That is actually more solved than you think. [1]ARED is basically equivalent to free-weight training under Earth gravity. Many problems with exercise in space is that elastic materials provide different resistance depending on how far they are deformed, meaning muscles that are used more at the beginning of your rep are worked less than those you use near the end. ARED uses a piston under vacuum to provide constant resistance through the entire stroke. There was a video of it in action on the SmarterEveryDay youtube channel but I can't seem to find it, I'm not sure if one has actually made it up to the space station yet either.

[1]https://www.nasa.gov/mission_pages/station/research/experime...

Yeah I've seen videos about that, its useful to keep the decay at bay but its still in issue.

Also there is blindness, cardiovascular issues, and more. I hope they can figure out how to mitigate or remove these issues.

Colonising is different to visiting.

When visiting, changes (like in bone density) are bad, but can be monitored and taken into account when planning the mission duration.

When colonising, some permanent changes may be fine, since the change in environment is also permanent. If a colonist visited Earth, they would have to take into account the health issues caused by high gravity.

I agree that much more research is needed though; even if some physiological changes are acceptable for colonising, we should make damned sure we know what they are, and what the unacceptable changes are.

True but the brittleness still remains, smash too hard into something and your bone will crack.

Also theres blindness, cardiovascular, and other issues as well.

Definitely needs more research. Also makes me wonder about Elon's plans to bring people back, might not be possible after a certain length of stay on Mars.