Muscle Memo

New research says you can't blame lactic acid for sore muscles anymore.

If your thighs and arms scream in protest during a vigorous yoga class, or you feel the painful aftereffects of your workout the following morning, traditional thinking would fault the build up of lactic acid—long considered a metabolic waste product—for your tender tissues. But that thinking has been proven wrong.


“Far from being a waste product, lactic acid is a valuable fuel that is critical to metabolism,” says exercise physiologist Thomas Fahey, a professor of kinesiology at California State University in Chico. Fahey points to research conducted by scientists at La Trobe University in Melbourne, Australia, the University of Aarhus in Denmark, and the University of California at Berkeley proving lactic acid can help forestall muscle fatigue. “It plays a role in muscle tissue repair,” says Fahey.

So who goofed on lactic acid? When scientists in 1915 noticed high levels of lactic acid in exercising muscles, they assumed the build up decreased muscle responsiveness, causing fatigue and irritation. But modern testing methods reveal that lactic acid regulates blood flow to muscles—and that muscles produce more of it during intense exercise because they need it to keep going.

To increase endurance, yogis should load up on fruits, vegetables, and complex carbohydrates; they provide the fuel to produce higher rates of readily usable energy for your working muscles, helping to sustain prolonged activity.