When you run very fast, you become short of breath and feel like you have to slow down, but if you keep on pushing yourself, you suddenly feel refreshed and can pick up the pace. Do you know what caused your sudden recovery?

It's called second wind and you don't even know that your recovery is caused by slowing down when you feel exhausted during exercise. When you exercise intensely, your muscles use large amounts of oxygen. You can run so fast that your muscles require more oxygen than you can breathe in through your lungs and you develop an oxygen debt that causes lactic acid to accumulate in your muscles. This makes your muscles acidic, which causes pain and burning/ and the lactic acid spills over into your bloodstream, causing you to gasp for air. You finally reach a point where you can't meet your needs for oxygen, which forces you to slow down. You don't notice that you have slowed down because you are working just as hard as you did when you were running at a faster pace. Not only must you bring in extra oxygen to meet your present needs, you must also use some of the oxygen to make up for the debt that you incurred when you were running at a faster pace.

Eventually, you catch up on your oxygen debt and feel better. So you pick up the pace and don't even know that it was the slower running that revived you. You think that there was a change in your metabolism and you call it "second wind."

By Gabe Mirkin, M.D., for CBS Radio News