After a long period in space and once back to earth, what do astronauts to get again used to the Earth's gravity?
NASA have an intensive rehabilitation program.
- Body fluid including blood moves from the legs to the head and upper body
- The total amount of blood in the body decreases
And from space.com,
- Bone density lessens at a rate of 1 percent a month
- Muscle mass shrinks
- Eyeball pressure changes, with roughly one-fifth of astronauts reporting vision issues
- The sense of balance is partially lost
These are all counteracted by exercise, both cardiovascular, and strength exercises. In order to regain the muscle mass and fluids, general exercise seems to work.
To increase bone density, calcium supplements are used, along with relatively high impact exercise such as running.
These exercises start off gently and increase in intensity.
The physical exercise requirements are fairly well understood (and described in part on their Non-exercise Physiological Countermeasures project page) but of perhaps more interest are the other areas listed there.