Summer is here and, as many athletes know, running isn't easy when the weather becomes sweltering. Runners, however, will run, and the best of them will adapt their workout routines to the heat. But new research is challenging long-standing beliefs about how the body responds to hot conditions. Alex Hutchinson takes a closer look at the latest science that affects how far - and how hard - you'll be able to go out there.
Brain versus body
Your brain slows you down in hot weather, not your body.
In a recent European Journal of Applied Physiology study, British researchers asked a group of cyclists to perform three 30-minute time trials. Not surprisingly, the cyclists covered about 5 per cent less distance under hot conditions (31.4 C) than under neutral conditions (21.8 C).
But in the third trial, which was also performed in the heat (31.6 C), the subjects were told that the room was only at 26 C, and the thermometer reading out their core temperature was altered to display 0.3 C lower than the actual reading. The result: They cycled just as far as in the neutral trial.
"Slowing down in the heat could be a subconscious regulation to protect us from damage, such as heat stroke," explains University of Bedfordshire researcher Paul Castle, the lead author of the study.
In other words, you don't slow down because your body has reached some critical temperature. Instead, your brain slows you down to prevent you from ever reaching that critical temperature. It's a subtle difference - but as the cyclists in the study discovered, it means that our physical "limits" are more negotiable than previously thought.
Our modern, air-conditioned lives may prevent us from properly acclimatizing to the heat.
The first hot days of the summer are always the toughest, but our bodies get used to the heat as the summer progresses. At least, that's how it should work. Ollie Jay and his colleagues at the University of Ottawa's Thermal Ergogenics Laboratory tested eight volunteers in mid-May and again in early September, measuring physiological variables during a 90-minute bike ride at 22 C. Despite a hot, muggy summer in Ottawa, the volunteers didn't show any of the typical signs of heat acclimatization - higher sweat rate, more blood flow to the skin, lower core temperature and heart rate - in the second trial.
"To obtain a substantial heat adaptation, core temperature must be elevated and high sweat rates need to occur," Dr. Jay explains. "The best way to do this is to combine exercise with heat exposure."
The study volunteers had averaged just 18 minutes a day of "moderate" or "intense" physical activity outdoors. In contrast, researchers have found that proper acclimatization takes 45 to 60 minutes of moderate-intensity exercise in the heat, either for seven to 10 consecutive days, or four to five times a week for two to three weeks.
Lowering the temperature of your palms allows you to exercise harder
In a strength-training study published last year, volunteers who used a special palm-cooling device between sets were able to bench-press 30 per cent more in the second set. Further studies into palm-cooling have been funded by the U.S. military, and a company in Texas is now selling a $30 palm-cooling device called the Bex Runner - essentially a gel pack that you freeze and then strap to your palms before heading out for a run in hot conditions.
Researchers initially thought that by cooling the blood flowing past your palms, you could actually cool down your core. But comparisons with U.S. Army cooling vests found that the vest could extract 55 per cent of the heat generated from exercise, while palm-cooling could only extract 14 per cent.
The primary benefit of palm-cooling isn't that your body is actually cooler. It's that you feel cooler, and thus trick your brain into being a bit less cautious. The result, according to Bex Runner's developers, is the same: You feel better and run faster.
So what does it all mean?
So if the slowdown due to heat is "in your head," does that mean you can just ignore it?
Not quite. For one thing, your brain's caution isn't a conscious response to heat - that's why the University of Bedfordshire researchers had to trick their subjects by telling them a falsely low temperature. That was possible in a short 30-minute lab study, but isn't likely to translate directly into the real world, or for longer bouts of exercise.
"I actually suspect that once individuals get into the upper ranges of core temperature, such a deception would not work," Dr. Jay said of Dr. Castle's study.
And of course, even if the brain is cautious, it's not infallible. "Clearly the body's systems don't always work, or they can be overridden, as unfortunately heat stroke does still occur," Dr. Castle points out.
But the results do carry an encouraging message: Slowing down in the heat doesn't necessarily mean that you're on the verge of physical collapse. Even simple cooling tactics like icing your palms or drinking a slushie can raise your limits.
So if you've taken the time to properly acclimatize, it's okay to push hard even in hot conditions. In fact, the more you sweat, the better you'll adapt.
Alex Hutchinson blogs about research on exercise at sweatscience.com. His new book - Which Comes First, Cardio or Weights? - is now available.