At first glance, the Adidas PureBoost X ($120) looks like a typical running shoe. Look more closely, though, and you’ll see the upper isn’t entirely attached to the sole. Where it would normally fuse together around the arch, there's open space.

This gap exists, Adidas says, because the shoe was designed specifically for women. That's unusual in and of itself: Most athletic footwear designed for women is adapted from athletic footwear designed for men, even though the anatomy of women’s feet differs considerably from that of men’s feet. Women tend to have narrower heels and higher arches than men. And according to motion-capture research Adidas did in-house, they also experience a lot of expansion in the forefoot. “The forefoot can expand up to 10 millimeters, width-wise, with every step,” says Ben Herath, who leads the company's running design team. “So the foot is constantly changing with every step.” Complicating things further, the degree of forefoot expansion varies more from woman to woman than it does from man to man.

Adidas

These observations led to the PureBoost X. “We wanted to create something that felt like a natural extension of the body,” Herath says. The floating arch functions like an extra-sturdy sock—when it's not glued to the sole, the arch can move while still providing support. The result, Adidas says, is a design that accommodates the range of motion and expansion observed in the female foot.

Research examining the differences between men's and women's feet goes back 20 years, says Benno Nigg, professor emeritus of kinesiology at the University of Calgary in Canada and an expert on the relationship between footwear design and biomechanics. Yet, 44 years after the passage of Title IX, the amount of research dedicated to improving women’s athletic gear still pales in comparison to that afforded men's equipment. Why that’s the case is “a political question,” says Nigg, but “it’s unbelievable. It’s only lately [in sportswear development] that if you have 200 subjects, that you have 100 female and 100 male. Typically you have 200 males.”

Adidas

Adidas's designers started studying women’s feet in earnest after spotting a trend. The athletics company sponsors a number of marathons, half-marathons, and shorter races like 5Ks. In recent years, the participation rate for those events has skewed more and more female—evidence that the marketplace for women runners is growing, globally. To cater to that, Herath and his team got their hands on Aramis, a motion-capture technology Boeing and NASA use to measure stress on aircraft. After obtaining data for what Herath says were hundreds of women’s feet, the team felt it had an accurate picture of how the female foot behaves. In addition to forefoot expansion, “we could trace a lot of movement in the Achilles area around the heel. And as the foot moves from heel to toe, there’s expansion in the midfoot,” Herath says.

While the floating arch design of the shoe is unique, it follows a broader trend in the running industry. Previously, the goal was to “put all the function in the sole, and little in the upper,” Nigg says. More recently, however, designers have begun flipping that equation by placing more functionality into the top of the shoe. Traditionally, it's the sole's job to provide arch support. Here, Adidas's designers have assigned that task to the upper. It's a solution that's rooted in biomechanics, and one that Nigg calls "clever" in theory.

The shoe launches today, and Herath says the feedback supplied from the "constant testing loop" with female athletes has been positive. And it turns out, that might be as much feedback as we get: Nigg says time and research have shown that there's an inverse relationship between athletes' reported comfort levels and the number of injuries they sustain—an observation he says holds true for men and women.