In the late 1980s, NASA researcher Robert Whalen was tasked with finding ways to help astronauts maintain their strength with new in-flight, in-space exercises. For a body in space to maintain its normal physical condition, exercising requires weights to simulate the effects of gravity on Earth. Whalen proposed using differential air pressure in space flight exercises. Weighing down astronauts with air while they exercised would mimic gravity, thus preventing bone and muscle degradation.
Up to that point, mid-flight exercise for astronauts was both difficult and imperfect. NASA's standard equipment was a steep, anti-gravity treadmill. To stay in place, astronauts were harnessed to the machine. Not only was that uncomfortable, but it stifled the movement of the muscles it was supposed to be aiding. Whalen altered the treadmill concept by applying high forces of air pressure specifically calibrated to match the effect gravity would have on that astronaut.
Whalen's G-Trainer resembles a treadmill encased in a bouncy castle. The user climbs into an airtight hole above the tread. Once there, the astronaut runs or walks, and the machine calibrates the inflatable sections, applying air pressure to the astronaut's weight, lifting the body as needed to push back on the lower body and providing the same service manually that gravity does naturally.
In 2005, NASA and Whalen licensed the patent for the G-Trainer to a company called Alter-G, Inc, which adapted the machine for both medical use and athletic training. Its AlterG Anti-Gravity Treadmill provides support for physical rehabilitation patients with diminished lower-body strength. But there's one big difference between it and the NASA model, because, technically, the treadmill does on Earth the opposite of what it does in space. In space, the air adds weight and provides healthy resistance. On Earth, it unloads weight. Today, the G-Trainer is used widely in rehab centers, military hospitals and in the training facilities of both professional and college sports teams.
NASA actively shares its knowledge with the athletic training community, even offering an athletic training internship. Open to students pursuing a career in athletic training, the internship provides valuable, real-world experience. Physical medicine and rehabilitation of muscles and bones are covered, as are actual physical evaluations of patients (including astronauts). Interns also have the opportunity to help design rehabilitation and training regimens.