Acclimating to Heat and Humidity Part II

28 06 2011

By Gale Bernhardt

In Part I of this two-part column, you learned how the body cools itself and what the risk factors are for heat illness. In this column we will explore strategies to reduce or prevent the affects of heat and humidity on your training and racing.

Heat Slows Pace

At the U.S. Olympic Training Center Heat and Humidity Conference, Georgia State University’s Laboratory for Elite Athlete Performance director, Dr. David E. Martin, noted that the body enjoys a very tight range of temperature control. From a resting average baseline temperature of 98.6 degrees Fahrenheit, our bodily enzymes function optimally during exercise at a temperature of 102.2 degrees Fahrenheit–this is one reason why we should include a warm-up prior to vigorous exercise.

” A mere 2.8-degree increase in core temperature from optimal performance temperature places us near our thermal death point. “

The most serious form of heat illness, heat stroke, is defined by a body core temperature greater than 105 degrees Fahrenheit. A mere 2.8-degree increase in core temperature from optimal places us near our thermal death point.

Because high intensity exercise contributes to increasing core temperature, pace must decrease when environmental temperatures increase in order to avoid heat illness. Dr. Martin estimates that overall marathon run time increases approximately one minute for every 7 degrees Fahrenheit above 54 degrees.

Jeff Galloway agrees that heat slows pace. He estimates the affects of heat on pace by assigning a percentage increase to various temperature ranges. In his example, he estimates that if you are an 8-minute-per-mile runner your pace will slow according to this chart:

55-60 degrees: 1% – 8:05
60-65 degrees: 3% – 8:15
65-70 degrees: 5% – 8:25
70-75 degrees: 7% – 8:35
75-80 degrees: 12% – 8:58
80-85 degrees: 20% – 9:35
Above 85 degrees: Forget it… run for fun

Interestingly, at least one study showed that warm weather has a greater negative impact on faster runners than slower runners. The study concluded that the slower running velocities from start to finish caused less pace degradation. This is likely due to the fact that higher paces require higher rates of cooling to keep the body out of the heat danger zone.

What Causes the Body to Gain Heat?

Certainly ambient temperature, relative humidity and exercise rate are factors that can cause your body temperature to increase. Part I included a partial list of risk factors for heat illness. Other factors that influence body temperature regulation and can contribute to heat illness include:

  • Basal metabolic rate (BMR)
  • Hormones
  • The thermic effect of consumed food
  • Age
  • Gender
  • Percentage of body fat
  • Level of heat acclimatization

You Can Improve Heat Acclimatization

Unfortunately, there are several items that you cannot control, which affect your ability to tolerate hot environments. The good news is that you can influence your level of heat acclimatization and improve your ability to tolerate heat in both exercising and non-exercising conditions.

Gradually exposing yourself to heat stress on consecutive days improves your ability to tolerate heat through several physiological changes. The adaptive changes improve heat transfer from the body core to the skin and ultimately to the environment. Secondly, the changes improve cardiovascular function. The majority, 95-percent, of changes take place in the first 10 days of heat exposure.

Some of the changes that occur during heat acclimatization include:

  • Decreased heart rate, skin and body temperature for a given exercise level
  • Blood plasma volume increases, keeping core temperature lower
  • Rating of perceived exertion decreases
  • Electrolyte concentration in sweat decreases
  • Sweat rate increases, allowing more effective cooling
  • Renal (kidney) electrolyte concentration decreases, preserving electrolyte levels
  • Cutaneous (skin) blood flow improves
  • Blood pressure stability improves
  • Circulation of blood to muscles improves
  • The threshold for sweating decreases  and evaporative cooling begins earlier in exercise
  • Less reliance on carbohydrate catabolism during exercise

Acclimatization Strategies

If you have the time and resources to travel to your race destination some 10 to 14 days early to facilitate the heat acclimatization process, fantastic. When you arrive at your destination, expose yourself to the heat on a daily basis, beginning with small time segments. Your exercise bouts in the heat should begin in the mornings or evenings at relatively cool levels, working your way toward training at the same time of day you will be racing. Your exercise sessions should not leave you feeling exhausted from the heat.

Dr. Armstrong, in his book Performing in Extreme Environments, suggests building your heat training sessions over the 14 days to some 90 to 110 minutes per day. For some athletes, this recommendation will be a challenge due to the taper process preceding race day. Do not sacrifice the taper process in order to train in the heat. Some heat acclimation is likely better than none, so train within your taper process.

If you are doing some of your training in a hot environment, recall that pace is affected by the heat. In an attempt to preserve as much of your hard-earned speed as possible, aim to do some of your quality training sessions at cooler temperatures during the heat training process.

If you do not have the time and resources to travel to the race venue early, you can begin acclimating to heat at your home location by wearing more clothing during your outdoor exercise sessions. Wear long-sleeved shirts, tights and maybe a hat and gloves during exercise. You may need to layer your clothing and wear a jacket that is made of breathable material as well, in order to simulate race day predicted temperatures.

Do not wear non-breathable sweat kits for exercise sessions. Your aim is to simulate warm exercise temperatures, not cook yourself.

Another option is to do some of your exercise sessions indoors on a spin bike, trainer or a treadmill. Depending on the temperature of the environment where the race will be held and your exercise room, you may or may not need to wear extra clothing during indoor training sessions.

An additional strategy is to use saunas or steam rooms. I have my athletes aim for non-exercising exposure in a hot box for 20 to 30 minutes, two to three times per week for the four to six weeks prior to the race. There is no specific research to confirm this strategy; however, I have used it for at least 15 years with good results. My theory is that because the athlete experiences shorter periods of intermittent heat exposure and the sessions are non-exercising, I believe they need longer than 14 days to acclimate.

I will often combine the non-exercising sauna or steam room situation with extra clothing during workouts for some four to six weeks prior to race day.

While I do know people that have set up an indoor trainer or a treadmill in a sauna, I have not had any of my athletes add exercise to a sauna situation and don’t recommend this strategy.

Improve Performance, Be Safe

By knowing how hot, or hot and humid, environments affect your body during exercise, you can take steps to minimize the effects. That written, you need to monitor your body and how the heat is affecting you both during training and on race day. Progressing from heat cramps to heat exhaustion and finally to heat stroke is serious. No workout or race is worth a heat injury; be safe out there.




Gale Bernhardt was the USA Triathlon team coach at the 2003 Pan American Games and 2004 Athens Olympics. Her first Olympic experience was as a personal cycling coach at the 2000 Games in Sydney. She currently serves as one of the World Cup coaches for the International Triathlon Union’s Sport Development Team. Thousands of athletes have had successful training and racing experiences using Gale’s pre-built, easy-to-follow cycling and triathlon training plans. Let Gale and Active Trainer help you succeed.

1. Armstrong, Lawrence E. PhD, Performing in Extreme Environments, Human Kinetics, 2000.
2. Ely, M.R., et al, “Effect of ambient temperature on marathon pacing is dependent on runner ability,” Med Sci Sports Exerc.,  Sep;40(9):1675-80, 2008.
3. Heat, Humidity and Air Pollution: Preparation for Athens 2004 Conference, U.S. Olympic Training Center,     September 17-19, 2003.
4. McArdle, Katch and Katch, Exercise Physiology, Energy, Nutrition and Human Performance, Sixth Edition,  Lippincott, Williams and Wilkins, 2007.
5. National Weather Service, Heat: A Major Killer
6. Noakes, Tim, MD, Lore of Running, Fourth Edition, Human Kinetics 2003.




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