As you exercise and your breathing becomes more strenuous, the oxygen level in your body gets depleted. That causes lactic acid to build up throughout, giving you a sluggish feeling and making you want to quit.
But as you get more comfortable with the exertion and are able to take more productive breaths, says Carol Ewing Garber, Ph.D., of Columbia University, your body starts to deliver adequate oxygen—which breaks down the lactic acid—as well as energy substrates (like phosphatidylcholine, glycogen, and triglycerides) to the muscles, both of which work to counterbalance that blah feeling.
That’s getting your second wind.
The best practice is to start slow, and work into catching the wind. If you overexert yourself too quickly, chances are you’ll crash and burn.