When U.S. swimmer Natalie Coughlin reached the final 15 meters of the 100-meter backstroke competition in Athens, she felt her body scream.
"That last 50 hurt a lot," she said, after winning the Olympic gold. "I got really excited that first 50 and I felt it the last 15 meters, definitely."
What was Coughlin feeling, exactly? Scientists still aren't sure about all the explanations, but they're getting closer to understanding most causes of muscle fatigue. One thing they now know for certain is that lactic acid — once labeled an unwanted waste product and blamed for the burning feeling in spent muscles — isn't as bad as people thought.
In fact, it helps muscles continue firing when they might otherwise putter out.
"Lactic acid production was viewed and is still viewed as a major factor causing muscle fatigue," said George Stephenson of La Trobe University's Muscle Research Laboratory in Melbourne, Australia. "It is now clear that it actually protects the muscle from becoming fatigued."
Lactic Acid’s Bad Reputation
The misconception that lactic acid is a harmful waste product began with the work of a Nobel laureate. In 1929, British physiologist Archibald Hill analyzed muscle fibers of frogs and suggested the lactic acid that pooled in the frog muscles after flexing causes fatigue. But researchers now realize the experiment was flawed. Hill studied the frog muscles in isolation, so he couldn't know that within the body, lactic acid is taken up and consumed as a fuel.
"It was a classic mistake in biology," said George Brooks, a biologist at the University of California at Berkeley and a pioneer in the evolving understanding of muscle fatigue. "But because he was a Nobel laureate and his students followed up on his work, the mistake was propagated for years."
Even today, some coaches tell their athletes to concentrate on cleansing their muscles of lactic acid after a hard workout, with the idea that it is the culprit of second-day sore muscles.
But people like Brooks have since shown that lactate from lactic acid is a valuable fuel derived as the body breaks down carbohydrates. Not only do muscles consume it eagerly, the brain and heart also suck it up from the bloodstream to keep systems running. High levels of the stuff can cause some sensation of muscle burn during exercise (thanks to the hydrogen ions that are released when lactic acid is broken down into lactate), but it doesn't stick around long after activity.
And now, new research shows that lactic acid serves as more than a fuel. In a study, appearing in the journal Science, Stephenson and others show lactic acid actually help keeps muscles running when they might otherwise become sluggish. To understand how, it's useful to look at what makes a muscle flex in the first place.
Thomas Fahey, an exercise physiologist at California State University in Chico, explains that muscle actions are triggered by a mechanism known as the sodium-potassium pump. The sodium-potassium pump moves sodium ions out of a cell while pushing potassium ions in. The difference in levels of each kind of ion creates an electrical charge. Muscle cells use this charge to respond to elecrical signals coming from nerves and to contract. For every two potassium ions that are pulled in from outside the cell, three sodium ions are moved to outside the cell.