The wall is a mechanical event we keep describing as a fuel problem

Runners slow badly in the final quarter of a marathon even when perfectly fuelled, because the legs stop returning energy long before the tank empties.

Carbohydrate is not the only reason people fall apart at thirty two kilometres. It is the reason we all repeat, because it is measurable and because there is an industry attached to it, and it is genuinely part of the picture. But feed a runner properly and time him honestly and he will still slow down. Something else is failing.

Look at what changes in the stride over the closing quarter. Contact time lengthens. Vertical oscillation goes up. The knee collapses a little further on landing and takes longer to come back. The runner is spending more energy per stride to cover the same ground, so the pace that felt sustainable at halfway now demands an intensity he cannot hold, and he drops to an effort he can. Nothing in that story requires an empty liver.

The cause is accumulated eccentric damage. Every landing asks the quadriceps to absorb load while lengthening, which is precisely the loading pattern that tears sarcomeres, and thirty thousand of them add up. The muscle loses some of its ability to store and return elastic energy. Running economy degrades by a few percent, and a few percent across the last ten kilometres of a marathon is several minutes.

If that is right, a lot of marathon preparation aims at the wrong system. The long run gets treated as an aerobic session with fuelling practice bolted on. It should also be a durability session, and durability is trained by loading the muscle the way the race will load it, which means running the closing miles at goal pace on tired legs rather than jogging them out.

Downhill running is the other underused tool and the one people fear most. Controlled, gradual, introduced early in a block, it produces exactly the damage that confers protection later. It also produces two days of misery, which is why almost nobody prescribes it.

I hold one further opinion that gets me into arguments. I think the plated shoe helps here more than anywhere else. Not because of any energy return story on a graph, but because a softer, better damped landing lowers the muscular cost of absorbing thirty thousand impacts, and the runner reaches thirty five kilometres with more leg than he used to have.

Fuel gets you to the last hour. Your quadriceps decide what happens inside it.