Running shoes are a foam, a last and a rubber compound

This is written to be used rather than admired. Each section below is a decision about athletic footwear construction, and each one has a default.

The midsole is most of the product

The midsole is the foam slab between your foot and the ground, and it performs almost all of the cushioning and energy return a shoe offers. Foam works by compressing air out of a cellular structure, absorbing energy as it deforms and returning a fraction of it as it recovers.

Density decides firmness, while the polymer chemistry decides how much energy comes back rather than being lost as heat inside the foam. Thicker foam allows more travel and therefore more cushioning, at the cost of raising your foot further from the ground and reducing lateral stability. Because the midsole cannot be replaced, its ageing sets the useful life of the shoe regardless of how the upper and outsole still look.

How foams age and why the feel changes

Repeated compression gradually collapses the cell walls, a process called compression set, and the foam recovers less of its original thickness each time. Ageing continues in storage as well, since the polymer network relaxes over time even in a box that was never opened.

Lighter, more responsive foams often use chemistries more sensitive to heat and moisture, so the softest ride is not usually the longest lasting. The change is gradual and largely invisible, which is why people notice a shoe has gone flat only after a period of unexplained discomfort. Alternating between two pairs gives each midsole recovery time between uses, which is a habit with a physical basis rather than a marketing one.

The last decides how a shoe fits

A last is the three-dimensional form the shoe is built around, and it sets volume, width, toe box shape and heel dimensions together. Two shoes labelled the same numeric size can fit entirely differently, because sizing describes length while the last describes the whole shape. Foot volume matters as much as width, since a high-instep foot can be uncomfortable in a wide shoe built on a shallow last.

In the small print, feet swell during activity and across the day, so the fit that feels correct in a shop is not the fit you will experience under load. A maker who reuses one last across a range gives you useful information, because a fit that worked before will probably work again.

Outsole rubber and the abrasion trade-off

Softer rubber compounds deform into surface texture and therefore grip better, while wearing away faster because that same softness allows material to be removed. Harder compounds last considerably longer on abrasive surfaces but transmit more vibration and grip less confidently on smooth wet ground. Many shoes use both, placing hard compound where wear concentrates and soft compound where grip is needed, which is visible as differently coloured sections.

Tread depth and lug spacing matter on soft ground and matter very little on pavement, where contact area does the work instead.

Exposed foam on the outsole saves weight and wears through quickly, which is a deliberate compromise rather than an oversight.

Uppers, and what holds the foot

The upper's job is to hold the foot on the platform, so its stiffness and structure matter far more than the material's name. Knitted uppers conform to the foot and breathe well while offering little lateral support unless reinforced with taped or welded overlays.

Judged against the category, the heel counter, a stiffened cup around the heel, controls how much the rearfoot can move, and its rigidity is easy to test by hand. Waterproof membranes reduce breathability, which is a reasonable trade in cold wet conditions and an uncomfortable one in warm weather. Seams and glue lines are the usual failure points, especially where flex concentrates across the ball of the foot.

Judging a pair without a laboratory

Handle the shoe: flex it, twist it, press the heel counter and squeeze the midsole, since all four tell you about structure directly. Weight is worth checking on a scale, because published weights are quoted for one particular sample size and rarely for yours.

On the bench, try shoes at the end of the day and with the socks you will actually wear, so the fit test happens under realistic conditions. Ask what the previous version was and whether the last changed, since a mid-life revision can alter fit without altering the name. Where a shoe is bought for a specific activity, the relevant evidence comes from people doing that activity over months rather than from a first impression.

The takeaway

Judge a shoe by the foam under it, the shape it was built around and the rubber on the bottom.

Anything you cannot buy a spare part for is a rental with a long term.