What if you only weighed a fraction of what you actually do? You could run so fast. You could take an immense amount of pressure off your knees while still getting a workout. You could focus entirely on your form.

And what if, as easily as you took the weight away, you could add it back on, gradually increasing resistance like you do with weights? Guess what? You can, and it could change your workouts forever.

We covered the AlterG Antigravity Treadmill back when it came out in 2008. Since then it's become a staple in everything from physical therapy offices to nursing facilities to hospitals to pro sports teams to the Olympic Village, and, of course, high-end gyms. There just happened to be one at the Equinox gym on 63rd and Lexington in NYC, so we headed over there to check it out.

Basically it works like this: You put on these very tight neoprene shorts that have a large zipper attached to them. The zipper zips into a large airbag which actually contains the treadmill inside it. Once you attach the shorts and seal up the pouch, the airbag is pressurized; since your hips are at the top of the bubble, you are lifted up as the the bubble expands. NASA developed this technology to train astronauts—to simulate walking on the moon, where you only weigh 16-percent of what you do on Earth. The AlterG can take you down to 20 percent, which is pretty damn close. (Fun fact: on the ISS they do the opposite, putting bungie cords on a treadmill to try to simulate earth's gravity.)

There are so many reasons this is a great thing. From the rehab standpoint, it has helped people recovering from injuries get back into shape. It has helped kids with cerebral palsy improve their gait mechanics. It has helped obese people who were ordinarily carrying too much weight to run get really good workouts. (Yes, it was featured on The Biggest Loser.)

From an athletics standpoint, there's a lot of great stuff, too: Relieving your body of some weight gives you a chance to really focus on form; once that's ingrained, you're more likely to carry it back into your regular running, allowing you to run more safely and efficiently.

Or you can just see how insanely fast you can run, because that's kinda awesome.

As you may remember from previous Fitmodos, I have never been a runner because I grew up with bad knees. But I'm training for a triathlon. This was a chance for me to get my mechanics checked out. It was an amazing experience.

If you watch elite distance runners, you might notice how high they kick their heels up. This is a very important part of a proper running stride called your recovery. It looks weird, but you actually want to keep you heels up high, because that effectively shortens the pendulum, the motion of your legs. It requires less energy for you to swing your leg forward for the next step. Makes sense, right? But I suck at this. I've only been running for a couple of months. My hamstrings aren't very strong yet, so I barely kick my heels up at all. When Lindsay (the trainer) took me down to 70 percent of my body weight, suddenly, I could run with excellent form. My heels campe up effortlessly, and I planted my feet in just the right place. It felt so amazingly good.

Lindsay could see a lot through the little windows in the airbag. For one, she noticed that I bring my right heel up more than my left. She asked if I'd had a injury to my left knee. Does falling out of a trapeze and landing on your left leg count? Apparently, yes. From that she was able to give me some tips about balancing myself out. I was also able to maintain speeds that I definitely can't it real life while still breathing easily and maintaining a conversation.

Oh, and if you're wondering if running at 20 percent of your body weight is fun, oh man, is it ever. I'd do that every day if I could.

The only real downside to AlterG is that, because you're lifted up by your shorts, it's rather snug around the man-bits. It didn't raise my voice an octave, but it was like a firm grasp from a robo-masseuse. The boys were happy to be freed. Also, because the device comes up high, you can't swing your arms naturally—you have to lift your elbows higher than you normally would. All that considered, I still want more.

AlterGs are all over the place, and you can find one near you via their website. If you have a chance, give it a shot. [AlterG Antigravity Treadmill]


Video by Michael Hession.

Big thanks to Lindsay Dettbarn, Nicole Moke, Equinox, and Devon Sibole!