Will running on hard surfaces like asphalt and concrete increase my risk of injury?
In a study to be published later this year, Brazilian researchers found that your feet feel about 12 per cent more pressure with each foot strike when running on asphalt compared with grass.
Thanks for that newsflash, Captain Obvious, you might say.
But the findings actually contradict several earlier studies, which - despite what our intuition tells us - have found that we seem to automatically adapt our running stride so that hard and soft surfaces administer roughly the same shock to the body.
In fact, it may be the smoothness of paved surfaces that makes them dangerous to runners, rather than their hardness. And softer, less even surfaces carry their own injury risks, so the best answer may lie somewhere in the middle.
"Understanding why and how runners get injured, and the role of the surface, would be like winning the Nobel Prize of sports science research," says Katherine Boyer, a Canadian biomechanics researcher now at Stanford University in California.
The surprising idea that your body can make adjustments for different running surfaces dates back to studies in the 1990s. Researchers found that when they varied the stiffness of a running surface, runners adjusted the effective stiffness of their legs in the opposite direction - by bending their knees slightly more or less and by tensing their muscles - so that their total up-and-down motion remained perfectly constant.
In support of this notion, a 2002 study by Mark Tillman of the University of Florida, using force-sensing shoe inserts, found no difference in the forces created by running on asphalt, concrete, grass and a synthetic track.
The 12-per-cent difference found by Vitor Tessuti of the University of Sao Paulo, which will appear in the Journal of Science and Medicine in Sport later this year, is still relatively small. And a companion study, yet to be published, found no difference between asphalt, concrete and a synthetic track.
Even when the forces on the feet are the same, though, the slight differences in knee angle and other parameters could theoretically translate into greater likelihood of injury on one surface compared with another, Dr. Tillman cautions.
The bottom line, for now, is that the simple picture - harder surface leads to more pounding leads to injury - isn't supported by the existing evidence. Indeed, on-the-ground studies such as one from 2003 that followed 844 runners preparing for the 10-kilometre Vancouver Sun Run have failed to find any association between running surface and injury rate.
But there are other factors to consider, Dr. Boyer says. Smooth, flat, paved surfaces will result in every stride being basically the same, so your muscles and joints are stressed in exactly the same way throughout the run.
On unpaved surfaces, in contrast, no two steps are the same, which provides slight variations in the impacts on your body, reducing the chance of an overuse injury. Too much unevenness, though, carries risks such as a turned ankle.
"The key is to find the balance between stress and overstressing the system," Dr. Boyer says.
The principle of specificity also applies: If you do all your training on one surface, your body may not be adequately prepared to run on other surfaces - especially in long, demanding races.
"I do make a calculated effort to run more on the roads when preparing for a marathon," says Dylan Wykes, Canada's second finisher at last summer's World Championships in Athletics marathon in Berlin.
Mr. Wykes's normal training regimen is about half on roads and half on grass and gravel trails, but he increases the road component to 75 per cent before marathons to make sure his legs are prepared for the unchanging impacts of 42.2 kilometres of smooth asphalt.
This approach, incorporating a healthy mix of different surfaces, is one most runners would do well to emulate - at least until biomechanics researchers reach a firmer conclusion. Still, the pile of conflicting research results suggests that, if your circumstances do force you to run exclusively on paved surfaces, it doesn't mean instant injury.
Alex Hutchinson blogs about research on exercise and athletic performance at www.SweatScience.com.