Fart Garfunkel wrote:
I don't believe that "energy return" has a major contribution to running, but theoretically it could.
The primary goal of the shoe is to provide cushion damping shock from impact forces. To provide the cushioning, the shoe must deform and absorb the force of your weight landing.
"Energy return" is then any return force that comes from the elastic return of the shoe. This exists to some extent with every shoe. I think of it as any reduction of you sinking down into the foam of the shoe. It doesn't propel you at all, but over greater distances it can reduce fatigue.
If you don't believe in energy return at all, then why don't you just buy the mushiest shoes you can find?
The human body is already able to absorb the force of landing on its own. That's why the arch pronates and supinates and the elastic tendons stretch and rebound.
Adding to that by putting a spring under them is like attaching a rubber band to another one. You don't get a stronger rubber band, merely one that goes farther. The shoe creates a false elastic surface under the heel, increasing its vertical displacement and prolonging footstrike. As this new surface isn't perfectly efficient (drop a ball on a trampoline and see if it bounces forever!) it actually results in less energy return overall.
The greater displacement could accelerate the heel upward to a higher speed off the ground, but that doesn't matter and could even be detrimental. The heel lifts naturally on its own, as I already mentioned, and the muscles don't start applying a lot of force until it's high enough to raise the arch via the windlass mechanism. Before then, the arch is too flexible to efficiently transfer force from the leg to the ground.