check in the download window... i bet it says "workshop" , so mods that got updated.

What was the source/cause of the muscle strain?  It's possible (and I'm only guessing here) that the option to turn it off relates to combat-induced strain, rather than climbing, exercise, woodcutting, etc.   Could that be relevant?

Tried on our branch, and switched to the current unstable with maximum sandbox for vehicles and road stories and fast move in Muldraugh (and Louisville to strain it as much as possible) and had no issues to be able to reproduce it unfortunately.   Maybe try reinstalling the game freshly on Steam if you haven't double checking mods are fully disabled, and/or verifying integrity of the game's files. It could be an issue on our side for sure, but one different circumstance we may not have accounted for might be not allowing me to reproduce it, or it may just be unclean files on your PC somehow triggering an issue from leftover mods.

I confirm that I have noticed the same thing and that it is reproducible.

I managed to to get the save back running, by comparing Folders!   I Fund the Folder: "C:\Users\MyUser\Zomboid\Saves\Sandbox\MyTimeStamp\apop"   was containing huge files. In every working savegame i have this folder contais small files apporx 3-5KB. But in the not running savegame some of thoose files are 3000KB! After deleting them i could run the game for 2 hours now without any problem or progress loss.