I thought such loss was seen all the way back to the initial long-duration space missions? They assumed that it was reduced stress on the bones and so introduc

I thought such loss was seen all the way back to the initial long-duration space missions? They assumed that it was reduced stress on the bones and so introduced various exercises. Is this merely recapitulating the older results or does it keep getting worse and worse the longer things are up there? If this, along with heart or other changes of zero-g, can't be dealt with they're going to need a (micro)gravity setup to make sure that astronauts can survive their return to gravity.