Heat Acclimation Training

I've done quite a bit of Running in the Heat, from North Carolina's brutal summers, to winning the 2010 Keys 100 or pacing Chris Moon at the Badwater 135.

Heat acclimation training can improve performance in hot and cold conditions. It also helps protect against heat injury and is particularly important when training for spring races. However, heat adaptation training can be dangerous, and care must be taken to avoid injury or death.


1 Introduction

Training for a spring race requires extra caution as you will have been training through the winter and be unprepared for warm conditions. While a spring race may be cool, there is also a risk of conditions that are warm enough (above 40f) to impair performance (see [Impact of Heat on Marathon Performance] for more details). Heat acclimation training, sometimes called heat adaptation training, can prepare you for these warmer conditions. This type of training is also valuable if you are traveling to a warmer climate for a race, or if you are training in the cool part of the day for a race in the warmer times. In addition, heat acclimation can improve cold weather performance. One study[1] showed that heat acclimation improved performance in the cold by 6% and by 8% in heat.

2 Background

Exercise becomes harder as the temperature rises, with 40 degrees Fahrenheit being close to optimal. Exercise in the heat causes blood vessels in the skin to expand to help with cooling. The demands of the extra blood for cooling creates added stress on the cardiovascular system[2]. The athlete's body will also sweat to produce cooling; in dry conditions evaporation of sweat provides 98% of cooling and in humid conditions 80%[2]. The loss of fluids due to sweating can lead to dehydration that also impairs performance. The impact of dehydration is in addition to the impact of the heat[2].

3 Danger of Death

A dedicated athlete can push themselves hard enough to raise their core temperature to dangerous levels, leading to heatstroke, which can be fatal[3]. Heatstroke can be the result of prolonged exercise in hot conditions, but it can also be the result of shorter periods of high intensity exercise, especially in the untrained or overweight. It is vitally important that heat acclimation training is started gradually. You must become aware of how your body is adjusting to the heat, and to learn the warning signs of elevated core temperatures. Training in heat suits (see below) is especially dangerous, as the heat will not escape even after you collapse! Generally, an athlete reaches 'voluntary exhaustion' when their core temperature reaches about 39c/102f[4], so never push hard with heat acclimation. I would take it as a personal favor if you could avoid killing yourself.

4 Symptoms of Heat Stroke

If you have any of the following symptoms while performing heat acclimation training, you should stop and cool off. Taking your internal temperature, ideally with an in-ear thermometer will allow you to double check if this is heat stroke. Heat stroke is caused by an internal temperature of >40.6 °C (105.1 °F), is extremely dangerous and can be fatal. The following can be symptoms of heat stroke:

  • Nausea or vomiting. These symptoms can occur before true heatstroke, as running makes digestion harder.
  • Weakness. An unusual muscular weakness could be due to low blood sugar, but elevated core temperature also creates weakness.
  • Headache. This can also be caused by dehydration, or low blood sugar. Having had headaches from each of the three causes, I have found the type of headache is different. My limited experience is that a headache caused by heat is particularly painful and intense.
  • Dizziness or confusion. This is a serious symptom that suggests either extremely low blood sugar or heatstroke.
  • Panting. My personal experience is that when my core temperature rises, I start to breathe more rapidly. The panting can occur at rest in a sauna, or during exercise where my breathing rate is far deeper and faster than would be typical for the exercise intensity. This panting may be a reflect that attempts to cool the body in the same way as a panting dog. I find this is the first warning of problems and a sign that I have to reduce the heat stress rapidly.

If you have any doubts, stop and check your temperature. Never do High Intensity Interval Training as part of heat acclimation; the intense work can spike your core temperature too high too quickly for you to recover.

5 Practical Heat Training

The following advice should be used as guidance for heat acclimation training. Please use caution and common sense.

  • Like any new training routine, start off slowly and build up both duration and intensity over time.
  • Measure your core temperature, ideally with an in-ear infrared sensor[5][6][7]. I use a Braun ThermoScan and I've been pleased with it.
  • Beware a rapid rise in core temperature. Your temperature could continue to rise for a while after you stop exercising, which can be dangerous.
  • Aim for a core temperature of 38.5c/101.3f and do not let your core temperature go over 39.0c/102.2f. This is based on my review of heat adaptation studies shown below.
  • Your core temperature should rise over about 30 minutes to your target core temperature.
  • In addition to monitoring your core temperature, be aware of how you are feeling and avoid pushing too hard.
  • Build up to exercising at 50% VO2max or above[8]. 50% of VO2max is about 70% of maximum Heart Rate[9] or "very slow running"[10].
  • Use gradually increasing periods from 30 to 100 minutes over 10 to 14 days[11]
  • Acclimation is fully developed after 7 to 14 days[8], but up to 75% of acclimation is reached after 5 days [11].
  • Reduce your training load to compensate for the added stress of the heat. The heat can make you far more tired than you would expect.
  • Consider alternating heat acclimation training and cooler training to preserve intensity[12]
  • Training in a warmer environment is ideal, but creating a microclimate (see Heat Suit below) by overdressing also works[12]
  • Exercise in heat produces better acclimation than passive heat[13], but passive heat (sauna) following exercise can also be quite effective[14].
  • Maintain hydration levels, as dehydration may impair key adaptations, such as increased sweat rate[15].
  • Once gained, heat adaptation can be maintained for at least a month by training in the heat every five days[16]. Without continued heat exposure, it's estimated that 2.5% of adaptation is lost each day[17].
  • Don't combine heat and altitude training as the altitude may impair heat adaptation[18].

6 Fellrnr Heat Suit

This 'heat suit' will allow for heat training even in quite cool conditions.

Ambox warning pn.svg

This heat suit works by preventing the body cooling itself, so it increases the risk of heat stroke. If you overheat wearing this heat suit, you may not cool off after you collapse. Please be careful taking this approach and start off with very low intensity exercise. Build up the duration and intensity very gradually and monitor for warning signs. Please be careful, as I've had some close calls using this technique and it is dangerous. See "Symptoms of Heat Stroke" above.

The key to the Fellrnr Heat Suit comes from two waterproof layers. A traditional sweat suit uses a single waterproof layer to trap your sweat and preventing it from cooling your body. The problem with the single layer is that the sweat soaks through any clothing and reduces the insulation. In cooler conditions the sweat soaked clothes can become chilly even with the waterproof layer. The Fellrnr Heat Suit approach traps the sweat away from the insulation layer, preventing this cooling effect. The Fellrnr Heat Suit has the following layers over both your top and legs.

A close fitting thermal layer near the skin. The purpose of this layer is to hold the sweat and keep your skin relatively comfortable. I wear a Halo hat to prevent the sweat getting in my eyes, plus a thin hat to prevent the waterproof hood sticking to my ears.
A waterproof layer that traps the sweat in the first thermal layer. Ideally this should be a non-breathable waterproof layer.
An insulation layer, such as fleece that prevents any heat escaping. Because it is trapped between the two waterproof layers, it never becomes wet.
A second waterproof or windproof layer that traps the body's heat in the insulation layer. In addition, wear hat, gloves and ideally a neck warmer or face mask.

This combination will prevent most of the heat escaping your body.

7 Steam Shower

My steam shower, with a nice mosaic and a bench to relax on.

I have a steam shower which I also use for heat acclimation, and it's wonderful. We put in the Thermasol Smart Steam system that was expensive, but well worth it. It provides rapid steam, with the initial steam after just a minute or so, and serious heat stress after about 5-10 minutes. If you turn it up, it will exceed your ability to cope quite quickly, or provide extended exposure at more moderate levels.

8 The Science of Heat Acclimation

For those who want to know more details about heat acclimation, here is a summary of the scientific data.

8.1 Core Temperature for Heat Acclimation

While most studies specified an ambient temperature and humidity, there are some that specified a target core temperature and time. This suggests that a core temperature of 38.5c/101.3f is a reasonable target and should be safe. You should keep your core temperature under 39.7c/103.4f and you should rapidly cool off if you hit that level. Your core temperature can continue to rise after you finish exercising, and if your core temperature goes too high you may be mentally impaired, so think ahead.

  • Target core temperature of 38.5c/101.3f for 90 minutes, with the target temperature reached over about 30 minutes[19][20][21][22].
  • Target core temperature of just over 39.0c/102.2f, for 2x 50 minutes with 10-minute recovery[23] or for 2 hours[24]
  • Target core temperature of 38.5c/101.3f or 39.0/102.2f for 90 minutes [25] (similar results.)
  • Passive heat exposure with core temperatures of 38.5c/101.3f to 39.0/102.2f for 2 hours[26]
  • One study explicitly set a core temperature limit of 39.7c/103.4f[27] but didn't give the rationale for that cut off. This is close to heat stroke temperatures, and it's not clear why the cutoff was chosen to be so high.
  • The diagnostic criteria for heat stroke is 40c/104f[28]/ Obviously this is a clear upper limit for core temperature.

8.2 Changes with heat acclimation

Heat acclimation will produce several benefits

  • Increased blood volume[29].
  • Sweating occurs at lower temperatures[12].
  • Sweat contains less electrolytes[30].
  • Sweating is more profuse[4].
  • Increased cardiac output in hot conditions[4].
  • Reduced core temperature for given exercise time and intensity[4].
  • The athlete becomes psychologically prepared for heat stress. [13].
  • The ability to consume and absorb more fluids (anecdotal evidence only).

8.3 Notes on Heat Acclimatization

  • Younger runners do better in the heat than older runners, but training can negate this[8]
  • Acclimation is faster in fitter athletes[8]
  • On return to a cool climate, acclimation lasts for about a week, then decays[8]
  • People who have always lived in hot climates are believed to have superior adaptation[12]
  • A sauna will produce similar heart rate and cardiac stress as 60-100w of exercise[31]. My experience matches that estimate for heart rate, though my respiration tends to be much higher as my body attempts to pant to cool off.

9 See Also

10 References

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  2. 2.0 2.1 2.2 DJ. Casa, Exercise in the heat. I. Fundamentals of thermal physiology, performance implications, and dehydration., J Athl Train, volume 34, issue 3, pages 246-52, Jul 1999, PMID 16558572
  3. HM. Binkley, J. Beckett, DJ. Casa, DM. Kleiner, PE. Plummer, National Athletic Trainers' Association Position Statement: Exertional Heat Illnesses., J Athl Train, volume 37, issue 3, pages 329-343, Sep 2002, PMID 12937591
  4. 4.0 4.1 4.2 4.3 B. Nielsen, JR. Hales, S. Strange, NJ. Christensen, J. Warberg, B. Saltin, Human circulatory and thermoregulatory adaptations with heat acclimation and exercise in a hot, dry environment., J Physiol, volume 460, pages 467-85, Jan 1993, PMID 8487204
  5. Roberta S. Erickson, Sharon K. Kirklin, Comparison of ear-based, bladder, oral, and axillary methods for core temperature measurement, Critical Care Medicine, volume 21, issue 10, 1993, pages 1528–1534, ISSN 0090-3493, doi 10.1097/00003246-199310000-00022
  6. ??lker Devrim, Ate?? Kara, Mehmet Ceyhan, Hasan Tezer, Ali Kerem Uluda??, Ali B??lent Cengiz, ??nci Yi??itkanl, G??lten Se??meer, Measurement Accuracy of Fever by Tympanic and Axillary Thermometry, Pediatric Emergency Care, volume 23, issue 1, 2007, pages 16–19, ISSN 0749-5161, doi 10.1097/PEC.0b013e31802c61e6
  7. Marianne M. Nimah, Khaled Bshesh, Janice D. Callahan, Brian R. Jacobs, Infrared tympanic thermometry in comparison with other temperature measurement techniques in febrile children, Pediatric Critical Care Medicine, volume 7, issue 1, 2006, pages 48–55, ISSN 1529-7535, doi 10.1097/01.PCC.0000185476.35550.B2
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  10. http://www.brianmac.co.uk/vo2max.htm
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  12. 12.0 12.1 12.2 12.3 Timothy Noakes, Lore of running, date 2003, publisher Human Kinetics, location Champaign, IL, isbn 0-87322-959-2, pages 188
  13. 13.0 13.1 Taylor, Nigel AS, and James D. Cotter. "Heat adaptation: guidelines for the optimisation of human performance: review article." International SportMed Journal: The Extreme Environment and Sports Medicine: Part 2 7.1 (2006): p-33.
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  15. Gavin Travers, David Nichols, Nathan Riding, José González-Alonso, Julien D. Périard, Heat Acclimation with Controlled Heart Rate, Medicine & Science in Sports & Exercise, 2020, pages 1, ISSN 0195-9131, doi 10.1249/MSS.0000000000002320
  16. J. Luke Pryor, Riana R. Pryor, Lesley W. Vandermark, Elizabeth L. Adams, Rachel M. VanScoy, Douglas J. Casa, Larry E. Armstrong, Elaine C. Lee, Lindsay J. DiStefano, Jeffrey M. Anderson, Carl M. Maresh, Intermittent exercise-heat exposures and intense physical activity sustain heat acclimation adaptations, Journal of Science and Medicine in Sport, volume 22, issue 1, 2019, pages 117–122, ISSN 14402440, doi 10.1016/j.jsams.2018.06.009
  17. Hein A. M. Daanen, Sebastien Racinais, Julien D. Périard, Heat Acclimation Decay and Re-Induction: A Systematic Review and Meta-Analysis, Sports Medicine, volume 48, issue 2, 2017, pages 409–430, ISSN 0112-1642, doi 10.1007/s40279-017-0808-x
  18. Erin L. McCleave, Katie M. Slattery, Rob Duffield, Philo U. Saunders, Avish P. Sharma, Stephen Crowcroft, Aaron J. Coutts, Impaired Heat Adaptation From Combined Heat Training and "Live High, Train Low" Hypoxia, International Journal of Sports Physiology and Performance, volume 14, issue 5, 2019, pages 635–643, ISSN 1555-0265, doi 10.1123/ijspp.2018-0399
  19. Andrew T. Garrett, Niels G. Goosens, Nancy G. Rehrer, Mark J. Patterson, James D. Cotter, Induction and decay of short-term heat acclimation, European Journal of Applied Physiology, volume 107, issue 6, 2009, pages 659–670, ISSN 1439-6319, doi 10.1007/s00421-009-1182-7
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  23. Matthew Kuennen, Trevor Gillum, Karol Dokladny, Edward Bedrick, Suzanne Schneider, Pope Moseley, Thermotolerance and heat acclimation may share a common mechanism in humans, American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, volume 301, issue 2, 2011, pages R524–R533, ISSN 0363-6119, doi 10.1152/ajpregu.00039.2011
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  30. Timothy Noakes, Lore of running, date 2003, publisher Human Kinetics, location Champaign, IL, isbn 0-87322-959-2, pages 214
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