When to replace running shoes

Almost everyone replaces running shoes when the outsole looks worn, and the outsole is the last part to fail. Rubber is durable, easy to inspect and slow to change, which makes it a comforting indicator and a useless one. The part that decides when a shoe is finished is the midsole foam above it, and foam does not announce itself.

What happens is mechanical rather than cosmetic. Foam cells compress and progressively stop rebounding, so the shoe returns less energy and, more importantly, absorbs less impact. The change is gradual enough that you adapt to it without noticing, which is why the first symptom is often not the shoe feeling different but your legs feeling worse after a run you have done a hundred times.

Why a single mileage number is a guess

Mileage ranges are the usual advice and they are a blunt instrument, because the range depends on the foam. Firmer, denser foams commonly last 700 to 800 kilometres. The lighter, softer, more responsive foams in modern trainers can be noticeably flat by 400. A heavier runner compresses foam faster than a lighter one. Someone who runs mostly on concrete wears shoes faster than someone on gravel paths. Any single number that ignores all four variables is a guess wearing a lab coat.

Three checks that beat counting kilometres

Three checks work better than a mileage count. Press your thumb hard into the midsole beside the heel: fresh foam resists and springs back, tired foam feels dead and stays dented for a moment. Put the shoes on a flat surface and look at them from behind: a midsole that has collapsed to one side is finished regardless of what the tread looks like. And note how your legs feel the morning after a familiar run — a new soreness in the shins or knees with no change in training is a foam signal more often than an injury signal.

There is a fourth, cheaper method, which is to write the date on the inside of the tongue in permanent marker when you first wear them. Not the mileage, the date. Most runners have a fairly stable weekly volume, so the date tells you what you need to know without logging anything, and it removes the temptation to negotiate with a number.

One thing not to do: keep a dead pair for short runs. The shoe is not less worn out because the run is shorter, and short runs are where most people run fastest.