Most ultra runners, especially the young and inexperienced, get hurt, but their overall health is good, researchers say.

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When it comes to at least one extreme form of exercise, it may pay to be older and more experienced, a new study of ultramarathon runners suggests.

The study, published Wednesday in the online journal PLOS ONE, is among the first to look at a small but growing group: people who run marathons longer than the traditional 26.2 miles. It finds that most of these runners are very healthy people, but most also do get hurt in their sport — though older, seasoned runners fare better than upstarts.

"It's just like teenage drivers being more likely to get into an accident. There's probably some learning curve," says Eswar Krishnan, a epidemiologist at Stanford University who is co-author of the study, led by Martin Hoffman, a professor at the University of California-Davis. Hoffman is an ultramarathon runner, but Krishnan is not.

Krishnan says he joined the study partly because he wanted to learn "if there is such a thing as too much exercise."

The new paper doesn't answer that, he says. Instead, he says, it's a baseline report on people who are pushing exercise endurance limits. The results are based on an online survey of 1,212 runners who had completed at least one race of at least 31 miles. Some had completed 100-mile races.

About 63,530 people in North America completed ultra-length races in 2012, according to UltraRunning Magazine. The survey participants should be fairly representative of that group, Krishnan says.

Among the findings, all based on runners' own reports:

• Ultra runners ranged in age from 18 to 81; half were older than 42, and 68% were men.

• Compared with the general population, they had low rates of most illnesses but high rates of asthma and allergy — something that also has been found in traditional marathon runners and may be linked to all the time runners spend outside.

• 77% had an exercise-related injury in the prior year, and 65% lost at least of day of training because of an injury. Knees were the most common trouble spot, and 5% experienced stress fractures, most often of the foot. Similar overall injury rates have been found in other marathon runners.

• Injury rates were highest among the youngest and least-experienced runners and those who spent the most time in intense exercise.

Follow-up studies will take a closer look at first-year ultramarathoners and those who drop the sport, Krishnan says. He says a learning curve may not be the only reason younger, less experienced runners have more injuries. It's also possible, he says, that those who get hurt drop out, leaving a band of stronger and older pros. Krishnan says he also wants to look at health effects over time.

None of the results so far are surprising, says Lewis Maharam, a sports medicine specialist in New York City and author of Running Doc's Guide to Healthy Running. "Anyone that exercises is going to get injured," he says. "It's proportional to the amount of exercise you do."

He says he doubts a long-term study would turn up new major health risk. People who can stick with ultramarathoning, he says, "are born to run."

But he says it's important for anyone planning to run marathons of any length to check with a doctor first to make sure their heart is healthy enough for endurance exercise.

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