Marathons are tough. Athletes push their bodies for miles and deal with cramping, dehydration and every runner's worst fear: that extreme form of fatigue called "hitting the wall." Why is distance running so difficult?
With the New York City Marathon kicking off this Sunday, Reactions runs through the science of distance running: why muscles burn, how sweat cools the body and the chemistry of runner's high.
Watch the video here:
Explore further: Mathematical model helps marathoners pace themselves to a strong finish