While on his third deployment to Iraq in March, U.S. Army Sgt. Damon Warren’s vehicle was hit by a roadside bomb that left him with fractured ribs, a torn rotator cuff, and a shattered left femur that required a rod be placed in his leg from his knee to his hip. Despite the severity of his injuries, Warren was back on his feet by July with the help of a NASA-inspired rehabilitation device allowing him to defy gravity while working out on a specialized treadmill.

Walking and, later, jogging on AlterG, Inc.’s Anti-Gravity Treadmill were a bit awkward at first for the 105-kilogram soldier. “I wasn’t used to something else holding my weight while I walked,” says Warren, 36, who is on leave but still on active duty in the Army. “It’s a rush of air that pushes you up.”

The Anti-Gravity Treadmill, which looks like a normal treadmill encased in a large, clear plastic bubble, uses differential air pressure to reduce the force of gravity on the mass of a runner or walker’s lower extremities. Patients, athletes and others must first don a pair of formfitting neoprene shorts that zip into the opening at the top of the treadmill’s shell and form an airtight seal. The treadmill then measures a user’s weight with the help of load cells located in its base. The area from the waist (where the shorts zip) down is actually a pressure-controlled chamber that fills with air, gently lifting the user and relieving the lower body of some of the effects of gravity. “From there, it calculates how much air is needed to unload a specific amount of a person’s weight,” AlterG CEO Lars Barfod says. “You don’t actually feel the air pressure at all because it’s uniform and low, between one and two pounds per square inch. You basically feel the same that you would feel in water, except you’re not wet.”

Warren works out on AlterG’s treadmill, which can alleviate up to 80 percent of his weight by lessening the gravitational pull on his mass, at Peak Physical Therapy and Sports Medicine in Plano, Tex., near his hometown of Carrollton. Initially, his rehabilitation involved walking on about 55 percent of his weight, but he is now able to reduce the treadmill’s differential pressure and walk or run on 70 percent.