Athletes, rejoice: We’ve officially entered marathon season. The New York City Marathon took place last Sunday, and in upcoming weeks, race participants will pound the pavement in Las Vegas, Brooklyn, Philadelphia, and Seattle. Months of training aside, how are these competitors capable of logging so many miles? The American Chemical Society’s latest Reactions video explains what happens to the human body on a chemical and cellular level during a long-distance running session—from the reason your legs sometimes burn to what's behind the highly sought after runner's high.

[h/t Reactions]

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