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At around 1 p.m. local time during the 2019 Boston Marathon, thousands of runners aiming for a finish over three hours ran smack dab into something that seemed far more difficult than Heartbreak Hill: heat stress.
The morning started out with high humidity, but it was cool and rainy. As the race continued, the temperature rose and the sun came out. Suddenly, according to CBS News, medical staffers were dealing with dozens of cases of heat stroke. Some athletes said it felt like they were being steamed, with the moisture from the ground below and the heat from the sun above making them into very unhappy broccoli. And according to most Boston weather stations, the high temperature was only 70 degrees Fahrenheit.
The dew point was around 60, meaning there was relatively high humidity, but according to a table compiled by Runner’s Connect, that should only impair performance by two or three percent tops, and only at the very end of the race. A 2012 article in the journal PLoS One looked at 1.8 million marathon results, and it indicated that the Boston temperature would have a moderate overall effect on performance later in the day. So what explains the big brick walls that some (but not all) Boston runners found themselves hitting with full force?
The easy answer is inadequate training or poor pacing. The even easier answer is that road marathons are designed by the Devil to remind us of the futility of our striving. The more interesting answer is that maybe some of the variation in late-race performance was related to how some athletes’ individual physiologies interacted with the conditions. The same might go for the Lake Sonoma 50-Miler, where a high sun and mid-70s temperatures felt warm for unacclimated athletes. As Bryon Powell of iRunFar said the day before the race in a tweet, “It’s currently 79F in Windsor, California. Feels like I’m on the surface of the sun.”
Studies show that heat can cause substantial performance reductions for unacclimated athletes, though the effects are highly dependent on individual physiology. That variation may be key here. Do you struggle with heat like I do? For example, a few years ago, my wife and I went out for a 10-mile run in California, with temperatures around 80F by the end, making it our first warmer run of the year. (Though it’s California, so there was no humidity and it was really beautiful and now I am remembering why every Bay Area two-bedroom house costs $8 million.)
We both had been running in the early mornings, so neither of us had been exposed to heat. At mile eight, our normal easy run pace had my heart rate shoot up to 170 beats per minute (around 30 to 40 beats higher than normal). My heartbeat rang through my earholes, and I had to finish the run with a brisk walk that led straight to the pool. Meanwhile, Megan didn’t even notice the heat. Perhaps my ancestors were from Antarctica. Possibly they were penguins.
Fortunately, as the weather heats up in the Northern Hemisphere, you don’t have to resign yourself to suffering on every lukewarm day. Important note: heat stress provides a big cardiac strain, so even more than other training discussions, heat acclimation should be discussed with a health professional before undergoing any protocol.
Heat stress can begin at relatively low temperatures for unacclimated individuals and that heat acclimation is useful. In addition, as outlined by numerous studies, those same adaptations may improve performance
in temperate conditions too.
What is heat acclimation?
A 2014 article in the Scandinavian Journal of Sports Medicine reviewed the physiological responses to heat, focusing specifically on athletes. To summarize, the body senses the amount of offset from normal in peripheral systems (like skin) and central systems (like the brain and spinal cord) to produce a graded response. Elevated core temperature produces a nine-fold greater heat response than elevated skin temperature, but skin can heat up way more before causing a physiological crisis, so both play a big role.
That article and others (like this one from 2014 in the journal Comprehensive Physiology handily titled “Human Heat Adaptation”) detail how physiology acclimates so that responses to repeated heat stress can be more effective. Let’s break it down.
The body becomes more efficient with sweating, decreasing the body temperature at which sweating is initiated, reducing the content of electrolytes to produce more dilute sweat, and actually changing the size and function of sweat glands. Those watery pit stains I get when I’m nervous? That’s just good physiology.
The sweat cools the skin’s surface through evaporative cooling, which combines with improvement in vascular function that essentially means that the blood that does flow to the skin’s surface better reduces heat stress. Simultaneously, the body rapidly increases blood-plasma volume, with increases of 15+ percent noted in some studies in around a week. That improves cardiovascular output and stability, which leads to an enhanced cooling effect and could supercharge performance in all conditions. There is some evidence that high-heat acclimation could even help performance at altitude, and researchers are now looking more at whether it can spur natural EPO production too.
On top of that, metabolic rate decreases from the heat strain and there is increased thermal tolerance at the cellular level (a phenomenon at least partially explained by genetic expression from repeated environmental exposure and proteins that respond to heat stress). Most of those adaptations are relatively persistent, with one 2018 review in Sports Medicine finding just a 2.5 percent decay in heart rate and core temperature adaptations per day without exposure. In addition, hydration status during short-term acclimation could affect adaptation responses (as outlined in this 2014 study in the American Journal of Human Biology).
That’s a lot of words to basically say that heat stress can begin at relatively low temperatures for unacclimated individuals and that heat acclimation is useful. In addition, as outlined by numerous studies, those same adaptations may improve performance in temperate conditions, too (see this groundbreaking one from 2010 in the Journal of Applied Physiology).
Coolest of all? It’s relatively easy to jump start the process, even if you live in a foreboding ice-scape like in the movie Frozen or the country Canada.
How can you acclimate to the heat?
The most straightforward way to acclimate to heat is to be exposed to it during exercise. Athletes have a higher amount of heat acclimation than sedentary individuals at baseline due to elevated body temperature (and possibly dehydration) during training, so we’re starting from a good place. Plus, many heat acclimation benefits come in just a week or two. For example, a 2015 study in the journal Medicine & Science in Sport & Exercise had unacclimated cyclists do time trials at around 100F at day 1, day 6 and day 14 of training in the heat, comparing that to baseline performance at around 50F. On day 1, they averaged 77 minutes. On day 6, they averaged a nice 69 minutes. And on day 14, they averaged 66 minutes. There was no difference between time-trial performance on day 14 and in baseline conditions, likely indicative of close-to-complete heat acclimation.
A 2015 consensus statement in Sports Medicine indicated that one to two weeks of exercise in the heat around an hour should be enough for many acclimation benefits (and even just a few days can make a substantial difference). However, the big benefits are not possible if it’s not that hot where or when you train. With that in mind, here are five evidence-backed ways to actively pursue heat adaptation that most athletes can use.
Method one: Sauna
A 2015 study in the European Journal of Applied Physiology (Europeans love their saunas) found that after just four 30-minute exposures to a 188F sauna after exercise increased blood plasma volume by a whopping 17.8%. Theoretically, that would improve heat acclimation substantially, and other studies on sauna use back that up. A 2007 study in the Journal of Science and Medicine in Sport exposed runners to a 190F sauna an average of 12.7 times over 21 days, which corresponded to a 1.9 percent improvement in time-trial performance. The authors theorized it was related to blood-volume expansion.
Before winning the 2017 Western States 100 on a day temperatures reached 95F, Cat Bradley used the sauna most days in the few weeks before race day. And there are tons of similar stories from the trail-running world. (Note: anecdotally, some athletes seem to respond worse to sauna and other heat-acclimation techniques, possibly due to how the stress of the activity interacts with their individual physiology. For example, sauna training seems to make Megan underperform. So always pay attention to how you feel, just like in training.)
The big takeaway is that you don’t need to sauna a ton or do it until you want to croak for there to be heat-acclimation benefits. When athletes I coach struggle with warmth or have a hot race coming up, I like them to consider using the sauna for seven to 10 days post-exercise for 20 to 30 minutes, waiting to rehydrate for at least 30 minutes afterward, being sure to consult with a doctor before undertaking the program (or any other in this article). I also ask them to get out immediately if anything feels strange. Or if there is a weird person in there grunting. Public saunas are strange social experiments.
Even a few sessions will probably help. That’s also what I’d say to the grunting person about therapy.
Method two: Hot tub
Hot-water immersion techniques accomplish many similar adaptations, but may be available to more athletes. Two studies, one from 2015 in the persistent Scandinavian Journal of Medicine and Science in Sports and one from 2018 in Frontiers of Physiology, both used a similar protocol of 40 minutes in a 104F hot tub after exercise for six days. Both showed improvement in heat-acclimation physiology like reduction in skin temperature and reduction in sweating onset, and both showed improved exercise performance in heat.
The protocols involve immersion up to the neck and no activity in the hot tub. That differs from the study from the classic Saturday Night Live sketch where Will Ferrell’s character warns the others in the hot tub, “We should mention that although the waters above appear calm, below the surface there is a frenzy of activity.” They were not practicing proper heat-acclimation protocol.
Method three: Hot yoga
I couldn’t find much on runners and hot yoga in the literature. But we are in luck! A fascinating 2018 study in the Journal of Strength and Conditioning Research had 10 international-caliber field-hockey players do six days of hot yoga and the results were what you’d expect if you have gotten this far in the article. The field hockey players had a 5 percent increase in blood-plasma volume and a small increase in speed when running at ventilatory threshold.
Hot yoga is a good way to get activity in hot conditions no matter where you live. Just be careful about potential downsides of yoga and stretching, like potentially reducing the power of post-stretching muscle contractions and being so ripped that people are jealous of you at the beach.
Method four: Wearing extra layers
Raising core temperature during exercise will lead to heat acclimation. That can be through training in heat or hot yoga, but it can also be through training in lots of clothing. Or it can be achieved through training in lots of clothing in the heat. Honestly, a hot-tub soak sounds nicer to me. There’s a reason Nelly didn’t sing, “It’s getting hot in here, so put on all your clothes.”
I don’t ask athletes to do this one, but it’s a common heat-acclimation strategy for lots of top athletes. In Boulder, you’ll often see the Japanese national team shuffling on the trails in the middle of the day, wearing all black tracksuits in 100F heat. You could also hike on a treadmill in a parka. Gosh, this one seems like it sucks.
Method five: Adapting training to do some sessions in peak heat
As warm weather approaches, probably the biggest key is just to not shy away from the heat. If you save your training for perfect conditions, you may be fit, but you’ll be unprepared for imperfect days no matter how much you hang out with grunting sauna strangers. See a 90F degree day upcoming? Smile devilishly at the forecast and prepare to run in it. Humid and nasty outside? The treadmill can be saved for ice and lightning. Don’t do many important workouts in the heat intentionally, since output will be lower, but don’t avoid it either.
Heat will never be easy. But then again, running training isn’t easy either. So learn to embrace the heat just like you embrace the daily run, as a chance to grow over time. Do that, and you might find yourself scouting out the long-term forecast before your next race, hoping for a scorcher.
—David Roche partners with runners of all abilities through his coaching service, Some Work, All Play. His book, The Happy Runner, is about moving toward unconditional self-acceptance in a running life, and it’s available now at Amazon.