We got some good news (or bad news) around here this past week and that is our tiny little community on the far edge of the city is getting sidewalks! It’s good news for many and people have been fighting hard for it for years but I’m not so pleased. For years I’ve believed and repeated the idea that running on concrete puts more stress on your joints than asphalt or trails. But is that even true?
I think I picked up this idea in passing but it seemed to jive with my experience. When I lived in the city and regularly ran on the sidewalk I did feel more beat up. Was that just my perception, old shoes or the fact I was less healthy, more stressed and oh so tired day to day?
Right now we have thin shoulders along the asphalt road but the shoulder is in pretty decent shape for the most part. I typically pick the even asphalt edge and pull over on to the shoulder when a car comes and dammit I like this system. Honestly though, I’m not against progress and if the majority wants sidewalks then sidewalks there will be.
But now looks like a great time to finally look into the science of running surfaces. Does it really matter and if it does how much does it really make a difference? So let’s dive into the scientific literature and find out!
In the community
I think that talking about running surface was a real trend in the running community a few years back. I didn’t drink the kool-aid when it came to barefoot running, those weird toe shoes, the dangers of heel-striking and so far I have resisted carbon fiber plates. Just like everything else trends come up and get everyone talking about the same thing at once. For some runners those trends work out perfectly and for others they come and go.
At the time there was a lot of conversation about what surface you should be running on. Ideally, at the time the argument was that grass was best, then trails, then hard trails followed by asphalt and the worst surface to run on was thought to be concrete.
We were told that this was an injury prevention thing, that it made our shoes last longer and our knees from wearing out. The counterpoint people argued at the time that the harder the surface the more energy was returned to your next step and so the harder the surface the physically easier it was to run on.
This all made a lot of sense to me at the time but I never changed where I ran because of it. When I lived where there were sidewalks I ran on concrete. When I felt most comfortable on the trail that’s what I did too. Maybe I used it as an excuse to buy new sneakers though.
Possible causes of running injuries
Runners spend a lot of time and energy trying to avoid injury but the reality is most regular runners deal with an injury for every 1000 km run. As much as we want to avoid it injuries they just keep happening. Some factors related to running injuries that are commonly discussed include:
- Age of shoes
- Style of shoes
- Running surface
- Arch height
- Running experience
- Previous injury
- Running frequency
Some of these things have in fact ben found to be linked to injuries while others have not. Some are linked to injury in some studies but not in others. Running surface is much the same. Honestly, whenever I look deeply into a certain aspect of running and injury in the scientific literature my most common takeaway is ‘how have we not figured this out yet definitively?’
Biomechanics of a runner’s foot
So let’s get back into the effects of different surfaces and their effects on runners. There is no question that different running surfaces are harder than others. Surfaces like natural grass and wood chips are the softest, artificial turf and rubberized tracks are moderately hard and traditional asphalt and concrete are very hard.
It is also well proven that the biomechanics of runners do vary with how hard the running surface underfoot is. Specifically the softer the surface the longer the runner’s foot tends to be in contact with the ground and the lower the forces in the leg tend to be. The physicists among us might recognize this as impulse (force x time) and equivalent to momentum. This equation represents the time of impact in an object experiencing a change in momentum. For example, when you hit a tennis ball against a wall it first has forward momentum, then experiences a change in momentum or impulse when it hits the wall and then has momentum back in the opposite direction.
A runner’s foot experiences that same impulse and change in momentum every time it hits the ground. That value is fixed and measured in joules. On a softer surface the time value increases meaning mathematically a lower force is incurred compared with a harder surface. On a harder surface the time a runner’s foot is in contact with the surface is decreased and the force is relatively higher.
It seems reasonable then that if the force felt in the runner’s foot and leg is higher on harder surfaces than there is an increased chance of injury, but is that the case?
Do harder running surfaces lead to more injuries?
Running on harder surfaces like asphalt or concrete has long been suggested for being responsible for a higher incidence of running injuries. However, the scientific evidence fails to prove that assumption.
Though softer surfaces like rubber tracks absorb more energy we don’t see higher forces in the joints of runners on harder surfaces. In some test subjects the researchers noted differences in the runner’s gaits that might be compensating for the harder surfaces (Dixon et al., 2000).
A second group found similar results. A force plate measured runners while impacting surfaces with a wide range of hardness. The researchers noted that even though the running surface got 12.5 times harder there was only a 29% increase in leg stiffness impacting the harder surfaces and running economy increased by 12%. Most of the variation was confined to the softest surface while all the stiffer ones were essentially unchanged (Kerdok et al., 2002). Put simply, the forces on the leg of the runner aren’t very different across hard surfaces but are reduced in very soft surfaces. It is also measurably easier to run on harder surfaces.
These results were confirmed by another study finding about 10% less force generated on grass compared to asphalt and concrete which produced identical levels of force in the runner’s leg (Tessutti et al., 2012)
A systematic review of several running studies found no link between running on harder surfaces and more incidences of running injuries (Mechelen 2012). Similarly a 13 week and year long study of training runners did not find that the predominant running surface was a factor in real world injuries (Taunton et al., 2003, Walter et al., 1989).
Taken together the consensus is that there is not a direct or linear relationship between surface hardness and forces felt in the runners leg. It is strongly suspected that runners naturally and quickly compensate using a combination of muscle activation and impact angles to limit the forces felt. It is however physically easier to run on order surfaces as we see an increase in running economy as the surface underfoot gets harder.
Runners compensate with their muscles and or form to compensate for a harder running surfaces. Exactly how they do this is either not that well understood or varies from runner to runner meaning that a harder running surface on it’s own isn’t more likely to leave a runner injured.
Running on harder surfaces is easier overall than softer ones. You can feel this for yourself by going for a run on the soft beach sand. Changes in running economy can be felt by the runner as perceived effort. You might feel, as I do, that running on the hard road feels light, easy and fast while running in the sand or soft trail feels slow, heavy and tough. Those feelings are bared out by the finding in these studies that softer surfaces are associated with lower running economies.
While harder surfaces may feel different under foot and may even cause us to ‘run differently’ there isn’t good scientific evidence that harder surfaces lead us to get injured more often. The measured forces in out legs when we run on very hard surfaces aren’t different then moderately hard surfaces and only slightly different then the very softest surfaces available.
So I guess there really isn’t a strong scientific reason for me to resist concrete sidewalks after all.
Great little science nuggets
- “The practice of running has consistently increased worldwide, and with it, related lower limb injuries” (Tessutti et al., 2012) is perhaps the truest sentence ever written about running injuries.
- The only factor conclusively linked to an increase in running injuries in one meta studie of studies was the number of miles run per week. (Powell et al., 2016)
Do you buy into the idea that running on concrete is bad for you? Do you feel better knowing that your race day route isn’t likely to be much more damaging to your joints than a trail run? Do you ‘feel’ like you run differently on harder surfaces compared to soft ones? Leave it in the comments below!
Read what I read:
Dixon et al., 2000. Surface effects on ground reaction forces and lower extremity kinematics in running. Biodynamics. https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=e55108c30217651f76ad2b0068c312ec82ec5620
Kerdok et al., 2002. Energetics and mechanics of human running on surfaces of different stiffnesses Journal of applied physiology. https://doi.org/10.1152/japplphysiol.01164.2000
Mechelen. 2012. Running injuries. Sports medicine. https://link.springer.com/article/10.2165/00007256-199214050-00004
Powell et al., 2016. An Epidemiological Perspective on the Causes of Running Injuries. The physician and sports medicine. https://doi.org/10.1080/00913847.1986.11709103
Taunton et al., 2003. A prospective study of running injuries: the Vancouver Sun Run “In Training” clinics. British journal of sports medicine. https://bjsm.bmj.com/content/bjsports/37/3/239.full.pdf
Tessutti et al., 2012. Attenuation of foot pressure during running on four different surfaces: asphalt, concrete, rubber, and natural grass. Journal of sports sciences. https://doi.org/10.1080/02640414.2012.713975
Walter et l., 1989. The Ontario cohort study of running-related injuries. Journal of American medicine. https://jamanetwork.com/journals/jamainternalmedicine/article-abstract/612508
okay science….I just find off road just more fun….
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I do avoid concrete surfaces when I can. And when there is a worn path beside a sidewalk, often I will run on that.
In running, the science always seems to be inconclusive or contradictory. what to believe?
When ever I read an article that says X is better than Y, I always take it with a few grains of salt.
As you mentioned, those toe shoes and barefoot running were going to revolutionize running. But I don’t see bare feet or those odd shoes anymore.
The hardness difference between pavement and concrete is probably not noticeable. At least not to me. I don’t think I change my running mechanics when I run on one or the other.
But I’m sure things change on dirt or sand.
I have noticed that running on softer surfaces, such as rubber tracks or sand, does require more effort. I’m not a scientist, but it would seem that more energy is absorbed by the softer surface and your body requires more energy for the kick off.
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