Tonight I got to the barn an hour ahead of schedule (yes, I actually escaped work early) with a goal in mind: try another saddle to see if I got the twitchy response from Bar.
We didn't quite get to the riding part of this exercise, but I got my answer pretty quickly in any case.
First, I brushed him down and poked at his neck in the places I suspected were causing the problem. He tolerated this with only mild irritation. Then I put the English saddle on his back, Calabar peeking around his shoulder at me with mild interest.
I pulled the cinch under his chest and buckled it loosely, intending to take him down to the round pen and warm him up.
The twitching started almost immediately.
I unbuckled the saddle and the twitching stopped. I probed the soft depression below his withers and the twitching started again. I tightened the cinch again with the identical result.
So I took the saddle off and explored his withers and the area at the top of his shoulders. I found lots of (as Karen calls it) "junk" in there. Trigger points and tightness, places that made the big brown head bounce up and down and swing towards me like I might get (but didn't) a nip.
Then I put a nice thick pad and Lena's saddle on him and that was okay, even after I cinched him up. Until we walked down to the arena and stood there awhile. Then the twitching started again.
Peter asked me whether I thought it was just Calabar not wanting to work at dinner time, but I told him I didn't think so since it's happened a not-dinner time and has not happened at dinner time.
I also dug my fingers in all the way around the top of his scapula, on both sides of the withers and in the shoulder and found lots of areas that felt like they could use some relaxing and attention. There were a couple areas where the slightest push would trigger the twitches, too.
So I did as much massage as he would tolerate, plus some stretching and liniment. (Sore No More is our favorite--Calabar is not fond of Vetrolin at all.)
My first thought is saddle fit, and Steve pointed to arthritis, too. Karen will be out on Tuesday and I'll have her look at our saddles, as well as watch him move with and without me on his back. Peter thinks he may even have some saddles I can try and will help me figure out padding, too.
I'll be relieved when I'm more sure, but I think I have a good answer to work from. And one that doesn't make me panic.
It may also be that trying not to worry about it for so long made me avoid trying to find an answer. And let that be a lesson to me.