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In 2018, runners wearing the new Nike Vaporfly 4% running shoe broke world records over the 100 km, marathon, half-marathon and 15 km distances. This shoe has advanced the benefit of footwear further and more substantially than any other previous shoe. When Nike compared its energetic cost (running economy) to contemporary elite racing shoes, the Vaporfly provided a 4% improvement in economy (hence the shoe’s moniker) and an estimated 3.4% increase in running speed.1 Subsequent independent laboratory testing2 and big-data performance analyses3 have corroborated the benefit.
The International Association of Athletics Federations (IAAF) rule 143.2 stipulates that shoes ‘must not be constructed so as to give athletes any unfair assistance or advantage’.4 While the Vaporfly gives runners an advantage, it does not violate current IAAF rules. However, many in the running community debate whether this shoe blurs the line between physiological and technological performance, and many have called for more explicit regulations.
Can we untangle the 4%?
The Vaporfly deviates from conventional running shoes in three ways: (1) an embedded carbon-fibre plate, (2) its midsole material, and (3) its midsole thickness (figure 1). Each of these components has design features that reduce energy loss in isolation and, perhaps more-so, in combination.
The full-length embedded carbon-fibre plate increases the longitudinal bending stiffness of …
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