Why do runners hit 'the wall'?

Answer: Glycogen stores deplete

Muscles run out of oxygen — Wrong. In a distance race you're running well below sprint pace, so oxygen delivery isn't the limiting factor; the wall comes from fuel, not air.

Glycogen stores deplete ✓ — Correct! The wall, often called bonking, is the sudden crushing fatigue that hits around 18 to 20 miles of a marathon. Your muscles normally burn glycogen for quick energy along with some fat, but once glycogen runs out you're forced to rely mostly on fat, which produces energy far more slowly. Your brain also runs on glucose, so low blood sugar brings on fog and weakness. That's why runners carb-load before a race, take energy gels along the way, and train with long slow runs to get better at burning fat.

Lactic acid accumulates — Wrong. Lactate clears from the blood fairly quickly and isn't what limits you in a long endurance effort; the wall is glycogen depletion forcing the body onto slower fat metabolism.