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Rod Dishman, director of the exercise-psychology laboratory at the University of Georgia, is annoyed when students enroll in a certain fitness class. Because it’s a class in walking.

“It is a sin for a healthy, capable young adult to enroll in a walking class,” he said. “ What they are getting credit for is avoiding making any effort.”

Therein lies a problem, Dishman and other researchers say. The public-health message about exercise is that any amount is good and that walking is just fine. Nearly all Americans say they have heard those messages. They know that exercise is good for them and that they should do it.

Yet they do not.

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About 40 percent of Americans report 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, investigators had to go to great lengths just to keep subjects in a walking program.

Now, with more recent studies 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 of physical activity the Department of Health and Human Services recommends: 150 minutes a week of moderate activity. Among those over 60, the percentage is even lower: 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 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.

In studies, Ekkekakis and his colleagues found that as exercise intensity increased to the point where a person was on the verge of breathing so hard that it was difficult to talk — the so-called ventilatory threshold — people had different reactions. Some say they felt more and more pleasure, while others felt less. Beyond the ventilatory threshold, though, most felt bad.

It’s complicated, though. A hard workout for one person can be ridiculously easy for another.

At one extreme are acutely sedentary people. “As soon as they get up and take a few steps, they are above their ventilatory threshold,” Ekkekakis said.

At the other extreme are athletes who cannot reach their ventilatory thresholds until their hearts are beating at nearly the maximum rate.

Some people actually feel their best when they surpass their ventilatory threshold.

It is not at all clear why at some level of intensity a workout starts to feel good. Nor, Dishman said, is it known why a long endurance workout can feel good in a different way than a short workout with intense bursts of effort.

But Ekkekakis has discovered a few clues. He borrowed from Daniel Kahneman, the Princeton psychologist whose research has found that people remember two parts of an experience: the peak, when the feeling was most intense, and the end.

Ekkekakis and his colleagues had volunteers exercise 20 minutes at a level they reported was unpleasant. In one session, the subjects had a five-minute cooling-down period afterward, which felt pleasant. In the other session, the subjects stopped exercising, without a cool-down.

A week later, the subjects were asked which workout they would repeat. “At a ratio of 2-to-1, they chose the one with the pleasant end,” Ekkekakis said. “But they cannot tell you why.”

There may be a public-health lesson there, researchers say.

To encourage exercise, perhaps people should be told to find one, and an intensity level, that makes them feel good, Ekkekakis said. “People like to do things that make them feel better, and they avoid things that make them feel worse. The idea is for them to have the motivation to exercise again tomorrow.”

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