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Exercising in the Heat

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

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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.

What Should Your Clients Drink?

Before Exercise. Drink 500 milliliters (ml) 2 hours before exercise (Armstrong 2000; Casa 1999b).
During Exercise. Drink about 200 ml every 15–20 minutes, aiming to match fluid intake to sweat loss. Maintain 400–600 ml of fluid in the stomach to optimize gastric emptying (Casa 1999b).
After Exercise. Drink 1 liter (L) per kilogram (kg) of weight lost during exercise (Armstrong 2000).

Sodium Sodium intake is necessary only if exercise lasts more than 60 minutes or if the client has a sodium deficiency.
Before, During and After Exercise. Consume 0.5–0.7 gram (g) per L of fluid. Glycerol Drinks that contain glycerol (the structural backbone of triglycerides) create an osmotic gradient in the circulation, causing fluid retention. This effect facilitates hyperhydration, protects against dehydration and maintains core body temperature (Robergs & Griffin 1998; van Rosendal et al. 2010).
Before Exercise. Ingest 1.2 g per kg of body weight in a 20% glycerol solution within a 30-minute period, followed by 26 ml of water per kg of body weight, distributed over the 90 minutes prior to exercise.
During Exercise. Ingest 0.125 g per kg of body weight mixed in 5 ml of fluid per kg of body weight.
After Exercise. Ingest 1.0 g per kg of body weight mixed in 1.5 L of fluid (van Rosendal et al. 2010).

Recommendations for Heat Acclimatization
  1. Attain adequate fitness in cool environments before attempting to acclimatize to the heat.
  2. Exercise at intensities > 50% of 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 lighter training during the hotter times of the day.
  4. Monitor body weight to ensure that proper hydration is maintained as sweat rate increases. Source: Armstrong 2000; Casa 1999b.


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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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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