A flight of stairs can leave the fittest people feeling out of breath.

It’s happened to all of us: We’re running late for a meeting and it’s only one flight of stairs away, so we dash on up. But when we arrive at the meeting, we’re embarrassed to be huffing and puffing as if we had just sprinted for a mile. It was just one little flight of stairs!

If you’re tempted to take this as a sign that you need to hit the gym more often, think again: Even marathon runners can get winded by the sudden task of vaulting a flight of stairs quickly, because physical fitness has little to do with it. It’s also tempting to assume it’s just a matter of not warming up. Eh, not really.

What happens when we approach a flight of stairs, with the intention of darting up them quickly, is that our brain tells our body to stop breathing.

Wait. What?

Humans (and many of our closest animal relatives) tend to stop or slow their breathing when concentrating on a specific task for a short period of time. When you’re running to make a meeting, you approach the stairs and aim to sprint up them quickly. This activates a specific program in your brain—let’s call it the “concentration on a small task” program. One result is that you slow or even stop your breathing as you approach the stairs, and maybe even continue this for the entire flight.

The outcome, of course, is that we combine a small burst of oxygen consumption by our muscles with a small burst of oxygen deprivation through our reduced respiration. Together, these two forces make our blood oxygen level plummet. After you’ve scaled the steps, the concentration program terminates. Your brain quickly notices the low blood oxygen level and it sends the opposite signal, which initiates rapid breathing to replace the missing oxygen. (It’s actually a spike in carbon dioxide in our blood that triggers this, but oxygen and carbon dioxide concentrations in our blood are inversely related in all normal circumstances.)

So why do we stop breathing right when we should be breathing more?

This reflex evolved to keep our bodies still when focusing on a physical task that requires concentration and precision. Imagine threading a needle, making a surgical incision, aiming a rifle, or throwing a dart. The key to being precise with these coordinated physical tasks is stillness and quiet concentration. By slowing or stopping our breathing, we reduce the background movements of our bodies and, hopefully, achieve better accuracy in the execution of our carefully planned action. That’s the idea, anyway.

Some people even report apnea (temporary suspension of breathing) when they are typing, chopping vegetables, looking for something in a drawer or refrigerator, drawing or painting, or any other task that requires momentary concentration.

One can imagine how useful this feature is for our animal cousins, who must make their living in the wild, as well as how useful it was for our ancestors who lived in the African savannah. From time to time, this trick likely made the difference between eating and not. That’s a clear evolutionary value and a clear selective pressure.

Try this: Next time you are dashing to a meeting, concentrate on your breathing, deliberately take deep breaths as you approach a flight of stairs, and force yourself to continue to breathe as you scale the steps, If you do this every time, it should become a habit and hopefully you’ll never arrive huffing and puffing again.