What changes
The first hours after returning are hardest because of that last change. Your brain starts receiving a signal about which way is down again, but it has forgotten how to use it, and when you turn your head, the ground seems to drift away.
How different the sense of balance feels immediately after landing, according to astronauts' own descriptions
A drop in blood pressure adds to the problem. As the heart and blood vessels adapted to blood no longer being pulled downward, they stopped keeping it in the upper half of the body - and when you stand upright on Earth, blood suddenly rushes to your legs.
How they cope
On the station, astronauts exercise every day: on a treadmill with straps pressing them against the belt, on an exercise bike, and on a resistance machine that simulates working with free weights. This takes about two hours a day.
Exercise slows the losses but does not prevent them. Bones recover more slowly than muscles, and restoring bone density takes considerably longer than the flight itself.
That is why people are carried out of the capsule after landing instead of being allowed to walk on their own. The first days on Earth are a recovery program in which they practice standing up, turning, and walking all over again.
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Sources
- During prolonged exposure to weightlessness, reduced bone mineral density, loss of muscle mass, fluid redistribution, and changes in vestibular function have been documented
- bone tissue takes longer to recover after return than the flight itself