Gravity Hurts (so Good) NASA Science

Gravity hurts: you can feel it hoisting a loaded backpack or pushing a bike up a hill. 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 per cent of their mass if you don’t use them. Muscle mass can vanish at a rate as high as 5% a week.