Your point that because of joint condition you prefer to sit to the trot (sitting trot) and find posting to the trot (posting trot) difficult to do, or possibly painful, brought to mind a training puzzle I've been thinking about.
I've been critical of the Reining Horse world, and stock horses in general, sitting trot that I've seen throughout equitation, reining classes, in fact all western disciplines that require showing or riding at the trot.
They, meaning those people involved at all levels, refer to it as the "collected," trot. Except in rare instances it is not "collected," but merely a shuffle, the horse simply taking shorter steps, with very low energy and very low lift from the ground.
The difference is like the letter "O," seen in both lower case and capital or upper case:
o and O.
It happens that this slower, lower trot is quite easy to sit. I wonder if it is more comfortable for the horse as well, as rarely have I found a green horse I've trained doing it spontaneously and willingly without training. In fact, it's fairly rare at liberty as well.
Most horses at liberty, when they trot, tend either to extend, or to collect with what we know as true collection, that is to say, moving their energy inward from either end, and then upward, lifting their hooves higher above the ground and retarding their forward progress speed.
So, though I've criticized this particular kind of western horse and western rider trot as not being truly collected there may be a place for it. I've seen no studies of it because I believe there has been no discrimination between the truly collected trot and this western shuffle trot. Could it be this is a much more comfortable gait for the horse over other kinds of trot? A horse trained to it seems to be able to pretty effortlessly do it for extended periods of time.
I've not heard of any particular hazards to the horse's body being connected to it, nor have I seen any myself, though most horses tend to, especially if professionally trained in this trot, bend at the fourth and fifth vertebrae, to me an unsightly and questionable practice for the horse's health. Some can lift the back, and some cannot in this posture or frame. Or I should say that some do and others do not. Though most do lift the back and attain the rounded up frame that most trainers/owners/riders/handlers try to attain.
I wonder what others experience has been with this faux collected trot?
Love is Trust, trust is All
So say Don, Altea, and Bonnie the Wonder Filly.