When you run very fast, you reach a point where you gasp for breath. You keep on pushing the pace and after a few seconds, you feel that you have recovered and that you can pick up the pace again. It’s called second wind and your apparent recovery is caused by lactic acid.
When you run fast, your muscles use large amounts of oxygen to burn carbohydrate, fat and protein for energy. You get the power to move your muscles from each of several successive chemical reactions, called the Krebs cycle. If you can get enough oxygen to meet your needs, food you have eaten is converted all the way to carbon dioxide and water that you blow off from your lungs when you breathe out. However, If you run so fast that your lungs cannot supply all the oxygen that you need, the series of chemical reactions slows down, you start to accumulate large amounts of lactic acid in your muscles and the lactic acid spills over into your bloodstream.
The lactic acid and carbon dioxide make your blood very acidic and the acid burns your muscles to make them feel hot and painful. You are desperately trying to breathe hard enough to get rid of the acidity in your blood by taking in enough oxygen to get rid of the excess lactic acid and blow off the excess carbon dioxide that is accumulating in your blood.
Researchers at the University of California in Berkeley have shown that lactic acid is the chemical that requires less oxygen to power your muscles than any other source of fuel. The marked accumulation of lactic acid in your muscles causes muscles to use more lactic acid as their primary source of energy. By doing this, your muscles require less oxygen and you catch up on your oxygen debt. This neutralizes the acidity in your blood, so your muscles stop burning and hurting and you can pick up the pace. You tell everyone that you suddenly got your "second wind", but actually you started to use huge amounts of lactic acid, which requires less oxygen for energy, and your blood became less acidic so you were able to run faster again.