“NOBODY KNOWS ANYTHING,” screenwriter William Goldman is said to have quipped about Hollywood. If Goldman had been commenting on running-shoe research, he might instead have said, “Nobody can prove anything.”
People trying to convince others of minimalism’s benefits or risks often cite research to back their claims. This study proves barefoot running is best, that study proves running shoes with heels cause injuries, and so on. Unfortunately, the presentation of the research is often lacking—findings are misrepresented, taken out of context, or given conclusions that aren’t in the research. Sometimes all three happen.
This occurs more because people are passionate about the topic and misunderstand the nature of research than because they’re being intentionally misleading. But it still doesn’t help runners make good decisions.
Here are a few reasons why research on running shoes should be taken in stride:
And from the everyday runner’s perspective, here’s the most compelling reason to keep all this in perspective: research that shows something doesn’t necessarily prove anything.
On that last point, consider one of any number of studies I could cite. A few years ago Kerrigan conducted a study in which she measured external joint torques at the ankle, knee, and hip of people running barefoot and in conventional stability running shoes. When they ran in shoes, the subjects had increased joint torques at the three sites compared with when they ran barefoot.
Greater external joint torques sound bad. It seems reasonable to think that they should lead to greater frequency of injury. But it also sounds reasonable to think that running on soft surfaces is always “safer” than running on hard surfaces, when that’s not necessarily so. (More on that below.) Researchers are almost always reluctant—sometimes maddeningly so—to draw big-picture conclusions from their work. Kerrigan hasn’t said her finding of greater torque in running shoes proves that conventionally shod runners get injured more often. But the rest of the world isn’t that careful, and the researchers aren’t in the business of sending out perspective-providing press releases when their work is simplified or taken out of context. So in the case of Kerrigan’s torque study, headlines such as “Running Shoes Worse than High Heels” are what people remember. To take another example, a study that Lieberman helped conduct found that heel-strikers on Harvard’s cross-country team got injured more than their forefoot-striking teammates. It gets presented by the minimalist shoe company Vivobarefoot’s Web site as “It’s official—barefoot is best.”
None of this helps the average runner make sense of these matters. We hear about these studies most often from nonscientists who cherry-pick research to support a conclusion they’ve already reached. The more ardent online barefoot advocates don’t mention studies like the one published in the British Journal of Sports Medicine in 2009 that showed that runners prescribed custom orthotics had less plantar pain over an 8-week period than a control group not running in orthotics. “Ah, but that was a short-term study,” the response might be, and that’s a valid point. But intellectual honesty compels reading peer-reviewed research consistently. You don’t get to tout the results of studies you like but then switch to a design critique of ones you don’t like.
Elite coach Steve Magness, who has a master’s degree in exercise science and spends his noncoaching hours immersed in this sort of stuff, says, “The research, honestly, hasn’t been done to the degree it needs to be done. There’s some decent comparative data but there’s nothing long-term related to performance or injury prevention that says, ‘Here’s what minimalism does compared to regular shoes.’”
Any honest assessment of research done to date has to include these three statements:
That doesn’t mean the research is worthless (or uninteresting). It’s better to be well informed than poorly informed, both in terms of facts and in how to put those facts in context. Below I’ve summarized some key findings in five topic areas. For a collection of links to the original research, go to www.runnersworld.com/minimalismlinks.
Research on running surfaces might not seem the most obvious place to start, but I’m leading with it because the research results lead to another matter of perspective to keep in mind. (As if everything I’ve laid out so far isn’t enough.)
Research consistently shows that runners run differently on different surfaces. Using sensory feedback, we try to regulate impact forces by adjusting footstrike, joint stiffness, and muscle activation based on the firmness of the surface. (Muscle activation refers to the degree of tenseness in muscles before landing.) On harder surfaces such as asphalt, we land more softly than on softer surfaces such as grass.
The most interesting study in this regard involved jumping, not running. People jumped off a bench onto mats of different colors; the colors, they were told, corresponded to different cushioning levels in the mats. When they thought they were jumping on a well-cushioned mat, impact forces were higher than when they thought they were jumping on a less-cushioned mat. This is consistent with the research on adjustments to varying running surfaces. Well, except for one detail: The athletes were lied to—the mats were all equally cushioned. But when they thought they’d be landing on a softer mat, they presumably made subtle adjustments in muscle activation and joint stiffness to let the “softer” mat do some of the work.
This same sort of errant adjustment is thought to occur from wearing cushioned running shoes. Perhaps because of blocked sensory feedback, runners in cushioned shoes stiffen their knees when landing compared to when running barefoot. Impact forces wind up being higher than would be expected; it’s as if running in the shoe causes your body to make adjustments that cancel out the supposedly beneficial cushioning.
So the research shows that we make complex, integrated adjustments to account for running surface and what’s between us and that surface. Barefoot running on asphalt probably isn’t as harsh as it might seem, and running in heavily cushioned shoes on soft surfaces probably isn’t as protective as it might seem. Now for the additional bigger-picture thought on research stemming from this.
Nearly all running research of this sort looks at one element in isolation, such as impact forces, joint torque, or running economy. Yet we just saw via one such isolated research focus how the human body while running is an organic whole, with mind and muscles working in sync. This increases the chances of misinterpreting and overemphasizing any one bit of research.
The onus for intellectual honesty in this regard is especially on the most ardent pro-barefoot adherents. They point out, rightfully, that the running body is an amazingly adaptive thing. So why can’t those adaptations be made in a healthy way when running in cushioned shoes? Sport podiatrist Stephen Pribut says, “Why do we assume the body suddenly becomes ‘dumb’ when you put on shoes? Shouldn’t we assume it stays smart and makes the correct adjustments?”
The findings on how biomechanics change when running barefoot versus shod are consistent. The best research, such as Lieberman’s, includes people who are experienced at running barefoot, rather than just having subjects who always wear shoes suddenly run barefoot. The findings are usually presented as how runners adjust their form when switching from shoes to barefoot, although if you wanted to be persnickety, you could argue it should be the other way around, given that barefoot is the natural state.
When runners ditch their shoes, stride length decreases (by about 6 percent), stride rate increases, and ground-contact time decreases. In Lieberman’s famous study published in 2010, runners who were accustomed to conventional running shoes (“the habitually shod,” Lieberman called them) were overwhelmingly heel-strikers in their running shoes. When they ran barefoot, they still mostly landed on their heels, but did so with a flatter foot than in their shoes—the angle of dorsiflexion (toes pointed toward the shin) of their feet went from 28 degrees to 16 degrees, and at the ankle went from 9 degrees to a bit over 0 degrees. That last bit means that their ankles were ever so slightly plantarflexed (pointing away from the shin) when they ran barefoot. Meanwhile, the angle at their knee upon landing went from 9 to 12 degrees.
That study also looked at these differences in runners Lieberman called “habitually barefoot”—that is, who regularly ran barefoot or in barefoot-style shoes like the Vibram FiveFingers. (Vibram helped to fund this study.) When running in shoes, half of them landed on their heels, the rest midfoot or forefoot. When barefoot, three-quarters of them used what Lieberman classified as a forefoot strike, and the remaining 25 percent landed on their heels. The joint angles were also quite different from those of the habitually shod. The only situation resulting in dorsiflexion was foot angle when running in shoes, and that was an angle of only 2.2 degrees. Compare that with the 28 degrees of dorsiflexion the habitually shod had in their feet when landing wearing running shoes. The angle of their knee at landing was much more similar barefoot versus shod than was the case with the runners used to conventional running shoes.
This is perhaps the most interesting finding of the study, suggesting that regular barefoot or minimalist runners have what are thought to be better foot and ankle mechanics regardless of what they wear on any one run. We’ll return to this notion in Chapter 8.
It’s worth remembering that this study, published in Nature and given huge amounts of mainstream press, had a small subject pool. There were eight runners in each of the habitually shod and habitually barefoot groups. So when you say half of the habitually barefoot were heel-strikers in shoes but only 25 percent were when barefoot, bear in mind you’re talking about changes in two runners.
Things seem pretty straightforward concerning how shoes affect running economy, or the oxygen “cost” of running a given pace.
One commonly cited finding is that every 100 grams (just more than 3.5 ounces) of weight added to a bare foot increases the oxygen cost of running 7:00 mile pace by 1.2 percent. Another is that shoes constituting 1 percent of a runner’s weight increase oxygen cost by 3.1 percent. (That would mean 12-ounce trainers for a 150-pound runner.) Another fun one to throw around is that models weighing just over 12 ounces per shoe increase the oxygen cost of running 8:00 mile pace by 4.7 percent. A recent study from Lieberman’s lab looked not at bare feet versus shoes, but at FiveFingers versus the Asics Cumulus (a typical conventional running shoe). Regardless of whether they were forefoot- or rearfoot-striking, the runners were 2 to 3 percent more economical in the FiveFingers.
Competitive runners have known this intuitively for years. It’s why someone sponsored by Asics might train in the Cumulus (11.2 ounces in a men’s size 9), wear the Hyperspeed (7 ounces) for a marathon, and switch to the Piranha (4.7 ounces) for a 5-K.
As for why more runners without shoe contracts don’t race barefoot, note that research on running economy hasn’t been conducted with large numbers of runners moving at race pace. It’s possible the shorter stride most runners adopt barefoot limits performance once you’re going faster than a certain race pace. We’ll consider this idea more in Chapter 7.
Also, recent research has suggested there’s not necessarily a predictable decrease in economy as you move from barefoot to light shoe to heavy shoe. A study from the University of Colorado published in 2012 measured running economy in a small (12) group of men who were midfoot-strikers and used to running barefoot. The researchers measured running economy when the runners ran 8:00 mile pace “barefoot” (actually, they wore thin yoga socks) and in the Nike Mayfly, a racing shoe that weighs a bit more than 5 ounces. Eight of the runners were more economical in the Mayfly, four weren’t, and when the results were pooled (as they always are), the difference in running economy when they were “barefoot” compared to in the Mayflys wasn’t statistically significant.
By now you’re probably tired of my offering five caveats for every research result. So I’ll just say that while the Colorado study is interesting and intriguing, it’s another example of a small study with precise parameters that gets spun into a supposed game-changer. For example, I’m chagrined to report that the Running Times Web site linked to the study with the headline “Here’s Proof Barefoot Isn’t Better.”
This is where things start to get really hazy.
To date, there have been no studies with satisfyingly clear conclusions on why runners get hurt. One research review—meaning that the study authors surveyed existing research to find commonalities—resulted in “strong evidence” for two culprits in lower-leg injuries: mileage and a history of injury. So, one of the leading contributors to injury is having been injured; that clarifies that!
The impact forces of running would seem to be a likely explanation for injury. Except that they’re not. Some studies have suggested that runners with what the researchers call “higher vertical loading rate” have more of some injuries, including stress fractures. But other studies have found fewer injuries in runners with a higher vertical loading rate than in those with lower impact forces. As Kirby points out, research has also found seemingly counterintuitive results—such as that running on hard surfaces doesn’t lead to more injuries than running on soft surfaces (see the earlier section “Running Surfaces”) and that cushioned insoles don’t appear to reduce the incidence of stress fractures. Further muddying things is that isolating a primary cause of one type of injury wouldn’t necessarily say anything about other injuries. Stress fractures likely have different causes than the rusty-coil sensation many longtime runners feel at their hamstring insertions.
The study on Harvard cross-country runners—the one that Vivobarefoot says makes it official that “barefoot is best”—looked at form, not forces, for insight on injuries. Of the 52 runners in the study, 36 were said to be primarily heel-strikers, the others forefoot-strikers. Nearly three-quarters of the team members were said to get injured every year, and the rearfoot-strikers had roughly twice as many repetitive stress injuries as the forefoot-strikers.
This sounds pretty clear-cut. (Vivobarefoot certainly thinks so.) But as usual, this really just leads to more questions. Maybe the heel-strikers are injured more often because they have some structural weakness that leads them to heel-strike and that would be present no matter how they run. Maybe heel-striking isn’t as good as forefoot-striking when you’re training for and racing in collegiate cross-country races, but is if your focus in running is elsewhere. Who knows, maybe if the Harvard cross-country team switched to road ultras, the forefoot-strikers would be the ones getting injured more often. And maybe we shouldn’t draw grand conclusions from a study on 52 thin runners in their early 20s.
Even if we could prove what causes injury, there’s another reason to be leery of leaning too heavily on research on the topic.
A common element of the most rigorous scientific paper and the most superficial newspaper article on minimalism is some version of the statement “Every year, between X and Y percent of runners get injured.” The numbers used for X and Y vary significantly, from less than 20 percent at the low end to more than 80 percent at the high end. The numbers are unsatisfactorily vague for a good reason: Nobody can say with confidence how many runners get injured every year.
There are several reasons why. First, there’s no commonly agreed-upon definition of an injury. Does it mean an overuse injury bad enough to merit time off from running? That’s a reasonable definition, but is it explained as such when runners are asked how often they’re injured?
Second, even if that were the universal definition, it’s flawed as a means of gathering meaningful data. What might lead you to take a week off from running could be the sort of condition I try to run through. Pushed to its extreme, the definition would mean that people who have running streaks never get injured, because they never miss a day. So, if you don’t want to get injured, start a running streak!
Third, this is by necessity self-reported data. Even if there were a universal definition of injury, and even if all runners had the same standard for taking time off because of injury, not all runners have accurate records of all their runs. Without a log, can you say how many days you ran in 2010 and how many days you missed to injury?
All of this drifts even farther away from accuracy when people try to compare injury rates over time. As we saw in Chapter 3, the average Runner’s World subscriber in the early 1970s was a 145-pound 29-year-old man who ran 50 miles per week. The demographics of running have fundamentally changed since then. Today’s Runner’s World subscribers are evenly split between men and women, with an average age of 42 and average weekly mileage of 20. Today there are more older runners, more new runners, more slow runners, and, let’s face it, more runners who weigh more than 145 pounds. Even if accurate injury rates for a given year were possible, comparing them from year to year, not to mention decade to decade, would be meaningless.
Do runners get injured? Of course. Do we know why? Not really. Do we know how many runners got injured last year, and how that compares with how many got injured 5, 10, or 30 years ago? No. Take with a healthy dose of salt any advice on how to proceed in running that’s based on injury research.
And now we enter the black hole of running research.
What impact forces the body incurs, and how those forces are incurred, is where online discussions of minimalism often deteriorate into people shouting past each other. For those who enjoy spending time arguing such things, there’s good news: Both sides can cite research to support their claims. Some studies show that vertical loading rates from impact forces are greater when you run in shoes. Other studies show that vertical loading rates from impact forces are greater when you run barefoot.
One part of this topic that’s often discussed is differences in what peak impact forces are like in a heel landing versus a midfoot landing. (This often gets interchanged with what happens when running in conventional shoes versus barefoot.) Lieberman’s famous study published in Nature included data that peak vertical impact forces were about three times less in forefoot-striking runners who were used to running barefoot, compared with heel-striking runners used to running in conventional shoes. Overall, Lieberman wrote, “in the majority of [forefoot-striking] runners, rates of loading were approximately half those of shod [rearfoot-striking] runners.”
But, referring to “Injury Causes and Rates”, these different impact-force measurements might not mean what we tend to think they do. Leading running mechanist Benno Nigg has produced research showing that people with low peak impact forces get injured as often as people with high peak impact forces. (And, of course, we have to consider all the notions about accuracy of reported injury rates. See what I mean about the black hole?) After surveying research on the topic, Nigg wrote, “One cannot conclude that impact forces are important factors in the development of chronic and/or acute running-related injuries.” Instead, Nigg proposed, impact forces are “input signals that produce muscle tuning shortly before the next contact with the ground to minimize soft-tissue vibration and/or reduce joint and tendon loading.”
One graphic you’ll see often if you delve into this topic compares the peak loading rate of a heel strike versus a midfoot strike. The heel-strike visual has a distinct spike upon initial contact, while the midfoot-strike visual shows a much more even distribution of force from landing to toe-off. It’s natural to look at the heel-strike visual and envision the heel crashing into the ground and imagine the bad things this causes compared with the visually pleasing midfoot-strike curve.
By now you won’t be surprised to hear it’s not that simple—some think more impact forces can lead to increased bone density. “What is the proven danger of the initial ‘spike’ in the rearfoot strike?” Pribut asks. “I’ve thought this is a ‘signal’ to the osteocytes [a type of cell found in bone] and bone in general to produce more bone.”
In the “Running Surfaces” section, we saw how the body makes complex, near-instant changes when it’s running. Research can offer valuable and interesting insights into isolated aspects of what happens when we run; it’s unclear that it’ll do more than that any time soon. Certainly runners shouldn’t feel compelled to change things because this or that study “proved” something. I’m not big on clichés, and I wish our society as a whole had more respect for science. But what shoes to run in is definitely an area where the “we’re all an experiment of one” bromide trumps the latest from the labs.