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Phys Ed Gretchen Reynolds on the science of fitness.

Regular exercise may alter how a person experiences pain, according to a new study. The longer we continue to work out, the new findings suggest, the greater our tolerance for discomfort can grow.

For some time, scientists have known that strenuous exercise briefly and acutely dulls pain. As muscles begin to ache during a prolonged workout, scientists have found, the body typically releases natural opiates, such as endorphins, and other substances that can slightly dampen the discomfort. This effect, which scientists refer to as exercise-induced hypoalgesia, usually begins during the workout and lingers for perhaps 20 or 30 minutes afterward.

But whether exercise alters the body’s response to pain over the long term and, more pressing for most of us, whether such changes will develop if people engage in moderate, less draining workouts, have been unclear.

So for the new study, which was published this month in Medicine & Science in Sports & Exercise, researchers at the University of New South Wales and Neuroscience Research Australia, both in Sydney, recruited 12 young and healthy but inactive adults who expressed interest in exercising, and another 12 who were similar in age and activity levels but preferred not to exercise. They then brought all of them into the lab to determine how they reacted to pain.

Pain response is highly individual and depends on our pain threshold, which is the point at which we start to feel pain, and pain tolerance, or the amount of time that we can withstand the aching, before we cease doing whatever is causing it.

In the new study, the scientists measured pain thresholds by using a probe that, applied to a person’s arm, exerts increasing pressure against the skin. The volunteers were told to say “stop” when that pressure segued from being unpleasant to painful, breaching their pain threshold.

The researchers determined pain tolerance more elaborately, by strapping a blood pressure cuff to volunteers’ upper arms and progressively tightening it as the volunteers tightly gripped and squeezed a special testing device in their fists. This activity is not fun, as anyone who has worn a blood pressure cuff can imagine, but the volunteers were encouraged to continue squeezing the device for as long as possible, a period of time representing their baseline pain tolerance.

Then the volunteers who had said that they would like to begin exercising did so, undertaking a program of moderate stationary bicycling for 30 minutes, three times a week, for six weeks. In the process, the volunteers became more fit, with their aerobic capacity and cycling workloads increasing each week, although some improved more than others.

The other volunteers continued with their lives as they had before the study began.

After six weeks, all of the volunteers returned to the lab, and their pain thresholds and pain tolerances were retested. Unsurprisingly, the volunteers in the control group showed no changes in their responses to pain.

But the volunteers in the exercise group displayed substantially greater ability to withstand pain. Their pain thresholds had not changed; they began to feel pain at the same point they had before. But their tolerance had risen. They continued with the unpleasant gripping activity much longer than before. Those volunteers whose fitness had increased the most also showed the greatest increase in pain tolerance.

“To me,” said Matthew Jones, a researcher at the University of New South Wales who led the study, the results “suggest that the participants who exercised had become more stoical and perhaps did not find the pain as threatening after exercise training, even though it still hurt as much,” an idea that fits with entrenched, anecdotal beliefs about the physical fortitude of athletes.

Because it did not examine physiological effects apart from pain response, however, the study cannot explain just how exercise alters our experience of pain, although it contains hints. Pain thresholds and tolerances were tested using people’s arms, Mr. Jones pointed out, while the exercisers trained primarily their legs. Because the changes in pain response were evident in the exercisers’ upper bodies, the findings intimate that “something occurring in the brain was probably responsible for the change” in pain thresholds, Mr. Jones said.

The study’s implications are considerable, Mr. Jones says. Most obviously, he said, the results remind us that the longer we stick with an exercise program, the less physically discomfiting it will feel, even if we increase our efforts, as did the cyclists here. The brain begins to accept that we are tougher than it had thought, and it allows us to continue longer although the pain itself has not lessened.

The study also could be meaningful for people struggling with chronic pain, Mr. Jones said. Although anyone in this situation should consult a doctor before starting to exercise, he said, the experiment suggests that moderate amounts of exercise can change people’s perception of their pain and help them, he said “to be able to better perform activities of daily living.”