Gravity hurts: you can feel it hoisting a loaded backpack (just ask my kids) or pushing a bike up a hill (when you aren't fit enough to actually ride it uphill). But lack of gravity hurts, too: when astronauts return from long-term stints in space, they sometimes need to be carried away in stretchers.

Gravity is not just a force. It's also a signal -- a signal that tells the body how to act. For one thing, it tells muscles and bones how strong they must be. In zero-G, muscles atrophy quickly, because the body perceives it does not need them. The muscles used to fight gravity --like those in the calves and spine, which maintain posture-- can lose around 20 percent of their mass if you don't use them. Muscle mass can vanish at a rate as high as 5 percent a week.

For bones, the loss can be even more extreme. Bones in space atrophy at a rate of about 1 percent a month, and models suggest that the total loss could reach 40 to 60 percent. These effects are physically devastating to astronauts.

Enter: The Vertical Treadmill. NASA is now using a new treadmill that allows people to run while suspended horizontally to help astronauts prepare for long-duration missions to the moon and beyond. The treadmill simulates zero gravity by suspending human test subjects horizontally to remove the torso, head and limbs from the normal pull of gravity. Participants are pulled toward a vertically-mounted treadmill system where they can run or walk. The forces against a test subject's feet are precisely controlled and can mimic conditions of zero gravity in low Earth orbit or conditions on the moon, which has one-sixth the gravity of Earth. In addition to simulating exercise protocols, the device may be used to imitate the physiological effects of spacewalking.