The data about human spines in space was first gathered from the Skylab missions. Six astronauts were studied. All six showed almost three percent growth. The spines returned to normal curvature as soon as the astronauts hit regular gravity. Then, they were back to normal within 10 days.

What happens in space is not an optical illusion, but one more example of how microgravity affects our bodies.There are two theories to explain why the spine gets longer, says Dr. Sudhakar Rajulu. He is a researcher at Johnson Space Center's Habitability and Environmental Factors Office. The first theory is that elongation only happens to the spine. It does not have a major effect on other areas of the body, such as the legs or arms. The reason is because those bones do not compress like the discs in the spine. The natural curve of the spine is straightened somewhat in space. Without the usual force of gravity pushing down on it, the spine is freer to relax.

The second theory says that the discs between each vertebra are pressed together in regular gravity. This compression is due to the pressure pulling the spine downward. When gravity is lowered in space, the discs are able to hold more spinal fluid. This makes them larger. It also puts more space between each vertebra.

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