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Astronauts experience some muscle atrophy for the simple reason that they don't have to use their muscles while they're in orbit.  On space shuttle flights this doesn't really matter, because muscles don't atrophy much in only a week.  This could be a problem for astronauts in orbit for a long time, and on long spaceflights it will be very important to exercise to maintain muscle tone.  Again, it is not a problem as long as the astronauts are in orbit, but as soon as they return to Earth they need the use of their leg and heart muscles. 

[S]truggle to regain equilibrium.  The room spins.  I remember fighting with myself, ordering my brain to accept "up" and "down".

brain: feedback/perception.  The body readjusts very quickly--in only 20 to 30 minutes.  That's not surprising, since humans evolved in 1-g, and astronauts all grew up in 1-g.  What's amazing is that in 1 week the body can readapt, change its feedback circuits, to accommodate weightlessness.
[C]hallenge is to get down the stairs without tripping--better yet without holding on to the rails[.]
post-landing: people waddle like penguins.  Some astronauts look like they're walking in starched pants--that's because they've inflated their g-suits.  About 50% of the astronauts elect to inflate them, to keep the blood from pooling in their legs.

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