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Fancy new midsole materials like PEBA make you faster, but at the cost of durability
The eternal question of how long your running shoes should last became suddenly topical a few months ago, when Tigst Assefa set a world marathon record in what Adidas billed as essentially a single-use marathon shoe. This development sparked some important discussions about sustainability, accessibility, and fairness. But it also served as a reminder of how little we actually know about the useful life of running shoes, especially in the new era of thick-foamed super shoes.
Fortunately, there’s some interesting new data in a study from researchers in Spain, led by Victor Rodrigo-Carranza of the University of Castilla-La Mancha. In the Scandinavian Journal of Medicine and Science in Sports, they compare shoes made with a modern super foam to a traditional EVA midsole before and after 280 miles, and find that the new midsole does indeed lose its magic more quickly—so much so, in fact, that the super shoe is no better than the regular shoe at that point.
Before digging into the specifics of the new study, it’s worth sketching in some context. The conventional wisdom that I’ve heard throughout my running life is that shoes typically last somewhere between 300 and 500 miles. This is based on some mix of cumulative experience and imperfect data, some of it published and some presumably locked away in shoe-company filing cabinets.
For example, there’s a 1985 study that measured shock absorption at frequent intervals between 0 and 500 miles, using a machine to simulate the impacts of running as well as two volunteers who racked up 500 real-life miles. Here’s an example of how shock absorption changed with mileage for one particular shoe:
The machine version of running obviously didn’t replicate the demands of in-person running very well, but the two human volunteers show similar patterns. Overall, the human testing found that shoes retained about 80 percent of the shock absorption after 150 miles, and 70 percent after 500 miles. You can see that the curve flattens out between about 300 and 500 miles, which is presumably where the conventional wisdom on shoe lifespan comes from. Conversely, you could argue that once you’re past 300 miles, the shock absorption isn’t going to drop much further, so if you’ve made it that far you might as well keep running in them until they (or you) fall apart. More on that below.
You might wonder whether shock absorption, as measured by a hydraulic machine, is really what matters to runners. More recent research has looked at other metrics: this 2020 study, for instance, used force-sensing insoles to measure the peak pressure on different parts of the foot, testing how it changed as a New Balance 738 shoe went from new, to 215 miles, to 430 miles. As the image below shows, the highest pressures are seen in the midfoot and forefoot:
Those pressures stayed virtually unchanged after 430 miles—but the midfoot peak pressure increased from 388 to 450 to 590 kilopascals, a 50 percent jump. It’s not clear exactly why the midfoot pressure increases. It could be that the relative lack of cushioning in the midfoot makes it less durable; it could be that the higher pressures on the rearfoot and forefoot break down the cushioning in those regions and shift load to the midfoot. Either way, it’s clear that the objective load on the foot has changed after 430 miles. And interestingly, the perceived cushioning reported by the volunteers picked up this change: those who reported feeling less cushioned in the midfoot in the worn shoes did indeed tend to have the highest peak midfoot pressures.
So the overall picture in the pre-super-shoe era was fairly muddy, but basically supported the view that shoes progressively lose their cushioning and reach bottom somewhere in that 300 to 500 mile range. Then came Nike’s Vaporfly in 2017, using a carbon fiber plate and a thick layer of cushioning to promise better running economy in exchange for a few hundred bucks—and, in the footnotes, less durability. The unsourced number that I frequently heard floating around after the Vaporfly’s release was that you should trust them for 100 miles or so.
It’s worth pausing here to say a few words about running shoe foams. Carlos Sánchez at RunRepeat has an impressive and exhaustive primer on the huge range of high-performance foams now on the market, which is worth digging into if you’re interested. A highly simplified history is that most running shoes used to use EVA (ethylene-vinyl acetate), then in 2013 Adidas’s Boost made TPU (thermoplastic polyurethane) popular, and then in 2017 Nike’s Vaporfly made PEBA (polyether block amide) the state-of-the-art. One study found that energy return was 66 percent for EVA, 76 percent for TPU, and 87 percent for PEBA.
While there’s been plenty of chatter about carbon-fiber plates, the weight of evidence is increasingly that most of the magic in super shoes comes from the foams. Unfortunately, the very property that makes them so good—their ethereal lightness—also makes them less durable. Of the more than three-dozen modern foams that Sánchez reviews, the main differences arise from the tweaks manufacturers make to trade off performance and longevity (along with price, of course). That makes it hard to issue general guidance about how long super shoes last: it depends on the shoe and the foam.
Still, one data point is better than zero, which brings us back to Rodrigo-Carranza’s study. The shoes he used were special prototypes manufactured for the study by On, almost identical except that one set had an EVA midsole while the other had a PEBA midsole. Both versions had a curved carbon-fiber plate. Twenty-two volunteers performed a running economy test to measure how much energy they burned at a given pace, once in fresh shoes and once in pre-worn shoes. (The researchers themselves ran exactly 280 miles in each pair of shoes to pre-wear them.)
One key finding: energy consumption in the new PEBA shoe was 1.8 percent lower than in the new EVA shoe. That bolsters the case that the foam itself is a big part of the magic, since both shoes had carbon fiber plates. (Not all PEBA foams are created equal, so we don’t know if On’s foam is as good as Nike’s ZoomX.) The other key finding: after 280 miles, there was no significant difference between the two shoes. The EVA shoe didn’t lose anything at all, whereas the PEBA shoe got 2.2 percent worse.
It’s not worth making any grand extrapolations from a single datapoint about when one type of shoe gets better than the other. Who knows what happens after 280 miles? For that matter, who knows what happened after 26 miles? Moreover, there are lots of other factors that might be relevant. One of the interesting claims in Sánchez’s review is that super foams maintain their properties even in frigid winter conditions, while EVA-based foams get much more rigid. In some shoes, firmness more than doubled after Sánchez’s freezer test. Maybe a worn-out super shoe is still a better bet than a newer EVA shoe for those of us who are latitudinally challenged. He also notes that shoe foams take more than 24 hours to recover from a run, and some recover faster than others, so it makes sense to alternate different pairs in order to maximize both comfort and shoe lifetime.
My own approach to shoe durability is, admittedly, not one that’s easy to generalize. I generally have two or three pairs on the go at any given time, and I run in them until they start to feel flat. It’s a noticeable sensation, especially if one of the other pairs in my rotation is relatively new and bouncy. Then, in the spirit of that graph above showing a plateau after about 300 miles, I keep running in them—but I pay more attention to how I feel during and after runs, and to whether the wear on the outer sole is changing the angle of my footstrike. Once I start to feel a few aches and pains, I retire the flat shoe. That approach works pretty well to keep me healthy, but the new data suggests that it’s probably not maximizing my performance. Anyone want to buy a pair of gently used six-year-old OG Vaporflys?(11/18/2023) Views: 146 ⚡AMP