Why Do I Feel So Horrible When It’s Humid? (A Scientific Answer)

Will and I were plodding through town the other day, lamenting how horrible our legs felt in the 61 degree dewpoint and oppressive sun when I whined/asked “I know why I don’t evaporate sweat in this weather but WHY do my legs feel so terrible.” Ever the well-read scholar, Will replied not with his own whining retort, but with an awesome concise answer on precisely why we all turn into plodding wrecks in the humidity.

We’ve long know that there is a threshold temperature at which our brains and bodies essentially shutdown. It’s why we worry about overheated runners in a marathon or people with uncontrolled fevers. The link between this threshold and running performance, however, seems to be a less direct link. Essentially, it’s the speed with which you APPROACH this threshold temperature that matters. An analogy could be drawn to accelerating in your car: you can get to 60 by slowly depressing the gas pedal or you can floor it. The end point will be the same, but the slope of the line is different. With body temperature, a steeper line means your central nervous system (CNS) puts on the warning lights earlier and tells your skeletal muscles that you’re exhausted to keep you from overheating. Or, as the author of this seminal work on hyperthemia notes “the organism can anticipate…and avoid a catastrophic outcome.”

The other piece of warm weather running that we’ve come to understand is that it’s a one shot deal; once the warning signal has come on for the day, no amount of external or internal cooling will turn that signal off and let you feel peppy again.

So where does this science hit the road? It’s summertime almost everywhere and we’re all dealing with heat and humidity on a daily basis. Since we can’t take off running until the temperatures cool down in October, the natural conclusions are as follows:

1. Take it easy in the heat. Sort of a no-kidding conclusion but it’s worth thinking about the slope of your temperature line when you do your workouts this summer. You can get them in, but make sure your warmup is slow enough so that you don’t flip the kill switch before you’ve even started working hard.

2. Consider shortening your warmup and external cooling in extreme conditions. Racing a 4th of July 5K? If the weather is forecast to be seasonably hot, it’s a good idea to shorten your warmup, keep it extremely easy and try techniques like an ice vest and cool compress to flatten out the line until the race.

3. Keep the faith. Everyone feels horrible in the heat and humidity, even people who claim to “run well in the heat.” While this may be due to making smarter choices in the heat, there is very little person to person variability in heat tolerance or critical temperature threshold.

How do you manage in the heat and humidity? Does it reassure you to know why hot, humid runs feel so darn tough?

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