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Injury Prevention

Running Hot: Part 2

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Tawnee_PrazakThe ins and outs of heat acclimatization.

Written by: Tawnee Prazak

In Part 1 of this series we discussed the physiological detriments to exercise in heat. Fortunately, the human body can adapt to the stress of heat with repeated exposure, resulting in less strain and increased comfort. So whether you’re training for a hot race or live in a hot climate, here’s what you need to know about heat acclimatization for optimal performance.

What happens with acclimatization?

1. You become a better sweater.

After acclimatizing to exercise in the heat we begin to sweat earlier, we sweat more and at a faster rate, our sweat glands fatigue less and the body better distributes sweat. There’s also less sodium content in the sweat, which helps with water retention and maintaining fluid levels to prevent dehydration. The result of better sweating is better cooling, meaning skin and core temperatures stay within reason and early fatigue, hyperthermia and heat illness are less likely. Fluid intake is essential to maintain these functions (see “Role of hydration” below).

2. Cardiovascular function improves.

Benefits include a decrease in heart rate, increase in plasma volume and improved blood flow. With more plasma, there’s more blood available to go to the skin’s surface for heat dissipation and to the working muscles. Most important, adaptations put less stress on the heart, resulting in a decrease in heart rate at any given intensity and increase in stroke volume (amount of blood pumped out of the heart chamber).

3. Exercise performance increases.

The non-acclimatized person will run slower and fatigue faster in heat; but, as acclimatization takes place, these negative effects will fade and running capacity and performance will increase due to improved thermal comfort and lower perceived exertion.

How to Acclimatize

1. Follow your own timeline.

Each runner is slightly different, but generally it takes about two weeks of continual training in the heat to acclimatize. Averaging at least one hour of moderate exercise daily in hot conditions is enough to get results in that timeframe. Gradually ease into heat training by keeping a low intensity at first (quality runs can be done in a cool gym). Depending on your normal running volume, fitness level, and natural heat tolerance, you may need to reduce your running volume slightly or significantly in the first days of heat exposure. As adaptations occur, intensity and volume can increase.

Certain adaptations occur quicker than others. Decreased heart rate can occur in as little as five days, while changes in sweating response could take weeks. Furthermore, trained endurance athletes usually adapt faster than untrained individuals.

2. How long does acclimatization last?

Unfortunately, adaptations to heat are reversed at twice the rate they are acquired once exposure to heat ends. Generally, most adaptations will be lost in about two weeks. The first to go are cardiovascular improvements.

3. Adapt your own environment.

Training in cool conditions for two to three months at intensities above 50 percent VO2max (66 percent of max heart rate) will evoke some physiological adaptations that are beneficial to exercising in heat. The key is maintaining an elevated core temperature during long, continuous exercise bouts. The adaptations aren’t as substantial as heat training itself, but they’re better than nothing. There’s also the option of creating an artificial hot and/or humid environment by wearing extra clothing during exercise, training in a hot room, using a humidifier, etc.

4. Hydrate properly.

Guzzling fluid does not enhance the effects of acclimatization. However, failure to hydrate enough to replace sweat loss has negative implications that are intensified in heat, negating adaptations to heat. Consequences of under-hydrating include decreased skin blood flow, decreased sweat rate and heat dissipation, reduced blood volume and increased core temperature. A decrease of just 2 percent in body weight via sweat loss means dehydration is setting in and performance will start to suffer.

Fluid requirements may double when exercising in heat. To find your specific needs, practice weighing yourself before and after exercise. Weight lost, if any, is weight that needs to be replaced via hydration.

5. Plan your race strategy.

Arriving at the event site at least two weeks before the race to train would be ideal; however, this is not always practical and trying to acclimatize right before a race can interfere with tapering. If heat training occurred at home, arriving three to seven days before a race is advised to allow further adjustments to the specific climate.

Also, research the weather conditions at the race venue. Dry heat and humid heat are very different, and adaptations made in dry heat may not provide necessary benefits for exercising in humidity (remember: humidity hinders sweat evaporation, and thus negatively affects cooling).

Bottom line: Be specific. Try to train in and exposure yourself to the specific climate in which you’ll be racing.


Tawnee Prazak is a certified triathlon coach, exercise science grad student and triathlete. Find out more at