So Dr. Schuelke and his colleagues decided to test the baby and his mother for mutations in the myostatin gene. The mother had one nonfunctioning copy of the gene. In the boy, both copies of the gene were inactive; he was making no myostatin at all. No other family members agreed to genetic testing.

The findings, researchers say, may help scientists pin down why some people find it easy to get strong while others can lift weights day after day to little effect. At least some of this natural variation, they suspect, may be a result of individual differences in myostatin levels.

''If you've looked at pictures of Arnold Schwarzenegger when he was a teenager, he just looked naturally muscular,'' said Dr. Robert Ferrell, a professor of human genetics at the University of Pittsburgh who in a small study found no major genetic differences between professional bodybuilders and ordinary people. ''Everyone has run into people like that who have great muscle definition and size. That's what I'm interested in.''

Certainly the baby's mutation was unusual, Dr. Schuelke said. He and his colleagues tested 200 people not related to the child and did not find it. But there are many ways to disable a gene, and it is possible, researchers said, that some naturally strong people have myostatin genes that function poorly, or not at all.

Eventually, experts say, it may be possible to use drugs to deplete myostatin. One way to do that could be with antibodies that block it, a path that Wyeth is pursuing. The company has begun safety tests in humans with the goal of treating muscular dystrophy and muscle wasting.

Dr. Elizabeth McNally of the University of Chicago, who wrote a commentary that accompanied Dr. Schuelke's paper, is hopeful. In mice with muscular dystrophy, blocking myostatin helped overcome muscle wasting, she said. There is also the potential to help people who have muscle loss from normal aging or from cancer and diseases like those of the lung or kidneys.

In the future, people may be able to have their myostatin genes tested to decide whether to train to become professional athletes.