When you exercise, your muscles burn sugar, fat or protein in the presence of oxygen to produce energy. If you exercise so intensely that you become very short of breath and your muscles cant get enough oxygen, lactic acid accumulates in your muscle fibers. A study from Australia shows that lactic acid buildup in muscles does not make muscles tired and may even make muscle contract more efficiently, which may increase your endurance (Science, August 26, 2004.) This startling research contradicts what most instructors teach in their exercise classes.

The old theory was that lactic acid makes the muscles more acidic which causes them to hurt and burn and interferes with their ability to contract, so you feel tired. This new research shows that rats muscles contact more efficiently when lactic acid accumulates in them. Electric currents cause muscles to contract. This electricity is generated by cell membranes causing potassium to move inside cells and chloride ions to stay outside. With vigorous exercise, potassium ions accumulate outside cells. As large amounts of potassium ions accumulate outside cells, electricity is not generated and the cells cannot contract. Another ion called chloride accumulates outside cells and prevents potassium from getting back inside cells. Lactic acid removes the chloride, so it is easier for potassium to get back inside cells. Therefore lactic acid increases the ratio of potassium inside cells to the amount outside, and this helps the muscle contract with more efficiency.

While this new 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.