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THURSDAY, Aug. 19 (HealthDayNews) -- If you happen to see an Olympic athlete felled by tired, cramping muscles, don't blame the usual suspect -- lactic acid.

That's because a study in the Aug. 20 issue of Science found that rather than causing muscle fatigue and cramps, lactic acid may actually prevent muscles from tiring. At least in laboratory rats.

"The etiology of muscle fatigue is very complex," said study authors George Stephenson and Thomas Pedersen in an e-mail interview. Stephenson is a professor of physiology at La Trobe University in Melbourne, Australia, and Pederson is a doctoral candidate at the University of Aarhus in Denmark.

"The most important message from this work is that lactic acid does not cause fatigue, but rather protects against fatigue that would otherwise develop from potassium build-up," said the authors.

Dr. Brett Silverman, a sports medicine and rehabilitation physician at the Hospital for Joint Diseases in New York City, said, "The findings in the study are very surprising.

"These findings turn what we think about muscle metabolism and the environment of muscles chemically on its head," he added.

But Silverman was quick to point out that this study was conducted on rats. "The study was done on rat muscle, which isn't human muscle, so we have to be careful in extrapolating to humans."

Stephenson, Pedersen and their colleagues developed a technique that can peel away the outer surface of muscle fibers, which are about half as wide as a human hair. This allowed them to see what was going on inside the muscle cells and to see what effect changing conditions had on those cells.

Electrical signals are sent from the brain to muscle cells continuously. These signals are what cause the muscles to contract, explained Stephenson and Pedersen. The signals are carried through currents of sodium, potassium and chloride ions, they said.

During intense exercise sessions, many such signals are sent. As this occurs, potassium is lost from inside the muscle cells and starts to build up outside the muscle cells, according to the authors. This build-up outside the muscle cells can then interfere with signals reaching inside the muscle cells and may cause muscle fatigue, the researchers speculate.

Lactic acid has long been suspected to be a key player in this process because it builds up around the same time potassium increases, according to the authors.

By stripping away the outer layer of the muscle fiber, however, the researchers were able to see exactly what was going on inside the muscle. They said that when potassium decreases inside the muscle, extra lactic acid may actually "counteract the deleterious effects of potassium build-up by reducing some of the short-circuiting currents."

Stephenson and Pedersen said this means that lactic acid actually protects muscles from fatigue.

Silverman said the findings were interesting, but would need to be duplicated in humans before they could be applied clinically.

He said that, if the findings hold up in humans, short bouts of intense exercise prior to a long exercise session might prime the muscle, and could potentially extend the tolerance of elite athletes.

That's exactly what the authors recommend. "Short sprints during the warm-up period before a bout of intensive exercise, such at the 100 meter sprint, are likely to be beneficial since the lactic acid thus generated in the muscle during the warm-up can prepare the muscle cells for the intensive potassium build-up during the 100 meter sprint."

For the average exerciser, Silverman said the best way to avoid fatigue is to do a proper warm-up and stretch your muscles before exercising intensively.

More information

The American Academy of Family Physicians offers tips on starting and maintaining an exercise program.

(SOURCES: George Stephenson, Ph.D., D.Sc., professor of physiology, La Trobe University, Melbourne, Australia; Thomas Holm Pedersen, M.Sc., doctoral candidate, University of Aarhus, Aarhus, Denmark; Brett Silverman, M.D., sports medicine and rehabilitation physician, Hospital for Joint Diseases, and clinical instructor of rehabilitation medicine, New York University School of Medicine, New York City; Aug. 20, 2004, Science)

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