What long-distance running does to the body

Energy, oxygen and water are all needed to keep slogging through a marathon. But did you know fatigue is caused by your cells struggling to keep up waste disposal? 

Lactic acid – which builds up when your cells inefficiently break down glucose – doesn’t directly make your muscles sore. But it does increase acidity inside cells and they don’t work as well.

That feeling that your legs are on fire? That’s your brain telling you to slow down.

See the video above, produced by the American Chemical Society’s Reactions team, for more on the science of distance running.