You can prevent injuries, Dr. Vorderer said, or, if you get them, can make them heal if you learn the right way to stretch and if you stretch regularly. And you should also learn the right way to run; in general, he said, runners should strike the ground with their heels first. If they strike with their midfoot or forefoot, he said, they are just asking for injuries.

Dr. Vorderer speaks from experience: he was a competitive runner for years and said he trained with fantastic coaches. And he says he has helped countless runners rid themselves of chronic injuries by figuring out why they were getting injured and teaching them, for example, the right way to stretch. He has e-mail messages from grateful patients, thanking him for solving problems that threatened to end their running altogether.

But exercise physiologists say none of what he espouses has been established by rigorous studies. Stretching evidence is so inconclusive that two large studies are now under way that randomize people to stretch or not and ask whether it prevents injuries, does nothing or increases injury rates. No one knows what the answer will be.

As for running styles, a credible study in 2007 showed that running form often depended on running speed. The slower people run, the study found, the more likely they are to strike the ground with their heel first. The same runners, going more slowly, run differently from when they run fast.

There is no right or wrong way to run, said Peter R. Cavanagh, a professor in the department of orthopedics and sports medicine at the University of Washington. And even if there is, he said, it is not clear that people can permanently change their natural stride.

Dr. Vorderer says that he knows experts often disagree but that the art of sports medicine is to understand individuals.

“It’s hard,” he said. “Anyone can put out a shingle and say, ‘I’m a sports medicine doctor.’ You basically want to go where you have to wait an hour in the office. Then you know it’s a busy office.” And, he adds, while it is easy to diagnose a problem like an Achilles tendon injury, what you really want to know is, “Why do I have this? What mechanical or structural or shoe problem gave it to me?”

So how are athletes supposed to know whom or what to believe? It’s a huge problem, researchers say. They have some tips, but their overall message is: Be wary and be careful. Look for credentials and rigorous science. And check the Web sites of reputable organizations, like the American College of Sports Medicine, which publishes position papers on exercise science. The papers are compiled by committees of experts and provide references and evidence to support their statements.

Yet bad advice is so ubiquitous that almost everyone, even experts themselves, has been foiled.

That happened, for example, to Dr. Paul Thompson of Hartford Hospital, a heart researcher and marathon runner. He warns that people should “be careful of advice that has a price tag.” The best example, he said, are orthotics, those shoe inserts that are supposed to prevent injuries.

“Podiatrists make money making them, and more power to them,” Dr. Thompson said. “But some podiatrists think they cure everything.”

“I once needed orthotics,” he continued. “The podiatrist wanted me to run with them. It completely changed my foot plant and produced lateral knee pain, which resolved when I used them only with my walking shoes.”

Something similar happened to me. After I got a stress fracture in a small bone in my foot last year, my orthopedist prescribed orthotics. For weeks I tried to run with them but felt slow, like I was running through sand. Then I pulled my hamstring. My coach watched me run with and without the orthotics and said he could see why I was having problems: the orthotics changed my foot plant so I was braking with every step. Now, like Dr. Thompson, I wear them only when I walk around.Those of us who are not experts can be especially vulnerable to bad advice. After my stress fracture, I had a biomechanical analysis by an exercise physiologist at a commercial studio in New York. Among other things, I was told to change the way I run. My heels never hit the ground; I had to learn to run so my heels struck first.

I knew I couldn’t do that, so I ignored the advice and never returned to those experts. But my friend Birgit Unfried tried to listen to a sports medicine doctor in New Jersey who analyzed her running stride. She had a painful iliotibial band, which stabilizes the knee, and was plagued with shin splints. The reason, she was told, was that her stride was wrong. She was a heel striker; she needed to learn to strike first with her midfoot.

BIRGIT tried and did so well that her doctor put before and after videos of her running on his Web site. But she never felt comfortable running that new way. As for her injuries, the new gait was no panacea. At first, Birgit said, her problems seemed to go away. But soon she pulled her quadriceps muscle and, despite her changed gait, her shin splints came back worse than ever.

“I haven’t been running at all,” Birgit said. “At times I think I’m getting better but then I touch my shin in certain areas, like down near the ankle, and it hurts like a bruise.”

Orthotics and running styles are the easier cases. All too often, there are no studies or scientific evidence to guide anyone, even the experts. When studies are done, they tend to be inadequate.

“Good experiments need tight experimental design, and they need control groups,” Dr. Cavanagh said. Without them, results are pretty much useless. And many exercise studies lack one or both of those crucial elements.

That may not matter to many who dispense advice. Often, they rely on a hunch or personal experience or on what they think makes a great athlete great.

Take pedal speed in cycling, said Michael J. Berry, a serious road cyclist and chairman of the department of health and exercise science at Wake Forest University. The studies, such as they are, say the best pedal speed is 60 to 80 revolutions a minute. But that is based on experiments with untrained subjects riding stationary bikes in an exercise lab. Those results may have no relevance for experienced cyclists riding on roads.

So, ignoring those lab studies, many experts counsel cyclists to pedal much faster. Their evidence? It’s Lance Armstrong, who pedals extraordinarily fast — 95, 100, 110 r.p.m.’s.

In the 2003 Tour de France, Armstrong consistently beat his rival, Jan Ullrich, and commentators, Dr. Berry noted, said it was in part because he could pedal so fast. Ullrich pedals slower but uses bigger gears. “What if Lance had never been there and Jan had won?” Dr. Berry said. “Would people say that the reason he is so good is that he pushes a big gear?”

Personal Best – For Athletes, a Road to Success Paved With Bad Advice – NYTimes.com.

There are training programs everywhere — in magazines, books, on the Internet. But eventually, exercise physiologists say, most people need guidance from a group with experienced coaches, like the one Mr. Gordon joined, or from personal coaches.

Training, though, can require such a commitment over so many years that many drop out. Not Mr. Gordon, who loves to train. And that love of serious training, coaches say, is often what distinguishes a good athlete from a mediocre one.

“Any great athlete who accomplishes anything, anywhere, loves to train,” said Tom Fleming, my coach and a former elite distance runner who twice won the New York City Marathon.

It’s not that talent is irrelevant. Truly talented athletes can be better without training than many average people can be with training. But most people are not in that elite athlete realm and, for them, training can mean the difference between a good performance and a poor one.

Carl Foster, an exercise physiologist at the University of Wisconsin, La Crosse, gave an example of what training can do. A man who has been running but not training might run a 5K race at a pace of 7:45 a mile. If he trained for six months, he could get his pace down to 6:10.

The biggest effects are in untrained people, said Hirofumi Tanaka, an exercise physiologist at the University of Texas. “Elite athletes’ performance is getting too close to the ceiling or upper limit,” he said. “There is not much space to improve.”

But even people who are starting reach a plateau within six months or so. After that, real gains are incremental and hard won. Yet people regress if they ease up on training.

Personal Best – Proper Training Is a Critical Element to Athletic Success – NYTimes.com.