The body is an all-terrain vehicle, born to run two million years before roads,1 and so maybe we suffer when we run for a long time on asphalt or concrete. Although running is an extremely healthy sport overall, and not as hard on the body as most people fear, running injuries are still common and frustrating and the risk factors for them remain mysterious. Could it be the roads? Is it insane to do exactly the same thing over and over again with your anatomy and expect to get away with it?2 It seems almost obvious, but no one actually knows. There’s a surprising lack of hard facts about the role of hard surfaces — and some of the scientific evidence we do have is surprising.
Although most runners fear the rigidity of concrete or ashpalt, the problem might actually be the continuity of the surface, the unrelenting same-ness of pavement. But there’s even less evidence about that possibility.
Important safety issue: for people with joints that may be unstable from previous injuries (e.g. ankle sprains), running on uneven or unstable surfaces (trails especially) may be the greater of evils. The evil-ness of roads is an unknown, but there’s no question that it’s all-too-easy to sprain an ankle on a trail run.
It seems common-sensical that running on hard surfaces is risky. Surely harder surfaces involve more impact, more biomechanical stress? Unfortunately, that “obvious” idea has a glaring citation needed problem. Is there any direct scientific evidence that running on hard surfaces is actually injurious? Has anyone ever gotten big groups of people to run for a long time on different surfaces, measuring injury rates in both groups (a prospective trial)? Incredibly, no: despite decades of running research, it’s still an untested idea.3
So it’s not proven that hard-surface running is risky, but it’s not exactly a crazy idea either. It’s a reasonable hypothesis, and there are arguments and evidence both for and against it.
Here are some of the clues and perspectives that cast doubt on the alleged “danger” of running on roads and sidewalks:
What can be asserted without evidence can be dismissed without evidence.
Christopher Hitchens, paraphrasing the Latin proverb “Quod gratis asseritur, gratis negatur” (What is freely asserted is freely deserted), in a 2003 Slate article
The science cited above is just about the only science that clearly casts doubt on the dangers of running on roads, none of it is actually direct evidence, and there are caveats and “yeah buts” galore.
The arguments in summary:
So the jury on this topic is definitely out, and it’s going to stay out for a long time. Having weighed all the arguments and evidence rather thoroughly, here is my opinion for now:
The impact of running is measured in many ways. Loading rate is the main technical way of measuring how jarring a runners’ steps are: how fast load is applied to tissues. Peak acceleration at various anatomical landmarks is another. There’s a lot of research about impact, some of it concerning different surfaces, just a few of those specifically about the relationship between impact and injury. As of the end of 2016, there were only about 18 decent experiments, with too many differences between them to clearly interpret. A review of these by van der Worp et al concluded just a single thing with confidence: a history of stress fractures is associated with a higher impact forces in running gait.10
That’s it. Every other kind of impact/injury connection is still a question mark. “Owing to the absence of prospective studies on other injury types” — the only kind of study that could actually prove that a higher loading rate causes an injury — “it is not possible to draw definite conclusions regarding their relation with loading rate.”
But where there is smoke there is fire! Of all running injuries, stress fractures seem the most obviously relevant to impact, and the evidence does support that assumption: the one established fact. Furthermore, there is a broad association between higher loading rates and runners with all kinds of injuries (no specific one).12 And that’s backed up by a good quality trial from just a little later in 2016: Davis et al found that “all impact-related variables were higher” in 250 women runners who got injured in a year after extensive gait analysis.13 Plus there’s the same implication from Altman 2016 (previously discussed).
So the common-sense idea that impact is injurious appears to have some scientific support.
There are flies in that ointment, of course. Most importantly, “impact” is not equivalent to “hard surface,” as you’ll see in the next section. The limited evidence at this late date in history is noteworthy. And there are some miscellaneous clues that suggest that impact is not straightforwardly injurious.
For instance, Zadpoor et al found that ground reaction forces (how hard you hit the ground) have no correlation with stress fractures, and loading rates (how fast you hit the ground) are only slightly correlated.14 That’s surprising for what seems like the most impact-related running injury. When van der Worp et al concluded that loading rate is associated with stress fractures, it’s probably not the whole story.
Maybe it’s because the stresses that fracture are not simple. The forces in normal running are mostly below the threshold at which we would expect them to cause stress fractures directly, but Milgrom et al demonstrated15 that there are much stronger forces involved in activities that involve greater shear strain,16 probably enough to cause fractures more directly/quickly. Thus it is runners who include a lot of stairs and jumps that are potentially at greater risk for stress fractures than just running, regardless of surfaces. This is just a good example of the thick layers of “it depends” obscuring the truth.
Maybe impact matters… but just matters quite a bit less than other factors, which makes it very hard to separate the impact-signal from the noise of bigger and badder causes. Because there are definitely other risk factors! A giant 2015 study of almost 1700 novice runners in a “Start to Run” program found that a lot of them got hurt (almost 11%), and of those that did get hurt were more likely to be older, heavier, have a history of previous musculoskeletal problems, and less prior running experience.17 Obviously this isn’t direct evidence about impact—it just emphasizes the presence of other “noisy” factors.
And that’s all I’ve got: I am not aware of any other evidence that impact is not an concern, just an absence of ample, conclusive evidence that it is.
And then there’s disconnect between “impact” and “surface.” If impact matters, that’s one thing. But do runners actually experience more impact on harder surfaces? This is really the key to this whole puzzle.
If you hit your head on it, there’s really no question, is there? But we must take nothing for granted! Some science does indeed support the obvious here: a straightforward 2012 experiment produced peak plantar pressures about 12% lower than hard surfaces.18 That’s not a huge difference, but I’m sure it adds up. After two hours of hiking with a 20kg pack, you’d probably be quite grateful for a 12% load lightening. And obviously not all grass is created equal.
Whether or not that 12% difference reduces injury risk is still anybody’s guess.
But hang on, this is way too straightforward for running science. There must be conflicting evidence. And there is: an excellent 2015 experiment by Fu et al found no difference at all in impact forces on any common running surface.19 Er, wut?Runners likely adapt their stride to cope with stiffer surfaces & that adaptation probably has a biomechanical cost. There’s no such thing as a free lunch.
There was one key difference between this experiment and Tessutti 2012: their subjects weren’t running as fast. It’s possible, perhaps even likely, that a difference would have emerged here at higher running speeds.
So Fu et al concluded that “these findings indicated that different running surfaces do not necessarily affect the peak plantar impact and, by implication, impact-related injuries in runners.” But their inference about injuries there is speculation: their findings cannot tell us anything about injury rates, and it’s equally reasonable to assume that runners likely adapt their stride to cope with stiffer surfaces, and that adaptation probably has a cost. There’s no such thing as a free lunch. That is, they may well reduce musculoskeletal stresses in the lower limb at the expense of greater stresses elsewhere — more evenly distributed, but they’re there somewhere.
So Fu et al found no difference in lower limb impact forces on different surfaces whatsoever, and Tessutti et al found only a 12% difference between pavement concrete and grass. I don’t know about you, but the last time I hit my head on concrete, it felt a lot more than 12% harder than grass. Indeed it is.
Measuring rubber ball bounces is a good way of getting a nice apples-to-apples comparison of surface hardness without all the messy complexity of running biomechanics interfering. The point of this is that running biomechanics do interfere. Fu et al did this for us:
A ball bounces 152 centimetres on concrete, but just 80 on grass, only slightly more than half as high. Clearly grass absorbs a lot of energy!
And so do runners. The take-home message from both Tessutti et al and Fu et al is that we adapt to different surfaces so well that the differences in forces on our lower limbs is either nil or negligible. Which is neat. But the real question is what that adaptation super-power costs us, and that is still unknown.
Is it possible that the issue isn’t the hardness of the surface, or not just that, but the relentless same-ness of the surface?
It’s possible, yes. There isn’t a scrap of evidence about it one way or the other, of course, but it’s an interesting hypothesis I hope someone will test someday: all other things being equal, a slightly uneven surface leads to more injuries than a perfectly smooth one.
Most recreational runners are running on sidewalks and paved paths. Any sunny morning, you can see hundreds of them on the seawall in downtown Vancouver, where I live. They never touch the grass or the sand. A hard, constant surface feels like the path of least resistance. But on an unvarying surface, your body is subjected to exactly the same forces with every strike of the foot. The biomechanics of each step are identical. If tissue ever fails under load — which obviously it does — it may fail sooner if the load is applied more consistently.
Also, the body is given little chance to adapt to any other stresses. Same-surface and hard-surface runners tend to become strong in one way, but weak in others — and therefore perhaps that is another way to become vulnerable to injury, particularly IT band syndrome.
The most classic runner’s injury is the repetitive strain injury known as iliotibial band syndrome. If pavement has anything to do with IT band syndrome, it’s probably the lack of variation in the surface, not the impact per se.
One possible cause of this condition is a relative weakness of the gluteus medius and minimus. This is a controversial theory, and I don’t quite buy it yet, but it’s looking firmer now than it did in the 2000s.20 It has gotten quite fashionable lately to strengthen hips to prevent knee IT band syndrome and patellofemoral pain syndrome. These gluteal muscles are lateral stabilizers; they control side-to-side movement of the hips. On a flat surface, they aren’t needed as much, because it’s relatively easy to stay upright on a flat surface. They don’t exactly atrophy, but the other leg muscles get disproportionately stronger. And maybe that’s a risk factor for IT band syndrome.
Another interesting idea is the possibility that the road camber (angle) creates relentless asymmetric forces that lead to injury. Citation needed but unavailable, of course.
As mentioned earlier, Milgrom et al showed that running (especially when it involves stairs, due to shearing forces) is stressful for shins, resulting in the triple threat of the three main kinds of shin splints: (1) medial tibial stress syndrome, (2) compartment syndrome, and (3) stress fractures. (The term “shin splints” is not diagnostically meaningful in itself: it just means “shin pain.”) All three can be show-stoppers for serious runners.
Although humans are great at adaptive shock absorption, there are limits, and highly repetitive pounding on a hard surface may break the tibia (stress fracture). The tibialis anterior and other shin muscles have the job of preventing the foot from “slapping” — if something didn’t hold the foot up a little bit after heel-strike, the forefoot would slap down loudly and awkwardly. On a hard surface, the transition from heel strike is particularly intense. It’s the tibialis anterior muscle that controls it, with strong and well-timed eccentric contractions that ease the foot down, somewhat like the biceps lowering a barbell — except it’s more like catching a barbell that’s being dropped from five feet up … hundreds of times in a row. You see the problem.
Eccentric contractions are a bit strange. How, exactly, does a muscle both contract and lengthen at the same time? There is obviously a need to lengthen muscle while still bearing a load, or you could never put anything down. But, believe it or not, despite a working theory about the chemistry of muscle contraction that’s been around for decades, no one really knows how eccentric contractions actually work.21 About all we do know is that they tend to cause much greater delayed onset (post-exercise) muscle soreness. Presumably, this also means that they are harder on the muscle.
When the tibialis anterior has been sufficiently irritated, things can get ugly. By an accident of anatomy, its muscle sheath is a bit small, creating a dangerous situation: without room to swell, it only gets more irritated, and tries to swell more, a vicious cycle. This is the physiological flaw that causes “compartment syndrome.” It is not self-limiting, and has the potential to literally kill the muscles of your shin … or even you via blood poisoning.
The same forces that can put the tibialis anterior in this sorry state may also start to simply strain the connective tissues wrapping around the bone and/or the underlying bone (medial tibial stress syndrome). Ouch.
Yet another common runner’s injury may be bothered by hard surfaces: patellofemoral syndrome. Unlike with shin splints, there’s no superficially obvious problem with impact forces. The actual problem isn’t hard to understand, though: the less give there is in the road, the more the legs have to do the job of shock absorption. The body does this well, but it means that you are using the joints more — a tiny little bit more flex with every step. It adds up!
When you step off the road, or even a slightly softer road, there’s just a little bit less for the joints to do.
The problem with patellofemoral pain is usually tissue fatigue around or near the joint between the patella and the femur. This joint is always working hard. Pressures under the kneecap are spectacular even when nothing spectacular is going on: when the knee is flexed, it’s naturally cinched up against the front of the knee so hard that you literally couldn’t get it off with a crowbar (the bone before you could move it. It’s amazing that the tissue mostly handles these pressures. But of course if we chronically demand maximum performance, they may stop coping so well.
Sandpaper your arches until they are raw and then go for a barefoot run: that’s what plantar fasciitis feels like. This common repetitive strain injury involves fatigue of the connective tissues of the arch, the plantar fascia, which are part of the system that makes the arch springy. The less give there is in the running surface, the more the arch has to do its thing. And the less variation there is in the running surface, the more exactly the same the loading on the plantar fascia with every step. While there’s no evidence that this actually a problem, we do know that plantar fasciitis is prevalent in manufacturing, where workers usually work on concrete, and “work stations that decrease the percentage of time walking or standing on hard surfaces may lower the risk for plantar fasciitis.”22 Chances are good that’s true for runners too, because they use hard surfaces even more intensely.
And a soft data point: people with plantar fasciitis really don’t like to stand on pavement, and find shoes with good arch support to be a great relief. These are classic features of the condition.
Softer and uneven surfaces have their own risks of course — like tripping! and twisting ankles — but if you’re prone to recurrence of any of the injuries discussed above, you may prefer some new risks for a while.
Even chip trails and other groomed trails may not be enough of a departure from paved surfaces — it may be soft, but it’s still same-surface running. We have evolved miraculously complex reflexes and musculature that can keep us upright on virtually any surface, even shifting surfaces like the deck of a ship. To develop and maintain a well-rounded fitness, all of those reflexes and musculature need to be constantly stimulated and challenged.
Ideally, your run should be on soft, constantly changing, and unstable surfaces — but not so unstable that your risk of tripping and spraining spikes absurdly high, of course.
I live in downtown Vancouver, which is runner’s Heaven: miles of scenic seawall running. The seawall itself is paved. But for most of its length, you can stay off of it, and run on beaches or grass, hop over logs and benches, go up and down hills, even scramble over rocks.
Alas, most people don’t have the option of running on the beach. The solution is what I call “urban cross-country.” The key to urban cross-country is creativity: do anything you can to vary your running surface, and to get off the concrete every chance you get. Put parks on your route whenever possible. If it’s a small one, run around it on the grass five times before continuing. No park? Run on people’s lawns! The sidewalk is not your path: everything else is. Look for stairs and steep hills, and put them in your route. Run with one foot on the curb and one foot off for a block.
Getting the idea? Just do anything you can think of to keep changing the stresses on your body.
But the devil is in the details. For instance, all-terrain running is probably a different kind of risk factor for iliotibial band syndrome specifically, because that condition is infamously irritated by running down hills.
Roger Davies, running researcher and medal winner in the 800-metre run at the 2005 World Masters Games in Spain, recommends a running technique in a similar spirit called “natural posture” running. He believes that adult runners need to imitate the running style of children, leaning forward with their arms swinging and feet flat. “Your body has to get back to its natural self,” Davies says. “Loose shoulders, loose hips. A lot of us are very tight.”23
The loss of well-rounded fitness in our society is in part the inspiration for the “core stability” exercising trend, and explains the burgeoning popularity of Pilates and Yoga. We probably lose core stability without a variety of exercise. While core stability exercise may have its place in our lives, core stability training for its own sake would probably be much less necessary if only we would walk and run on the sand or the grass more often.
I am a science writer, former massage therapist, and I was the assistant editor at ScienceBasedMedicine.org for several years. I have had my share of injuries and pain challenges as a runner and ultimate player. My wife and I live in downtown Vancouver, Canada. See my full bio and qualifications, or my blog, Writerly. You might run into me on Facebook or Twitter.
Nine updates have been logged for this article. All PainScience.com updates are logged to show a long term commitment to quality, accuracy, and currency. more
When’s the last time you read a blog post and found a list of many changes made to that page since publication? Like good footnotes, this sets PainScience.com apart from other health websites and blogs. Although footnotes are more useful, the update logs are important. They are “fine print,” but more meaningful than most of the comments that most Internet pages waste pixels on.
I log any change to articles that might be of interest to a keen reader. Complete update logging of all noteworthy improvements to all articles started in 2016. Prior to that, I only logged major updates for the most popular and controversial articles.
See the What’s New? page for updates to all recent site updates.
— Lots of new information about impact forces on different surfaces, mostly based on Fu et al. This is the sixth substantial update so far in 2017. Phew: what a rabbit hole!
— Many miscellaneous improvements. I’ve now mostly completed the process of eliminating the assumption that pavement is risky from the old second half of the article.
— Widened and deepened the discussion of the science of impact and injury. Added citations about stress fractures and grass versus concrete hardness.
— Polishing of the arguments for and against road running. Added much more information about the science of the relationship between injury and impact forces.
— More introduction polish and much more thorough rebuttals the the arguments against the riskiness of running on pavement.
— Another wave of revisions: the scientific uncertainties now permeate the whole intro; all the arguments against “running pavement is risky” are now much more thorough; title is now a question: “Is Running on Pavement Risky?”
— New section, “Citation badly needed! Are roads really risky?” This now introduces the uncertainties on this topic more thoroughly, with some relevant links and citations. More citations and more thorough analysis of them coming soon.
— The premise of this article needs questioning. I’ve added a prominent, important caution to the introduction about the lack of evidence that any running surface is actually risky. I also removed and changed some a few particularly overconfident statements about injury risk. Major revisions forthcoming.
— Added a new section about plantar fasciitis.
This analysis of about a zillion runners versus walkers found that “running significantly reduced arthritis and hip replacement risk”…but due in large part to the fact that runners were typically skinnier. So weight was a trump factor here.
This data flies in the face of the common assumption that running is much harder on the joints. Instead, what it clearly shows is that running is either neutral or helpful, and weight is a much more important factor regardless of whether you walk or run.BACK TO TEXT
This simple experiment showed that runners adapt to changes in the hardness of the surface they are running on with amazing speed — just a single step — as measured in terms of maintaining the height of their centre of mass. Importantly, this nearly instantaneous adaptation only occurs with an expected change on familiar surfaces, but we are probably pretty quick with unexpected and unfamiliar surface changes as well.BACK TO TEXT
For this study, fifteen experienced male hikers walked down a 36˚ test ramp 30 times with poles and 30 times without, and with three different loads: nothing, a light pack, and a heavy pack (30% of bodyweight). A force plate in the ramp measured the intensity of their foot impact, and they were videotaped to get measurements of their joint movement. Consistent with other cited research, the use of poles resulted in significantly reduced forces, movement, and power around the knees and ankles. Interestingly, it didn’t matter how heavy the pack was: “packs only resulted in a larger power generation at the hip.”BACK TO TEXT
Even though Altman et al showed that injury rates were the same in barefoot runners, there’s an incredibly important caveat: the barefoot runners they tested put in just 24km/week, while runners in shoes ran 41km/week! Injury rates invariably go up with training volume. So what would the injury rate have been for the barefoot runners if they had almost doubled their distance to match the shod runners? Probably higher! As Alex Hutchinson put it for Runner’s World, “The only way the comparison has any relevance is if they’re arguing that barefoot running reduces injuries by preventing you from running as much as you’d like.”
It’s all still debatable, but in my opinion I think both common sense and some evidence now suggest that pounding the pavement without padding is almost certainly more injurious — which suggests that pounding pavement is probably more stressful than pounding trail.BACK TO TEXT
This was a study of the relationship between the loudness of foot strikes in running and several technical measures of forces on the lower limb. Twenty-six runners were tested when instructed to run quietly versus normally. Most runners (77%) switched to a forefoot running style. The surprise finding is that natural variation in footstrike volume has no direct relationship with smaller, slower impact forces when running normally. In other words, there are some quiet runners with a jarring gait, and some surprisingly silent runners who are really pounding the ground. Odd.
Not so surprisingly, actually trying to run quietly does soften footstrike.
This science brought to you by the Department of Well Okay Then Thanks I Guess?BACK TO TEXT
This study of studies tries to determine if stress fractures are connected to ground reaction forces (the force of your strike) or with loading rates (how fast the force is applied, i.e. more slowly or more jarring). They found that the force you are striking with has no connection with stress fractures, but the “the vertical loading rate was found to be significantly different between the two groups.” So it’s not how hard you hit the ground, but how fast you hit it. However, the science was murky on something important: the correlation identified is statistically “significant,” but the size of the correlation is not impressive. So it’s how fast you hit the ground, but probably only to a modest degree. Presumably there are quite a few variables involved, which reduces the importance of even the most seemingly obvious risk factors.BACK TO TEXT