There are some theories out there about why you get a second wind , though, according to a British Journal of Sports Medicine blog. One hypothesis is that your body is switching from burning carbohydrates to fat, but there are a few holes in the theory. Other experts think it could have to do with lactic acid and oxygen in your muscles. Research shows the chemical lactic acid requires less oxygen to power your muscles than other sources of fuel. When you have a build up of lactic acid in the muscles, you may be able to go harder and feel less pain as you do, explained sports medicine doctor and fitness expert Gabe Mirkin, M.D . in a blog post. "You tell everyone that you suddenly got your 'second wind,'" he writes. "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."