Quick question
If humans lived on Mars and came back hundreds of years later, what would they look like?
Rather...
They’d be tall, weak, and extremely vulnerable
See martians when returning to Earth would be like extremely tall, and weak. The moment they trip, you can expect to hear a couple bones crunching in the process.
The reason for them being so tall is because Mars’ gravity is two times weaker than that of Earth, so the gravitational force pulling down on their spine would be less. Their spines would take the chance to grow, and they’d become …
Just like this man (Who by the way, is the world's tallest man), they’d also need canes to walk around on Earth. In fact, he’s a very good example of how Martians would look like.
Their bone structure would not be strong enough to allow them to walk on Earth without waddling, or at least stressing their bones toward fractures. Either way, they won’t be able to go on hiking trips without a medic team nearby.
This weakening of bone structure is called: “Osteogenesis Imperfecta” - and it’s real.
So When you play with your Martian friend, be very careful because, they’re so fragile, that the moment you slightly pat them on the back with too much force, the back of their rib cage would fracture or even straight out break, if you are one of those who back-pats a bit too enthusiastic.
The human body adapt to environmental changes, so when gravity on Mars is two times weaker, their bones will follow suit. This also means that you’re two times stronger than an average martian.
The next is that they’ll probably be dead by cardiac failure after a while.
Martians, or rather Human living on mars would have an enlarged heart, due to the low gravity on Mars. This would mean that Martians hearts needs less exercise to pump blood throughout the body than their Earth counterpart.
However, when they return to Earths gravity, their hearts would be overwhelmed and unprepared. The amount of Oxygen being delivered throughout the body won’t be enough to sustain thems, and soon enough …
They’ll probably break a couple of bones in the process, too.
Either way you look at it, if they don’t do surgeries to artificially strengthen their bodies in preparation for their Earth trip. They’d be either dead or injured, and they probably would need an adult’s supervision in their day to day activities.