Concrete is a much harder surface than asphalt or macadam. It's the worst commonly encountered surface that you can run on and should be avoided like the plague. To compare the "hardness" of concrete and asphalt, hit each surface with a hammer and see how it feels to your hand and arm. You will find quite a difference. You will leave a dent in the asphalt, but not in the concrete.

When running, your feet strike the surface with a force of up to 6 times your body weight. And unless you land dead midfoot all that force is concentrated on a very small landing surface. For a typical heel striker, it's maybe a square inch or two. Let's assume that a person who weighs 120 pounds lands at 5 times body weight with a heel strike that covers two square inches. That's equivalent to an initial strike force of 300 pounds/sq in (equivalent to 3600 pounds/sq ft) upon contact. If asphalt is really 10 times "softer" than concrete, as the study that Bill mentioned said, that would make a big difference in initial energy dissipation vs that which shoes, normal pronation and body structure have to absorb. (BTW, that's also the problem with a non-overpronator using stability or motion control shoes. They unnecessarily restrict normal pronation, which is a natural shock absorption biomechanic, and result in an increase in the force that the body's skeleton and joints have to dissipate.)

I can tell a very distinct difference in how running on concrete feels compared to asphalt. There are a couple of wide concrete sidewalks that I cross during an 8 mile run on the B&A Trail. I really know it when I cross over them. I would not want to run a marathon that is mostly on concrete. I don't even like to run 10k's on concrete.

Actually, there are a few other surfaces that are even harder than concrete, such as brick, stone and steel. Fortunately, they are seldom encountered when running. I've always thought the people who run regularly on the brick waterfront promenade in Baltimore's Inner Harbour were being foolish. And steel is a surface that is usually encountered only when running on the deck of a ship.....like a few hardcore marathoners did in the 2001 Antarctica Marathon when weather conditions prohibited running on the snow and ice.

The state of Georgia announced plans to build a concrete walking/running trail a year or so ago in the Atlanta area. A Mervite who is a very good runner (Emorydoc) led an effort to try to get them to construct it of asphalt or macadam instead of concrete, but a construction company that does a lot of business with the state "donated" the concrete. So the state chose to go the cheap route instead of considering what was best for the intended purpose.

If most of your running is on concrete, I would put that at or near the top of the list of suspects for recurring injuries. You would be much better off running on grass or dirt alongside the concrete, if you have that option.

Jim2