Putting Muscles to the Acid Test

Acid aid. Lactic acid may help rather than hamper muscle performance, a study suggests.

Olympic athletes competing in Athens this week will be painfully aware of the "burn" caused by lactic acid buildup in their muscles. But how many of them realize that lactic acid is helping, not hindering, their performance? This shocking discovery was made 3 years ago; now, researchers have figured out what's going on.

The electrical signals that make our muscles contract are controlled by the balance of sodium, potassium, and chloride ions inside and outside muscle cells. During intense exercise, potassium flows out of the cells and accumulates, which reduces muscle contractions. For decades, physiologists ascribed muscle fatigue to something else, the buildup of lactic acid. But doubts began to emerge over the last 15 years, explains physiologist Ole Nielsen of the University of Aarhus in Denmark, because studies failed to show any effects of lactic acid on muscle function. In 2001, Nielsen and colleagues showed that lactic acid in fact offsets the depressing effect of potassium, but they could not explain the mechanism by which it exerted this effect.

In experiments reported in the 20 August issue of Science, Nielsen and graduate student Thomas Pedersen teamed up with researchers at La Trobe University in Melbourne, Australia, to explore the effects of acid on muscle fibers removed from rats. After stripping the outer membrane from the fibers, they used chemical reagents to modulate the pH levels and chemical balance within the muscle cells.

The team discovered that lowering pH to a level typical of a hard-working muscle counteracts potassium's reduction of muscle contractions. The acidity does this by reducing the dampening effect of chloride ions, probably by stemming their flow in and out of the muscle cell, which would normally inhibit spontaneous muscle contractions, explains Nielsen. The discovery could provide insights into conditions such as cramps, chronic fatigue syndrome, and myotonia, in which sufferers cannot relax their muscles, he says. It may also explain why warming up helps an athlete's performance, if this boosts acid levels in the muscles.

Such applications are a long way off, because we don't yet know if the mechanism operates in humans, notes physiologist George Brooks of the University of California, Berkeley. It's no revelation that lactic acid is beneficial, he adds, but the research reveals "an aspect of muscle function we need to know about."

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