The fitness movement has bypassed four out of five American adults, especially those who are older, have low incomes and are less educated, a new survey says.

Only one in five U.S. adults gets significant exercise at least twice a week, and 45 percent don't participate in any fitness activity, according to the survey of 12,500 people commissioned by the National Sporting Goods Association.Lowest rates of participation are among people over 55 years old, those with incomes under $15,000 and those lacking college educations, according to the survey.

"You would have had to be living under the proverbial rock to miss hearing about the need for frequent participation in a fitness activity," said Thomas B. Doyle, research director for the sporting goods group.

The scientific evidence linking physical activity to health is growing, according to Dr. J. Michael McGinnis, director of the federal Office of Disease Prevention and Health Promotion in Washington.

"We know that regular exercise can reduce the risk of heart disease and help maintain a healthy weight," he said in a statement. "And it can also make people feel better."

The survey defined significant exercise as any of seven activities: aerobic exercising, cycling, calisthenics, exercising with equipment, exercise walking, running-jogging and swimming. It didn't say how long each activity must be performed.

The survey's findings parallel those of previous studies by the national Centers for Disease Control and other groups.

Inactivity threatens the health of a far greater proportion of Americans than any of the much-publicized evils of smoking, high blood pressure or high blood cholesterol, one researcher said.

Yet a sedentary lifestyle can double the risk of a fatal heart attack or stroke, said the researcher, Harold Kohl, an epidemiologist with the Institute of Aerobics Research in Dallas.

"It's almost the same (risk) as for . . . smoking, high blood pressure, high cholesterol," he said.

"An overwhelming majority of the population is not exercising currently at the levels we believe most optimal for health," he said.

Kohl said he believes people are discouraged by the belief that they must exercise at "optimal" levels - 20 minutes a day, three times a week, at 65 percent of the heart's maximum capacity to pump blood.

But latest findings indicate that the highest risks of death from fatal heart attacks or strokes occur only in the most inactive people, and substantial benefits result from activities such as brisk walks and gardening.

"People think you've got to go out and do marathons, triathlons, 10 K's (10-kilometer races)," Kohl said. "What we need to say is, `Turn off the TV, do some gardening, take a walk after dinner.' "

The survey was conducted by the independent polling organization N.P.D. Research in Port Washington, N.Y.