Maybe a cheap $20 smart watch can be repurposed for that. You would need to shave the fur at one spot and fix the smart watch there. Then either use the normal smart tracker integrations or a custom firmware (e.g. there are at least two competing custom firmwares for the Colmi P8, one of them python-based). If you go with a custom firmware you also have access to the raw acceleration and gyroscope data which might be useful for detection.
Keeping anything attached to a cat is an issue though. Even as a collar it might be a bit bulky
This is useful! Someone else mentioned PineTime which is pretty similar to the P8 and writing custom firmware in Python definitely sounds less daunting. I'll buy one since it's pretty cheap and see if Yuri can tolerate it on his collar. Thanks a lot!
All the solutions I found online seem pretty bulky and mostly used during surgery or for short-time monitoring. However, thanks to your comment, I found PetPace and I've already contacted them to see if their collar could help. Thanks a lot for your answer!
Keeping anything attached to a cat is an issue though. Even as a collar it might be a bit bulky