What the Shoe Actually Does
Running is a repeated cycle of storing and returning energy. On each step the leg and foot absorb the impact of landing, storing some of that energy elastically in tendons and muscle, and return part of it during push-off. The rest is lost, mostly as heat, in tissue and in whatever the runner is standing on. A traditional midsole foam absorbs a meaningful share of that energy and gives back relatively little, which means the shoe is a small but persistent leak in the cycle.
The new foams changed that. They are far more resilient, returning a substantially higher proportion of the energy put into them, and they can be made thick without becoming heavy. Thickness matters because a deeper layer of compliant material can absorb and return more energy per step than a thin one. The combination, a much better return rate applied over a much greater depth, reduces the energy a runner spends to hold a given pace. That is running economy: the metabolic cost of moving at a particular speed. Improving it does not make anyone able to run faster than their physiology allows. It makes their existing physiology reach further, which over a long race is the more valuable of the two.
