Heat Acclimation for Runners: A Coach’s Analysis

Trail runners and ultramarathoners are groups that like to take on challenges. The tougher, the better, right? In a quest […]

By on February 26, 2009 | Comments

Trail runners and ultramarathoners are groups that like to take on challenges. The tougher, the better, right? In a quest for new and tougher challenges we go longer, higher, hillier, and sometimes hotter. If we choose to face the heat, it may be appropriate to apply various degrees of heat acclimation. The type and degree of heat acclimation call for depends on the race. When heat quietly plays a factor, such as at the Western States 100 or the Vermont 100, running during the heat of the day or driving around with the heat on might be enough to take the edge off on race day. However, when the heat is more likely than not to be a headline feature of the event, such as at Badwater… or the Marathon des Sables, more strenuous heat acclimation. In this post you’ll find some background on heat acclimation, additional heat acclimation resources, and my personal experiences with heat acclimation for MdS.

Background Information on Heat Acclimation for Athletes

Considering the only heat acclimation I’d ever done before this week was driving from Park City to Moab, Utah and back with the heater on in June (for the record, it was quit toasty in the car), I went to an expert for advice. Greg Loomis was the man I called on, as he has advanced medical training, was involved in researching heat acclimation for the Army, and ran for a year in the Sinai while serving in the Army. Here’s what Greg had to say:

Human’s ability to sweat allows us to cool our bodies even when running in extremely hot environments. However, the need to circulate blood out to the skin periphery for this cooling draws this much needed blood away from working skeletal muscles and causes a lower cardiac filling and stroke volume leading to higher heart rates at any given work load. The loss of electrolytes and fluid via the sweating (without adequate replacement) will lead to a decreased blood volume and add additional demand on an already taxed heart.

Heat acclimatization is one way to improve ones ability to run well in a hot environment and in extreme cases is necessary for survival. Heat acclimation is merely when an individual has been conditioned to maintain a higher blood plasma and volume level, increased sweat rate, a decreased salt amount in the sweat produced, decreased fatigue rate of sweat glands, and quicker onset of sweating when placed in the heat. These changes are all needed to meet the demands on the body mentioned above. Heat acclimation is produced via repeated exposure to heat sufficient to raise body core temperature. This is most effectively done by exercise (skeletal muscle contractions) vs. sitting in a hot room. Only a few sessions of one hour of moderate exercise in the heat will produce an effect in un-acclimated individuals with changes being seen in a few days.

Some interesting highlights to acclimatization:

  • One can become acclimatized to heat and cold at the same time. Even with training bouts being on the same day in the different environments.
  • Most of the improvements in heart rate, core and skin temp, and sweat rate are acquired in just ONE week of heat exposure. Heart rate adaptations are seen in just 4-5 days! However increases in sweating and a feeling of “ease of walking” in a hot environment can take up to one month to occur.
  • More is gained from a 100 minute bout of heat exposure exercise than one 50 minute bout, but adding bouts beyond 100 minutes of exposure did not quicken adaptation.
  • Heat exposure adaptations have been studied to disappear as quickly as one week if the subject is not re-placed in the heat, but may last as long as 3 weeks in some individuals.

All facts taken from “Human performance physiology and environmental medicine at terrestrial extremes” Pandolf, Sawka, Gonzalez. ISBN: 1-884125-02-6

Additional Heat Acclimation Resources

While Greg provides some great background on heat acclimation, you may want to jump even deeper into the physiology and process of heat acclimation – I know I did. Fortunately, the military has published Medical Aspects of Harsh Environments, Volume 1. Section 1 of this publicly-available tome contains over 300 pages of the physiology of and adaptation to heat. Of particular utility is Chapter 2 – Human Adaptation to Hot Environments (pdf) in which I found the discussions of thermoregulatory control, thermoregulatory responses during exercise, and factors that alter heat tolerance to be wonderfully useful. These discussions taught me how the human body reacts to heat and how these reactions will change with heat acclimation. Heavy with references – Human Adaptation to Hot Environments will put you in the know on everything from how non-acclimated whites have a very high concentration of sodium in their sweat to the variation in adaptions to dry and humid heats.

After you’ve conquered the Army’s heat acclimation resource or if you are looking for something that more directly addresses various processes for heat acclimation, check out the International SportMed Journal’s Heat Adaptation: Guidelines for the Optimisation of Human Performance. This article discusses the advantages and disadvantages of heat adaptation in athletes before diving into various methods of heat acclimation, including:

  • Natural acclimatisation
  • Passive heat acclimation
  • Exercise-induced heat adaptation
  • Combined exercise-heat acclimation with exploration of
    • Constant work-rate regimens;
    • Self-regulated exercise regimens; and
    • Controlled hyperthermia regimens

While less “scientific” than the previously noted resources, Arthur Webb’s article Heat Training in the Sauna found on the Badwater website worth reviewing. While less of a research piece, this article offers insight into the actual process someone uses to heat acclimate for Badwater. There’s even a day-by-day sauna time and temperature schedule… which reassured me that folks can survive the type of sauna sessions I wanted to attempt.

My Heat Acclimation Experience

Enough about about smart people with penchants for authoring well-referenced academic medical publications and more about me. Most people would follow the conventional wisdom about heat acclimation and do something along the lines of hitting the sauna for two or three 10 minute periods during their first heat acclimation sessions, but not me. Instead, I saw Greg’s mention of 50 minute sessions and jumped right in.

On Monday, I went to Gold’s Gym on a free trial membership. After waiting around I was pleasantly surprised to learn they had a dry sauna rather than a steam room. “Perfect,” I thought to myself. Not long thereafter, I found myself sitting in a 160 F room. No, that’s not a typo there, it was one-hundred and sixty oven-like degrees in the room. I felt like a roasting turkey. All I lacked was a thermometer sticking out of me. It took a couple minutes for the sweat to start rolling off me and it wasn’t long after that when I began feeling slightly nauseous. By about 10 minutes, I was setting into a groove. I just sat there or stood up and did some light stretching. By 20 minutes, I was quite uncomfortable and not at all enjoying myself. I did some more stretching and was amazed at how much greater my flexibility was than normal – I felt like rubber man. When not stretching during this time, I was trying to read a fellow MdS runner’s analysis of the race’s nutritional needs and was having a really hard time comprehending it. My lack of comprehension was not because the material was overly difficult or poorly written (the analysis has been incredibly approachable and useful at normal temperatures); rather, my mental faculties were clearly deteriorating. By 25 minutes, I was kinda scared and really pressing to stay in the sauna. Around 27 or 28 minutes, I was freaked out and settled on bailing at 30 minutes. Those last two minute seemed to take forever. Upon leaving the sauna, I went straight for a cool shower to lower my core temperature.

Less than 10 minutes later, I was back in the sauna to finish up my 50 minutes. These 20 minutes went by much like the first except I was slightly more in control of my emotions. I can’t say the same for my cognitive ability. Reading and understanding took my full concentration and even then I felt like I was missing things. I was very happy for the session to be over, but felt very, very weak afterwards. Within 15 minutes, I was at Chiptole. Sure, I was picking up dinner, but what I really wanted was a huge iced beverage with some quick calories to restore my strength.

I decided to be all gung-ho and jumped back in the sauna again on Tuesday, this time with the intention of remaining for the full 50 minutes. I didn’t try to read at all during this session and listened to music instead. Even without the need for deep thought, I once again caught my mind faltering. This was really eye opening in that before I head to Morocco I need to have my nutrition and hydration decisions made and have them down pat. I also have to be on the lookout for heat-induced mental retardation and only slowly and deliberately make decisions once I notice this effect.

Another thing I need to be on the lookout for is panic. I was seriously on the verge of panicking a couple times during this second session. The panic ultimately lead to me leaving the sauna after 45 of the 50 minutes that I had intended to be in the sauna. This was a good lesson to learn before hand. Knowing that this could be a real problem on the course, I’ve already set out to find ways to combat the panic.

Aside from becoming aware of the slowed cognition and panic that heat can induce, this second session was damn stupid. I felt horrible went I was done. I stood in the cold shower for 7 or 8 minutes immediately afterward, all the while feeling very weak and nauseous. On Wednesday morning I was completely without energy on my run to work. I continued to feel “off” throughout the day and into the evening. Stupid, stupid Bryon.

One fun thing that did come out of Tuesday’s sauna session was a heart rate profile that clearly shows the effect of heat exposure on heart rate over time. The first 45 minutes show my time in the sauna with a steadily increasing heart rate despite the fact that I rarely moved much. From 45 to 52 minutes, I was standing in a cold shower attempting to lower my core temperature as quickly as possible during which time my heart rate plummeted. From 52 to 60 minutes, I was getting to and leaving Gold’s. After that, I was running home. Enjoy … I sure didn’t! ;-)

The effect of heat (160 F dry heat) on heart rate over time.


  • Have you ever tried to heat acclimate for a race?
    • If so, what did you try?
    • Do you felt like it worked?
    • Have any interesting stories?
  • Do you know of any other useful heat acclimation resources?
Bryon Powell

Bryon Powell is the Founding Editor of iRunFar. He’s been writing about trail running, ultrarunning, and running gear for more than 15 years. Aside from iRunFar, he’s authored the books Relentless Forward Progress: A Guide to Running Ultramarathons and Where the Road Ends: A Guide to Trail Running, been a contributing editor at Trail Runner magazine, written for publications including Outside, Sierra, and Running Times, and coached ultrarunners of all abilities. Based in Silverton, Colorado, Bryon is an avid trail runner and ultrarunner who competes in events from the Hardrock 100 Mile just out his front door to races long and short around the world, that is, when he’s not fly fishing or tending to his garden.