First things first: not lame!
I explained the things I'd been noticing to T. before we even got the lesson started, and he watched us warm up - head-jerking, inconsistent contact, etc. - and put us through our paces veryvery thoroughly, with improvement in spots. Then we talked about it for a while.
The upshot is: shortness in the right hind. Which is not unusual; it's always been his weakest limb. T. did not see any discomfort or pain, more of a mechanical stiffness/habitually limited range of motion. Which is to say, he's fifteen, and this is the first time in his life he's being asked to really truly swing his hind end. Probably evenly split between his stifle and his hock. He can reach and extend without pain, he'd just really rather not. So add a dash of laziness on top of it all.
So what's happening is he's coming up short with the right hind - not necessarily a problem tracking right, but tracking left he's not getting the thrust he needs to (as T. explained it, most of the lifting power comes from the inside hind, most of the thrust from the outside hind), then offloading the problem onto his left front, which is throwing him off balance and making him jerk his head up.
Solution is, as always, kick-kick-kick-kick. Bend him inside, half-halt outside, push him onto the outside rein and then make damn well sure the outside hind is doing its fair share. Supple with the left wrist, bend around the left leg, half-halt the right hand, kick-kick-kick-kick the right leg. Several circles of come-to-Jesus and we were going 2-3 strides evenly and with power; once we could get that more or less consistently, we had a bit of a walk break. Picking up again, I worked HARD on the same problem in the walk, where it was a bit easier to convince him to swing through. He wasn't exactly pleased at all of this, mind, but once I closed off every available exit door, he sighed and farted and acquiesced.
By the end of the lesson, he'd loosened up nicely, and we had some really glorious canter complete with spiralling in and out both directions, and a big powerful swingy trot on a long rein a few times around to stretch out.
So: old horse, new tricks. Though as T. points out, we're kinda victims of our own success. It's not like this is new for Tris, more like by the time we used to get to this level of the work it was the last five minutes of the ride, and he was already warmed up and loosened. So he wasn't really having to work very hard to muscle through it. Now, we're warming up with the work we used to finish with, and he's not quite supple enough to support that work so early in the ride. It's just a time-patience-work thing, though; strengthen that right hind, get the joints used to moving, loosen up the joint fluid, and blow his little mind. He's already on glucosamine and I just started him on MSM about 3wks ago. We're getting more bute on Friday from the vet; for the next little while, I might make him a little bran mash with bute after days he's worked hard.
Horses are so neat sometimes; every small little thing adding up and figuring it out and working on it is always like putting together an incredibly intricate lifelong puzzle. I love it.