Seattle ? Gene injections in rats can double muscle strength and speed, researchers have found, raising concerns that the virtually undetectable technology could be used illegally to build superathletes.

A University of Pennsylvania researcher seeking ways to treat illness said studies in rats showed that muscle mass, strength and endurance could be increased by injections of a gene-manipulated virus that goes to muscle tissue and causes a rapid growth of cells.

“The things we are developing with diseases in mind could one day be used for genetic enhancement of athletic performance,” Lee Sweeney of the University of Pennsylvania said Monday at the national meeting of the American Association for the Advancement of Science.

Sports officials said the gene therapy had the potential of betraying the very essence of sport — athletes using their natural talents and training to compete.

It would, said Tom Murray of the Hastings Center, a research organization, be like allowing an athlete to compete in the Boston Marathon wearing roller blades.

“Performance-enhancing drugs have been a concern in sports, and gene therapy has the potential to kick it up a notch,” said Murray, who has studied the issue of doping in athletics. “It makes the challenges greater (of controlling performance-enhancing measures).”

Murray said he “has no doubt athletes will be in touch with Sweeney” when they learn of his research.

Sweeney said that already half the e-mails he receives are from athletes or sports trainers.

Richard Pound of McGill University and the World Anti-Doping Agency, which controls doping in athletics, said the sports community lost control of drugs for performance-enhancement in the 1960s to 1990s, and “we’ve been playing catch-up ever since.”

Now gene therapy looms as an even more serious threat, he said.

“Sport is and should be an effort to see how far you can go with your natural talents honed by exercise and skill perfection,” he said, and not by manipulating genes to build muscle.

He said the international sports community already had regulations forbidding gene therapy for performance improvement, and his agency hopes to be active in efforts to control use of the technique as the science develops.

Sweeney said his laboratory studies showed that injecting into muscles a manipulated virus that carries a gene for insulin-like growth factor 1 (IGF1) caused target muscles in rats to grow in size and strength by 15 to 30 percent. The inserted gene causes formation of extra IGF1 which, in turn, prompts the growth of muscle cells.

When the technique was used on rats that also were put through an exercise program, the animals doubled their muscle strength, he said.

“If a normal person would inject this, their muscles would get stronger without them doing anything,” Sweeney said. “If they are athletes in training, the rat study indicates that their training would be much more effective, injury would be overcome more easily, and the effect of the training would last a much longer time.”

The effect appeared to last throughout the life of the rats.