Untangling Running’s Shoe Cushioning Paradox

Published May 25, 2022 04:02AM

Over the past four years, a new breed of heavily cushioned running shoes modeled on Nike’s Vaporfly line have rewritten the road-running record books. Soft-bottomed shoes, it turns out, can be exceptionally fast. But the hot new debate about the Vaporfly’s performance advantages has managed to push aside an older conundrum that should be of greater interest to 99.9 percent of runners: Does the cushioning in your trainers protect you from shin splints, stress fractures, and other impact-linked running injuries? Or, as barefoot runners insisted during their brief ascendency a decade ago, is it the cause of such injuries?

In the period following the 2009 publication of Christopher McDougall’s bestseller Born to Run, one of the key claims made by the barefoot and minimalist running movement was that the design of conventional running shoes was not evidence based. Elements like raised heels, pronation control, and thick midsoles might make intuitive sense, barefooters argued, but no one had ever tested their injury-prevention benefits in properly administered randomized trials.

Even shoe-industry insiders were forced to admit that the critics had a point. As a result, the past decade has seen a flurry of research centered on the contribution that shoe materials and construction make to running injuries, including a study published earlier this year that helped resolve a long-standing riddle about the role of cushioning.

The new study was published in the European Journal of Sport Science by Laurent Malisoux of the Luxembourg Institute of Health. Malisoux has emerged as the running-shoe world’s foremost advocate of placebo-controlled studies. If a researcher randomly assigns you to run in either a thick-bottomed Hoka or an ultralight sandal made from an old tire, your expectations about these two options will affect how you run—and perhaps how you perceive any aches and pains that crop up during the study. And if one option turns out to be better, it will be impossible to tell whether it was the cushioning, the heel-to-toe drop, or any of dozens of other differences between the two shoes that accounts for why.

To navigate this problem, Malisoux and his colleagues collaborated with the French sporting-goods giant Decathlon to produce a number of custom running-shoe designs that are visually indistinguishable and differ in only a single technical detail for each trial. That enables Malisoux to run blind studies, where neither the subject nor the researcher knows which type of shoe is being worn. One such study found that shoes limiting pronation (an inward rolling motion as your foot strikes the ground) reduced injury risk; another found that heel-to-toe drops ranging from zero to ten millimeters had varying effects depending on how much running subjects did each week.

Malisoux’s most recent randomized trial investigated shoe cushioning. Decathlon manufactured two prototypes, each with an inch-thick layer of midsole cushioning made from EVA foam. The chemistry of the EVA was manipulated so that half the shoes had soft cushioning while the other half had firmer foam. The shoes were randomly assigned to 848 healthy runners, who completed a treadmill test to assess their stride characteristics and were then monitored for six months for signs of injury.

The initial results, which were published in 2020, found that those given the firmer shoes were 52 percent more likely to develop an injury during the follow-up period, which seemed to confirm the injury protection offered by soft cushioning. But the stride analysis presented a riddle. Measurements from the force-sensing treadmill showed that runners sustained greater peak impact forces in their lower legs when wearing the soft shoes—a finding that echoed previous studies, but which you’d intuitively expect to cause more injuries, not fewer. Malisoux’s European Journal of Sport Science study digs deeper into the biomechanical data in an attempt to resolve what has become known as the shoe-cushioning paradox.

The key to the riddle? The timing of the impact forces. When your foot smacks the ground as you run, there are two distinct impacts: the first is the result of your lower leg decelerating abruptly; the second, a few milliseconds later, is the larger force delivered by the rest of your body. That first impact is the one researchers suspect is linked to injuries, since the force is applied all at once. It’s also the one that, in Malisoux’s study, seemed to be greater for runners in soft shoes.

That turned out to be an illusion, however. One of the effects of soft cushioning is that it slows down that first jolt, when the lower leg abruptly decelerates, spreading it over a longer period of time and causing it to overlap with the second. The combination of the two impacts makes the total force appear greater, giving the impression that softer shoes produce more force. But when Malisoux used mathematical techniques to separate the first and second impacts into distinct values, he found that the first impact—the one linked to injury—was actually smaller in softer shoes.

That’s vindication for the much maligned notion that shoe cushioning can lighten the load on your joints and reduce injury risk. It’s also vindication for some of the ideas that emerged during the barefoot boom. One of the benefits of switching from heel striking (common in cushioned shoes) to midfoot or forefoot striking (common, after careful transition, in minimalist shoes) is that it delays that initial impact peak and spreads it over a longer period of time—just like cushioning. There’s even evidence that a couple of weeks of gait retraining, when runners are instructed to “run softer” and given feedback on the size of their impact forces, can lower injury risk. In other words, there isn’t one right answer. You can pick your path.

As for the new generation of thick-soled, Vaporfly-style running shoes in vogue today, their injury risk remains a topic of heated debate. All that plush cushioning should, in theory, reduce leg impact, Malisoux says. But, he adds, most of these shoes also feature a stiff carbon-fiber plate embedded in the midsole, and its effect on injury risk remains untested. The height of the shoes may also make runners more vulnerable to overpronation and reduce stability, particularly when cornering. Only time—and properly designed studies—will tell.

In fact, despite all his research, Malisoux believes we still know very little about the complex links between shoes and injuries. So for now, his advice is to stick with whatever you’re running in if you’re happy with it. You might consider alternating between styles to vary the stress on your body. If you do decide to switch to a different shoe, think carefully about why you’re changing, try several different designs, and take your time making the transition. Finally, and perhaps most important, ignore everyone else’s advice. “I’m serious,” he says. “Each runner is unique, and the shoe that works well for your friend may not fit your foot.”