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In the four decades since the invention of the running shoe, most of the industry’s attention has focused on the midsole. After all, conventional wisdom maintained that the sole determines how the foot interacts with the ground, cushioning and controlling as necessary to protect and enhance your stride. Differences in sole density, shape and materials have defined the categories we talk about to differentiate running shoe models.

Recently, however, voices in science and medicine have questioned whether our prescriptions based on soles and the devices we build into them actually work. A new model posits that the best way to choose a shoe is to find one that feels the most comfortable throughout the stride, enhancing and supporting your natural, preferred movement path.

With the role of the sole in decline as the single distinguishing factor between running shoes, the upper is receiving increasing attention — and rightfully so. In his on-foot shoe testing at his Heeluxe lab in Santa Barbara, Geoffrey Gray has observed that uppers affect function far more than previously understood. Given two shoes that have identical midsoles, outsoles and insoles, but different upper shapes, he finds, “People will always identify that the fit is the same, what they identify is different is flexibility, cushioning, stability.” And its not just perception: A 2017 study out of Brazil revealed that different shoe uppers created more consistently-measurable changes in runner’s biomechanics than different midsole materials.

Holistic Comfort

“The upper is about fit,” says podiatrist, biomechanist and shoe consultant Simon Bartold. “Which means it is incredibly important, because fit equals function. If it doesn’t fit properly it doesn’t work properly.” Jonathan Tiepan, Senior Manager, Global Footwear Product Line Management at Brooks, adds, “The upper might have more to do with your preferred motion path than the bottom.”

Designers are increasingly realizing that the shoe works as a unit. “You can’t look at the bottom as completely separate from the upper,” says Dave Dombrow, experienced running shoe designer and Co-Founder at Speedhack. “How you solidify some zones, how you let other zones stretch, and how that interacts with the forces on the bottom — it’s all one thing.” Kurt Stockbridge, Footwear Development Vice President at Skechers Performance, agrees. “It’s much more holistic than people know,” Stockbridge says. “It’s not about this and that, but how everything comes together. Comfort: you can’t define it — it is everything.”

Moving Parts

At the very basic level, the upper’s task is simple. “The primary purpose of an upper, where it can’t fail, is to hold you on the footbed,” says Stockbridge. “The rest is all bonus. It should contribute to doing that in the most comfortable way possible.”

Complicating this seemingly simple task is the fact that the shape of the foot changes as it moves through the stride. The midfoot arches and elongates with each step. The ball splays and flexes. The toes lengthen. Muscles all around the foot contract and relax. “The variables are just massive,” says Tom Garza, Product VP of Global Footwear at 361°. “There are a ton of moving parts, and the foot operates completely different for everybody. How do we allow mobility to occur, but provide structure at the right places?”

The first key to creating this dynamic fit is to know where you want to hold and where you want to give. Experts and designers agree that primary location you need control is in the midfoot. “If you can lock down the fit over the navicular — the instep — everything else kind of works,” says Gray. Stockbridge defines the key location more precisely as just behind the ball of the foot. They point to a sandal or flip-flop strap as a simple example of where the foot needs support to keep aligned on the sole during stance and push-off.

Importantly, however, this hold cannot extend forward around the ball of the foot, which needs space to expand and bend. Allowing more freedom for the front of the foot is one of the positive changes that came out of the minimalist movement. “If you compare shoes from a decade ago to now, one of the biggest changes in the upper is the fit from right behind the met-heads up through the toes,” says Spencer White, VP of Saucony’s Human Performance & Innovation Lab. “There’s more room there, and that’s a good thing. It never needed to be as restrictive as it was. It made the shoe less comfortable, and for some people, having that grip made it impossible to run in the shoe.”

Altra’s foot-shape design has taken this trend to its logical end, and co-founder Golden Harper believes people are beginning to understand the difference needs in shoe tightness during running compared to other athletic pursuits. While a basketball shoe, for example needs to lock your foot down to keep you from rolling off during violent lateral movements, running is different. “It is not the same movement,” Harper says. “Running is about efficiency, relaxation, forward movement.”

Nothing is Standard

Beyond knowing where to hold and where to relax, good fit is further complicated by the huge variety of foot shapes, even at rest. Arch shape, instep height, toe length, heel width — all of these and more vary from runner to runner. White points out, for example, that the location of the flex point along the length of the foot can vary by up to 10mm for the same size foot, so designing a controlling strap to fall precisely behind that point can be problematic.

Designers use multiple strategies to create an adaptive fit that can accommodate the spectrum of feet. Laces are the traditional solution and continue to work admirably. “You have the ability to do almost an infinite number of different combinations that somehow will make the shoe fit for you,” says Bartold. Companies continue to refine lace thickness, stretch and friction, along with the placement of eyelets to create the desired hold in the proper location. The most extreme example is Puma’s Netfit design that allows laces to connect and wrap anywhere on the upper.

Some are playing with independently moving sections of the upper to adapt to and support varied regions of the foot. Astra’s Provision 4, for example, features an interior arch wrap snugged to the foot by fingers that each tie separately into the laces. Illustrating the new ascendence of the upper over the midsole, Altra is promoting this as a , designed to “establish a connection between mind, body and feet which encourages a natural foot placement in each step.”

Others are using stretchy materials to adapt to different foot shapes, providing comfort at the cost of snugness. “Stretch makes it easier, like sweat pants,” says Gray. “But a tailor can take normal pants and make them fit right, and you can move.”

Vive la Différence

Those tailored pants, however, would only fit and move on you, not on your neighbor. In the end, no matter how adaptive uppers become, the true source of fit comes from the shape, which is designed by the last the shoe is built upon. “Some adaptive fit strategies can accommodate a lot of foot types, but certain foot types are not going to fit certain lasts,” says Brice Newton, Director, Global Footwear Merchandising at Brooks. “It’s why we have more than one shoe. Everybody is shaped differently.”

Gray agrees, and thinks companies and retailers should celebrate the unique fit of each brand. “Not everybody likes to fit the same,” he says. “This is where there is opportunity for brands to have a signature fit. Rather than trying to fit everybody, good brands know, ‘This is our customer, this is our fit, this is what we’re going to make.’”

Boa is working with several companies to create uppers that adapt to your foot shape and then cinch down precisely with their cable-and-dial system. “What are the parts of the foot that want that feel of being glued down, and what parts want freedom,” says Clarke Morgan, Run & Athletic Fit Specialist at Boa. The answer to that will vary for each brand, and for a relaxed training shoe compared to a sprint spike. “Each shoe should be different, based on use case,” Morgan says.

One area where we’re seeing a more tailored approach is around the heel, which designers identify as the second-most important location for a good fit. “If you can secure the heel, it acts as a guide for the rest of the foot,” says Dombrow. Using apparel industry techniques and materials, Under Armour is playing with contour and geometry to surround the heel for a secure and comfortable hold.

Many brands are moving away from traditional puffy, foam collars and rigid heel counters and instead molding a heel shape out of thinner materials that conform closer to the foot, eliminating gaps and slop. “The shoe and foot are moving together. It gives you that extension of your foot to the ground,” says Heather Pieraldi, Head of Running Footwear at Salomon.

U-Curve of Support

Just as one “perfect” upper won’t fit every runner, neither can one upper construction work for every shoe in the product line. The level of desired control or freedom varies with the type of shoe.

“One of the focuses we have is how we can marry the shoe to the foot better,” says Colin Ingram, Director of Product at HOKA ONE ONE. Ingram points out how the right upper complements the sensation of the sole: a plush upper can enhance a soft sole, for example, while it might ruin the fast feel of a racing flat. “We’re trying to get more to the full, organic softness or firmness, holistically across the shoe,” Ingram says.

As designers discuss models in their line, one notices a u-curve of desired support along the “more shoe” to “less shoe” axis. Heavier shoes, designed for runners who have excess foot movement, need a more controlling upper. Lighter, less structured shoes tend to have uppers that allow the foot to move more—until you cross into racing shoes, where you again find more controlling uppers with tight, non-stretchy fabrics designed to eliminate slip and transfer all power to forward momentum.

White points out that this variation not only reflects desired feel but the reality of forces being created by different soles. “The more flexible the sole is, the more it moves with your foot, and the less structure you need in the upper to hold your foot on,” he says. Their highly-flexible PWRRUN+ sole, for example, compresses and conforms on landing, rather than torquing, and the sole stays with the foot during the toe off, which allows for flexible, free upper designs.

Trail shoe uppers have the same design challenges as road models, with the added complication of needing to hold the foot on the sole despite lateral forces from uneven terrain and sharp changes in direction. Plus, trail uppers need to protect feet from a greater array of elements and physical dangers. Innovations include the introduction of Kevlar fibers into upper weaves for durability, and a new waterproof membrane technologies that are bonded to the shoe’s upper so they don’t affect fit and flexibility nearly as much as the old inner “bootie” construction.

Image is Everything

Not to be overlooked is the role the upper plays from the moment the customer sees a shoe. “The upper is also about looks and signaling,” says David Allemann, co-founder of On shoes. “We always talk about function, but there is a psychological factor as well — what does it do for your perceived notion of how the shoe runs.” A well-designed upper will not only be congruent with the sole and its function but also in what type of psychological profile it signals with its appearance. “Even before they run a step, even in their hands, people are making a judgment,” says White.

Knitting and engineering processes that reduce the need for overlaid layers are also helping to create cleaner designs that match the demand for product that doesn’t scream “running shoe” but can be worn all day. At the same time, designers have more freedom and ability to create uppers that say “plush” or “fast” or “tough.”

Letting the Foot Lead

More than anything, shoe designers are recognizing how every thread in an upper can affect the entire feel of a shoe, and new materials and processes are allowing them to focus on each individual thread. Additionally, the focus of upper design is increasingly moving away from control to functional comfort. “We want it to be as intuitive as possible — that’s how it is supposed to feel, that’s what my foot is shaped like,” says Ingram. “We want the best foot hold possible while allowing natural motion, to allow your foot to be a foot.”

As designers play with new materials, construction and geometry, fit and function are moving into a new era. “We’re now speaking to what the foot really wants. It’s about proper movement, being efficient, and not restraining the foot,” says Garza. “It’s going to be a very beautiful experience.”