The conventional wisdom is deeply entrenched: constant pounding on hard surfaces will beat up your legs and lead to injury. But proving this has turned out to be remarkably tricky—in fact, a growing number of studies suggest that, despite our intuition, we’re able to automatically adjust our running stride so that hard and soft surfaces administer roughly the same shock to the body. Instead, researchers suspect that the crucial difference between running surfaces may be how smooth or uneven they are.
The surprising idea that your body can make adjustments for different running surfaces dates back to studies in the 1990s.
Scientists 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 in-shoe forces felt by runners on asphalt, concrete, grass, and a synthetic track. This result has been called into question by a 2010 Brazilian study that found 12 percent more pressure with each foot-strike when running on asphalt compared with grass. Still, even that difference is surprisingly small given the large difference in the softness of the two surfaces.
Forces and pressures are only part of the picture, though, Tillman cautions. For example, slight differences in knee angle as your leg adjusts to a different surface could theoretically translate into greater likelihood of injury on one surface compared with another. But the simple picture—harder surface leads to more pounding leads to injury—isn’t supported by the existing evidence.
It may be that our focus should be on how smooth the surface is rather than how hard it is, according to Stanford University biomechanics researcher Katherine Boyer. Flat, paved surfaces will result in every stride being almost identical, so your muscles, joints, and bones are stressed repeatedly in exactly the same way throughout the run.
That sets the stage for overuse injuries like shin splints, which typically occur when you try to increase your training too quickly. (The same factors would presumably be present on treadmills, though researchers have yet to investigate this.) On unpaved surfaces, in contrast, no two steps are the same, which provides slight variations in the impacts on your body. Too much unevenness, though, carries risks such as a turned ankle. “The key is to find the balance between stress and overstressing the system,” Boyer says.
So far, on-the-ground studies such as one from 2003 that followed 844 runners preparing for the Vancouver Sun Run 10K have failed to find any association between running surface and injury rate. That doesn’t mean no relationship exists, just that it’s more complicated than we initially suspected. One thing we do know is the principle of specificity: if you do all your training on one surface, your body may not be adequately prepared to run on other surfaces. So don’t prepare for a marathon by running only on grass, and don’t prepare for a backcountry trail race by running only on roads.