Just past the finish line of the Ottawa Marathon last week, a volunteer with a spray bottle in each hand spritzed grateful runners in the face as they staggered past. Some stopped to beg for more – a reminder that, with the long-awaited arrival of summer heat, those who exercise outdoors will be looking for every possible trick to stay cool.
Two of the key summer strategies that top athletes rely on are heat acclimation, which involves deliberately exercising in hot conditions to trigger internal adaptations, and precooling, which involves starting a game or workout with as low a body temperature as possible. A new study in the Journal of Strength and Conditioning Research tests these two approaches head-to-head, both separately and combined, with results that offer some useful takeaways for athletes of all levels.
Researchers at the Environmental Extremes Laboratory at Britain's University of Brighton put nine amateur runners through a series of five-kilometre time trials in 32 C heat.
Before one of the runs, the runners spent five days acclimating to the heat with 90-minute training sessions designed to keep their core temperature above 38.5 for the last 60 minutes. This typically involved about 30 minutes of cycling in a heat chamber kept at 37 degrees, followed by another five minutes of cycling every half-hour to keep core temperature up.
Immediately before a second trial, the runners precooled by applying wet, iced towels to their head and neck, immersing hands and forearms in nine-degree water, wearing an ice vest and strapping ice packs to the front of their upper legs for 20 minutes.
The results? Precooling led to a 3.7-per-cent improvement in five-kilometre5-km time, acclimation led to a 6.6-per-cent improvement and doing both led to a 7.0-per-cent gain.
That suggests that acclimating to hot-weather exercise for at least five days is the most effective strategy, thanks to physiological adaptations such as increased blood volume that help your body shed excess heat more easily, decreased core temperature and increased sweat rate. Precooling is also effective to a lesser extent, but doesn't seem to add much if you're already adapted to the heat.
Applying these strategies outside the laboratory is logistically challenging, admits Carl James, the study's lead author. James recently took a post at the National Sports Institute of Malaysia, where his focus has shifted from theory to practice.
"Sitting on the other side of the fence now, I'm firmly of the opinion that practical heat acclimation strategies are desperately needed," he says.
For example, those who aren't eager to insert a rectal temperature probe can use tympanic (in-ear) thermometers to monitor core temperature. In that case, James suggests, aim to keep your temperature above 38.0 degrees for an hour, rather than 38.5, since ear measurements typically underestimate temperature.
That's the approach James and his colleagues used to help a British soccer referee prepare for the 2014 World Cup, which was held in Brazil. The referee completed five sessions of intermittent running while monitoring in-ear temperature, and also added seven "passive" heat sessions, soaking in a hot bath for 30 minutes after a workout to maintain elevated temperature. The bath was at 48 degrees, and the referee got in and out to keep his in-ear temperature above 38.0.
"It can be done safely outside of a controlled research environment," James says, "but we educated him a lot around the risks before he did anything unsupervised."
For those without much experience of hot-weather exercise, or with little desire to fiddle around with thermometers, James suggests 60 minutes of moderate-intensity exercise in warm conditions for five straight days as a simpler starting point.
Ideally, he suggests, you should make sure you have company – a coach, training partner or friend – when undertaking these deliberately hot acclimation sessions. You can drink to thirst during the sessions, which will leave you mildly dehydrated, but make sure to continue rehydrating throughout the rest of the day.
For precooling, the benefits may depend as much on how hot you feel as on your core temperature. That's why the precooling protocol involved cooling as much skin surface as possible, since skin temperature plays an important role in your perception of thermal comfort – which, in turn, affects how fast you choose to run.
The logistics of precooling can be daunting, depending on where you're exercising. A cooler with iced, wet towels and ice packs is relatively mobile. A cool bath (or jumping in a convenient lake) can also drop your temperature effectively; that's the approach the Australian Olympic team used at the muggy 2004 Olympics in Athens. And drinking a slushie can provide an effective dose of internal precooling.
One caveat, James notes, is that it's difficult to judge your pace properly when you're tricking your perceptions by altering skin temperature. If you try precooling, make sure to experiment with it several times in practice before trying it in competition, to get used to the disorienting sensation of quickly going from comfortably cool to unpleasantly hot.
Of course, none of these tactics grant you immunity from heat. If you're planning to push your limits on a hot day, start conservatively, prepare appropriately and take advantage of sprinklers – or volunteers with spray bottles – wherever you find them.
Alex Hutchinson's latest book is Which Comes First, Cardio or Weights? Follow him on Twitter @sweatscience.