College athletic departments, physical therapy offices, and even the occasional high school are purchasing AlterGs for their widely lauded ability to allow runners and other athletes to continue to train pain-free even with significant injuries. By reducing your effective body weight, the AlterG allows you to run at normal training speeds with drastically reduced impact and active forces. With careful modulation of the body weight settings, you can often maintain running fitness even during the rehab period of formerly season-ending injuries like a stress fracture.
The AlterG achieves its anti-gravity effects using a pressurized "bubble" that encapsulates the runner's lower body. Special compression shorts with an airtight skirt zip securely into a thick vinyl bubble that surrounds a standard running treadmill. The heart of the AlterG, a computer-controlled air pump, inflates the bubble to above atmospheric pressure, applying an evenly-distributed force from air pressure to the runner which counters the force of gravity. By adjusting the air pressure inside the bubble, the AlterG can adjust your effective body weight. The AlterG uses a force plate to correlate changes in the bubble's internal air pressure and your effective weight while standing on the treadmill.
In the past few months, I've been fortunate enough to have access to an AlterG. I've also been fortunate to not have to use it for any injuries (knock on wood...), so I used the opportunity to look into a question that I've been wondering since learning about the AlterG: How much easier is running on an anti-gravity treadmill compared with running on land?
Because the majority of the metabolic cost of running comes from absorbing impact and accelerating your body weight against the force of gravity to propel yourself forward, it's axiomatic that reducing your effective body weight while maintaining the same running speed will reduce the energetic cost of running.
Notably, this is not the same situation that occurs when a runner loses weight normally—if a 150 lb runner decreases his weight to 140 lbs by restricting his caloric intake, muscle loss is inevitable (this is part of the problem with the idea of "racing weight"). Though he now weighs less, and thus the energetic cost of running a given speed is decreased, he has also lost some muscle, so his ability to produce energy is reduced as well.
There's no good bio-energetic equation to predict the metabolic cost of running; the only way to get a good answer would be with an experiment, which I set out to conduct.