The "anti-gravity treadmill" was originally invented by Robert Whalen, a biomechanics researcher at NASA Ames Research Center, in the 1990s.
Whalen knew that astronauts on the International Space Station have to exercise for hours each day to combat the loss of bone mass and muscle in microgravity. But the treadmill on the ISS has always left a lot to be desired. In lieu of gravity, it uses straps around the shoulders and hips to anchor the astronaut to the treadmill. The bungee system doesn't do a good job of replicating the magnitude or types of force that runners experience here on Earth. And to make matters worse, it's pretty uncomfortable to run in. Astronaut Sunita Williams, who was the first person to run the Boston Marathon in space, described her experience in a NASA press release: "During the marathon my foot sometimes went numb and tingly from the straps' pressure on my hip. Also, I had to use moleskin where the harness rubbed my neck raw."
Whalen designed a treadmill that would let astronauts run in a more natural way. The design, patented in 1992, encloses a treadmill and the astronaut's lower body in an airtight chamber. Lowering the air pressure inside the chamber pushes the astronaut down, simulating gravity. Whereas the ISS's old treadmill allowed Williams to run on about 60 percent of her Earth weight, Whalen's treadmill would have allowed her to exercise at her normal Earth weight. That's important for keeping the muscles and bones healthy for when astronauts get home.
But Whalen's idea never made it off the ground. In 2005, the technology was licensed to a company called AlterG, which appears to have coined the term "anti-gravity treadmill." Instead of adding weight to astronauts in space, AlterG uses the technology to take the weight off of rehab patients recovering from leg and foot injuries.
AlterG's product looks like a bounce house for your lower body. To use it, you put on a pair of tight neoprene shorts. The shorts have a sort of skirt attached, and the skirt is lined with zipper teeth. You step onto the treadmill, inside a hole in its plastic casing, and zipper yourself in so that, from the waist down, you're encased in an airtight plastic bag. As you stand there, the treadmill measures your weight, and you tell it how intense you want your workout to be. The the machine uses "unweighting technology" to make you feel up to 80 percent lighter—so if you weigh 100 pounds, you could feel as light as 20 pounds on the treadmill. The terms "anti-gravity" and "unweighting technology" are enthusiastic descriptions for what the machine actually does, which is inflate the plastic bag around your lower body to lift you off the surface of the treadmill.
Despite its perhaps overhyped name, the anti-gravity treadmill seems to be doing good things in physical rehab clinics, because it allows patients to exercise without exacerbating an injury. Here's NASA's glowing review of the anti-gravity treadmill:
Professional and college sports teams across the United States feature the AlterG treadmill in their training facilities. Injured soldiers walk and run with the technology’s assistance at military hospitals and rehabilitation centers. Seniors get essential exercise using the support the machine provides, as do people with bariatric weight issues who cannot normally support their own weight. The treadmill has been a proven option for neurological uses as well, including helping patients re-learn proper balance and gait and transition to independent movement after traumatic brain injury.
A variety of peer-reviewed studies also suggest it helps people get back on their feet again.
A true anti-gravity machine—one that is not acted on by gravity—would of course have even more exciting applications, particularly in spaceflight. Unfortunately, for now at least, those machines only work in science fiction.