Why do marathon runners suddenly hit the wall?

Marathon runners hit the wall because they run out of glycogen, the sugar fuel stored in their liver and muscles. When those stores are empty, fatigue arrives suddenly and energy drains away. In a marathon it typically happens around kilometre 30, which is why so many races fall apart in the last quarter.

The maths is brutal. An untrained person on a normal diet stores about 380 grams of glycogen, roughly 1,500 kilocalories, and not all of it is available to the running muscles. Hard running can burn 600 to 800 kilocalories an hour or more. Without refuelling, those reserves are gone after about 24 kilometres of running, well short of the 42 km finish.

The body has another fuel, fat, but the harder you run, the more of your energy comes from glycogen. When the glycogen runs out, the muscle cells run low on ATP, the molecule that powers them, and the heart races to try to deliver more fuel.

Runners fight back in three ways: they start full, with training and big carbohydrate meals like the classic pasta dinner the night before, which can raise glycogen stores to as much as 880 grams; they eat or drink carbohydrates during the race; and they pace themselves so they burn less glycogen. German athletes have the best name for what happens if you don't: der Mann mit dem Hammer, the man with the hammer, who catches up with you and strikes.