From an article by
After months in space, your crew has finally made it to Mars, but there’s a nasty readjustment awaiting all of them.
Living in zero gravity during the Earth-Mars transit will have some pretty harsh side effects, but most urgently they’ll have to readjust to gravity again. And those first few days on Mars will be busy as the first Mars explorers get their bearings, prepare their habitat for the stay and begin thinking about setting up experiments.
You can therefore see why NASA is so intent on figuring out how to help astronauts stay healthy and happy after long stays in space so they can adjust quickly for life on Mars. We already know that a six-month stay on the International Space Station wreaks havoc with bone and muscles, but at least when the space station crew return to Earth they have a team of medical professionals to look after them. The first Mars astronauts will have no such luxury.