Ultrarunners’ Heat Acclimation Cheat Sheet

By Jason Koop,
Head Coach of CTS Ultrarunning

With what looks to be a very hot edition of the Western States race coming up in just a few weeks, this is a good opportunity to review heat acclimation protocols. It is a little bit late in the game to start the “Repeated Exposure” protocol described below, but you still have time for the “Single Exposure” protocol. And if you are already engaged in a heat acclimation strategy, there is information below that may help you do it better. If you are not doing Western States, consider these heat acclimation protocols before any of the hot weather races you have coming up this summer.

The science and practical recommendations related to heat acclimation has come a long way in recent years, in part due to the Tokyo Olympics and Paralympics, which was one the hottest Olympics on record. Following the Games, many national teams and sports scientists divulged the heat acclimation and mitigation strategies they used. About the same time, Julien Périard out of the University of Canberra Research Institute for Sport and Exercise release a treasure trove of science-based practice in this behemoth of a review paper in the American Physiological Society Physiological Reviews. The paper (I triple dog dare anyone to read all 108 pages) comprehensively went through every single detail of our response and adaption to hot weather environments. Based on this great research on heat acclimation, here’s what you need to know before your next hot weather race.

Why does heat acclimation matter?

Ultrarunners did not always get their heat training protocols right. My first exposure to this was in the early 2000’s working with athletes training for the Badwater 135. At that time, it was commonplace for just about every athlete participating in that race to partake in this bizarre ritual of wheeling their treadmill into the laundry room, disconnecting the dryer vent, pointing said dryer vent at the treadmill and turning the dryer on full blast to simulate the searing winds of Death Valley in July. But the contrivance did not stop there. Athletes would also crank up the heat in the room via a space heater (maybe two for good measure) and, finally, layer up with long sleeves, pants and Rocky-esque sweatpants… as if somehow all the heat adaptation strategies were greater than the sum of their parts.

What these early athletes failed to realize, and what we now can derive from sports science since that time, is that it takes relatively little effort to achieve a reasonable degree of heat acclimation. Yet, athletes still have a selection to make when choosing their heat acclimation protocol of choice. Do you use a hot bath? A dry sauna? What about layering up in down jackets as advocated by a few in the ultrarunning space? The research and practice mentioned above provide some straightforward guidance on which protocols athletes should adopt and avoid.

Benefits of Heat Acclimation

What adaptations are we trying to achieve with heat acclimation? Well, you can read more about preparing for exercise in heat in this article, but as a short synopsis, the benefits include:

• Start sweating sooner: To stay ahead of rising core temperature, your body kicks your primary cooling system (sweat) into action earlier than when not heat acclimated.
• Sweat more profusely: Your body learns to open the floodgates to get more fluid onto the skin surface for evaporative cooling.
• Sweat more evenly: You have sweat glands all over, and your body needs to increase evaporative cooling you’ll start sweating from everywhere.
• Retain more sodium: Your sweat glands adapt to retain more sodium and electrolytes even as sweat rate and fluid losses increase. This, along with regulation from your kidneys, helps maintain sodium concentrations in the body.
• Increase plasma volume: Your blood is what’s transporting heat from your core to your skin for radiant cooling as well as evaporative cooling. Increased plasma volume increases your capacity for heat transfer, and provides fluid for sweat.

Priority of Methods

A quick scan of the paper by Périard reveals a litany of different protocols one could undertake to gain an edge in hot weather environments (see chart below). Options range from exercising in the heat, using a combination of exercise and environment to achieve a predetermined internal temperature, or simply sitting in a hot bath or sauna. If you comb through the literature, each protocol has its own merits and detriments. Some have the potential to cause large disturbances in your day to day run training while others are intended to cause as few conflicts as possible. Some take as few as 20 minutes to complete while others will take a full 90 minutes per session.

To cut through the clutter, I developed the simple hierarchy below, based on the protocols that produce the best effect with as little compromise to your day-to-day workload as possible. Think of it as a cheat sheet the next time you prepare for a hot weather race. In summary, if you are preparing for a hot weather race and have access to a sauna, use it. If you don’t have access to a dry sauna, use a hot water immersion bath. If you don’t have access to a hot water immersion bath, use a wet sauna. And finally, if you don’t have access to any of these, consider using a climatic chamber (like the contrived ones mentioned earlier that Badwater runners used). As a last resort, run in that grey sweatsuit you have from college.

Figure 7.7 from “Training Essentials for Ultrarunning”

Things to keep in mind:

• Any heat acclimation protocol will induce a training stress. Therefore, you will have to take some training off the table somewhere else when undergoing any of these.
• You need seven to ten days of heat exposure in a row, ideally, to elicit positive adaptations.
• After heat acclimation is achieved, you can maintain these adaptations by incorporating exposure to heat again once every three days.
• The average degradation of heat acclimation is ~2.5 percent a day after the first forty-eight hours without heat exposure. If you were to do a two-week taper with no heat exposure, you could expect a decay in heart rate control of ~35 percent, a decay in core temperature control of ~6 percent, and a decay in sweat rate improvements of ~30 percent. If you are traveling to an event in a hot environment, it is important to know you can generally reacclimate in as little as four days.

Practical implementations

Knowing what protocols work best is one thing, practically implementing them into a training program is an entirely different matter. This is an area where the literature fails to present a comprehensive picture. From marathoners, ultramarathoners, race walkers and even this podcast with USOPC Physiologist Lindsay Golich, a loose consensus of heat acclimation strategies emerged, utilizing either a sauna or hot water immersion, both of which are outlined below (note I am using the word ‘sauna’ below to be both a dry sauna or a hot water immersion bath). Depending on how regularly you have access to a sauna or hot water immersion bath, you can either use a single or repeat exposure as described below.

Protocol #1: Single Exposure Leading up to the Race

If you have limited time and resources to gain access to a sauna, adding in a heat acclimation protocol in the several days leading up to an event is going to be your best choice. However, take extra precaution that you taper more than you normally would, as the sauna will add additional stress on your system, which is contraindicated for the taper.

Protocol #2: Repeated Exposure

If you have more regular access to a sauna and if your training leading up to an event is going particularly well, this protocol might be more effective for you. This protocol, while more complicated, mitigates the risk of overdoing the sauna exposure close to the race. For this protocol, you take full advantage of a prolonged period by acclimating in two separate bouts. One occurs approximately six weeks out from the event, and the second occurs in the final week of the event. In between, you will have several heat “maintenance sessions” approximately every third or fourth day so that when you are ready for the final re-acclimation period it can be faster and more effective.

These maintenance sessions also partially mitigate the ~2.5 percent per day decay in heat acclimation mentioned previously. Additionally, we know that repeated exposure to hot environments have an additive effect. So, you might get a slightly better adaptation from this protocol. The downside of this protocol is that the first acclimation period (about six weeks out from an event) may coincide with heavy and important training. If this is the case for you, consider protocol #1 so you are not adding so much stress all at the same time.

This summer promises to be a hot one and more ultramarathons are being run in extreme temperatures each year. Research and practice always drive our understanding of effective training interventions, and heat acclimation is one area where the convergence of these two is necessary to achieve best results. Regardless of when your next hot weather race is, keep this cheat sheet handy so that you can utilize it for your best performance.

References:

Périard, Julien D et al. “Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies.” Physiological reviews vol. 101,4 (2021): 1873-1979. doi:10.1152/physrev.00038.2020