Investigators have begun two large studies of stretching, asking about its effectiveness in much the way scientists might ask about a new drug or medical device. They’re actively recruiting thousands of volunteers to participate, in the United States and elsewhere, and randomly assigning participants to use the method, or not. That is the only way, researchers say, to detect the subtle effects that most treatments and exercise interventions might be expected to evoke.

The studies are being done independently, one by researchers based in Norway and Australia and the other by a group in the United States.

The studies are not identical, reflecting perhaps the different views of stretching worldwide. People in Norway and Australia stretch for different reasons than people in the United States and do slightly different stretches. Yet exercisers and coaches everywhere, the researchers report, tend to have passionate convictions about the merits of stretching, or lack thereof.

Researchers say those strong feelings pro and con show in themselves that it will take a large study to see what, if anything, stretching really accomplishes. If stretching were remarkably effective, athletes would notice its effects right away and everyone would agree on when to stretch and what stretching does.

The study in Norway was the inspiration of Dr. Andy Oxman, a senior scientist at the Norwegian Knowledge Center for the Health Services. He had just completed what he calls a public clinical trial. It was a sort of reality show on public television that asked whether the nutritional supplement Valerian helped with insomnia; 405 people signed up to receive Valerian or a placebo and reported on their sleep by logging onto a Web site. Some participants insisted that because they slept so well they were taking Valerian. Or they said they knew they had taken the placebo because their sleep didn’t improve.

Then, the results were announced on the TV show and published: Valerian had little or no effect on sleep. Some who maintained they had the supplement actually had the placebo and vice versa.

After that, Dr. Oxman and his colleagues cast about for another good subject for a public clinical trial. A colleague in Australia suggested stretching, pointing out that there were no good data on its effects. The studies that were done were almost always small and inadequate.

The first question, though, was what is stretching supposed to help with? Dr. Oxman’s group surveyed people in Norway and Australia and heard that they tended to stretch to prevent soreness and injuries and enhance well being and performance. In Norway, people stretched after they exercised; in Australia, they stretched before exercise.

The investigators designed a 13-week study that would include walkers, runners, cyclists, swimmers and soccer players. That way, said Gro Jamtvedt, the principal investigator and a researcher at the Norwegian Knowledge Center, their findings will not be specific to any one sport — they should apply across the board.

And they decided that to get meaningful results they needed at least 2,300 volunteers who exercised at least once a week and were willing to be randomly assigned to stretch or not.

Participants enroll on the Internet and report back once a week on the study’s Web site, telling about muscle soreness, injuries and feelings of looseness. Those who are randomized to stretch are asked to do a 10-minute program before and after exercise, stretching muscles in their legs, hips and back. Those randomly assigned not to stretch must continue their normal exercise routine but refrain from stretching for 13 weeks. (http://stretchingstudy.nokc.no/study-stretching-web/innhold/welcome.faces)

Subjects include Luciana Macedo, 28, a physiotherapist in Sydney. She plays soccer once a week for an hour and a half and was assigned to the stretching group. She believes stretching helps her loosen up. “It’s almost like if after a stretch you were able to do a bit more,” she wrote in an e-mail message.

Another participant is Christian Rafn, 59, a management consultant in Oslo. He plays tennis once or twice a week and occasionally rides a mountain bike. He thinks stretching prevents soreness. In the study, though, he was assigned to the group that does not stretch.

So far, the investigators have more than 1,700 participants and are looking for more — there is no limit to the number they can accept, Ms. Jamtvedt said.

The stateside study is sponsored by USA Track & Field, but is the inspiration of Alan Roth, a 65-year-old runner, who is a marketing consultant and a partner at a start-up company, and his orthopedist, Dr. Daniel Pereles of Montgomery Orthopedics in Kensington, Md.

Dr. Pereles, a marathoner, a triathlete and a member of an advisory board for Runner’s World magazine, mentioned to Mr. Roth that he wanted to do a clinical trial of stretching but didn’t know where to find enough subjects.

Mr. Roth was intrigued and conferred with the track and field association. The group agreed to cooperate and Mr. Roth also enlisted the Road Runners Club of America. He found a statistician in his local running club and, he said, he got help from the Centers for Disease Control and Prevention in designing questions for participants.

THE study (http://www.usatf.org/stretchStudy/) began last August. It continues to recruit people who run at least 10 miles a week. It is not asking about soreness — that’s not why people in the United States stretch, Mr. Roth said. Instead, it is asking whether stretching before exercise affects the injury rate.

Participants are given a static stretching program for their hamstrings, quadriceps and calf muscles, the sort of routine done by most runners who stretch, Mr. Roth said. Participants agree to do the stretches before running, or to abstain from stretching before their runs, for three months. They also have to report their injuries during that time. So far, the study has enrolled 1,905 runners of whom 795 have completed their assigned regimens and submitted reports.

The researchers will keep recruiting participants until they see at least a 5 percent difference in injury rates between the stretchers and nonstretchers or until it becomes clear, because they have accumulated data from so many thousands of runners, that there is no difference.

In the meantime Mr. Roth, at least, still stretches the way he always has: never before exercise, nor afterward. Instead, he stops and stretches during runs and “at odd times during the day,” he said.

Nevertheless, he said he was agnostic about stretching and was looking for runners who feel the same way.

“If you listen to coaches or trainers, they are very opinionated about stretching,” Mr. Roth said. “Some say, ‘Don’t ever stretch before you run.’ Others say, ‘You just have to stretch before you run.’ We say, if you feel strongly that you must stretch or you must not stretch, don’t join the study.”