What exactly makes it difficult for astronauts to walk immediately after landing after extended stays in space?

Question

Photos and videos of astronauts returning from the ISS being taken out of their capsule show them being mostly lifted out by hand and lowered into chairs, where they rest for a little while, then they are taken to what looks like a recovery tent for a little while, then a helicopter ride to a reception center.

By the time they arrive at the reception center they appear to generally have adjusted sufficiently to Earth gravity to walk at least short distances with minimal or no help.

During this short adjustment period of the first few hours, what is it biologically that needs to adjust? A few hours is not enough time to build muscle strength, so this must me at least partly neurological, but is it related to balance (semicircular canals suddenly experiencing 1 g after microgravity for several months) or motor neurons developing sufficient signaling to produce enough muscle power, or the brain remembering how to coordinate, or all of the above, or something else? Has this been studied?

Answer 1

The extremely frank discussion of the post-landing period found in long-duration crewmember Clayton Anderson's "The Ordinary Spaceman" makes it clear that, for him at least, the transition was largely vestibular and regaining habit patterns.

Upright for the first time on Earth in over five months, the entire middeck of the orbiter began to spin counterclockwise at an incredible rate....

...Success was achieved until I moved my head to look at the open hatch and freedom. The spinning began again....

He goes on to detail the venting of every possible substance from his body over the next few hours. I mentioned it was extremely frank, right?

The next morning

I had to carefully position myself for each distinct movement as though my body was a marionette and my brain a puppet master.