i had read on another post recently that a manager with a player with higher stamina (i guess 8+) struggled to keep their GS at strong or proficient.
I havent noticed anything personally, but that doesnt mean there isnt something to it.
The logical thing (though what is logical isnt always what is in the game engine!) would be if maintaining game shape and stamina are linked in some way, for example to illustrate my point:
with stamina 1 a player could play 4 quarters a week to improve ad maintain top GS,
stamina 4: 5 quarters a week,
stamina 7: 6 quarters a week (which ties to 72 minutes, what we think is upper acceptable limit for all players)
and stamina 10: 7 quarters a week.
the interesting thing about this would then be that the value of stamina is just about how well a player plays during the game/when they need to take a breather but also about how many minutes you can get out of them in a week and still keep them in good shape