In 2019, long-distance runner Eliud Kipchoge ran the marathon in under 2 hours, shattering all known records, wearing Nike Vaperfly shoes. Soon people wondered how much Kipchoge’s triumph had to do with his running talent versus his running shoes.

Modern running shoes have three main parts. The upper needs to fit securely, the sturdy outsole hits the ground, and the midsole is where all the action, and controversy, happens. Learn about energy return in the midsole and why some believe it gives runners an unfair competitive advantage.