On January 31st, World Athletics released new technical rules officially qualifying what a 'legal' running shoe is. A host of brands, led by Nike with its Vaporfly series, have forced the hand of the regulatory agency by developing shoes containing new types of lightweight foam and shock-absorbing carbon plates that function like springs. These types of shoes have been proven in studies to improve running economy over conventional marathon racing shoes. 

Because Nike has led the development of these shoes, their sponsored runners have enjoyed a relative advantage over their competitors dating back to the 2016 Olympic Trials, where 4th place finisher (Sketchers-sponsored) Kara Goucher compared the Vaporfly shoes to a doping advantage. Distance-running scientists weighed in on the resulting kerfuffle, proposing limits on sole height to future-proof the stacking of carbon plates resulting in bigger and bigger springs. 

It appears that World Athletics agrees: they've ruled that no more prototypes are allowed in competition; shoes must have a maximum sole height of 40 millimeters; and they may have no more than a single carbon plate. Those rules allow runners to wear Nike's popular Vaporfly 4% and Next% (pictured) shoes, but it appeared they would ban their Alphafly shoe, last year's prototype worn by both Eliud Kipchoge (1:59:40) and Brigid Kosgei  (2:14:04) when they shattered record marathon times in Vienna and Chicago. However, upon official measurement the Alphafly just barely cleared the height limitation at 39.5 millimeters. 

Shoe brands will continue to innovate with new materials and designs, but whether they manufacture exclusively within those limits from a commercial aspect remains to be seen: is there a market for shoes that may not be competition-legal, but that could allow amateur athletes to reach the performance levels of more gifted athletes? That is a question that science cannot answer.