fisky wrote:
Even a slight deviation could negate the benefits. Energy released before the foot reaches push off position is lost. Energy released after the shoe leaves the ground is lost. Energy released at the wrong point on the shoe is reduced.
All the acceleration in running comes from the body pushing against the plant foot. If a spring or whatever energy restorer pushes the plant foot upward or forward, the muscles pushing the body away from the plant foot must exert extra force to compensate, otherwise the foot would simply move closer to the body. It is not a free bounce.
The forward or upward motion in the toe of any shoe - the last point of contact - is insignificant in any case, and I don't believe any shoe could time its springiness precisely enough to make any difference. Bounciness in the heel could possibly relieve fatigue in some muscles, but I doubt any shoe does that. A certain well-tuned track might, on which MD runners with a particular form are able to set records. But this relies on adapting to a prolonged footstrike and lower cadence, and I think it's useless over long distance and harmful to sprinters.
The world record for a mile on a pogo stick is over 10 minutes. Running isn't about bouncing.