer, but your participants may also get a better workout. Research has shown that endurance exercise capacity in the heat is significantly greater in the morning than in the evening. If a client must train with you during the hotter part of the day, do the workout in the shade and recommend loose-fitting, moisture-wicking, light-colored exercise clothing that reflects the sunlight.

The next time your clients run in the heat or take part in a summer outdoor boot camp, make sure they follow these guidelines. If they take the necessary precautions, they will get more out of their workouts and greatly reduce the risk of heat illness.

For more tips on training in the heat, please refer to the full article, “Exercising in the Heat,” in the online IDEA Library or in June 2010 IDEA Fitness Journal.


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Jason Karp, PhD

A professional running coach, freelance writer, fitness consultant and PhD candidate in exercise physiology at Indiana University. He coaches runners of all levels through RunCoachJason.com.

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