By Jason R. Karp, PHD MBA
We asked Dr. Karp to write something on how athletes should deal with hot weather.
Last year, I took a sabbatical from my entrepreneurial life in California and became a college professor and assistant cross country and track coach at Georgia Southern University. It would be an understatement to say that running in southern Georgia in the summer is a challenge. The summers are downright sticky, as stepping outside your air conditioned house feels like walking into a steam room. Most people running in the Georgia summer wish they could handle the heat as well as Apollo, the Greek god of the Sun, who was known for bringing heat and light to the Earth. But even Apollo would have a hard time running in the Georgia heat, as it presents a number of thermal and cardiovascular challenges. Many places in the country also experience harsh summer conditions, so knowing how to handle it will protect your athletes.
Physiology of Environmental Heat and Dehydration
You run outside on a sunny, hot, humid day. The crimson, mid-August 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 of 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 to 0.2 degrees Celsius for every one percent of body weight lost due to sweating.
Water is vital for many chemical reactions that occur inside your cells, including the production of energy for muscle contraction. Therefore, dehydration influences your athletes’ workouts. Indeed, exercise performance declines with only a 2 to 3 percent loss of body weight due to fluid loss. Since the effects of heat and dehydration on physiological function summate to have a greater effect than either one alone, being dehydrated when running in the heat causes performance to decline even more, and can even be a recipe for disaster, with the risk of heat-related illnesses rising dramatically. The problem, as you discover about three miles into your run, is that running in the heat makes it very difficult to prevent dehydration, since your sweat rate exceeds your ability to ingest and absorb fluid while running. While mild to moderate exercise typically results in sweat losses of 0.8 to 1.4 liters per hour, high environmental temperature combined with intense exercise can increase sweat rate to 1.4 to 2 liters per hour. However, your gastrointestinal system can absorb only about 0.8 to 1.2 liters of fluid per hour. Thus, heat stress and dehydration often occur together.
Humidity presents 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 the inability to continue exercise in the heat. Heat stroke, which is a medical emergency, occurs when body temperature rises to a level that causes damage to the body’s tissues (>103-104 degrees F). In an attempt to prevent body temperature from rising to dangerous levels while running, 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 the skin’s surface. It may seem somewhat counterintuitive that as your core body temperature rises while you run 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 running pace to decrease and the perception of effort to increase. When your body has a choice between maintaining the exercise intensity and cooling itself so you don’t overheat and die, it’s going to choose the latter. 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 drinking any fluid and continue running.
As if trying to prevent you from overheating on your run weren’t enough, accompanying the increase in thermal strain when running in the heat 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 oxygen flow to your muscles is then compromised, and the running pace decreases. To compensate for the decreased stroke volume, your heart must work harder to pump blood, and 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 to 5 beats per minute for every one percent of body weight loss from dehydration.
Due to both the thermal and cardiovascular strain of running in the heat, the ability to run declines linearly with an increase in environmental temperature. While most research has examined the effect of dehydration on prolonged cardiovascular exercise, resistance exercise performance has also been shown to decrease when dehydrated, however, it seems to take a greater amount of dehydration (at least a 5 percent loss of body weight) to see strength decrements.
After you complete your run fully exhausted, dehydrated, and a little lightheaded, your T-shirt drenched with sweat, you walk into your air conditioned house and ask yourself, “How can I prevent this from happening to my athletes?”
Recommendations for Exercising in the Heat
The two most important things your athletes can do to prepare themselves for their summer outdoor training sessions are hydrate and acclimatize.
Hydrate
Because of the decrease in running 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 “hyper-hydrating” before a workout can delay dehydration when running, maintain running 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 athletes should drink fluids before they exercise in the heat so they begin every workout fully hydrated, and they should continue to drink during workouts longer than one hour. For specific recommendations on how much and which ingredients to drink, see “What Should Your Athletes Drink?”
A good indicator of your athletes’ hydration levels is the color of their urine. While it may be outside your coaching scope of practice (and may seem a bit weird) to obtain a urine sample from your athletes, you can educate your athletes about how to monitor their hydration status. The lighter the urine color, the better the level of hydration, so tell your athletes their urine should look like lemonade rather than apple juice.
Acclimatize
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 to 6 days, while core body temperature and electrolyte concentration changes take 9 to 10 days. Full acclimatization is complete after two weeks, as the increased sweating response catches up to the other adaptations. Therefore, your athletes should take two weeks of slowly introducing themselves to the heat to be fully acclimatized and prepared for prolonged training sessions. When preparing for intermittent exercise (e.g., interval workouts, resistance training), however, your athletes may not need as long to acclimatize. Research has found that just four 30- to 45-minute sessions of intermittent exercise in the heat was enough to cause acclimatization and resulted in an improvement in intermittent running capacity. Furthermore, subjects who went through the acclimatization protocol had a lower core body temperature and an increase in thermal comfort during exercise compared to subjects who did not acclimatize. While exercising in the heat will always present a stress, acclimatization has a moderate prophylactic effect, minimizing the stress and reducing the risk of heat-related illnesses. For specific recommendations about how to acclimatize to the heat, see “How Should Your Athletes Acclimatize to the Heat?”
Other Strategies for Running in the Heat
If your athletes have a choice of when to run, the best time is the early morning, when the temperature is lower. Not only is it cooler and thus safer to run in the morning than in the afternoon or evening, your athletes 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 a lower initial core body and skin temperature. If your athletes must meet you for their runs and workouts during the hotter part of the day, try to maximize time in the shade, wear sunscreen, and recommend loose-fitting, moisture-wicking, light-colored clothes that reflect the sunlight.
Next time your athletes run in the heat, make sure they follow these guidelines. If they adequately hydrate and acclimatize, not only will they crush their workouts and eliminate their risk for heat illness, they may even challenge Apollo’s heat.
A competitive runner since sixth grade, Dr. Jason Karp quickly learned how running molds us into better, more deeply conscious people, just as the miles and interval workouts mold us into faster, more enduring runners. This passion Jason found as a kid placed him on a path he still follows as a coach, exercise physiologist, author of 15 books and 400 articles, and recent professor and assistant cross country and track coach at Georgia Southern University. His TED talk, How Running Like an Animal Makes Us Human, inspires people all over the world. He is the 2011 IDEA Personal Trainer of the Year and two-time recipient of the President’s Council on Sports, Fitness & Nutrition Community Leadership award. His run coaching certification was obtained by coaches and fitness professionals in 26 countries before being acquired by International Sports Sciences Association. In 2021, he became the first American distance running coach to live and coach in Kenya. His book, Coaching the Kenyans, as well as his others, are available on Amazon.