Exercising in the Heat

by Jason Karp, PhD on May 18, 2010

Ex Rx

Thermal and cardiovascular challenges to be aware of during hot summer days.

Anyone who grew up as a runner in New Jersey, as I did, would tell you that running during the summer in the Northeastern United States is no ordinary challenge. Some days are downright sticky; stepping outside your air-conditioned house can feel like walking into a steam room. Similarly, many places in the country experience harsh summer conditions that carry thermal and cardiovascular challenges. Knowing how to handle these will protect your clients.

The Physiology of Environmental Heat and Dehydration

You go out for a run on a sunny, hot, humid day. The crimson mid-July sun hangs overhead against the azure sky like the blade of a guillotine. A couple of miles into your run, your body temperature, already on the rise from muscle contraction, increases even more. Since your primary mechanism for cooling your body is through the evaporation of sweat from the skin’s surface, your sweat rate increases. As a result, you lose body water and begin to become dehydrated.

Despite the occasional compliment you get in the gym about your well-defined muscles, water—not muscle—is the major component of your body. So when you lose water, there are consequences. A major consequence of dehydration is an increase in core body temperature during exercise, with body temperature rising 0.15–0.2 degrees Celsius for every 1% of body weight lost due to sweating (Casa et al. 2000).

Water is vital for many chemical reactions that occur inside your cells, including the production of energy for muscle contraction. Therefore, dehydration influences your ability to exercise. Indeed, exercise performance declines with only a 2%–3% loss of body weight due to fluid loss (Armstrong 2000; Casa et al. 2000). Since the effects of heat and dehydration on physiological function together have an even greater effect than either one alone, being dehydrated when exercising in the heat causes exercise performance to decline even more (Casa 1999a) and can even be a recipe for disaster, with the risk of heat-related illnesses rising dramatically.

The problem, as you discover about 3 miles into your run, is that preventing dehydration is very difficult when exercising in the heat, since your sweat rate exceeds your ability to ingest and absorb fluid while exercising. While mild to moderate exercise typically results in sweat losses of 0.8–1.4 liters (L) per hour, high environmental temperature combined with intense exercise can increase sweat rate to 1.4–2.0 L per hour (Armstrong 2000). However, your gastrointestinal system can absorb only about 0.8–1.2 L of fluid per hour (Armstrong 2000). Thus, heat stress and dehydration often occur together.

Hot and humid days present an even greater challenge. When it’s humid, the air is already saturated with water, limiting the amount of sweat evaporating from your skin. As a result, the ability to dissipate heat is minimized and core body temperature rises rapidly, leading to hyperthermia. In extreme cases, hyperthermia can lead to heat exhaustion and heat stroke.

Heat exhaustion, the most common heat illness, is defined as the inability to continue exercise in the heat (Armstrong 2000). Heat stroke is a medical emergency and occurs when body temperature rises to a level that causes damage to the body’s tissues (> 103–104 degrees Fahrenheit, or > 39–40 degrees Celsius) (Armstrong 2000).

In an attempt to prevent body temperature from rising to dangerous levels during exercise, your central nervous system orchestrates a complex response in which blood vessels supplying your inner organs constrict, while blood vessels supplying your skin dilate, causing blood to be diverted away from inner organs and directed outward to the skin to increase cooling through the convection of air over your skin’s surface. It may seem somewhat counterintuitive that as your core body temperature is rising during exercise in the heat, skin temperature decreases as a result of convective cooling. More blood being directed to the skin means less blood (and therefore less oxygen) going to the active muscles, causing exercise intensity to decrease and the perception of effort to increase.

When your body has a choice between cooling itself and maintaining exercise intensity, it’s going to choose the former. So on this hot, humid day, your running pace slows and you feel fatigued. You notice a sprinkler on a neighbor’s lawn and run past it, hoping to cool yourself, but you quickly realize that spraying water on your body, while refreshing, is not effective for decreasing body temperature. To decrease body temperature, you need to ingest the fluid. Since you don’t want your neighbors to see you trying to drink from their sprinklers, you forego that idea and continue running.

As if trying to prevent your body from overheating weren’t enough, accompanying the increase in thermal strain is a greater cardiovascular strain. Profuse sweating to increase evaporative cooling causes a loss of plasma volume from the blood, and total blood volume decreases. When blood volume decreases, stroke volume (the volume of blood pumped by the heart with each beat) decreases. A decreased stroke volume means that oxygen flow to your muscles is then compromised and the exercise intensity decreases. To compensate for decreased stroke volume, your heart must work harder to pump blood, and your heart rate drifts upward in an attempt to maintain cardiac output (the volume of blood pumped by the heart each minute) and blood pressure. This rise in heart rate during prolonged exercise without an increase in intensity is called cardiac drift. Heart rate rises 3–5 beats per minute for every 1% of body weight loss from dehydration (Casa et al. 2000).

Owing to both the thermal and cardiovascular strain of exercising in the heat, the ability to exercise declines linearly with an increase in environmental temperature (Casa 1999a). While most research has examined the effect of dehydration on prolonged cardiovascular exercise, resistance exercise performance has also been shown to decrease when the body is dehydrated (Kraft et al. 2010); however, it seems to take a greater degree of dehydration (at least a 5% loss of body weight) to cause strength decrements (Casa 1999a).

As you complete your run fully exhausted, dehydrated and a little lightheaded, your cotton T-shirt drenched with sweat, you walk into your air-conditioned house and ask yourself, “How can I prevent this from happening to my clients?”

Recommendations for Exercising in the Heat

The two most important things your clients can do to prepare themselves for their summer outdoor training sessions are hydrate and acclimatize.


Because of the decrease in exercise performance and the potential health danger of dehydration, there has been plenty of research (and an onslaught of sports drinks) on strategies 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 your ability to control body temperature and increases plasma volume to maintain cardiac output (van Rosendal et al. 2010). Clients should drink enough fluids before exercising in the heat to begin every workout fully hydrated, and they should continue to drink during workouts longer than 1 hour (see the sidebar “What Should Your Clients Drink?”). Since rehydrating while running or cycling necessitates carrying fluids, your clients should plan some way of drinking during prolonged exercise in the heat. Given the growing popularity of fitness boot camps and portable resistance training equipment (e.g., resistance bands and the TRX® Suspension Trainer) that can be taken outside, making sure your clients rehydrate during resistance training workouts is more important than ever.

A good indicator of clients’ hydration levels is urine color. While it is probably outside your scope of practice as a personal trainer (and may seem a bit weird) to obtain a urine sample from your clients, you can educate your clients about 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, while rectal temperature and electrolyte concentration changes take 9–10 days. Full acclimatization becomes complete after 2 weeks as the increased sweating response catches up to the other adaptations (Casa 1999b). Therefore, your clients should take 2 weeks of slowly introducing themselves to the heat to be fully acclimatized and prepared for prolonged continuous exercise.

When preparing for intermittent exercise (e.g., cardiovascular interval training, resistance training), however, your clients may not need as long to acclimatize. For example, Sunderland, Morris and Nevill (2008) found that after just four 30- to 45-minute sessions of intermittent exercise in the heat, subjects were acclimatized and saw improvements in their intermittent running exercise capacity. Furthermore, subjects who went through the acclimatization protocol had a lower rectal temperature and an increase in thermal comfort during exercise compared with subjects who did not acclimatize. While exercising in the heat will always present some stress, acclimatization has a moderate prophylactic effect, minimizing the amount of stress and reducing the risk for heat-related illnesses (Casa 1999b). For specific recommendations on how to acclimatize to the heat, see the sidebar “Recommendations for Heat Acclimatization.”

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 and is accompanied by lower initial rectal and skin temperatures (Hobson et al. 2009). 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 clothes that reflect 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 the workouts and greatly reduce the risk for heat illness.

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Armstrong, L.E. 2000. Performing in Extreme Environ­ments. Champaign, IL: Human Kinetics.

Casa, D.J. 1999a. Exercise in the heat. I. Fundamentals of thermal physiology, performance implications, and dehydration. Journal of Athletic Training, 34 (3), 246–52.

Casa, D.J. 1999b. Exercise in the heat. II. Critical concepts in rehydration, exertional heat illnesses, and maximizing athletic performance. Journal of Athletic Training, 34 (3), 253–62.

Casa, D.J., et al. 2000. National Athletic Trainers’ Association position statement: Fluid replacement for athletes. Journal of Athletic Training, 35 (2), 212–24.

Hobson, R.M., et al. 2009. Exercise capacity in the heat is greater in the morning than in the evening in man. Medicine & Science in Sports & Exercise, 41 (1), 174–80.

Kraft, J.A., et al. 2010. Impact of dehydration on a full body resistance exercise protocol. European Journal of Applied Physiology [Epub ahead of print]. Retrieved Mar. 10, 2010.

Robergs, R.A., & Griffin, S.E. 1998. Glycerol: Bio­chemistry, pharmacokinetics and clinical and practical applications. Sports Medicine, 26 (3), 145–67.

Sunderland, C., Morris, J.G., & Nevill, M.E. 2008. A heat acclimation protocol for team sports. British Journal of Sports Medicine, 42 (5), 327–33.

van Rosendal, S.P., et al. 2010. Guidelines for glycerol use in hyperhydration and rehydration associated with exercise. Sports Medicine, 40 (2), 113–29.

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About the Author

Jason Karp, PhD

Jason Karp, PhD IDEA Author/Presenter

It started with a race around the track in sixth grade in Marlboro, New Jersey. Little did Jason know how much it would define his career and life. A Brooklyn, New York native (you can take the boy out of Brooklyn, but you can't take Brooklyn out of the boy), he grew up playing baseball and soccer and running track. It was intoxicating. The passion that Jason found as a kid for the science of athletic performance (one of his earliest questions was how baseball pitchers throw curveballs) placed him on a yellow brick road that he still follows as a coach, exercise physiologist, author, speaker, and creator of the REVO2LUTION RUNNING™ certification program for coaches and fitness professionals around the world. Dr. Karp has given hundreds of international lectures and has been a featured speaker at most of the world’s top fitness conferences and coaching clinics, including Asia Fitness Convention, Indonesia Fitness & Health Expo, FILEX Fitness Convention (Australia), U.S. Track & Field and Cross Country Coaches Association Convention, American College of Sports Medicine Conference, IDEA World Fitness Convention, SCW Fitness MANIA, National Strength & Conditioning Association Conference, and CanFitPro, among others. He has been an instructor for USA Track & Field’s level 3 coaching certification and for coaching camps at the U.S. Olympic Training Center. At age 24, Dr. Karp became one of the youngest college head coaches in the country, leading the Georgian Court University women’s cross country team to the regional championship and winning honors as NAIA Northeast Region Coach of the Year. As a high school track and field and cross country coach, he has produced state qualifiers and All-Americans. He is also the founder and coach of the elite developmental team, REVO2LUTION RUNNING ELITE. A prolific writer, Jason is the author of eight books: The Inner Runner, Run Your Fat Off, 14-Minute Metabolic Workouts, Running a Marathon For Dummies, Running for Women, 101 Winning Racing Strategies for Runners, 101 Developmental Concepts & Workouts for Cross Country Runners, and How to Survive Your PhD. He has more than 400 articles published in numerous international coaching, running, and fitness trade and consumer magazines, including Track Coach, Techniques for Track & Field and Cross Country, New Studies in Athletics, Runner’s World, Running Times, Women’s Running, Marathon & Beyond, IDEA Fitness Journal, Oxygen, PTontheNet.com, and Shape, among others. He also served as senior editor for Active Network. Dr. Karp is a USA Track & Field nationally certified coach, has been sponsored by PowerBar and Brooks, and was a member of the silver-medal winning United States masters team at the 2013 World Maccabiah Games in Israel. For his work and contributions to his industry, Jason was awarded the 2011 IDEA Personal Trainer of the Year (the fitness industry’s highest award) and is a two-time recipient of the President’s Council on Sports, Fitness, & Nutrition Community Leadership Award (2014, 2019). Dr. Karp received his PhD in exercise physiology with a physiology minor from Indiana University in 2007, his master’s degree in kinesiology from the University of Calgary in 1997, and his bachelor’s degree in exercise and sport science with an English minor from Penn State University in 1995. He is currently pursuing his MBA at San Diego State University. His research has been published in various scientific journals, and he serves as a journal expert peer reviewer.