Lactic acid is the most efficient fuel that your muscles can use, even more than sugar. When you exercise as hard as you can, it helps you to go harder. A paper from Aukland University in New Zealand reviews the latest research showing that lactic acid is good for you (Sports Medicine, Volume 36, 2006). Your muscles use carbohydrates, fats and proteins for energy. Enzymes in muscles break down carbohydrates in a series of reactions that release small amounts of energy at a time. More than 80 percent of the energy used to power muscles is lost as heat, so burning fuel instantly for energy would produce so much heat that it would burn your muscles.
Enzymes require oxygen to turn food into energy. When you exercise so hard that you can’t get all the oxygen you need to break down food for energy, lactic acid accumulates in muscles and spills over into the bloodstream. This makes muscles acidic and it is the acidity that makes muscles burn and forces you to slow down. However, muscles require very little oxygen to turn lactic acid into energy. So when your muscles produce lots of lactic acid, they use this chemical for energy and require less oxygen. As soon as you slow down, you catch up on your oxygen debt and recover. So lactic acid is good for you. It helps you to exercise with less available oxygen.