Heat Acclimation

I am not an exercise scientist, but I do have a strong interest in the science of endurance exercise, and every once in a while I speculate on the kinds of questions exercise scientists like to explore experimentally. For example, back in 2004 I found myself wondering if training in a hot environment might improve endurance performance in a temperate environment, sort of like how training at high altitude improves endurance performance at low altitude. My curiosity led me to put the question to famed sports science researcher Tim Noakes, who, in his prompt and courteous reply, dismissed the idea as “too bizarre to consider.”

Six years later, sweet vindication came my way in the form of a study appearing in the Journal of Applied Physiology under the title “Heat acclimation improves exercise performance.”

Led by Santiago Lorenzo of the University of Oregon, the study involved 20 highly trained cyclists, who were asked to complete a performance test in temperate conditions on two occasions separated by 10 days. Between the tests, all 20 cyclists completed a prescribed training program, but 12 of them did it in a controlled, hot environment (100 degrees Fahrenheit) while the other eight performed their workouts in the same temperate conditions (55 degrees) as the performance tests. The 12 cyclists who underwent heat acclimatization improved their performance in the temperate performance test by a massive 7 percent, while the control group showed no improvement.

Lorenzo’s team attributed the performance-boosting effects of heat acclimatization on endurance performance in cool conditions to improved efficiency in heat dissipation and increased blood volume. They also found evidence that it caused some changes in muscle cell enzymes, which may have contributed to the effect as well.

Several subsequent studies have mined the same vein vein more deeply. The most recent study on heat training in endurance athletes, published in the European Journal of Applied Physiology, offers important guidance on how best to use this method in real-world settings. Led by Mark Waldron of Swansea University, the experiment aimed to track the time course of adaptations to heat training.

Twenty-two male cyclists were separated into experimental and control groups. Members of the experimental group cycled indoors at 100 degrees Fahrenheit while members of the control group did an equal amount of cycling at 68 degrees. Waldron’s team measured VO₂max in both groups before the intervention, on days five and ten of the intervention, and on days one, two, three, four, five, and ten afterward.

The results are interesting. Both groups exhibited an initial decrease in VO₂max during the 10-day training period that was followed by a rebound beyond baseline afterward. The peak increase was higher in the heat-training group, but not until four days after the last heat-training session, with some variation between individuals. VO₂max then began to trend toward decline in this group, though the amount of decline that occurred between day four to day 10 post-acclimation did not reach statistical significance.

In a nutshell, these findings suggest that if you’re going to use heat training to increase your endurance performance, you need to time it to end about four days before you race. This means that your heat training is likely to overlap with your pre-race taper. Is this insane? It might sound so, but there’s a difference between sound and substance. While training in 100-degree heat might be uncomfortable, it’s not going to kill you, and which would you rather do: 10 days of heavy, peak training in 100-degree heat or 10 days of lighter, taper training?

That being said, I don’t recommend that you try heat training for the first time before an important race. Instead, test it out early in a training cycle to see how it affects you. It won’t be wasted even then, because if it works it will give your subsequent training a nice boost.

I can’t help but wonder if doing one hot workout every week or so throughout a training cycle might have similar benefits. Personally, I would find this approach easier to manage. Heat training could then be used in much the same way carb-fasted workouts are, and perhaps the two methods could even be combined to minimize the number of training days that need to be set aside as “special” sessions. Can I get a real exercise scientist to look into this?