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 nowhere near 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. Just as with running barefoot or minimally shod — a sort of mirror image of this topic — there’s a surprising lack of hard science about running on 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.
Citation badly needed! Are roads really risky?
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.
Hard surfaces are innocent! The case for the defense
Here are some of the clues and perspectives that cast doubt on the alleged “danger” of running on roads and sidewalks:
- Runners’ joints are in good shape. A 2018 study showed that runners probably have half the rate of knee and hip arthritis than non-runners.4 This generally undermines the popular idea that running is “hard on the joints,” and suggests instead that it’s actually stimulating adaptation, making joints tougher. If true (and it probably is) it undermines the obviousness of hard surfaces being problematic.
- Humans have amazing shock absorption features. For instance, when we run onto a new surface, we adjust the spring in our step on the first step — by adjusting our leg stiffness.5 We dynamically adjust our shock absorption, and we’re extremely good at it.6 And which suggests that surface hardness might not be a big deal.
- Impact forces are not strongly associated with injuries. “The evidence of the link between injury and impact related factors is either just not there or far from compelling,” writes Craig Payne on RunResearchJunkie.com, summarizing the significance of a review of studies.7
- Shoes don’t make much difference. If surface matters, then what we put between our feet and the surface probably matters too — a proxy surface — but no kind of shoe (or lack of shoe) has been clearly shown to make any important difference in injury rates. It was only in 2016 that we finally got good data on barefoot running compared to shod, and they were quite similar — different injuries, but the same overall injury rate.8
- Ignore fear-mongering claims made without evidence. “Common sense” is often suspect, and Hitchen’s razor cuts deep here: “What can be asserted without evidence can be dismissed without evidence,” and probably should be dismissed if it discourages people from participation in what is clearly a healthy activity. In other words, until we actually know, let’s err on the side of not making people scared of a risk that may not exist. A positive attitude truly matters in rehab.9
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
Hard surfaces are guilty! The case for the prosecution
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.
- Maybe trails are even better! Just because runners’ joints do surprisingly well doesn’t mean they wouldn’t be in even better shape with off-road running. While the evidence on arthritis can tell us that road running clearly isn’t wrecking people, it is simply mute on the difference between harder and softer surfaces. Injury and arthritis are not imaginary; although bodies thrive by adapting to manageable loads, they do struggle to adapt and fail if the load is excessive. For all we know, off-road running is the loading sweet spot for most runners.
- Shock absorption isn’t “free.” We may be good at adjusting our gait for shock absorption, but that’s mean that there are no costs or consequences.10 When I hike down a mountain, I’m sure my legs very cleverly do everything possible to compensate for the stresses of taking thousands of steep steps downwards, but steep descents are still holy hell on my knees, and adding extra protection in the form of walking poles definitely helps.11 Clearly there are limits to our ability to absorb shock, we don’t know where that line is drawn. Why not let the surface do some of the shock absorbing for you? Speaking of that …
- There’s evidence that springier surfaces are easier on bodies. We know that more spring in the surface means less spring and bending in the joints.12 Hips, knees and ankles all bend less when you walk or run on a springier surface. This is a highly plausible mechanism for increasing the rate of overuse injuries running on non-springy surfaces. And what could be less spring than pavement?
- The impact evidence is just not direct enough. Studies showing a weak link between injuries and impact forces are the closest thing to relevant science, but they are nowhere near as relevant as actually comparing the results of running on different surfaces. They are “circumstantial evidence.” They cannot definitively answer the scientific question. Nevertheless, I will look at this evidence in more detail below.
- Actually, barefoot running is a problem. The best evidence on barefoot running and injuries actually supports (or is consistent with) the original common-sense notion that impact is a problem and the cushioning of running shoes actually does meaningfully protect us from it.13
- Erring on the side of caution is reasonable. Erring on the side of not worrying sounds great to me — I want to champion everyone’s peace of mind — but there’s an obvious rebuttal. In the absence of evidence, how about erring on the side of caution? It would be ridiculous to advise total abstinence from the road — that would be fear-mongering nonsense. But minimizing it? That just seems like a good use of the precautionary principle… based on a risk that’s plausible.
My conclusions on the risk question for now
The arguments in summary:
- Road-runners’ joints are in surprisingly good shape, but maybe they’d be even better off on trails.
- We adapt to surfaces deftly, but there are probably limits and consequences to shock absorption.
- Studies show no clear association between impact forces and injury, but they are not the right studies to answer the real question here.
- Different types of running shoes don’t seem to have anything to do with injury, but a lack of shoes does.
- Although it’s nice to avoid fear-mongering about a risk that may not exist, a better-safe-than sorry policy is reasonable.
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:
- Running on pavement is probably a bit risky, but much less risky than lots of other popular things, like basketball, baseball (crazy injury rates in those sports, believe it or not) and hungry bear poking. Everything’s relative!
- We should beware of fear-mongering, but it’s wise to gently apply the precautionary principle, and minimize hard surface running and explore the alternatives.
Impact and running injuries
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.14
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).17 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.18 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.19 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 demonstrated20 that there are much stronger forces involved in activities that involve greater shear strain,21 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.22 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.
Is grass softer than pavement?
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.23 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.24 Er, wut?
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 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.
Pavement seems more than 12% harder than grass
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.
Impact and intervertebral disc health
Slow running and fast walking are surprisingly good for intervertebral discs, but they can’t adapt to faster and more jarring running.25 This is a great (and interesting) example of more indirect evidence that running on pavement is risky.
Back pain isn’t usually considered a common running injury, but it is — many runners struggle with it. The spine is part of the spring shock-absorption system, both flexing and compressing, and the tough little jelly-filled donuts of connective tissue between the vertebrae are a key component.
For decades, experts assumed that the jarring impact of running (at any speed) constitutes a source of relentless wear and tear on the spine, and that the discs in particular probably cannot keep up with the onslaught, and aren’t able to adapt and recover — a slow losing battle. This assumption was mostly based on data low tissue “turnover rates” — how slowly disc tissue is replaced. But new evidence, the first of its kind, suggests exactly the opposite: “tissue adaptation will occur in the intervertebral disc with exercise.”
Discs are much “juicier” in runners: “better hydration and glycosaminoglycan content” by a wide margin. Discs are also thicker in runners, but only a little. These results were found in runners aged 25-35 with a five year history of running at about the same level, and the benefits were slightly greater in long-distance runners (more than 50K per week).
But additional data collected from 10 runners revealed a critical caveat: the improvements were associated with “fast walking and slow jogging,” while the link weakened at both slower and faster speeds (under 1.5m/s and running over 2.5m/s), or with high-impact jumping. It was a just-right intensity of the stimulus to the discs that mattered. And discs appear not to adapt to too much.
This is about a hundred scientific miles from good support for the claim that “running on pavement is harder on your discs,” but it is nevertheless quite suggestive. Clearly discs do absorb shock, and adapt to that stimulus, but there is a speed limit … and I’m betting that limit is little higher on springier surfaces. Again, the more shock absorption is provided by the surface, the less the body has to do.
Lack of variety in running surface
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 seawall around Stanley park in Vancouver, Canada, one of the most popular running routes on Earth — & deserted when I took this photo, because it had just been re-paved & it wasn’t officially re-opened. Shh.
IT band syndrome on the road
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.26 It has gotten fashionable lately to strengthen hips to prevent knee IT band syndrome and patellofemoral pain syndrome. In addition to being surprisingly powerful primary running muscles,27 these gluteal muscles also control side-to-side movement of the hips, a part of core stability. On a flat surface, they probably aren’t needed as much, because it’s much easy to stay balanced on a flat surface. They don’t exactly atrophy, but the other leg muscles get disproportionately stronger. This may be 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.
Shin splints on the road
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.28 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.
Use a muscle hard enough, and it will start to hurt. This may be the only problem with so-called “compartment syndrome,” where supposedly the tibialis anterior swells in its muscle sheath and gets oxygen starved. Frankly-Miller et al. have argued that there is no strong evidence of a connection between pressure and chronic pain, and chronic exertional compartment syndrome is misnamed: it’s just irritated muscle, and it can be fixed just by tuning running technique to make life easier for the tibialis anterior.29 They call it “biomechanical overload syndrome.” If they are right about the nature of the beast, running on a less rigid surfaces would probably also be helpful.
Finally, 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”).
Patellofemoral pain on the road
Both kinds of runners knee, IT band syndrome & patellofemoral pain, are probably aggravated by running on hard & even surfaces.
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.
Plantar fasciitis on the road
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.”30 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.
Alternatives to running on hard, even surfaces
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 and natural posture running
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.”31
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.
Impact reduction take-home points
Stay off concrete as much as possible. Prefer tracks and treadmills for the bounciest of all options, the best at giving you some of your impact energy back and reducing the load on your biomechanical spring. (Grass, sand, and chip trails are much softer than pavement, but are more like running on foam: mushy, not springy.)
I work just a couple blocks from a running track. I don’t usually walk there, because I’d far rather stick to tree-lined streets and parks. But if my plantar fasciitis flared up, for example, I’d absolutely make walking on that track part of my rehab regimen, because that surface is obviously springier than anything else I can walk on regularly. Not everyone has such a convenient option, of course, but it’s an instructive idea.
If you can’t find a springy surface to run on, put a spring in your step?
Run more gently. Avoid that jarring feeling! Experiment with your running style; do anything that feels less like you’re slamming your feet down. We can definitely run more softly if we try to,35 most easily by shifting to a forefoot impact and basically running more like a tiptoeing cartoon thief; barefoot running or minimal shoes help with this, as they strongly discourage heel impact and so we shift our impact to the forefoot. But there are trade-offs! Forefoot impact undoubtedly shifts the burden between tissues, which might help solve patellofemoral pain while risking new injuries. More about this in the next section.
Alternatively, another strategy for running softer is probably less prone to dicey trade-offs: slow down and take smaller steps! Basically just run less aggressively, period.
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About Paul Ingraham
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.
- Save Yourself from Plantar Fasciitis! — Plantar fasciitis explained in great detail, including every possible treatment option, and all supported by recent scientific research
- Save Yourself from Shin Splints! — Causes and treatment options for shin splints explained and discussed in great detail, especially shin pain caused by myofascial trigger points, compartment syndrome, medial tibial stress syndrome, and stress fracture
- Does barefoot running prevent injuries? — A dive into the science so far of barefoot or minimalist “natural” running
- Civilization Survival Tips (#9) offers another take on the same subject, with some audio as well.
- Are Orthotics Worth It? — A consumer’s guide to the science and controversies of orthotics, special shoes, and other allegedly corrective foot devices
- Massage Therapy for Tired Feet (and Plantar Fasciitis!) — Perfect Spot No. 10, in the arch muscles of the foot
Most of the knee pain information on PainScience.com is about patellofemoral syndrome and iliotibial band syndrome, the two most common runner’s knee conditions, typically causing pain on the front and side of the knee respectively (learn more about the difference). Many related articles:
- Massage Therapy for Your Quads — Perfect Spot No. 8, another one for runners, the distal vastus lateralis of the quadriceps group
- IT Band & Patellofemoral Pain Defy Common Sense — The science shows that you can’t blame runner’s knee on structural quirks that seem like “obvious” problems
- Do Women Get More Knee Pain? — The relationship between gender and knee pain, especially runner’s knee (IT band syndrome, patellofemoral pain)
- What Can a Runner With Knee Pain Do at the Gym? — Some training options and considerations for runners (and others) with overuse injuries of the knee
- Patellofemoral Pain Diagnosis with Bone Scan — If you have anterior knee pain, should you bother x-ray, MRI, CT scan, or bone scan?
- Does Cartilage Regeneration Work? — A review of knee cartilage “patching” with autologous chondrocyte implantation (ACI)
- The Runners Knee Diagnostic Checklist — How to tell the difference between the two most common kinds of runner’s knee
- Should You Get A Lube Job for Your Arthritic Knee? — Reviewing the science of injecting artificial synovial fluid, especially for patellofemoral pain
- Does Hip Strengthening Work for IT Band Syndrome? — The popular “weak hips” theory is itself weak
- Patellofemoral Pain & the Vastus Medialis Myth — Can just one quarter of the quadriceps be the key to anterior knee pain?
- Patellofemoral Tracking Syndrome — The beating heart of the conventional wisdom about patellofemoral pain is mostly nonsense
- Knee Surgery Sure is Useless! — Evidence that arthroscopic knee surgery for osteoarthritis is about as useful as a Nerf hammer
What’s new in this article?
Fourteen 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.
2018 — Added a substantive point to the arguments against running on hard surfaces, about the effect of surface springiness. Also, a nice citation about low rates of arthritis in marathoners (see Ponzio 2018.
2017 — Science update: changed the description of the nature of chronic exertional compartment syndrome based on Franklyn-Miller et al (because it may be mis-named), and added a citation to Lenhart et al regarding the function of the gluteus medius and minimus.
2017 — New section: “Impact reduction take-home points.”
2017 — New section on intervertebral disc adaptation to running as indirect evidence that running on pavement may be risky, based on Belavý et al.
2017 — Added a footnote about the importance of psychological factors in rehab (such as not being scared of the surface you run on). Added beefy footnotes about springing and muscle tuning: two biomechanical shock absorption tactics in human biology.
2017 — 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!
2017 — 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.
2017 — Widened and deepened the discussion of the science of impact and injury. Added citations about stress fractures and grass versus concrete hardness.
2017 — Polishing of the arguments for and against road running. Added much more information about the science of the relationship between injury and impact forces.
2017 — More introduction polish and much more thorough rebuttals the arguments against the riskiness of running on pavement.
2017 — 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?”
2016 — 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.
2016 — 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.
2016 — Added a new section about plantar fasciitis.
- Lieberman DE, Bramble DM. The evolution of marathon running: capabilities in humans. Sports Med. 2007;37(4-5):288–90. PubMed #17465590 ❐ “Human endurance running performance capabilities compare favourably with those of other mammals and probably emerged sometime around 2 million years ago in order to help meat-eating hominids compete with other carnivores.”
- The original quote is from Benjamin Franklin: “The definition of insanity is doing the same thing over and over and expecting different results.”
- NYTimes.com [Internet]. Kolata G. For Runners, Soft Surface Can Be Just as Hard on the Body; 2016 December 29 [cited 17 Jan 19]. “Exercise researchers say there are no rigorous gold-standard studies in which large numbers of people were assigned to run on soft or hard surfaces, then followed to compare injury rates. … It’s too hard to recruit large numbers of people willing to be randomly assigned to one surface or another for their runs.”
- Ponzio DY, Syed UA, Purcell K, et al. Low Prevalence of Hip and Knee Arthritis in Active Marathon Runners. J Bone Joint Surg Am. 2018 Jan;100(2):131–137. PubMed #29342063 ❐
In this survey of 675 marathoners, there was no link between current arthritis symptoms and their running history, and they had a lower rate of arthritis than the general population. That is, no matter how much they ran, they had the same low rate of arthritis: about 9%, compared to 18% in non-runners. Obviously this is nice news that challenges the assumption that relentless “pounding” on the road is hard on joints, but for better evidence based on longer-term data, see Lo.
- Ferris DP, Liang K, Farley CT. Runners adjust leg stiffness for their first step on a new running surface. J Biomech. 1999 Aug;32(8):787–94. PubMed #10433420 ❐
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.
We have a couple of main biological shock absorption tricks:
- Muscle tuning is the dynamic dampening of impact vibrations with precisely timed muscle contractions — a very cool system (Boyer et al). And quite exotic (and likely not embraced by all experts).
- Springing is the more obvious one: we adjust the springiness of our entire body by being bendier. Harder surface? More bending! Softer surface? Less bending! (Ferris et al) It’s obvious in an extreme example, like bouncing on a trampoline, where you can keep your knees straight; but jump down just one metre onto concrete, and you’ll have bend your knees quite a lot. We do the same thing much more subtly when we walk and run.
- van der Worp H, Vrielink JW, Bredeweg SW. Do runners who suffer injuries have higher vertical ground reaction forces than those who remain injury-free? A systematic review and meta-analysis. Br J Sports Med. 2016 Apr;50(8):450–7. PubMed #26729857 ❐
- Altman AR, Davis IS. Prospective comparison of running injuries between shod and barefoot runners. Br J Sports Med. 2016 Apr;50(8):476–80. PubMed #26130697 ❐ For this test, 200 experienced runners were studied over the course of a year. The results are clear and unsurprising: there was no important difference in injury rates, just the types of injuries. Each was better in some ways, worse in others. Although the paper emphasizes “fewer overall injuries” for barefoot runners, injury rates are what matters — the number of injuries per 1,000 kilometres, say — and they were “not statistically different between groups due to significantly less mileage run in the barefoot group.”
- Several papers by Clare Ardern et al. have shown that a positive view towards return to sport is important for a successful return (see Ardern 2013, Ardern 2014, Ardern 2015, Ardern 2015). Unnecessarily fearing the surface you run on is the opposite of a “positive view.”
- Bodies have to work to minimize the effect of jarring steps on any one anatomical structure. As described above, we absorb shock mainly with two tricks: muscle tuning and springing. Every precisely timed vibration-dampening contraction takes energy and yanks on our anatomical rigging; every bit of extra springing takes joints a little further into flexion, with a little more muscle power to control the movement. We’re good at it … but it’s work.
- Bohne M, Abendroth-Smith J. Effects of hiking downhill using trekking poles while carrying external loads. Med Sci Sports Exerc. 2007 Jan;39(1):177–183. PubMed #17218900 ❐ PainSci #56827 ❐
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.”
- Humans have a couple main biological shock absorption tricks: one is to dampen vibrations with precisely timed muscles contractions (which is very cool, see Boyer 2004), and the other is to bend like a complex spring (Ferris 1997). The springing is really obvious in an extreme example, like bouncing on a trampoline, where you can keep your knees straight; but jump down just one metre onto concrete, and you’ll have bend your knees quite a lot. We do the same thing much more subtly when we walk and run. We’re amazing at making much finer, faster adjustments to surface rigidity when running (see Ferris 1999).
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.
- van der Worp H, Vrielink JW, Bredeweg SW. Do runners who suffer injuries have higher vertical ground reaction forces than those who remain injury-free? A systematic review and meta-analysis. Br J Sports Med. 2016 Apr;50(8):450–7. PubMed #26729857 ❐
- Phan X, Grisbrook TL, Wernli K, et al. Running quietly reduces ground reaction force and vertical loading rate and alters foot strike technique. J Sports Sci. 2016 Sep:1–7. PubMed #27594087 ❐
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 surprisingly jarring gait, and some loud runners who aren’t pounding nearly as hard as you’d think. 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?
- Which is, by the way, a nice demonstration of an interesting training principle: it’s easier to modify technique by focussing on an external or abstract goal, rather than the biomechanics of the technique itself. In this case, the abstract goal of being quiet or “sneaky” evokes forefoot running almost like magic, without having to devote the slightest attention to the specifics of how to run more quietly.
- van der Worp 2016, op. cit. “The loading rate was higher in studies that included patients with a history of stress fractures and patients with all injury types, both compared with controls. Only studies that included patients with a history of symptoms at the time of kinetic data collection showed higher loading rates overall in cases than in controls.”
- Davis IS, Bowser BJ, Mullineaux DR. Greater vertical impact loading in female runners with medically diagnosed injuries: a prospective investigation. Br J Sports Med. 2016 Jul;50(14):887–92. PubMed #26644428 ❐
- Zadpoor AA, Nikooyan AA. The relationship between lower-extremity stress fractures and the ground reaction force: a systematic review. Clin Biomech (Bristol, Avon). 2011 Jan;26(1):23–8. PubMed #20846765 ❐
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.
- Milgrom C, Burr DB, Finestone AS, Voloshin A. Understanding the etiology of the posteromedial tibial stress fracture. Bone. 2015 Sep;78:11–4. PubMed #25933941 ❐
- The bone resisting bending rather than resisting longitudinal compression. Sheer strain could explain the oblique stress fractures more often seen in young adults.
- Kluitenberg B, van Middelkoop M, Smits DW, et al. The NLstart2run study: Incidence and risk factors of running-related injuries in novice runners. Scand J Med Sci Sports. 2015 Oct;25(5):e515–23. PubMed #25438823 ❐
- Tessutti V, Ribeiro AP, Trombini-Souza F, Sacco IC. Attenuation of foot pressure during running on four different surfaces: asphalt, concrete, rubber, and natural grass. J Sports Sci. 2012;30(14):1545–50. PubMed #22897427 ❐ A study of the relationship between “in-shoe pressures” and asphalt, concrete, and natural grass in 47 recreational runners. Each of them ran 40 metres at about 12kph with Pedar X insoles, which measure pressures on the bottom of the foot. Running on asphalt and concrete produced the same pressures, but pressures on were about 9–16% less. Note that measuring forces only in the foot can only tell us so much.
- Fu W, Fang Y, Liu DM, et al. Surface effects on in-shoe plantar pressure and tibial impact during running. Journal of Sport and Health Science. 2015 Dec;4(4):384–390. PainSci #53552 ❐ This paper with surprising results is unusually well-written, with a good introduction reviewing the subject background. They measured two key impact variables in 13 male recreational runners (all heel-strikers) at 12 km/h velocity on concrete, synthetic track, natural grass, a normal treadmill, and a treadmill equipped with a cushioning. Plantar pressures were measured with an in-shoe pressure system, and tibial shock (peak positive acceleration) was measured with an accelerometer at the top of the shin. Almost no differences were observed in these forces on any of the surfaces!
- Belavý DL, Quittner MJ, Ridgers N, et al. Running exercise strengthens the intervertebral disc. Scientific Reports. 2017 Apr;7:45975. PubMed #28422125 ❐ PainSci #53606 ❐
- But you’ll also hear it from countless physical therapists these days, so let’s run with it for the sake of this point. For a full discussion about this, see Does Hip Strengthening Work for IT Band Syndrome?.
- Lenhart R, Thelen D, Heiderscheit B. Hip muscle loads during running at various step rates. J Orthop Sports Phys Ther. 2014 Oct;44(10):766–74, A1–4. PubMed #25156044 ❐ PainSci #53657 ❐ Using computer modelling of their own design, the authors claim to have produced evidence of “substantially” more powerful gluteus medius and minimus contractions than any other hip muscle: “The sum of peak forces from the gluteus medius and minimus, 2 primary hip abductors, was 3.5 times that of the gluteus maximus, a primary hip extensor.”
- For more detail, see another article on PainScience.com, Eccentric Contraction: A weird bit of muscle physiology.
- Franklyn-Miller A, Roberts A, Hulse D, Foster J. Biomechanical overload syndrome: defining a new diagnosis. Br J Sports Med. 2014 Mar;48(6):415–6. PubMed #22983122 ❐ PainSci #53656 ❐
- Werner RA, Gell N, Hartigan A, Wiggerman N, Keyserling WM. Risk factors for plantar fasciitis among assembly plant workers. PM R. 2010 Feb;2(2):110–6; quiz 1 p following 167. PubMed #20193937 ❐
- Roger Davies was quoted in a Canadian Broadcasting Corporation article published Mar 3, 06.
- The rationale for this product is discussed thoroughly in my orthotics article, but basically the idea is that shoes of this type “reduce lower limb muscle forces,” (Wunsch et al) because the surface is giving you some energy back.
- Verdejo R, Mills NJ. Heel-shoe interactions and the durability of EVA foam running-shoe midsoles. J Biomech. 2004 Sep;37(9):1379–86. PubMed #15275845 ❐
Science news flash! Shoes wear out: “Scanning electron microscopy shows that structural damage (wrinkling of faces and some holes) occurred in the foam after 750 km run. Fatigue of the foam reduces heelstrike cushioning, and is a possible cause of running injuries.”
- Kong PW, Candelaria NG, Smith DR. Running in new and worn shoes: a comparison of three types of cushioning footwear. Br J Sports Med. 2009 Oct;43(10):745–9. PubMed #18801775 ❐
When shoes wear out, the biomechanics of running do change. Kong et al tested 24 runners before and after 200 miles of road-running in the same pair of shoes. There were a few minor changes: longer stance phase, less forward leaning, and less ankle flexion. Hip and knee angles were unchanged. (Also, 200 miles is not much — a strangely low number for this study, actually — and the impact on biomechances may only just be getting started by then.)
I do recommend replacing your shoes when they begin to show obvious signs of wear. The risk of running in decrepit shoes may be small, but there’s not much reason to take that risk — just the modest cost of buying shoes somewhat more often. It’s not like you weren’t going to buy new shoes eventually! On the other hand, this data makes it pretty clear that replacing shoes while they still look fine isn’t really going to make much of a difference.
- Baggaley M, Willy RW, Meardon SA. Primary and secondary effects of real-time feedback to reduce vertical loading rate during running. Scand J Med Sci Sports. 2017 May;27(5):501–507. PubMed #26992659 ❐ “However, forefoot strike and cues to reduce average loading rate also increased eccentric ankle joint work per km. Potentially injurious secondary effects associated with forefoot strike and cues to reduce average loading rate may undermine their clinical utility.”