Also: Everyone should know this. There are still huge issues that need to be figured out before this becomes a commercial product. The 7% figure is bound to change depending on the speed you walk at (diff. speeds will likely require springs with diff. stiffnesses for optimal efficiency). Both Steve and Greg talked about future incarnations that could switch between multiple springs.
...and I didn't write that headline, or the dek (the thing under the head)
...and I didn't write that headline, or the dek (the thing under the head)