Aug. 9, 2024 – Turns out Paris is no Tokyo, weather-wise.
Heat and humidity were the big storylines of the 2020 Tokyo Olympics. According to a 2023 study, roughly 110 Olympians suffered heat-related illnesses as temperatures at some outdoor venues exceeded 95 F and humidity neared 70%.
Team USA and New Balance distance athlete Emily Sisson ran in that Tokyo heat, placing 10th in the 10,000-meter race. And she will run again on August 11 in Paris – this time in the marathon.
“I actually almost hope it is [hot] a little bit,” she said. “I'm not going to ignore it. I'm going to prepare for it. I want to go in with confidence that I can handle those conditions.”
Temperatures for the Paris games have been balmy in comparison, mostly hovering in the 80-degree range with occasional 90-degree pops, but it could’ve been even worse than Tokyo: The hottest 3 years in France have all occurred since 2020 – with the capital registering a blistering 109 F during a heat wave in 2022.
Here in the U.S., we’re not as lucky. Summer 2024 has been the hottest on record in 30 cities and is on track to become the hottest in history.
All this to say that ever-rising heat is an alarming new norm for anyone exercising outdoors in the summer. It has also fueled a crop of committed researchers who are doubling their efforts to work with athletes and coaches to ensure they are prepared.
“It is well known that environmental heat stress impairs performance in as little as 5-7 minutes of heat exposure,” said Luke Pryor, PhD, associate director of elite athlete performance at the University of Buffalo’s Center for Research and Education in Special Environments. “At the highest level of competition, any potential source of performance decrement like heat strain could be the difference between a podium finish or not.”
It looks like Sisson will get her wish: Paris temps are expected to hit 94 degrees on August 11, the women’s marathon day.
She’s right to prepare. An ever-growing body of research shows that a good performance in the heat – no matter who you are – begins long before you step onto the court, track, or field.
Prepare for 'Thermal Impact'
Our body’s core temperature is a highly regulated thermostat, constantly being tweaked and tinkered with to stay within a tight range of 97.7 to 99.5 F, where our metabolic processes function properly.
When we exercise, our muscle contractions produce internal heat – a lot of it. After just a few minutes, that core temperature goes up; it will keep rising based on how intense the workout is and how long it lasts.
Dial up the heat around you, and that workout becomes even more taxing. (Case in point: Temps above 77 F have been shown to impair running performance at distances at and over 800 meters, or about half a mile.) The heart has to pump blood to your muscles as well as your skin, where heat can be released through your pores. This inter-body competition puts added stress on your heart and blood vessels, while high heat and humidity weaken natural cooling at the same time.
When people can’t remove the heat they produce, it’s stored in the body. That’s where problems start, said Jessica Mee, PhD, a research and innovation fellow at the University of Worcester. Staying in that state too long puts athletes at a higher risk of heat illnesses, ranging from muscle cramps and dehydration to exhaustion, fainting, and heat stroke.
When it comes to extreme weather, even the most elite athletes cannot escape the impact. “The attributes we see affected the most by heat are stamina, power, and concentration,” said USA Beach Volleyball head athletic trainer Scott Mackay.
Other team sports like rugby, field hockey, and soccer are routinely impacted by the drop in mental functions like decision-making and skill execution caused by brutal conditions. Pole vaulters have slipped when their grip gets too sweaty. An archer’s collapse at the Tokyo Olympics proved even just standing in the heat for long periods of time can harm blood pressure control.
Humans Can Adapt to the Heat
Luckily, scientists have been hard at work figuring out a hack to the heat – and they have a pretty convincing solution.
An analysis of 96 studies showed that after following a heat acclimation protocol, time-to-exhaustion and time trial performance improved up to 23% and 7%, respectively. People are also less likely to suffer a heat-related medical event when they are properly acclimated to hot weather conditions.
What is heat acclimation, exactly? Generally, it involves exposing athletes to a series of “thermal impulses,” or increases in core body temperature. The objective of each heat exposure is simple: Sweat a lot and increase core temp to at least 101.3 F for at least 60 minutes per day. Within 10-14 days, this helps the body adapt, making it easier to withstand (and perform in) intense temps.
The heart and blood vessels adapt first, Pryor said. Usually within 2-4 days, plasma volume (the fluid part of our blood) expands, helping to lower our resting and exercising heart rates. The adaptation that takes the longest is increased sweat rate, which for some athletes can take up to 2 weeks of daily exposure.
It’s not a bulletproof approach. “Thermoregulation has a cost,” said Jo Corbett, PhD, an associate professor of environmental physiology at the University of Portsmouth and co-author of the 2024 paper “Rings of Fire: Heat Risks at the 2024 Paris Olympics.” Someone’s best day in brutal temps will likely not be as good as their best day in cooler ones. But Corbett stresses it is by far the most powerful tool we have for adapting our bodies to physical activity in summer heat.
“If two identical athletes are competing in an event that is adversely affected by high heat, the athlete who has most effectively prepared for the high heat will win,” Corbett said.
How Olympic Athletes Prepare
It should come as no surprise that our country’s top athletes and coaches are embracing this evidence-based approach.
“As a team, we try to have an intentional plan,” said Team New Balance Boston coach Mark Coogan, whose Paris-bound athletes have thrived in some seriously steamy races. “We know it takes a couple of weeks to acclimate, and you really must have a good plan in place to be successful.”
Ideally, this involves focused heat training – a minimum of four times per week for 60-90 minutes per session – about a month or so before an athlete’s big race.
“The best preparation is to live in the environment that you will race in, and that is what we’ve been doing,” Coogan said. “Luckily right now, it is extremely hot and humid in Boston where the team trains. One of the reasons we did not go back to altitude was so we could do our final preparations in the heat of Boston.”
To prepare for the marathon, Sisson scheduled her tough workouts during the coolest part of the day (early morning) and saved her easy runs for when it’s warmer. This helps her adapt to heat without sacrificing the training improvements she wants from her workout sessions.
Adding Layers
There are other ways athletes can increase core temperature, stimulate profuse sweating, and increase skin blood flow.
One of the most surprisingly effective strategies is “passive warming,” said Olympian and running coach Dathan Ritzenhein, head of the On Athletics Club in Boulder, CO. Along with hot summer days, his runners use layering – literally bundling up in extra clothes – and they control the temperature of their homes. “It sounds like it wouldn’t do much, but we try to make sure they don’t feel cold during the day while not training.”
Team USA beach volleyball uses a different tool: the sauna.
Frequent sauna exposure has shown promise for helping both trained men and women get used to the heat. (One study found adding 30-minute post-exercise sauna sessions over 2-3 weeks increased time to exhaustion by 32% in trained men.)
“This protocol includes either active (stationary cycling) or passive sauna use at varying frequencies, durations, and intensities, depending on training, competition schedules, and the amount of time available to acclimate,” Mackay said.
Experts say this kind of dual approach may work best. “Adapting in a superficial heat stress environment comes with a guarantee of inducing thermal strain but a lack of real world specificity,” said Oliver Gibson, PhD, senior lecturer in exercise physiology at Brunel University. “By doing both, you can utilize their respective strengths and minimize the potential limitations.”
Then It’s Time to Chill Out
Once the focused prep work is complete, athletes can maintain their gains through intermittent exercise-heat exposure, or getting used to the heat again just before competition with 2-4 consecutive days of exercise-heat training.
Then it’s time to put the heat away. “The accumulative heat exposure in the days leading up to the Games has the potential to cause a decline in performance,” Mee said.
This is where short-term heat management solutions can make an impact:
- Proper hydration. “The biggest thing I do for summer races is making sure I get enough fluids,” said Team USA’s Yared Nuguse, who won the bronze in the 1,500-meter run earlier this week. “I try to switch between water and electrolyte drinks so that I’m not just flushing out my electrolytes with water.”
- Cool sleep environment. Times change: From 1924 to 1933, Paris only observed four “tropical nights,” when temps didn’t drop below 68 F. In the last decade, they’ve seen 84 of them. So when it was announced that organizers chose not to install air conditioning in the Olympic Village, Team USA decided to bring their own.
“If you sleep in a warm to hot environment like a dorm room without air conditioning, your core body temperature remains elevated above normal resting values.” The problem? Lower-quality sleep and starting your day at a higher core temp, with less room to increase before it reaches dangerously high levels, Pryor explains.
- Pre-cooling. Slurping an ice slushy pre-exercise can lower your core temperature. Dousing yourself with water increases evaporative heat loss without additional sweating. Placing ice towels on your neck for as little as 1-2 minutes can help lower perceived “thermal strain,” meaning you don’t feel as hot and can exercise longer.
- Coogan’s team is using ice vests in Paris during warmups and cooldowns. “It’s a great way to maintain your core temp and not overheat,” he said.
Physically Hot, Mentally Cool
Call it the heat-adapted paradox: Many of the athletes heading to Paris had to earn their spots in really hot races. These same athletes say they just handle heat well. Is that simply a coincidence?
“There certainly is some degree of genetic underpinning” when it comes to heat tolerance, Gibson said. But he thinks there’s something more going on: Athletes who routinely use heat adaptation in their training seem to adapt better. “There is a molecular memory at play, and athletes appear to be using this to their advantage by more frequently seeking thermal stress in their training.”
Take Sisson. Her coach thinks she’s just naturally good in the heat. “And maybe that's true,” she said. “But when I was getting some opinions on how to heat train, a lot of the advice was stuff I already did. I just didn’t realize that it was considered heat training.”
That repeated exposure – whether intentional or not – pays dividends. “Heat is a factor at most major championships, so experience matters a lot,” said Ritzenhein, who coaches Nuguse. “Knowing what works and controlling things that you can are helpful. It's also a psychological stressor, so being prepared but not overthinking is very important.”
An athlete’s feelings about heat stress matter, because the mind and body are connected, Pryor said. “Sports psychology tactics like positive self-talk have been shown to improve endurance performance in the heat.”
That certainly has been true for bronze medalist Nuguse. “I think embracing the heat is an important part of performing well in it,” he said. “I naturally handle heat very well, but even when it’s excessive, I will remember that everyone is dealing with the heat and it’s not just me. That helps me stay cool about it and not let it impact my performance too much.”
