I'm not active in prostheses anymore, but have done a smidge of work in them. The propioceptive positioning does occur in muscles, but, as far as I remember, i

I'm not active in prostheses anymore, but have done a smidge of work in them. The propioceptive positioning does occur in muscles, but, as far as I remember, it's mostly in the tendons. In that, your brain/spine knows the stretch of all the tendons and then back calculates that to a position based on experience. So, people with very recent tendon injuries or differential loads on the muscles will have a somewhat hard time knowing the position of their body (by very recent, I mean within a few minutes/seconds). Blood pressure would be an interesting way to do the math, but as blood pressure changes a lot, it would be doing a lot of work here. Also, most of these systems are inherited from very ancient times when blood was a bit different and before hearts like ours. Animals want to know where they are more than they care to have much of a heartbeat. 'Fun' little ways that we have discovered these things are mostly when someone has a disease that causes them to do wacky stuff. Phantom limb diseases are a bit like that. There are some other ones, but I can't remember then, sorry! If you'd like to learn more about propioception and all the little biological fun times, I'd suggest just about any intro neuroscience textbook. They're kinda fun once you get into them. For grad level learning, Kandel's Principals of Neural Science is the go-to book. Lots of case studies in there.