by Dr. Gabe Mirkin, M.D.
When you exercise, sugar is broken down into different chemicals, to produce energy for muscles. As long as you get all the oxygen you need, the final products are carbon dioxide and water, but if you exercise so vigorously that you can't get the oxygen that you need, the reactions stop, causing a chemical called lactic acid to accumulate in your muscles and spill into you bloodstream.
Lactic acid buildup in muscles does not make muscles tired and may even make muscle contract more efficiently, which may increase your endurance. This concept contradicts what most instructors teach in their exercise classes.
While this concept of how muscles use lactic acid for energy is reasonable, it is not likely to change the way athletes train or the way exercisers become more fit. Healthy people are supposed to exercise vigorously and feel a burn in their muscles during exercise, which signifies buildup of lactic acid in muscles. They feel sore on the next day, go easy for as many days as it takes for muscles to feel fresh again, and then exercise intensely again.
When lactic acid causes your muscles to hurt, you breathe harder and faster, and slow down to catch up with your oxygen debt. This converts lactic acid into carbon dioxide and water that are blown off as you breathe. Blood levels of lactic acid lower and your muscles stop hurting. A pace that you can hold breathing fast and deeply, but not gasping for breath, is called the lactic acid threshhold and is the training level for most competitive athletes.