Almost every time I manage a national team the conditioning of the players simply stays brutal. At first I thought that it was because I managed smaller nations but now that I'm both in the Finnish league and on the Swedish national team it can't be due to the choice of national team.
It seams way more likely to affect the best players available for the nation and if anything the players seems to drop from around 50-60% at the start of the season down to single digits at the end of it, free agency doesn't seem to affect it either. Age isn't it either as I've seen everything from 22 year old's to retiring players. I almost every time go with all leagues active so it's not that either.
This is an example when running as a Kärpet/Sweden GM. I'm at the selection for the Olympic games and players looks like this:
