Springolator mules were developed by Beth Levine for Herbert Levine in NY. SHe took the idea from an elasticized insole designed for workboots to provide comfort and fit, and applied it to mules because high heeled mules do tend to fall off, especially if the wearer is in a hurry! Mules were never big sellers in North America previous to the 1950s, they had only ever found favour in Italy and France.
I believe she first produced them in 1956 and first advertised them by running across the lobby of a hotel at a shoe manufacturer's convention wearing them! (I heard that story directly from the her mouth!) Apparently the other shoe manufacturer's were impressed that her mules had not fallen off! Because she borrowed the technology from another source, couldn't patent the idea, so many makers globbed onto the idea and by 1957, they were being made by numerous different makers. They were popular into the early 1960s.