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Proof: asphalt or concrete doesn't matter


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Author Topic:   Proof: asphalt or concrete doesn't matter
posepa
Cool Runner
posted Nov-06-2001 07:13 PM     Click Here to See the Profile for posepa   Click Here to Email posepa     Edit/Delete Message   Reply w/Quote
Here, once an for all, is the proof that running on asphalt
has no advantage over concrete, since they are both
at least a thousand times harder than your running shoe.

References:
http://www.chemcosystems.com/epoxy.html
http://physics.uwstout.edu/strength/tables/cyoungs.htm

Comp. modulus of elasticity, or Young's modulus for materials:
Rubber (average) = .4 (k.p.s.i.)
Composite Asphalt = 380
Wood (compression along grain) = 1,500
Concrete = 4,500
Steel = 30,000

Using these numbers, this is the percentage of energy that is
absorbed by a rubber-based shoe running over each of
these surfaces:

Shoe Surface
Asphalt 99.89% 0.11%
Wood 99.97% 0.03%
Concrete 99.99% 0.01%
Steel >99.99% 0.001%

In running shoes, training on concrete is like adding one
extra stride's worth of shock for every every thousand
strides that you would take on asphalt, or about one stride per mile.

Since the cushioning difference between any two shoe models
is much more that 0.01%, I submit that shoe choice, and not
surface choice, is the only thing that matters for injury prevention
on hard surfaces.

------------------
Paul Osepa
posepa@earthlink.net

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jcumming
Cool Runner
posted Nov-06-2001 08:30 PM     Click Here to See the Profile for jcumming   Click Here to Email jcumming     Edit/Delete Message   Reply w/Quote
Very interesting stats, but I know my brain tells me that asphalt is softer which is probably good because I tend to train on concrete and race on asphalt.

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jc
Born to Run

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tigger
Cool Runner
posted Nov-06-2001 08:50 PM     Click Here to See the Profile for tigger   Click Here to Email tigger     Edit/Delete Message   Reply w/Quote
There's a simpler, albeit somewhat intuitive test. Bounce a golf ball off concrete, asphalt, and dirt. The rebound distance will give you a good idea of which surface is relatively easier to run on. I think you'll find that asphalt and concrete are pretty close to the same.

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Schrauf
Cool Runner
posted Nov-07-2001 12:28 AM     Click Here to See the Profile for Schrauf   Click Here to Email Schrauf     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by posepa:

Rubber (average) = .4 (k.p.s.i.)
Composite Asphalt = 380
Wood (compression along grain) = 1,500
Concrete = 4,500
Steel = 30,000

I don't know - it makes sense, and I have always had this theory in my mind (it is nice to see it spelled out somewhere), but there is a lot to be said for actual experience.

Using this theory, there should also not be a big difference between packed dirt and asphalt, since a running shoe is much softer than both. But I normally train on dirt, and when I go for a longer run on asphalt I am always more sore. One person does not negate a theory, but I get the impression that many runners have this experience with dirt versus asphalt.

But like I said, your argument that there should NOT be a difference makes sense to me . . .

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centipede
Moderator of Advanced Running
posted Nov-07-2001 12:41 AM     Click Here to See the Profile for centipede   Click Here to Email centipede     Edit/Delete Message   Reply w/Quote
I've been trying to tell her what a "sensitive" guy i am. Now this proves it.

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posepa
Cool Runner
posted Nov-07-2001 02:40 PM     Click Here to See the Profile for posepa   Click Here to Email posepa     Edit/Delete Message   Reply w/Quote
To Schrauf:
You are certainly correct that dirt is MUCH softer than any road surface. Dirt must be close to the softness of rubber. You can run barefoot on dirt and grass, afterall.

As far as asphalt "feeling" like it's softer: that's because it has a rougher surface than concrete, so it absorbs sound better. Your shoes don't make quite the loud slapping noise on asphalt that they do on concrete, so it may "sound" softer.

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Sue in NYC
Cool Runner
posted Nov-07-2001 02:47 PM     Click Here to See the Profile for Sue in NYC   Click Here to Email Sue in NYC     Edit/Delete Message   Reply w/Quote
My knees don't believe this.

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Shrek
Member
posted Nov-07-2001 03:01 PM     Click Here to See the Profile for Shrek   Click Here to Email Shrek     Edit/Delete Message   Reply w/Quote
I'm not buying this. Physics aside I once read concrete is as much harder on the legs than asphalt as asphalt is harder than dirt or grass, and I believe it - when I am stuck in the city for more than a few days and forced to run on concrete sidewalks I always get sore knees but I can train on regular pavement just fine.

Keep in mind that back in the 1920s physiologists "proved" man could never run under 4:00 for a mile using mathematical formulas. Experience however proved otherwise!

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posepa
Cool Runner
posted Nov-07-2001 04:29 PM     Click Here to See the Profile for posepa   Click Here to Email posepa     Edit/Delete Message   Reply w/Quote
So, the two of you are telling me that your knees can tell the difference between surfaces that are 00.1% different in hardness? That your feet can somehow distinguish between rock hard surfaces that a golf ball would bounce off of in the same way in both cases? Physics aside, eh? Can't do that in this universe.

You brain may perceive a difference, but your feet can't.

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hooptie64
Member
posted Nov-07-2001 05:10 PM     Click Here to See the Profile for hooptie64   Click Here to Email hooptie64     Edit/Delete Message   Reply w/Quote
One thing that was not considered in the original statement is the flexability of concrete vs asphalt. Concrete is considered a rigid section and is typically reinforced with steel. Asphalt is a flexible section and is not reinforced. This could affect how "hard" something feels when you run on it.

I don't have numbers to relate to a person on concrete and asphalt, but I'm betting the flexibility of the two surfaces should be considered when comparing which is better to run on.

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the texas hammer
Cool Runner
posted Nov-07-2001 05:33 PM     Click Here to See the Profile for the texas hammer   Click Here to Email the texas hammer     Edit/Delete Message   Reply w/Quote
Something else to consider, it takes concrete nearly 100 years to get to its hardest state. The concrete tested may have not been very old. Asphalt reaches its hardest state within days, and may even get much softer on days with heat and sunlight.
Interesting study, however.
I still prefer asphalt. Put the numbers and studies aside and do a 20 mile run on each surface and you be the judge.

[This message has been edited by the texas hammer (edited 11-07-2001).]

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runningart
Cool Runner
posted Nov-07-2001 05:51 PM     Click Here to See the Profile for runningart   Click Here to Email runningart     Edit/Delete Message   Reply w/Quote
Another potential problem with asphalt is the crown of the asphalt road. The road in the center will be higher then the road at the edges to allow for drainage. Your legs don't like this. One leg is always higher than the other. Still, I would much rather run on an asphalt road than a concrete sidewalk because the road is usually more even. Plus in some places there are no sidewalks so it is annoying to go from sidewalk to road back to sidewalk again.

Alan
http://www.geocities.com/runningart2004

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posepa
Cool Runner
posted Nov-07-2001 06:16 PM     Click Here to See the Profile for posepa   Click Here to Email posepa     Edit/Delete Message   Reply w/Quote
RunningArt is right. The real reason why asphalt is usually better is that it has less cracks and curbs to trip over. However, both street and sidewalk can have potholes and slopes, which are the major causes of injury.
I've done faceplants on sidewalks, but never on a street. Yet.

As for the flexibility of asphalt: The top layer of asphalt has to rest on top of a completely rigid substrate (check my reference above). If it didn't, it would deform and crack every time a heavy vehicle rolls over it.

------------------
Paul Osepa
posepa@earthlink.net

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CJ-BLDR
Cool Runner
posted Nov-07-2001 06:44 PM     Click Here to See the Profile for CJ-BLDR   Click Here to Email CJ-BLDR     Edit/Delete Message   Reply w/Quote
I'll buy this when you take two groups of ~1000 runners, group A trains on concrete, group B trains on asphalt ... and compare the injury rates at the end of 1 year. So, any volunteers for the 'crete group?

[This message has been edited by CJ-BLDR (edited 11-07-2001).]

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posepa
Cool Runner
posted Nov-07-2001 07:47 PM     Click Here to See the Profile for posepa   Click Here to Email posepa     Edit/Delete Message   Reply w/Quote
Ok, so you folks are still not convinced? I did a little more researh.

The thickness of the padding of running shoe as about 1 centimeter. The thickness of an average latex condom is about 0.07 mm.
Check here if you don't believe me.
http://www.condomania.com/cgi-bin/SoftCart.exe/catalog/condoms/sensitivity/stinspire.shtml?E+reload
Therefore, running the difference between running on concrete (harder) and asphalt (softer?) is like adding about 1/7 the thickness of a standard condom to the bottom of your running shoes.

Now, which of you is going to tell me that you can tell the difference?

------------------
Paul Osepa
posepa@earthlink.net

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CJ-BLDR
Cool Runner
posted Nov-07-2001 07:54 PM     Click Here to See the Profile for CJ-BLDR   Click Here to Email CJ-BLDR     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by posepa:
Ok, so you folks are still not convinced? I did a little more researh.

The thickness of the padding of running shoe as about 1 centimeter. The thickness of an average latex condom is about 0.07 mm.
Check here if you don't believe me.
http://www.condomania.com/cgi-bin/SoftCart.exe/catalog/condoms/sensitivity/ stinspire.shtml?E+reload
Therefore, running the difference between running on concrete (harder) and asphalt (softer?) is like adding about 1/7 the thickness of a standard condom to the bottom of your running shoes.

Now, which of you is going to tell me that you can tell the difference?


Our ears hear what you are saying, but our old bones aren't listening.

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oldcolonial
Cool Runner
posted Nov-07-2001 08:20 PM     Click Here to See the Profile for oldcolonial   Click Here to Email oldcolonial     Edit/Delete Message   Reply w/Quote
Just a fly in the ointment of posepa's post. Its actually not the proportion of energy returned that matters as much as the elasticity of the surface that matters. A highly elastic surface that returns all of the energy over a longer period of time would be easier on your legs than one that is not very elastic an returns a lower proportion of the energy. Since both asphalt and concrete return virtually the same amount of energy its the amount of time that it takes to return the energy that matters. If asphalt is more elastic and it takes even a little bit more time to return the energy it can be appreciably easier on your body. All else equal, the force applied to your body it essentially proportional to the ratio of the square of the amount of time it takes to return the energy.
So if we take a numerical example, say 0.1 seconds to return the energy of your foot strike on asphalt and 0.09 seconds on concrete, the force applied to your body by running on asphalt is 81% of that applied by running on concrete. That 19% for 1/100 of a second really matters.
Another example of the impact of time on acceleration is the difference between what going from 60 mph to 0 mph in 2 seconds vs. 0.2 seconds. The first case is fun, the second case will kill you. Both deal with exactly the same amount of "energy".

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posepa
Cool Runner
posted Nov-08-2001 01:27 AM     Click Here to See the Profile for posepa   Click Here to Email posepa     Edit/Delete Message   Reply w/Quote
Sorry oldcolonial, but Young's modulus (the modulus of elasticity) for asphalt is nearly constant over a broad range of sound frequencies, so the value that I cited is true for a broad range of collision types, and most certainly all the energy coming out of a collision with a running shoe. If you don't believe me I can dig up one of the many civil engineering asphalt testing standards that can be found on the web.

The main point is just that the damping caused by the rubber and plastic in your running shoe is so much greater than any surface made of rock-hard material, that the differences between those rock-hard materials is insignificant. You would need an incredibly sensitive rig just to be able to pull a measurement out of the noise.

Some time ago, a myth was started by someone writing an article in RunnersWorld, who clearly has never had an entry level class in Physics, Mechanical Engineering, or Materials Science. Don't perpetuate the myth. If you are using surface "softness" as a reason to put yourself in harm's way by running on a road instead of a sidewalk, then forget that silly article, please. There are plenty of other RW myths you can still believe in, with much less risk.

------------------
Paul Osepa
posepa@earthlink.net

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streakmarine
Cool Runner
posted Nov-08-2001 06:21 AM     Click Here to See the Profile for streakmarine   Click Here to Email streakmarine     Edit/Delete Message   Reply w/Quote
posepa:
You can't 'prove' that asphalt is no better than concrete unless you have evidence that applies to runner's concerns. Young's modulus for materials means nothing to us. Until you have statistics on injury rates etc, us runners aren't going to believe you because running on asphalt *does* feel easier on the knees than concrete, especially on the downhills.

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tigger
Cool Runner
posted Nov-08-2001 08:11 AM     Click Here to See the Profile for tigger   Click Here to Email tigger     Edit/Delete Message   Reply w/Quote
posepa,

So where does a software development engineer learn so much about engineering and physics? You're making yourself out to be an expert here and I'm not so sure you are. Oldcolonial's point is a good one that shouldn't be dismissed so easily. You say that Young's modulus is constant over a wide range of frequencies. What do frequencies have to do with anything anyway? Young's modulus is not constant over a wide range of temperatures. If you run on 100 degree asphalt your legs will react differently than on freezing ashpalt. One is soft and yielding where the other is hard and elastic.

Speaking as a mechanical engineer, asphalt is a slightly better shock absorbing running surface than concrete. As it warms up it becomes even better. At low temperatures the difference can be more than offset by purchasing shoes with more cushion, however with the same shoes one is better off on asphalt than concrete. The more you run the more important the issue of running surface becomes, but only because we should avoid both surfaces and stick to dirt or grass.

[This message has been edited by tigger (edited 11-08-2001).]

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j2ee
Cool Runner
posted Nov-08-2001 08:54 AM     Click Here to See the Profile for j2ee     Edit/Delete Message   Reply w/Quote
Most of my running is on concrete, sometimes as much as 120 miles per week and really, I have had no issues what so ever. I personally cannot feel much of a difference between concrete and asphalt even wearing ASICS Gel-Lyte’s.

Instead of blaming the surface I would suggest that most injuries are caused by poor biomechanics and can be improved with strengthening and form drills. When I’m passing people all I normally hear is the “Slap, Slap, Slap….” of their feet on the ground, no matter what surface they are running on. This is what causes injuries.

Next time you’re at a marathon with a bunch of Kenyans or Ethiopians, listen to these elite guys as they pass. They are almost silent.

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oldcolonial
Cool Runner
posted Nov-08-2001 09:59 AM     Click Here to See the Profile for oldcolonial   Click Here to Email oldcolonial     Edit/Delete Message   Reply w/Quote
Paul;

First let me say that I applaud you for bringing some reason and analytic thinking to the examination of this issue. Its been a long time since I took Physics (freshman year in College), but I still have to disagree with you on your basic point, that running on concrete is no harder on your legs than running on asphalt. Further, you did not address my point which has to do with the amount of time it takes (actually how smooth this transition is which I admit I did not make clear) to go from falling into the pavement to jumping off the pavement.
To examine this point, lets consider the indoor track that I run on in the winter. It is built on corrugated sheet metal with concrete on top of it. Steel and reinforced concrete, just about as hard as you can make something large without spending a lot of money. It also has a thin (3 - 5 mm at most) layer of rubberized paint on it. Basically, this has about impact as wearing an extra pair of socks. This track though, because of its construction is quite "bouncy" deflecting and bouncing back with each stride a runner takes. You can actually see it deflect as people run by. You can also feel the floor move (ever so slightly as they run by). Its not a slow track, nor is it hard on your legs. My assertion is because it deflects and then returns the energy back into the runners stride at approximately the right time. Some people might say its well tuned for running. If its well "tuned" it returns the energy just as you need it, somewhere in the middle of your push off is my guess. If its poorly tuned it returns it while you are still trying to stop your fall into the track (too soon) or perhaps after you've already pushed off most of the way (sort of like trying to run on a trampoline). The well tuned track would provide you with a smooth acceleration from falling to jumping a poorly tuned track would not. A smooth transition means less stress on your body. In fact if you run out of phase with another runner you will notice that the track seems to not quite be all there for you as you push off and come up at you a little quicker than you might expect as you make your foot strike. Because of this I almost never see people running out of phase with each other on the track. Sort of like kids on a swing set tend to swing in phase with each other because of the feedback provided by the swing set it self.
Oh, sorry about the rambling there.. my point? its not the surface hardness that matters as much as the timing of how it returns the energy of the foot strike.
A brief aside, I notice that my legs are often sore after I run in the dark. This could be because in the dark I'm on the concrete sidewalk more than during daylight or that I have a harder time coordinating my foot strike with the ground in the dark. I suspect its a combination of both. Don't know how much to attribute to each though. Anyone else notice this?

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posepa
Cool Runner
posted Nov-08-2001 11:53 AM     Click Here to See the Profile for posepa   Click Here to Email posepa     Edit/Delete Message   Reply w/Quote
Thanks Tigger & oldcolonial, for your thoughtful responses.

On the issue of the suspended sheet steel & concrete track: Suspended tracks like that one, and the famous indoor track at Harvard, are like basketball and aerobics floor, where there is an effective spring under your foot, because the material is thin enough to flex on top of supporting struts. The benefits of this are well known. But this is never the case on streets or sidewalks, which are very thick sheets.

Tigger's point is an excellant one: as asphalt heats up it does soften. But the reports that I came across when I did my search did not show much variance until the surface temp reached 70C, which would only happen on scorching hot days. My point about Young's modulus being nearly constant is that
IMHO, asphalt is _not_dispersive_ to sound waves from impacts with rubber so that it will not absorb the energy of a running shoe collision. Doesn't the golf ball test confirm this? What else should I consider?
What factor could possibly overcome the 1000x difference between rubber and asphalt?

BTW, I'm a physics guy in software engineer's clothing, so I'm open minded. Show me the mechanism by which we can go from 00.1% to something significant, and I will withdraw my claim.

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the texas hammer
Cool Runner
posted Nov-08-2001 12:23 PM     Click Here to See the Profile for the texas hammer   Click Here to Email the texas hammer     Edit/Delete Message   Reply w/Quote
I have always been a little confused about the golf ball dropping test. I am no scientist, but there just seems to be more variables that we are missing when comparing asphalt and concrete. Ex. Instead of using a golf ball lets use a person instead. First drop him onto the pavement from 3 feet high. Next, drop him on a trampoline. Which one has the highest bounce? Obviuosly it was not the hardest surface. Now do the same with a golf ball. Now the pavement has the highest bounce. The human body and golf ball have absolutely no similar properties (ie. hardness, softness, weight distribution,shape,etc.,etc.)There just seems to be more to it than measuring hardness. Like everyone else has said, your knees don't lie.

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tigger
Cool Runner
posted Nov-08-2001 12:31 PM     Click Here to See the Profile for tigger   Click Here to Email tigger     Edit/Delete Message   Reply w/Quote
TH, you're right! The knees don't lie!

The golf ball test determines how much energy from the dropping of the ball is reflected back into the ball, thus making it bounce up again. This is a function of the surface you drop it on. For ashpalt and concrete the energy reflection is high. That's why your knees are sore after running on those surfaces. For dirt the energy reflection is low. Most of the energy in the ball is absorbed by the dirt. Same thing when you run on dirt. The energy from your foot impacting on the ground is absorbed rather than being reflected back into your leg.

However, posepa's original claim was "Here, once an for all, is the proof that running on asphalt has no advantage over concrete...." We now know this to be false. There are many advantages to running on asphalt, including smoother surface and fewer piles of dog turds to step in. And if you consider the minuscule difference in reflectivity to be an advantage then it's just one more.

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