Why it’s harder to exercise in the heat, and what you can do about it

Summertime, and the livin’ is easy; but the exercise is much harder. 

You’re dripping sweat, your heart is racing, your skin is flushed (or is that a sunburn?). What’s making your body work so much harder? It’s important to understand how our bodies regulate temperatures during exercise to train better and stay safe.

When we exercise, our bodies produce metabolic heat. When we increase our energy expenditure, we are actually “burning calories.” Making more energy to get our muscles moving also releases heat. In fact, we’re not very fuel efficient vehicles: we really only capture about 15% of the energy we produce to get moving; the rest gets released as heat!

And all that heat needs to go somewhere to prevent us from spontaneously combusting. Unlike our poor dogs that can only release heat by vigorously panting, we release heat by pushing water onto our skins via sweat, which then evaporates and cools us down.

In order to sweat, we have to direct bloodflow to our skin. The hotter we get, the more blood we need to send to our skin to release heat and get the evaporative cooling effect from sweating. And when external temperatures are high, our bodies get hotter and need to sweat more to cool off. Do you exercise in a humid environment? It’s harder for you to cool down: sweat evaporates less in humidity, so you need to sweat more to get the same cooling effect.

Sending all this blood to our skin makes it harder to exercise. You want as much blood going to your working muscles as you can, to carry the oxygen and energy needed to produce movement. But instead, your skin is stealing that blood from your working muscles so your engine doesn’t overheat.

When blood goes to your skin instead of your muscles to help cool your body, your heart has to work harder. Compared to exercising in cool weather, your heart rate is higher at the same exercise intensity because you need to release all that extra heat in addition to moving your muscles. That means you’re not really burning more calories in hot weather.

And what about all that sweat, where does it come from? Your blood is about 60% water. When you send blood to your skin to release heat, you are also pushing water out from your blood, onto your skin. It gets there by passing salt out – water follows salt. That’s pretty cool(ing). That’s also why you need to stay plenty hydrated, and get enough electrolytes. Sports drinks are usually considered superior to water for staying hydrated, but unless you’re exercising for over an hour or in a really humid environment, you’re probably getting enough electrolytes in your diet, and you may not need to guzzle electrolyte drinks.

But while this loss of plasma may be great for cooling your body, it makes your heart work even harder still, especially if you’re not properly hydrated. That’s because when you lose water from your blood, your blood gets thicker, or more viscous. Thicker blood doesn’t flow as well through your vascular pipes, so it needs stronger pumping from your heart’s left ventricle.

In addition all those cardiovascular adaptations to the hot environment, your cells can’t work as well. Your muscles fatigue sooner as they get hotter. And your brain, which starts to accumulate a lot of heat since heat rises, weakens its ability to recruit your muscles to move.

If you’re consistently exercising in a hot environment, you should get acclimatized to the heat, which takes several days. As you become more acclimatized, you’ll store more water in your blood, you will lose less electrolytes, and your skin won’t need as much bloodflow to sweat. And you’ll be able to sweat more (a good thing to cool you down), at a lower intensity. So just because you’re dripping wet, that doesn’t mean you’re working harder. Instead, you’re becoming better at regulating body temperature.

But whether you’re acclimatized to the heat or not, follow these tips to train at your best, and stay safe and happy while you’re doing it:

  • Stay hydrated! Before, during, and after exercise. Remember, if you’re thirsty, you’re already dehydrated
  • Exercise during the coolest part of the day, either first thing in the morning, or right before it gets dark
  • Minimize exercise intensity and duration on hot, humid days
  • Expose as much skin as possible for evaporation, but don’t get burned! Sunburned skin can’t sweat as well
  • Take plenty of breaks if you can, and rest/cool down in the shade

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