Relentless Forward Progress_ A Guide to Running Ultramarathons - Bryon Powell [72]
Aside from the increase in plasma volume, there are some other physiological adaptations that occur during heat training— changes in sweat rate, changes in sweat sodium concentration, and changes in core resting temperature, to name a few. The various adaptations occur with different amounts of acclimatization. Here’s a graphical representation of the times over which an athlete can gain these benefits:
In my own low-intensity heat acclimation program for the 2009 Marathon des Sables,* my heart rate at the same speed and temperature decreased by about 9 percent—a very significant improvement—following just nine sessions. Now, it didn’t get as low as what I would expect it to be in “normal temps,” but the improvement was significant, and could be expected to translate into a tangible benefit in competition.
* During the acclimation program, I ran on a treadmill at 60 percent VO2 max for 45 to 75 minutes for nine sessions. The first three sessions were at 95 degrees F (35 degrees C); the next five were at 113 degrees F (45 degrees C). The last (ninth) session occurred at 95 degrees F again—this allowed us to compare my physiological data from the first and the last sessions.
Heat Acclimation Methods and Considerations
The work needed to achieve the benefits of heat acclimation is reasonable. Most laboratory-based heat acclimation protocols have athletes spend about one hour a day in a heat chamber for 7 to 10 days. Importantly, this needs to occur as close to the time of the competition as possible, as the adaptations conferred by acclimation decay rapidly without ongoing exposure. So there’s no point in spending two weeks in a heat chamber a month before the race without intervening heat exposure—the effects will decay in one to three weeks.
Estimates of heat acclimation decay vary, but it’s possible that you could lose half of the benefit in 10 days without ongoing heat exposure. This raises some logistical problems for athletes living in cold environments who are attempting to acclimate for a hot-weather event. To benefit maximally from acclimatization, the heat training sessions should occur as close as possible to the event. That seems pretty straightforward. The problem is that acclimation is quite physically demanding, and most athletes attempt to taper in the week(s) prior to a big race. So if you want to acclimatize optimally, it needs to occur during your taper— which may cause overtraining, or at least minimize the benefits of tapering.
As with all training, the more specific, the better. When it comes to heat acclimatization, this means that your training climate should reflect the competition environment as closely as possible—the same temperatures as well as the same humidity. Why is humidity important? As anyone who has survived an East Coast summer knows, humidity makes it harder to lose heat via sweating. Training in a humid environment does confer some benefit if you are training for a dry, hot race, but not as much as training in a dry, hot chamber. Interestingly, there is better “transfer” of acclimation if you train in a dry, hot climate and then race in a humid, hot race than the other way around. So as much as possible, match humidity and temperature of your acclimation phase to your race environment.
What about passive acclimation? That is, will sitting in a sauna at the YMCA get us ready for running in Death Valley? Essentially—a bit, but not much. Acclimatization is vastly greater (and more rapid) if you exercise during the heat exposure. Whether this is again the principle of specificity, or whether it’s simply that core temperature rises faster with active acclimation (increased core temperature is probably the stimulus for the adoptions that occur), is not clear.
Stay Hydrated!
One of the most important ways to prevent heat injuries and maintain performance in extremely hot environments is to drink adequate amounts of fluid. This seems obvious, but it is actually surprisingly hard