Foam is the whole story. Not the leather upper, not the rubber outsole, not the woven collar lining. The reason a modern running shoe or lifestyle sneaker feels like stepping onto a compressed mattress, while an older pair from the 1990s or early 2000s feels comparatively rigid and unforgiving, comes down to what is packed inside the midsole and how that material is made.
The midsole is the thick layer between the upper and the outsole. For most of sneaker history, that layer was made from EVA, ethylene-vinyl acetate, a foamed plastic that was cheap, light enough, and adequate. It did the job. Brands competed mostly on upper design and colorway, because the foam underneath was essentially the same across the industry: same process, similar density, similar feel. A Nike from 1997 and a New Balance from 1997 could look completely different and feel almost identical underfoot.
What Changed in the Foam Itself
The shift began in earnest in the early 2010s and accelerated sharply after 2013 or so, when Adidas launched its Boost midsole technology, made from expanded thermoplastic polyurethane (TPU) pellets. The pellets are fused under pressure and heat into a midsole that looks, from the outside, like a dense cluster of small white beads. That structure is not cosmetic. It changes the mechanical behavior of the foam entirely.
Traditional EVA foam is made by injecting a material into a mold with a blowing agent, which creates gas bubbles inside the foam as it cures. Those bubbles are irregularly shaped, and the walls between them can degrade fairly quickly under heat, compression, and UV exposure. Pack a pair of EVA-soled sneakers in a hot car trunk for a summer, wear them daily for eight months, and the midsole loses a measurable share of its original cushion. The foam compacts. It stops returning.
Expanded TPU works differently. Each individual pellet is itself a small closed-cell foam bead, and when the pellets are fused together, the resulting structure has a much higher capacity to store and release energy. The material also resists heat degradation better than standard EVA, which is why a pair of Boost-soled shoes worn for a year often still feels noticeably springier than a traditional pair worn half as long. The energy return is genuinely higher, and the softness does not bleed out at the same rate.
Thickness Has Climbed Alongside Chemistry
The material science accounts for part of the softness gap. The geometry accounts for the rest.
Midsole stack height, the vertical distance between the ground and the foot, has grown substantially since the mid-2010s. Lifestyle sneakers that once sat 20 to 25 mm off the ground now commonly run 30 to 40 mm. Running shoes that used to top out around 28 or 30 mm of cushion have crept past 35 and, in some maximalist models, past 40. More foam simply means more compression is available before the foot reaches a hard surface. Softness is partly physics: deeper foam cushions more.
This trend created an interesting aesthetic side effect. Chunky, thick-soled sneakers stopped looking like a niche running category and became the dominant silhouette across casual wear. When something feels better, people wear it more, and when people wear it more, the look normalizes. The stack height that once read as clinical running-gear excess now reads as standard proportions on a street sneaker.
Density Tuning and Zone-Based Cushioning
Modern midsoles are rarely a single uniform block of foam. Brands have learned to vary the density of foam across different zones of the same shoe, placing softer, lower-density foam under the heel where the foot first absorbs impact, and firmer foam under the forefoot where propulsion happens. Older shoes used a single-density midsole throughout, or at best a simple dual-density construction with a visible wedge of harder foam on the medial side.
The multi-zone approach means the underfoot sensation has become more calibrated. A step does not just compress evenly; it compresses more where the load is heaviest and offers more resistance where stiffness actually helps. From a pure perception standpoint, the shoe feels softer on impact and more supportive through the stride, and those two qualities used to be in tension with each other. More cushion meant less control. The density tuning has largely resolved that trade-off for everyday use.
The Plate Era and What It Added
Starting around 2017 and moving aggressively into mainstream models after 2019, brands began embedding stiff plates (typically carbon fiber in performance shoes, nylon or TPU in more casual models) inside the midsole. The plate’s purpose is to control how the midsole bends through a stride, storing energy at one point and releasing it at another. In performance running, the carbon plate in a shoe like Nike’s Vaporfly became famous for its role in marathon times. In everyday sneakers, the same principle filtered down with softer materials.
A nylon plate inside a thick foam midsole does something counterintuitive to perceived softness: it makes the shoe feel more cushioned, not less, because the plate prevents the foam from deforming in ways that reduce its energy return. Without any structural element, a very soft foam midsole can feel pillowy at first but mushy and unstable under sustained weight. The plate stiffens the bending axis while leaving the compression axis open, so the foam does what it is supposed to do without bottoming out or twisting unpredictably. Softness becomes controlled softness, which reads as premium.
Why Older Shoes Feel Hard by Comparison
Pull out a pair of athletic shoes from the mid-1990s, even a well-preserved pair, and the difference is immediate and a little startling. The midsole feels almost boardlike. Some of that is age: EVA degrades over decades regardless of use, and foam that sat in a box through multiple summers has already lost compression capacity. But even accounting for age, the original specification was simply firmer. The design assumptions were different.
Running science in the 1980s and 1990s was deeply concerned with motion control and overpronation, the inward rolling of the foot after heel strike. The dominant theory held that excessive pronation contributed to injury, and the industry response was to build firmer, more structured midsoles that resisted foot motion. Stability features like medial posts (denser foam wedges along the arch side of the midsole) were standard on most mid-range running shoes. The result was cushioned by the standards of its time but controlled in a way that sacrificed softness.
That design consensus has largely reversed. Research over the past two decades has complicated the straightforward pronation-injury link, and the industry has moved toward more flexible, less structured designs that allow the foot to move more naturally. Softer foams became acceptable partly because the obsession with controlling foot motion loosened. The midsole could prioritize feel over restriction.
The Brands That Still Make Hard Shoes, and Why
Firmer midsoles have not disappeared. A handful of brands, particularly those selling to runners who prefer ground feel and proprioceptive feedback, deliberately keep their foam dense and their stack heights low. Some trail running categories favor firm, protective midsoles because the irregularity of terrain makes a highly compliant foam unpredictable and potentially destabilizing on rocks or roots.
Court shoes for basketball and tennis have also stayed comparatively firm, though the gap has narrowed. Lateral movement and cutting put different demands on a midsole than straight-ahead running. A very soft, thick foam midsole that feels incredible on a morning walk can actually compromise stability during a sharp lateral cut, since the ankle is fighting to find solid ground through an inch and a half of compliant material. Most basketball shoes are still noticeably firmer than their running counterparts.
Those exceptions clarify something important about the softness trend: it is not universal, and where it has taken hold most aggressively is in the exact categories where softness is least likely to cause problems. Straight-line running and casual everyday wear are forgiving environments for compliant foam. Categories that demand lateral stability have held back.
What the Foot Actually Registers
There is a sensory dimension to this that the material specs alone cannot fully capture. The plantar surface of the foot is densely packed with mechanoreceptors, sensory nerve endings that respond to pressure, texture, and deformation. When a shoe places a thick layer of soft foam between those receptors and the ground, the signals they receive become muted and delayed. The foot is, in a real sense, receiving less information about the surface it is walking on.
Most people find this pleasant. Fewer sharp signals means less discomfort, and modern life involves a lot of standing on hard tile and concrete. The sensory attenuation that comes with a thick foam midsole is part of what people actually mean when they say a shoe feels comfortable. But a smaller group of wearers, and many podiatrists and biomechanics researchers, point out that some of that ground-feel is functionally important. The foot adjusts its mechanics in real time based on what it senses, and reducing that input changes how the foot, ankle, and knee respond to each step. Whether that matters for an ordinary person doing ordinary things is genuinely unsettled territory. For high-mileage athletes, the conversation is more active.
A shoe from 1994 transmitted the sidewalk directly. A modern maximalist shoe transmits an interpretation of it, filtered through 35 mm of engineered foam. The 1994 shoe hurt more. The modern shoe tells the foot less. Both of those facts are true, and the industry has placed its bet on which one consumers care about. So far, the cushion is winning, one compressed EVA cell at a time.