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Stay hydrated. Increase your mileage gradually. Don't run into oncoming traffic.

Certain assumptions about running are still safe to make. But as game-changing studies puncture myths about everything from the benefits of pre-run stretching to the merits of expensive orthotics, runners had better stay on their toes.

Here's the lowdown from the research labs:

Myth: Stretching before running prevents injuries

In fact, all those pre-run contortions may be a waste of time. In February, a clinical trial involving 2,700 runners concluded there was no difference in the risk of injury for those who stretched before running and those who didn't. After three months, the injury rate for both groups was about 16 per cent, says the study's author, Daniel Pereles, an orthopedic surgeon in Kensington, Maryland.

The findings were the same for bothultra marathoners and recreational joggers, he adds. "If you don't feel like doing a five-minute pre-run stretch," he says, "it's probably not going to make much difference."

The study didn't look at the effects of gentle warm-up exercises or dynamic stretches such as walking lunges, which rely on motion to engage muscles. Both are popular among elite athletes. In the trial, runners were asked to do static hamstring, quad and Achilles stretches before they ran, and told not to alter any mid- or post-run routine.

Although a five-minute stretch did not protect against injury, runners who were used to stretching but stopped as part of the study had a disproportionately higherinjury rate.

Researchers aren't entirely sure why, Dr. Pereles says. But he advises against making any sudden changes in routine.

Myth: Orthotics correct alignment problems

If only it were that simple. After decades of looking for evidence that orthotic inserts can help treat mechanical-alignment problems, researchers have come up with zilch, according to Benno Nigg, a professor of biomechanics and co-director of the Human Performance Lab at the University of Calgary. "You cannot align the skeleton by using orthotics," he says.

One reason is that patients with, say, flat feet or overpronation, don't react in the same way to orthotics designed to correct the condition. In fact, no one can accurately predict how an orthotic will perform, Dr. Nigg says. "The specialists don't know."

His research suggests that custom inserts may not be worth the expense. But that doesn't mean orthotics are useless - including the drugstore variety. On the contrary, in a study of more than 200 Canadian soldiers, Dr. Nigg found that those who wore shoe inserts for four months had half as many injuries as soldiers who wore none. The study used six types of inserts designed to correct specific problems. But there was no good news for podiatrists: The study found no apparent relationship between soldiers' individual biomechanics and the type of insert they chose.

"Comfort was the selection criterion," Dr. Nigg says. "It was fantastic."

Myth: Marathons ruin your knees

Not so, according to a long-term study of endurance runners conducted by researchers at Stanford University. When the study began in 1984, about 7 per cent of the runners - aged 50 to 72 - had mildly arthritic knees. No one in the age-matched control group was affected. But almost two decades later, the runners were in the lead: 32 per cent of the non-runners had arthritic knees, compared with 20 per cent of the long-distance runners.

More research is needed, according to Anthony Luke, director of primary care sports medicine at the University of California, San Francisco. Dr. Luke led a 2010 study using magnetic resonance imaging technology to look at biochemical changes in knee cartilage among beginning marathoners with normal knees. Researchers found increases in a biomarker called T1rho, which is associated with osteoarthritis. Three months after the race, the levels were declining but not back to normal.

It's unclear whether the changes were permanent or whether a longer rest period is needed, says Dr. Luke, noting that his study found no major structural injuries in the knees.

Previous injuries, such as torn ligaments, are known risk factors for osteoarthritis, he adds. But "for people with good alignment and healthy knees," he says, "marathon running is certainly safe."