When a heat wave hits, how do I keep my workout on track?
Hot weather is more than a mere inconvenience. In the past 50 years, at least 128 football players at all levels have dropped dead from heat exhaustion during gruelling summertime practices, according to data compiled by the American Football Coaches Association.
There are some obvious steps that will help you avoid the worst outcomes: Stay hydrated, schedule workouts for the coolest parts of the day, keep out of the sun and reduce intensity. But recent studies also add insight into how Canada's northern latitudes affect our susceptibility to heat stroke, and suggest some new techniques for beating the heat.
One reassuring fact to note is that we're generally pretty good at automatically adjusting our effort under hot conditions. In fact, when researchers at the University of Cape Town asked volunteers to cycle for 20 kilometres in 35C temperatures, they found that the volunteers' brains were automatically signalling fewer muscle fibres in the legs to contract than in cooler conditions - right from the start, before they'd had a chance to start overheating.
"When it's hot, you don't wait 20 minutes to slow down; you slow down within a minute," says Ross Tucker, the lead author of the study.
Still, when the conditions are unusually hot, many people do manage to push beyond their limits. Earlier this year, the medical director of the Twin Cities Marathon in Minnesota, William Roberts, published an analysis of eight recent marathons in which hot conditions caused "mass casualty incidents" that overwhelmed local health facilities and caused unacceptable delays in emergency care.
Dr. Roberts found that most of these races started with weather conditions deemed acceptable under the guidelines published by the American College of Sports Medicine. The problem was that the races took place in cities such as Boston, Chicago, London, Rotterdam and Rochester, N.Y. - all northern cities where temperatures are cool for all but a few months each year.
As a result, mass casualty incidents occurred when the Wet Bulb Globe Temperature was a relatively modest 21C at the start of the race. Crucially, these races all took place in the spring or fall, rather than in the middle of summer, when runners would have been used to the heat.
In other words, context matters: The actual temperature is less important than how well you're prepared for it. So by all means exercise caution this summer - and stay cautious this fall, when an unexpected hot day could catch you by surprise.
Alex Hutchinson blogs about research on exercise at sweatscience.com.