Upper
Everything above the midsole: mesh, knit, synthetic overlays, laces, and lining. It controls breathability, lockdown, and how roomy the shoe feels.
Reference
Running shoe glossary
Plain-language definitions for the specs, parts, and jargon you'll see on StackTracker and across brand sites, so you can compare shoes with confidence.
Upper + anatomyMidsole + cushioningSupport + stabilityOutsole + tractionRide + geometryFit + sizingTraining + racing rolesTerrain + listed specs
Upper & anatomy
How the shoe wraps your foot
Toe box
The front volume where your toes spread and flex. A narrow toe box can cause black toenails or numbness; a roomy toe box helps wide feet and long miles.
Heel counter
A firmer structure cupping the heel (sometimes external clip). It limits heel slip and adds stability without necessarily making the whole shoe a “stability” model.
Often paired with padding in the heel collar for comfort.
Gusseted tongue
A tongue sewn or bonded to the sides of the upper so debris stays out and midfoot wrap feels more uniform; common on trail shoes.
Knit / engineered mesh upper
Knit or layered mesh uppers stretch differently than stiff synthetics. They can improve comfort but vary in durability and rain absorption.
Midsole & cushioning
Foam, height, and how the shoe feels underfoot
Midsole
The foam (and sometimes plate or inserts) between outsole and foot. It drives cushioning, weight, durability, and how “soft” or “responsive” a shoe feels.
Stack height
Foam thickness underfoot, usually measured at the heel and forefoot (in mm). Higher stacks often mean more cushion and protection; lower stacks can feel quicker and more grounded.
Brand measuring points can differ slightly; compare within one site or review lab when possible.
Heel-to-toe drop (offset)
Difference in height between heel and forefoot stack (e.g. 8 mm drop). Higher drops load calves/Achilles less for some runners; lower drops ask more of calves but can feel natural for midfoot strikers.
Midsole foams (EVA, TPU, TPEE, A-TPU, PEBA & more)
Midsoles are gas-expanded polymers. EVA is ethylene-vinyl acetate; TPU is thermoplastic polyurethane; TPEE is thermoplastic polyester elastomer; A-TPU is aliphatic thermoplastic polyurethane, premium versus commodity TPU; PEBA-type materials are polyamide elastomers (Pebax® is a familiar trade name).
EVA (ethylene-vinyl acetate)
Brands lean on EVA as the workhorse foam in entry-level and mid-tier daily trainers, walking shoes, and lifestyle sneakers: cheap, easy to tune soft→firm, with decent vibration damping for easy miles. Trade-offs: modest rebound versus upscale elastomers, stiffens in the cold, and many blends pack out over hundreds of miles.
TPU (thermoplastic polyurethane)
Brands deploy commodity molded or sheet TPU in durable daily trainers and many stability shoes, where it steps up from EVA with better longevity and more consistent rebound. Trade-offs: denser than halo foams (cushioned builds add grams) and rides skew firm-to-lively rather than marshmallow soft.
TPEE (thermoplastic polyester elastomer)
Brands use TPEE in uptempo daily trainers and tempo/workout shoes: polyester elastomer chemistry tuned for energetic rebound and fatigue resistance without full supershoe pricing. Trade-offs: brand recipes vary widely; many implementations skew dense-lively or firmer rather than ultra-soft.
A-TPU (aliphatic thermoplastic polyurethane)
Brands use A-TPU as a premium foam in super trainers and select race shoes; aliphatic hard segments instead of the aromatic MDI-style chemistry of cheaper molded TPU give livelier rebound and cleaner aging. Trade-offs: cost jumps versus standard TPU, and shoes built around it are usually heavier than PEBA-class racers at the same stack height.
PEBA / Peba-type polyamide elastomer
Brands use PEBA-class foams as the headline material in racing supershoes and halo super trainers; polyamide block copolymer elastomers (Pebax® grades sit here) deliver the lightest, springiest rides on the market. Trade-offs: expensive, cosmetic creasing or localized wear can show faster, and tall compliant stacks want plates or sidewalls for stability.
Supercritical foaming (process)
Brands apply supercritical foaming to premium midsole compounds (often PEBA or A-TPU) for super trainers and racers to drop weight and tighten cell uniformity; supercritical CO₂ or N₂ is infused into the polymer then decompressed to nucleate fine microcells. Trade-offs: it is a process rather than its own chemistry, demands costly tooling, and marketing hype outruns identical formulas across models.
Energy return / compression
Marketing and lab language for how much foam rebounds after landing vs absorbing shock. Higher return can feel “springy”; softer foams may damp vibration better.
There’s no single consumer standard; use feel and reviews alongside specs.
Rocker / toe spring / meta-rocker
Curved sole geometry that rolls you forward, reducing work at the ankle and toes. Helpful for efficiency or stiff big toes; some runners dislike reduced ground feel.
Carbon plate (and plates generally)
A stiff layer (often carbon-fiber in racing shoes) paired with light foam to stabilize the midsole and improve turnover. Training plates may use plastic for durability or lower cost.
Dual-density midsole
Two foam firmnesses in one shoe, often firmer medially for stability or softer heel / firmer forefoot for transitions.
Sockliner (insole)
Removable foam insert under your foot. Swapping it can tweak volume fit slightly or accommodate orthotics.
Support & stability
Pronation, posts, and “neutral vs stability”
Neutral shoe
No deliberate extra posting or guide rails for inward rolling; cushioning is relatively symmetric. Suitable if you don’t need corrective guidance.
Stability / support shoe
Adds structure (firmer foam on the inside, rails, or wedges) to gently resist excessive inward roll (pronation). Mild stability feels close to neutral; max stability is rarer today.
Pronation
Natural inward rolling of the ankle through stance phase. Everyone pronates to some degree; shoes intervene only when movement is painful or clearly excessive for that runner.
Supination (underpronation)
Outward rolling of the foot away from the midline during stance, with more load on the outer (lateral) edge. High arches or limited inward roll are sometimes called underpronation; very stiff patterns can show faster lateral outsole wear.
Neither pronation nor supination is bad by itself; pain, uneven wear, or unstable landings are the reasons to adjust shoe choice or see a professional.
Medial post
Classic stability feature: a wedge of firmer foam on the inner midsole to slow inward collapse.
Guide rails
Firmer rim foam or plates at heel/midfoot edges that only engage when you stray toward unstable mechanics, often marketed as less intrusive than old-school posts.
Outsole & traction
Rubber, lugs, and durability where you touch the ground
Outsole
Rubber (or minimal rubber coverage) contacting pavement or dirt. Traction pattern and compound affect grip, noise, and wear life.
Lugs
Raised knobs on trail outsoles that bite into loose dirt and mud. Deeper lugs grip soft terrain better but feel clunky on roads.
Rubber compound / blown rubber
Rubber chemistry trades grip vs durability vs weight. Some road shoes expose foam without full rubber coverage to save grams.
Ride & geometry
How transitions and rocking shape each stride
Responsiveness
Colloquial for how quickly a shoe returns energy or snaps back, often linked to foam type, plate stiffness, and flex pattern.
Ground feel
How much road texture and proprioception you sense through the shoe. Lower stack and firmer foams increase ground feel; max-stack racers mute it.
Transition
How smoothly the shoe moves from heel/midfoot landing through toe-off. Rockers, plates, and flex grooves all shape transition.
Fit & sizing
Lasts, toe box, and picking the right size
Last
The foot-shaped form around which a shoe is built. Determines overall volume: narrow vs wide platform, arch height bias, and heel taper.
True to size (TTS)
Community shorthand for ordering your usual running-shoe size and expecting a reasonable fit, though it still varies by brand and foot shape.
Wide / 2E / 4E
Manufacturers offer wider lasts labeled Wide, 2E, 4E, etc. If the lateral pinky or bunions rub, width matters more than half-sizing.
Training & racing roles
Daily trainers, workouts, race-day shoes
Daily trainer
High-mileage workhorse for easy runs and general volume; prioritizes durability and comfort over absolute lightness.
Tempo / workout shoe
Snappier and lighter than most daily trainers for intervals, threshold runs, or fartleks, often less padded than race supershoes.
Racing shoe / supershoe
Purpose-built for PR attempts: light foam + plate, aggressive rocker. Less durable and sometimes unstable for daily miles.
Reserve for key workouts or race day unless your budget and ankles disagree.
Trail shoe
Protective upper, grippy lugged outsole, sometimes rock plate; intended for off-road surfaces.
Recovery / easy-day shoe
Soft, forgiving cushioning when legs are trashed, often overlapping with max-cushion road trainers.
Zero-drop / minimal shoes
Zero-drop means heel and forefoot are level (0 mm offset). Minimal shoes reduce cushioning or structure. Both demand gradual adaptation to avoid calf/Achilles overload.
Terrain & listed specs
Road vs trail and numbers you’ll see on product pages
Road vs trail designation
Road shoes prioritize smooth grip on pavement; trail shoes add protection and traction. “Door-to-trail” hybrids compromise both directions.
Listed weight
Manufacturer weight for a men’s or women’s sample size; real pairs vary slightly. Lighter isn’t automatically better if cushion or stability suffers.
Breathability vs waterproof (GTX)
Waterproof membranes block rain but trap heat. Breathable mesh dries faster in warm weather but soaks through in storms.