PITTSBURGH -- Some consider 20-year-old Fred Grzybowski the best pogo-stick rider in the world, able to leap over a minivan, among other feats. But his days on top may be numbered.

This past weekend, at Pogopalooza 6, the world championship of the fledgling sport of extreme pogo, a pair of "xpogo" athletes who are 15 and 16 years old beat him in key competitions and set high-jump records.

"They're...