The eternal question of how long your running shoes should last became suddenly topical a few months ago, when Tigst Assefa set a world marathon record in what Adidas billed as essentially a ...

These shoes, which have fully entered the mainstream of the running world, are expensive. The Nike Alphafly 2 shoes currently retail at $285. The latest Vaporfly shoes are “just” $260.

Klein, however, is concerned about the long-term consequences of the true stars of super shoes—the midsole foam. Fly Now, Pay Later? “My big question has to do with tendons,” Klein says.