Biology
What happens to marathon runners' bodies at mile 20?
Quick fact
Hitting the wall happens when your body depletes its liver glycogen stores, starving the brain of glucose and forcing it to switch to emergency metabolic pathways.
Why this is interesting
At mile 20, your muscles still have enough oxygen, but your brain runs out of its preferred fuel—triggering a sudden, overwhelming urge to stop.
Read the full explanation
Understanding What happens to marathon runners' bodies at mile 20?
Glycogen is stored in the liver and muscles. The liver supplies glucose to the brain. When it drops below a critical threshold, the brain perceives a life-threatening energy crisis and drastically reduces muscle activation to force you to slow down.
A deeper explanation
This isn't just fatigue—it's a central nervous system shutdown called 'central governor theory.' The brain intentionally reduces motor neuron firing rates to protect itself from glucose starvation. At the same time, it releases neuropeptides that amplify feelings of effort and pain, making every step feel like wading through concrete. Interestingly, elite runners 'hit the wall' not just because of glycogen depletion, but because their brain anticipates the depletion ahead of time—proving that endurance is as much a cognitive challenge as it is a cardiovascular one.