About 40 percent of Americans report that they never exercise, a figure that has remained steady for decades. They will not even do the easy stuff. In studies of moderate exercise to help prevent diabetes, for example, investigators had to go to great lengths just to keep subjects in a walking program.

Now, with more recent studies using accelerometers that measure actual movement rather than relying on self-reports, the data are even more dismal. Only 3.5 percent of Americans between 18 and 59 do the minimum amount recommended by the Department of Health and Human Services: 150 minutes a week of moderate activity. Among those older than 60, it's 2.5 percent. "It is stunning," said Panteleimon Ekkekakis, an exercise researcher at Iowa State University.

If Americans know exercise is so good for them, why don't they take the message to heart as they did the exhortations against smoking? And if exercise makes people feel so good, why don't they just do it?

Maybe, some researchers say, the problem is the message. It obviously has not had much of an effect. The idea now is to make use of tools that psychologists have developed to assess people's moods during exercise, asking how good or bad it feels as the intensity varies.

Simply giving people an exercise prescription, like walking for 20 minutes a day, five days a week, is clearly not working. Nor are programs that claim very intense, very short bouts of exercise are all that is needed. To encourage exercise, perhaps people should be told to find an exercise, and an intensity level, that makes them feel good, Dr. Ekkekakis said.

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