When soccer star Oguchi "Guch" Onyewu ruptured his patellar tendon last October during the United States team's game against Costa Rica, it seemed unlikely that the defender would recover in time for the FIFA World Cup in June. But recover he did, helping the U.S. tie England 1-1 on June 12. Sean Whalen likes to think that his company, AlterG, which makes the anti-gravity treadmill that Onyewu used for rehab, had something to do with the win. "To recover from that in seven months, and be fit for the World Cup, is simply an amazing feat," says Whalen. "To us, it really validates the power and effectiveness of our technology."

AlterG owes a debt of gratitude to Whalen's father, Robert, a former NASA research scientist who built the first prototype of the anti-gravity treadmill when his son was just eight years old. "Everyone who walked past our house thought we were building a hot tub," recalls Whalen. Robert Whalen's original idea was to build an exercise machine that astronauts could use in space, but it wasn't until Sean began a masters program in entrepreneurship at Stanford in 2004 that father and son began exploring commercial possibilities. The treadmill features a pressurized chamber that encloses the lower body and enables a user to walk or run at just 20 percent of his or her actual body weight.

Through friends, the Whalens contacted former marathon runner Alberto Salazar, who now trains elite runners through the Nike Oregon Project. Salazar was so impressed with the treadmill that he put up $15,000 to help the Whalens refine the machine. They raised another $300,000 from friends and family. A year and a half later, the treadmill was ready to be tested. Salazar bought three for the Oregon Project, and the Washington Wizards provided AlterG with its first sale to a professional sports franchise, followed by the Golden State Warriors and the Oakland Raiders. "We looked to sports as a high-visibility way to get the product out there," says Whalen. "But medical is the big market for us."

Last October, AlterG launched a new product for the medical market and sold 30 of the treadmills for an average price of $28,000, compared with $75,000 for previous models. Just last month, the company's physical therapy product won a Medical Design Excellence Award.

The company has also caught the eyes of venture capitalists. A firm called Red Planet Capital invested $2.5 million in 2006 and, three years later, Versant Ventures invested another $8 million.

Whalen, who is now CTO, says AlterG doubled sales last year and is on track to triple them this year. "From the beginning, we made a conscious decision to start a big company," he says. "We see both acquisition or going public as options. We have first mover advantage."