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Phys Ed Gretchen Reynolds on the science of fitness.

Tell yourself during exercise that you’re not as tired as you think you are and you could make that statement true, a new study shows, reminding us that the body intertwines with the mind in ways that we are only starting to understand.

For the new experiment, which was published last month in Medicine & Science in Sports & Exercise, researchers from the University of Kent in Canterbury, England, and other institutions turned to a group of 24 healthy, physically active young men and women and asked if they would be willing to ride a bicycle to the point of limp exhaustion, repeatedly.

Physical fatigue is a surprisingly enigmatic condition. Scientists don’t fully understand how the body knows when it has had enough. Many of us might guess that activity ceases once our muscles have run out of fuel or fluids. But in studies with rodents, even after they are pushed to run until they drop, scientists have found reserves of fuel in the animals’ muscles. Physiologically, they remain capable of more exercise, although their bodies don’t seem to think so.

Such experiments have prompted some scientists recently to propose a different theory of exercise-related fatigue, in which the brain, rather than the muscles, initiates exhaustion after receiving and analyzing inputs from the body. An attractive element of this theory, often called the psychobiological model, is that it allows more wiggle room. If exhaustion is determined by the brain and is, to some degree, subjective, then theoretically the right tweaks during training could convince your brain that you can go farther or harder than it would otherwise allow.

That possibility motivated the new experiment, which was designed specifically to determine whether verbally encouraging yourself during a draining workout can affect your mind’s calculations and stave off fatigue.

To test that idea, the scientists first took a series of baseline physical measurements of their volunteers. Then, during a separate lab visit, the volunteers were asked to pedal a computerized stationary bicycle at about 80 percent of their predetermined maximum force until they felt that they could pedal no more and quit.

Throughout, the scientists measured each rider’s heart rate, pedaling power and pace. Having attached electrodes to the riders’ foreheads and cheeks, the researchers also monitored their facial muscular contractions — i.e., grimaces — an accepted physiological indicator of increasing physical exertion. And they asked the riders several times during and at the conclusion of the ride how hard the exercise had felt, on a scale of zero to 10.

Once each rider’s measurements had been recorded, they were randomly divided into two groups. One group was told to continue with their normal exercise routine for the next two weeks. Those in the other group were coached in “self-talk,” the kind of verbal banter that many athletes engage in during workouts, whether done aloud or silently.

For many of us, self-talk is haphazard and, if the banter turns berating, it can be demotivating. In this case, however, the chosen volunteers systematically learned how best to talk to themselves in an encouraging way. Provided with phrases that psychologists previously had found to be motivating, such as “You’re doing well,” the volunteers were asked also to jot down any expressions that they had used during exercise in the past. A popular choice was “feeling good.”

Each volunteer then chose four phrases that appealed to him or her, and was told to start repeating these frequently during subsequent, normal exercise sessions. The volunteers practiced this self-talk during exercise for the next two weeks.

Then each group returned to the lab and underwent another cycling test to exhaustion, during which the riders in the self-talk group studiously repeated their mantras; some aloud, some silently.

Afterward, it was obvious that self-talk had bolstered riders’ feelings and performance. The group that had talked to themselves had pedaled much longer before succumbing to exhaustion than in their first rides and reported that the pedaling had felt easier, even though, objectively, their heart rates and facial expressions had remained the same, indicating that the physical exertion had been just as great as in the initial ride.

The riders in the other group, meanwhile, generally repeated their performances from before, lasting about the same amount of time before quitting and feeling about the same degree of discomfort.

On one level, these findings indicate that “motivational self-talk improves endurance performance compared to not using it,” said Samuele Marcora, the director of exercise research at the University of Kent and senior author of the study.

But a deeper reading of the data, he continued, buttresses the idea that physical exhaustion develops, to a considerable degree, in your head. “If the point in time at which people stop exercising was determined solely biologically,” he said, self-talk would have no effect. But it did.

To be effective, though, self-talk probably has to be consistent and systematic, he said. Some of the riders in the control group muttered or silently exhorted themselves during the cycling — they weren’t told not to — but they tended to do this haphazardly, and without discernible benefit. Better, Dr. Marcora suggested, to deploy phrases that particularly encourage you and repeat them often, even on a schedule, especially as a workout or competition wears on. It is likely we all could stand to hear that, despite intimations to the contrary, we’re “feeling good.”