A poll on the Running Times forums asked which is the softer surface on which to run, asphalt or concrete? My reply was:

“Definitely asphalt. Concrete is much denser. To ‘feel’ the difference, strike both with a hammer.”

Another forumite, Hillrunr, replied with the following:

“Jim, this is possibly the only time I can ever recall disagreeing with you. Are you really comparing the running motion to striking the ground with a hammer? Even if so, since well cushioned running shoes are nearly universal, you better strike those surfaces with the hammer through the midsole of a running shoe.”

My reply to Hillrunr was:

You are right on two counts, Ryan:

1) We haven’t disagreed on running related matters in the past....well, there was the "drafting in calm air" issue, but we sorted that one out.
2) We do disagree this time.

I used the hammer example to illustrate a principle, not to suggest that running on concrete subjects the body to similar shock. However, the principle is still valid. Use a rubber mallet and you can still tell the difference in your hand and arm between striking concrete and asphalt, although it is considerably less than when using a ball peen hammer. I have done both....plus a plastic mallet.

Actually, it isn't even necessary to swing at the surfaces with a hammer to detect the mechanics involved. Just hold the hammer 3 inches above the surface and let the head of the hammer fall freely while holding the handle loosely to control it's path and see what happens. The hammer bounces off of concrete halfway back to the height where it started because the concrete absorbs very little energy, but reflects almost all of it back into the hammer head. However, on asphalt the hammer head strikes with a thud and stays there because most of the energy is absorbed by the asphalt, which momentarily depresses at the spot struck. It literally acts like a shock absorber while concrete acts like a shock reflector.

The same principle, but to lesser extremes, applies to a running foot striking the ground. Yes, shoe cushioning absorbs some of the force. But not all of it. What's left must be absorbed into the running surface and the runner's body. The runner might not consciously "feel" the difference because his body is already busy dealing with absorbing shock. But his musculoskeletal system is absorbing even more force on concrete than on asphalt. A common result can be stress fractures. Remember, even if the difference is small (like just a percent or two), there are approximately 37,000 strides in a marathon and more than twice that in a 60 mile week.

Another variable is running shoes. Not all types are the same in terms of cushioning. For instance, I have to run in stability or motion control shoes to control my overpronation. That type of shoe is more rigid and less cushioned than some other types. Thus, my body has to deal with a greater percentage of force/shock than it would have to if I could use cushioned shoes. I appreciate every little bit of help I can get from the surface I run on....and I can really tell it when that surface is concrete.

In terms of risk injury, concrete is rated 2.5 on a scale of 1 to 10 (where 1 is the greatest risk of injury and 10 is the least risk of injury) and asphalt is rated 6…..that’s a significant difference. Further, these ratings include the environment in which the running surface is most commonly found. For instance, most asphalt running surfaces are roads, many of which are cambered, which induces biomechanical nuances that can lead to ITBS. OTOH, most concrete running surfaces are sidewalks, which are flat. Still, despite the potential ITBS risk of running on asphalt roads, running on concrete is rated significantly more likely to result in injury than running on asphalt. Why? Because concrete is 10 times more dense (harder) than asphalt. (See http://www.runnersworld.co.uk/news/article.asp?UAN=152 or http://www.ssc.gov.sg/SportsWeb/sw_c...t=33&cat=1 36)

Bob Glover in “The Competitive Runner’s Handbook” says, “Concrete used on sidewalks and some roads is the worse surface in terms of shock absorption. If the choice is between concrete and asphalt, take asphalt since it is much more forgiving.”

A podiatrist who is a marathoner says that, for injury prevention, soft surfaces are best and concrete is worst. About asphalt, he says, “So what’s a good compromise? I like asphalt. In fact, I love asphalt! I can immediately tell the difference between concrete and asphalt during the marathon. After running on asphalt, my legs shock and strain, whereas running on concrete batters my calves, hamstrings and knees. (Of course, if you think these surfaces are tough, try running across steel/concrete bridges at the N.Y.C. Marathon. All the carpet in the world on that bridge doesn’t soften the worst surface I’ve ever run on.)” (See http://www.merlinofitness.com/sub_pa...g_surfaces.php)

A chiropractor says “Concrete sidewalks may provide safety, but also represent the hardest surfaces to run on. Asphalt is less hard and man-made tracks are generally preferable. The forces generated at heel strike are dissipated through the musculoskeletal system. Harder surfaces result in increased pounding and subsequent deleterious effects. Many runners do not have access to a track or treadmill and the irregular surfaces of cross-country running provide obvious hazards; therefore, most take to the roads. These surfaces are typically asphalt, and less hard than concrete, but are often crowned or banked like some indoor tracks. Banked surfaces result in overpronation and should be avoided. As this may be impractical, runners on banked surfaces should run on the same side of the road on their way out and on their return. This effectively alternates the foot subjected to the more banked surface." (See http://www.chiroweb.com/archives/10/18/17.html)

I think there is a lot of evidence, albeit much of it anecdotal, that running on concrete presents a greater risk of injury than does running on asphalt. And I think both logic and physics support that. However, the benefits of asphalt over concrete might not end there. Asphalt might offer yet another advantage over concrete to the runner….better performance.

Studies have shown that, to a point, materials with less surface stiffness reduce the metabolic cost of running…..one can run faster or sustain a pace longer for a given amount of energy. I am not aware of any studies comparing concrete and asphalt in this regard. However, one study conducted at Harvard demonstrated a significant variation in "metabolic cost of locomotion" (as measured by oxygen consumption) among five surface stiffnesses within a stiffness range used for running tracks. The metabolic energy variation was 12% between the most and least stiffness values tested. The improvement at the lower stiffness value is due to the running surface acting like a spring and returning energy to the runner at toe off. Thus, the runner has to generate that much less energy to propel himself forward. Obviously, there is a point where this benefit is maximum and further reduced surface stiffness beyond that point becomes detrimental because the material can’t return the energy to the runner. (Obvious examples are running on grass or sand where surface stiffness is very low.) However, this study didn't identify that optimum surface stiffness or a material that provided it. (See http://biomech.media.mit.edu/publica...Metabolism.pdf)

Bottom line….I stand by my original post in which I recommended that, in order to reduce the risk of injury, one should avoid concrete like the plague and opt for asphalt if even softer surfaces aren’t available or it means running on an irregular surface.

Jim2