While astronauts are in space they gain anywhere from 1-2 inches in height. This is because in microgravity (without the force of gravity), the spinal column straightens out, and looses it normal curvature. Astronauts often have backaches, which, it is thought, are caused by relaxation of the muscles and ligaments of the back. Back on Earth, the force of gravity will cause the astronaut’s spine to return to its normal length.
Source: NASA, Science World
