3D Systems recently assisted Loughborough University in the UK to produce highly customized shoe soles for olympic athletes. Researchers observed that the margin of victory in recent Olympic track events has been vanishingly small, sometimes as low as 0.01 seconds. You can't get much closer than that.
But a close margin of victory means that even the smallest advantage could be parlay a second place finish into first place. Thus a detailed investigation of longitudinal bending stiffness took place, with many trials. Evidently it is actually possible to "double the amount of mechanical energy they generate at the ankle."
The benefit of this approach, of course, is to manufacture shoe soles that are specifically designed for the individual runner to maximize their performance.
Thanks to Julie Graham of 3D Systems