Tell me if these suggestions sound familiar: Find a soft grassy field to do sprints. Stick to trails instead of pavement when you can. Enjoy the springy surface of a stadium track rather than pounding your feet against concrete. The advice often concludes ominously: Your knees will thank you.

It kind of makes sense: harder surfaces should mean more impact on our legs, including our knees. But is that impact meaningful enough to cause injury?

Putting stress on your body isn't necessarily a bad thing. We stress our muscles and tendons to make them stronger, and the same happens to bone.

While running has been blamed for knee problems (mainly by non-runners who like to say, "isn't that bad for your knees?" as you sail by), people who exercise tend to have healthier knees than people who don't. They have stronger bones and stronger muscles. The impact from running might be bad for people who already have arthritis, but experts agree that a fear of damaging your knees shouldn't keep healthy folks away from running.

Forget arthritis. What about the more common nagging injuries so many of us get as part of our running habit? One study of elite runners, published in Foot and Ankle International, found that Achilles tendinopathy (what many of us would call tendonitis) was actually more likely to happen when the elites ran on soft sand than when they ran on asphalt.

So is it better to run on soft or hard surfaces? It turns out that when we decide to run on one surface rather than another, we're overthinking it. Our bodies already know how to adapt to each surface.

How We Adapt

A Cleveland Clinic study shows how little surface matters. Investigators asked 12 men to run on a force-measuring treadmill in soft and hard shoes. In a second experiment, they had 10 men run in the same shoes on a treadmill that could be adjusted to be harder or softer.

The results: Runners used their body differently with each surface or shoe, in a way that suggested their body was compensating for the changed surface. Runners on a hard surface bent their knees a little more at the landing, so that the knee angle—which indicates force on the knee—was nearly identical no matter the surface.

Other studies have gotten slightly different results, leaving scientists to work out the details. For example, do we run differently on a soft surface like sand versus a soft and springy one like a rubber track? Do we sometimes put up with a higher-impact gait if it helps us save energy while we're running? Watch for more findings in the coming years as new research is done.

One thing is clear: we definitely change our stride based on the type of surface we run on. Researchers asked 40 experienced runners to run barefoot over hard and soft surfaces in a study published in Footwear Science. The vast majority landed heel-first when the surface was soft, but most of those people switched to either midfoot or forefoot landings when asked to run on concrete.

Likewise, a study of barefoot Kenyans—not professional runners, but people who just happen to go barefoot most of the time—found that they used a mix of running styles depending on surface and speed. The myth that there's a single "natural" stride, or that barefoot people never heel-strike, was definitively busted.

The bottom line: people don't have just one way of running (or even one way we should run). We adapt to different surfaces. So far, we can't say that one surface is safer than another. Most likely, your injury risk has more to do with your muscles and your genes than with whether you choose roads or trails.