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.
When a muscle is working hard, potassium ions can start to leak from the muscle cells and accumulate on the outer membrane. This creates an imbalance and stifles muscles from firing. Chloride ions also start to act as a natural braking agent. By testing muscle fibers of rats, Stephenson and colleagues showed the presence of lactic acid counteracts this braking effect and keeps muscles firing.
"It is a very clever trick," said Stephenson, explaining the acid enables "impulses to keep exciting the muscle when they would otherwise fail."
The finding is one more step away from old misperceptions about muscle fatigue and lactic acid and could be used in ways to help athletes prepare their bodies for competition. Stephenson suggests, for example, that doing short sprints during warm-ups for an event could help an athelete's performance by increasing levels of lactic acid in the muscle and preparing the muscle for a buildup of potassium ions.
"In fact, many sprinters do this already prior to competition," says Stephenson.
Indeed, coaches often know how to influence their athletes' physiology without really understanding why. They learn what works, not by science, but through training and trial and error. Sometimes it can take a while for science to catch up to coaching and explain why some techniques work. Fahey, who is a coach and champion discus thrower, as well as a physiologist, points out it can also take a while for coaching to catch up with science.
For example, swimming coaches (including those of swimming phenom Michael Phelps) commonly measure lactic acid levels of their athletes to determine how fit they are. In Fahey's opinion, this is a misguided test because a fit athlete could have both high or low levels of the acid, depending on their diet, how fast their bodies are consuming lactic acid and a huge array of other factors.
"It doesn't really tell you much, in the end," he said.
Whether or not such tests are truly instructive, they may offer something else — confidence. As all athletes know, what goes on in the mind during competition is often what counts most. So, if it helps build confidence to take a lactic acid test or a sugar pill — why not?
"Athletes try all kinds of crazy things. If Michael Phelps ate jelly beans, you'd probably have a bunch of swimmers suddenly eating jelly beans," said Fahey. "That's how we do sports — we look at what works, whether it's grounded in science or not."