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When Is It Too Hot To Exercise Outdoors?

It’s hot out. Too hot. But is it too hot to exercise?

This is the question facing both professional and amateur athletes as July rolls across the Northern hemisphere. Recently, players in the Italy vs. Costa Rica match at this year’s World Cup in Manaus, Brazil needed to take the Cup’s first-ever “hydration break” to deal with the heat and humidity of the Brazilian rainforest surrounding the pitches.

Much of the World Cup coverage has focused on what the athletes should be drinking to remain properly hydrated, which is part of an ongoing conversation in athletics about the benefits of water vs. sports drinks and vice versa. When LeBron James had his recent cramp-up during his finals game, the cramping was attributed to his poor hydration or dehydration. Pundits wondered if he was drinking the “right” fluids, or simply not enough, causing his body to enter what’s called “osmotic shock,” or a critical imbalance of electrolytes in the body. Electrolytes help the nervous system communicate with the rest of the body, and an imbalance in those electrolytes can cause stiffness and cramping. But one problem with the LeBron James story is that James was drinking Gatorade, an electrolyte-enhanced sports drink, at the time he cramped up.

The other problem? Gatorade sponsors most of the research into the effects of dehydration on athletes. So everything we think we know about dehydration, cramping, and electrolytes was at one time funded by people trying to sell us more electrolytes.

This doesn’t mean you should stop hydrating during exercise. Water is good for you. It plumps the tissues in the skin and the brain, and helps your lymphatic system push toxins and bacteria out of the body. And electrolytes are also good for you, as anyone who has survived a particularly indulgent bachelorette party probably already knows. But hydration isn’t the alpha and omega of athletic performance, and it can’t stand up to other very important factors: heat and size.

Able-bodied adults can perform athletically within a range that extends up to 104F/40C. Beyond that point, the brain starts to have trouble sending signals to the body, just like it does when you have a dangerously high fever. (104F/40C is also the temperature that should send you to the emergency room, if it shows up on your thermometer.) Basically, at that temperature the brain tells the body to stop moving. But that exchange happens faster when you’re a bigger person, because larger bodies don’t have a favourable surface area : volume ratio. Smaller bodies disseminate heat faster, which is why people with smaller frames are always cold. But larger athletes (like LeBron) can’t disseminate that heat as quickly, so they cramp up faster.

So what does this mean for exercising outside? It means that unless you’re a tiny marathon runner, you’re better off hitting the gym and exercising indoors. We keep our gyms cool and our classes hot, so you can get the best workout possible without worrying about sunscreen, cramps, dehydration, or needing to take a rest on the side of two-lane blacktop. We’d love to see you, this summer. Come by and say hello!