Too Darn Hot?

The Effect of Heat Stress on Marathon and Road Race Injury and Performance is Predictable. Knowing When It’s Too Hot for You to Run Could Save Your Life.

By WILLIAM O. ROBERTS, MD

WE ALL know runners who never miss a day out on the roads. Torrential downpour, three feet of snow, fever, cold—it doesn’t matter. The Run or The Race rules. You may even be this runner. As much as you don’t want to hear it, there are days when you would be better off skipping your run—or cross-training or exercising indoors. A brutally hot and humid day may be one of those days. The effect of heat stress on marathon and road race injury and performance is predictable, and this article aims to make you a more informed competitor so you know your personal limits regarding heat stress and can answer the question, “When is it too hot for me to run safely?”

Body heat is distributed in the core and shell of the body. The body shell is the surface for heat exchange and varies in “thickness” based on the need to lose or conserve heat. Cellular heat is accumulated during running along with a corresponding rise in body temperature. The rise in cell temperature in the essential organs causes the syndrome of exertional heat stroke (EHS).

Core temperature is a sum of exercise-produced metabolic heat plus environmental heat added to the body minus heat lost to the environment. Metabolic heat is a function of intensity and duration of exercise. Environmental heat is gained when the ambient air temperature is greater than skin temperature or from the sun’s radiant heat. Heat is lost from the body by evaporation, conduction, convection, and radiation.

Evaporation is the most powerful means of heat loss to the surrounding environment. High humidity limits evaporation heat loss, and high ambient temperature limits conduction and convection heat loss. Dehydration limits heat transport to the body surface and also limits sweating.

Heat-related running injury and heat stroke caused by the exertion of running can be prevented with proper planning by the race organizing committee and proper preparation and good judgment by you. Preventing all exertional heat stroke (EHS) is probably not possible because we cannot control the individual risk variables during running. Primary prevention of EHS is the responsibility of the race administration, but each of us also has the responsibility of knowing how well we cope with hot running conditions.

Definitions of exertional heat stroke and exertional hyperthermia are not chiseled in stone and include a “floating” temperature based on the presence of heat stroke symptoms. These symptoms are normally associated with abnormal brain function. In general, a rectal temperature greater than 104 degrees F is considered exertional hyperthermia.

The clinical picture of EHS is best described as elevated body temperature and altered central nervous system function. EHS victims will most often be sweating and will not always feel hot to the touch. The only adequate assessment of body temperature in the suspected EHS casualty is to take a rectal temperature.

The symptoms and signs to watch out for with EHS include these: ashen skin color, vacant stare, dizziness, fatigue, weakness, impaired judgment, hyperventilation, flushing, chills, and intense thirst. None of these symptoms or signs is specific for heat stroke, but they should raise the suspicion that heat stroke may be present.

The most ominous sign of heat stroke is the presence of abnormal brain function involving bizarre behavior, memory loss (especially event details and name), loss of hind limb function, inability to walk alone, collapse, delirium, stupor, or coma. The presence of the symptoms and signs should alert runners and medical staff of a potential medical emergency. Immediate evaluation and cooling measures should be started.

High humidity associated with high temperature poses the greatest risk to the body’s ability to dissipate heat and cope with elevated body temperature, especially if it is associated with intense activity or fast-paced races. The risk of heat injury begins to rise rapidly above 65 degrees F and 50 percent relative humidity. With intense work, you can generate 1,000 Kcal of heat per hour of activity, which can raise the body temperature of an averaged-sized runner into the hyperthermic range in 20 to 30 minutes.

It is important to underscore the role of fluid in countering this process. How much fluid you take in while you run or race affects your body’s ability to transport heat through the circulatory system and to lose heat through sweating. There are numerous scientific studies graphically illustrating the effect of dehydration on heat storage and performance. A dehydrated runner will have a higher core temperature, an increased heart rate, a lower cardiac output, and an increased perceived exertion at the same workload compared to a well-hydrated runner.

Drinking fluids during races will not only improve your performance, it will also protect the cooling system during activity. Taking in fluids during your training is essential to being able to tolerate taking in fluids during a race. You have to “practice” this fluid replacement strategy on a regular basis on training runs.

Acclimatizing, or physiologic adapting, is critical to performing successfully in the heat and takes at least 10 days of exposure before it kicks in. Gradual and progressive exposure to increasing heat loads during training is the safest way to acclimatize your body to hot conditions. You can induce acclimatization more rapidly with harder training, and athletes who have a higher cardiorespiratory fitness level at the beginning of heat training and runners with a greater VO2max can acclimate to heat faster.

The physiologic effects of acclimatization include decreased heart rate, increased plasma volume, earlier and increased sweating, decreased skin blood flow, and decreased Na+ (sodium) losses with activity in the heat. The end result of acclimatization is decreased core body temperature, increased exercise tolerance time, and decreased perceived exertion in the heat. The greatest improvement in acclimatization occurs after 8 to 12 weeks of exposure, and nearly daily exposure to heat is required to maintain this adaptation.