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It has been known since the 1960s that astronauts oftentimes have trouble standing and walking after returning from space.
Their vestibular system, the body’s sensory system that keeps us balanced and oriented in the right direction, is often out-of-whack after coming from the microgravity environment of space (where the perceived force of gravity is very much reduced) when compared to the regular environment of gravity here on Earth’s surface.
NASA scientists have also learned over the past forty years or so that bones and muscles also take a beating when subjected to the weightlessness (“perceived” lack of gravity) of space.
In fact, U.S. cardiologist Benjamin Levine, of the University of Texas Southwestern Medical Center, states in the NASA media release, "When astronauts land back on Earth after a long time in space, not only is their vestibular system mixed up and their kinesthetic sense thrown off, but also their bones and muscles have deteriorated." [NASA: “The Beating Heart, Minus Gravity” (July 10, 2009)]
The most troubling effects of long time orbiting around Earth are muscle atrophy (muscle deterioration) and spaceflight osteopenia (skeleton deterioration). However, astronauts counter many of these problems through exercise.
Other major problems caused by visits to outer space include fluid redistribution (fluids of the body tend to not flow evenly throughout the body and, instead, collect in specific areas); slowed cardiovascular system (reduced blood flow and reduced heart function); decreased red blood cell production; and weakened immune system.
More symptoms include loss of body mass, nasal congestion, sleep disturbance, and puffiness of the face. When in the confines of the Space Station or the Space Shuttle, another symptom that can cause embarrassment is excess flatulence: "gas."
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