When It Comes To Running Shoes, How Light Is Too Light?

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On September 24, 2023, Ethiopia's Tigst Assefa obliterated the women's world record in the marathon by two minutes and 11 seconds, clocking an astonishing 2:11:53. And she did it in the brand-new Adizero Adios Pro Evo 1, a groundbreaking super shoe Adidas announced just 10 days earlier.

RELATED: Tigist Assefa Shatters the Marathon World Record in Berlin

The main selling point of the Evo 1: It weighs just 138 grams, or 4.9 ounces. That's 40 percent lighter than any other racing super shoe Adidas has created, 25 percent lighter than the Nike Vaporfly (which is 184 grams, or 6.5 ounces), and 32 percent lighter than the ASICS Metaspeed Sky+ and Saucony Endorphin Elite (both of which are 204 grams, or 7.2 ounces).

In running, lighter equals faster--a notion the sport has finally started to move away from in terms of body size, but one that absolutely still applies to gear, as Assefa proved in Berlin.

Adding 100 grams per shoe impairs running economy and performance, a study published in 2020 in the journal Frontiers in Physiology confirmed, reinforcing findings dating back to the 1980s. The inverse is also true: "For every three and a half ounces you remove from a shoe, you get about a one percent improvement in running economy," says Matthew Klein, a professor of Physical Therapy at West Coast University Center for Graduate Studies and founder of Doctors of Running. Improved running economy doesn't directly translate to faster speeds; it just means you can maintain a harder effort for longer.

"Lighter weight is always going to add to performance, as long as you don't sacrifice some other area of shoe performance," explains Geoffrey Gray, founder of Heeluxe Footwear, an independent shoe research lab.

Clearly, in Assefa's case, the Evo 1 did not sacrifice any performance elements. Adidas cut serious weight by using a different manufacturing process to create a bouncier, lighter version of their Lightstrike Pro foam; replacing Continental rubber on the outsole with a much thinner rubber that has no tread; and slimming down the upper to a translucent shell.

Are Ultralight Running Shoes For Every Runner?

But just because Assefa had an incredible race in the Evo 1 doesn't mean this type of ultralight shoe is right for all runners. Other Adidas athletes wore the new shoe in Berlin and did not break any world records. "I do think this shoe is going to be important for certain people, but I don't think there will ever be a shoe that works great for everyone," says Gray.

It's sort of like how different runners respond differently to carbon plates. When the original super shoe, the Nike Vaporfly 4%, was first introduced in 2017, it improved running economy (at paces between 5:22 and 6:54 per mile) by an average of four percent, but the individual results ranged from 1.59 to 6.26 percent, a study published in Sports Medicine determined. And slower paces will see less of a benefit, research published in 2023 in The International Journal of Sports Physiology and Performance found; runners clocking 8:03 miles saw a 1.6 percent boost while 9:40 milers saw just a 0.9 percent increase, on average.

RELATED: Still The King, Nike Vaporfly 3

Similarly, a lightweight shoe can mess with your biomechanics in a way that will make sustaining a certain effort harder. Given the weight and the stack height of this particular shoe (which is 39 millimeters in the heel and 33 millimeters in the forefoot), "I'm assuming it's going to be very soft because the foam is going to be very low-density," says Klein.

He's not wrong. It's not like a spring; it's like a foam pad or trampoline. "It requires controlling your landing with every step," Klein explains. "If you can't, all of your energy, instead of going forward, is going to be going off to the side and your intrinsic muscles will have to work really hard to make sure you're stable on the platform and able to adequately absorb that impact." Translation: Your quads, hips, and ankles are going to be toast long before you get to the end of the race.

You also have to understand how to use a shoe like this, and, at this point, Adidas isn't leaving a lot of room for trial and error. A shoe is a tool, and you may not have the capacity to use this particular tool, especially without practice. "Not anyone can drive in a Lamborghini," says Klein.

The Lamborghini analogy is on point in several ways: For one, at $500, the Evo 1 is the most expensive super shoe ever released. And Adidas says it's only good for one marathon and a familiarization period (the company doesn't specify the exact mileage), making it essentially a single-use shoe--not great from a sustainability standpoint. Finally, only 1,000 pairs will be released, which puts it out of reach for the vast majority of runners, even if they could afford it.

Fortunately, there are more shoe options than ever that are designed to meet the different experiences of the wearer--and they're all getting lighter, too, says Gray. "We've been collecting data for 13 years now, and we've seen the average weight of a men's size nine running shoe decrease from 10.2 ounces to 9.1 ounces," says Gray. "Cushioning, durability, stability--all of these things are getting better at less weight."

The Super Shoe Evolution Continues

The Evo 1, however, is an important step in the evolution of super shoes. "What used to limit us was a balance of weight and cushioning--there was only so light you could go before you start sacrificing cushioning and things started to suffer," says Klein. "This shoe is kind of like a proof of concept--Adidas is saying we're able to combine super lightweight with super cushioned, and now they've just got to develop it to a point where it becomes more accessible."

But remember: Even at 138 grams, this isn't the lightest shoe that's ever been made for performance running. "Not even close," says Gray. "But it's an opportunity for brands to rethink the materials they use and how a shoe is constructed, and that kind of outside-of-the-box thinking is absolutely going to create more innovation." And innovation means more options and more opportunities for every runner to find a shoe that works best for them.

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