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Ryan Hall, one of the world’s best distance runners, used to pride himself on wearing his running shoes into nubs. No more. Now he assiduously replaces his shoes after running about 200 miles in them. He goes through two pairs a month.

“I know that my shoes could probably handle a couple of hundred more miles before they are worn out, but my health is so important to me that I like to always make sure my equipment is fresh,” he said.

Of course Mr. Hall, sponsored by Asics, does not have to pay for his shoes. Most of the rest of us do, and at around $100 a pair they aren’t cheap. Yet we are warned constantly to replace them often, because running in threadbare shoes may lead to injuries that can take months to heal.

So here’s a simple question: How do you know when your shoes are ready for those discard bins in gyms? And if you do get injured, is it fair to blame your shoes?

My friend Jen Davis runs more than 100 miles a week, like Mr. Hall, but has a different set of criteria for getting rid of shoes. One is that if they smell bad even after she washes them in her washing machine, it’s time for a new pair. She estimates she puts 500 miles on each pair of shoes.

Henry Klugh, a running coach and manager of The Inside Track, a running store in Harrisburg, Pa., says he goes as far as 2,000 miles in some shoes. He often runs on dirt roads, he said, which are easier on shoes than asphalt is and do not compress and beat up the midsole as much.

My coach, Tom Fleming, has his own method. Put one hand in your shoe, and press on the sole with your other hand. If you can feel your fingers pressing through, those shoes are worn out — the cushioning totally compressed or the outer sole worn thin.

As for me, my practice has been to keep track of the miles I run with each pair and replace them after 300 miles. Who is right? Maybe none of us. According to Rodger Kram, a biomechanics researcher at the University of Colorado, the theory is that you must change shoes before the ethylene vinyl acetate, or E.V.A., that lines most running shoe insoles breaks down.

“Think of a piece of Wonder Bread, kind of fluffy out of the bag,” he said. “But smoosh it down with the heel of your palm, and it is flat with no rebound.”

A moderate amount of cushioning improves running efficiency, he has found. But as to whether cushioning prevents injuries, he said, “I doubt that there are good data.”

Dr. Jacob Schelde of Odense University Hospital in Denmark, has looked for clinical trials that address the cushioning and injury question — and has found none. He’s applying for funds to do one himself, a 15-month study with 600 runners.

Dr. Schelde did find a study on injury rates among runners, published in 2003, that had some relevant data even though it was not a randomized clinical trial and shoe age was not its main focus. The study was large and regularly tested runners in a 13-week training program. The researchers failed to find any clear relationship between how long running shoes were worn and a runner’s risk of injury.

It also is difficult to find good data on how long E.V.A. insoles last. But one exhaustive study, led by Ewald Max Hennig of the biomechanics laboratory at University of Duisburg-Essen in Germany, involved 18 years of shoe testing from 1991 to 2009. The researchers measured the performance of 156 shoe models worn by runners. Dr. Hennig and his colleagues wrote that the sort of mechanical testing that shoe manufacturers do to evaluate cushioning materials does not reflect what happens when people actually run.

Over the years, running shoe quality steadily improved, the researchers reported. The shoes also changed as running fads waxed and waned. Shock attenuation, for example, diminished starting around 2000, when there was talk of shoes providing too much cushioning.

Then, when cushioning became fashionable again, it returned. But so did minimalist shoes designed for the barefoot running fad, which have almost no cushioning.

In Europe, the researchers reported, people typically wear shoes for about 600 miles. But their studies indicated that shoes could last much longer.

Most shoemakers, of course, would prefer to see us trade in sooner. Kira Harrison, a spokeswoman for Brooks, said shoes should last for 400 to 500 miles. The very light models last about 300 miles, she said.

Biomechanical studies have shown that after those distances the shoes lose their bounce, she said: “Everyone in the industry knows that standard.”

Gavin Thomas, a Nike spokesman, said a shoe’s life span depended on the type of shoe — lightweight or more heavily cushioned — and on the runner’s weight and running style. Those who are light on their feet can wear shoes longer than those who pound the ground. Those who run on soft surfaces can keep their shoes longer.

After 300 or 400 miles, Mr. Thomas said, a typical shoe worn by a typical runner will not feel the way it used to, a sign it is worn out.

But Golden Harper, developer of Altra running shoes and founder of the company, said any advice on mileage was “a lot of malarkey.” Mr. Harper, a distance runner, said most runners could feel when their shoes need to be replaced. “You get a sense for it,” he said. “Nothing hurts, but it is going to soon.”

So when should you retire those faithful running shoes, and what happens if you don’t? Despite the doomsday warnings, no one really knows. And with so many variables — type of shoe, runner’s weight, running surfaces, running style — there may never be a simple answer.

But we can take comfort in Dr. Hennig’s work. Even people like Henry Klugh, who put in many more miles than most guidelines suggest may still be fine. Their shoes may still be performing.