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Scientific discoveries can be serendipitous, and so it was when Jay L. Alberts, then a Parkinson’s disease researcher at Emory University in Atlanta, mounted a tandem bike with Cathy Frazier, a Parkinson’s patient. The two were riding the 2003 RAGBRAI bicycle tour across Iowa, hoping to raise awareness of the neurodegenerative disease and “show people with Parkinson’s that you don’t have to sit back and let the disease take over your life,” Dr. Alberts said.

But something unexpected happened after the first day’s riding. One of Ms. Frazier’s symptoms was micrographia, a condition in which her handwriting, legible at first, would quickly become smaller, more spidery and unreadable as she continued to write. After a day of pedaling, though, she signed a birthday card with no difficulty, her signature “beautifully written,” Dr. Alberts said. She also told him that she felt as if she didn’t have Parkinson’s.

Impressed, Dr. Alberts, who now holds an endowed research chair at the Cleveland Clinic in Ohio, embarked on a series of experiments in which he had people with Parkinson’s disease ride tandem bicycles. The preliminary results are raising fascinating questions not only about whether exercise can help to combat the disease but also — and of broader import — whether intense, essentially forced workouts affect brains differently than gentler activity does, even in those of us who are healthy.

Scientists have known for some time that in lab animals, forced and voluntary exercise can lead to different outcomes. Generally, mice and rats enjoy running, so if you put a running wheel in a rodent’s cage, it will hop aboard and run. That activity, obviously, is voluntary. But if you place an animal on a treadmill and control the speed so that it must keep pace, often with help from a finger prod or electrical shock, the activity becomes forced.

Interestingly, in animals, the effects, especially on the brain, typically are more beneficial after forced exercise. In one study from 2008, rats forced to run wound up with significantly more new brain cells after eight weeks than those who ran when they chose, even though the latter animals ran faster. And in another, similar experiment, mice that were required to exercise on treadmills subsequently performed better on cognitive tests than those given access to running wheels.

Before Dr. Alberts’s work, there had been few comparable experiments in humans, primarily because no one had known how, ethically, to “force” people to exercise. Dr. Alberts solved that problem by placing volunteers with Parkinson’s on the back seat of a tandem, which had been modified to ensure that the back rider would have to actively pedal; he or she could not just passively let the pedals turn. First, though, he had each volunteer ride a solo stationary bicycle at his or her own pace. Most chose a pedaling cadence of around 60 revolutions per minute, an unstrenuous level of exertion.

But on the tandem, the rider in front had been instructed to pedal at a cadence of about 90 r.p.m. and with higher force output or wattage than the patients had produced on their own. The result was that the riders in back had to pedal harder and faster than was comfortable for them.

After eight weeks of hourlong sessions of forced riding, most of the patients in Dr. Alberts’s study showed significant lessening of tremors and better body control, improvements that lingered for up to four weeks after they stopped riding.

These findings are exciting, Dr. Alberts says, because they contrast with some earlier results involving voluntary exercise and Parkinson’s patients. In those experiments, the activity was helpful, but often in a limited, localized way. Weight training, for instance, led to stronger muscles, and slow walking increased walking speed and endurance. But such regimens typically did not improve Parkinson’s patients’ overall motor control. “They didn’t help people tie their shoes,” Dr. Alberts says.

The forced pedaling regimen, on the other hand, did lead to better full-body movement control, prompting Dr. Alberts to conclude that the exercise must be affecting the riders’ brains, as well as their muscles, a theory that was substantiated when he used functional M.R.I. machines to see inside his volunteers’ skulls. The scans showed that, compared with Parkinson’s patients who hadn’t ridden, the tandem cyclists’ brains were more active.

Why forced exercise would have a greater effect on brain functioning than gentler regimens isn’t clear. Scientists have speculated that in animal experiments, being forced to work out may cause the release of stress-linked hormones in rodents’ brains, which then prompt various reactions in the cells and tissues. But Dr. Alberts suspects that in Parkinson’s patients, the answer may be simple mathematics. More pedal strokes per minute cause more muscle contractions than fewer pedal strokes, which, in consequence, generate more nervous-system messages to the brain. There, he thinks, biochemical reactions occur in response to the messages, and the more messages, the greater the response.

Whether forced exercise would similarly affect healthy brains is unknown at this point, he says, as is the question of whether riding on the back of a tandem behind a stronger cyclist is the only qualifying exercise. “Pedaling at 90 r.p.m. is a fairly intense activity,” he says.

“It seems likely,” he continues, that intense exercise of any kind should produce comparable brain reactions. “There is data showing that people who exercise intensely have less risk” of developing Parkinson’s and other neurological diseases, he says. So perhaps, if you have no access to a tandem (or no stomach for being harangued to pedal harder by the rider in front), try cranking up the speed on your next treadmill session, until you are outside your normal running comfort zone.

Dr. Alberts remains most enthused, though, about the implications of his findings for people with Parkinson’s and other brain-related conditions. He has partnered with Y.M.C.A.’s in several cities to offer special tandem cycling programs for Parkinson’s patients and is hoping to expand the program nationwide. He’s also planning studies with patients who’ve suffered strokes, in hopes that the brain changes following forced exercise could ease the relearning of physical skills.

“This is not a cure” for Parkinson’s or other brain conditions, he cautions. “But it seems to help significantly” with tremors and other symptoms, “and it gives people a chance to be active participants in their own treatment.”

He plans to return to the Iowa bike event next summer, as a representative of a program he founded, Pedaling for Parkinson’s, and expects, he says, to be joined by Ms. Frazier, who still frequently rides a tandem and signs her name legibly.