The lactic acid myth
A common misconception is that an excess level of lactic acid buildup in the muscles causes fatigue and soreness the day after a workout.
Lactic acid is a byproduct from the body burning carbohydrates during exercise. The harder you work, the more lactic acid you produce, said Dain LaRoche, an associate professor of exercise science at the University of New Hampshire.
But within 45 minutes, lactic acid levels return to normal, LaRoche said. “Which means it cannot be responsible for the pain and discomfort you have a day or two after exercise,” he said.
Exercise — lengthening and especially contracting muscles — damages the muscle cells, creating microscopic tears that are believed to cause the pain and soreness felt a day or two later.
LaRoche and other exercise specialists said post-exercise soreness is also triggered by inflammation and the release of chemicals such as histamine, which causes fluid to accumulate, producing swelling and pain.
“It has absolutely nothing to do with lactic acid,” LaRoche said.