I'M ON PACE to run a three-minute mile, which will obliterate Moroccan Hicham El Guerrouj's world record of 3:43. And I'm barely sweating. How is this possible for an every-other-Sunday jogger who is usually psyched to run a nine-minute mile? I'm harnessed into the new G-Trainer, a high-tech treadmill that's suspending me on a cushion of air, allowing me to run harder and with less effort. Eat my waffle soles, Guerrouj!

The G-Trainer was developed by Alter-G, a company that's been working with elite coach Alberto Salazar. Spun off a NASA design that was created to help astronauts combat muscle atrophy, the G-Trainer works in the opposite manner: It lightens your load instead of adding weight. The result is that the shock of each stride is drastically reduced. The decreased impact is a boon for runners who batter themselves, and it supposedly cuts injured athletes' rehab times by as much as half. Running without gravity doesn't sacrifice your cardio workout, either: Jog four miles at 75 percent of your body weight and you get about the same workout as you would over three fully loaded miles, with a lower chance of injury.

The G-Trainer is one of those remarkably simple contraptions that makes you wonder why no one thought of it sooner. You pull on a pair of neoprene shorts embellished with a flange of fabric and zip into an inflatable bag that buoys you above the whirring treadmill. A bank of controls allows you to offset up to 80 percent of your body weight. Running on it was a bizarre sensation at first, like I imagine jogging on the moon must feel like. But soon, striding along at half my weight, I was cruising at world-record pace (did I mention that yet?) and awaiting the day I could set one up in my personal gym.

That might take a little while. At $75,000 a pop for this first-generation model, the G-Trainer is not cheap. There are just 40 units currently in use nationwide (mostly in exclusive gyms and military training centers), so access is an issue. But if antigravity running's popularity among world-class athletes is any indication, the G-Trainer should be a mainstay in every gym in the country within a few years: When I arrived at Alter-G's Menlo Park, California, headquarters, I had to wait my turn behind Bryon Friedman, a U.S. Ski Team racer who shattered his leg in January 2005. "I really haven't been able to run at all, until now," he said as he cruised along. "This thing could be my ticket back."