Thanks for the response, everyone
I understand the reaction, I always though of unicycling as nothing but beneficial for the spine. I had some onsetting back problems (sitting in front of pc too much during winter) - but completely forgot about them after I bought skiis and unicycle. The following 2 years I rarely, if ever, left home w/o unicycle. And never felt anything except what you supposed to feel after a perfect workout
Last winter, after exacerbating a leg injury, I had to avoid walking, i.e. had to sit a lot. And my only outdoor activity was unicycling. Sitting at home and then sitting - in saddle - outside. And jumping a little while sitting. My back got worse. After reaching the point when I could hardly sit or lay on my back, I skipped my daily unicycling once and I felt like resurrected the next day. I also had to drop one exercise from my morning gymnastics (the one where you raise your both legs while lying down… It’s called “defeated dragon”) After getting better, I regularly tried to resume riding, but every time, after a few minutes I felt discomfort, after a longer ride - aftereffect for the several hours.
I was scared that I wouldn’t be able to ride a bicycle - luckily it was not the case. After the summer season has ended, I resumed my attempts, with the same result.
I do not hold any of my hands on the handle/seat (why?)
The discomfort I feel is from the up-down rocking - 2-3 oscillations that happen after each gentle slope on a generally even road - probably the result of the tire contracting/expanding on them, this also may or may not be affected by how pedalling combines with those waves.
I tried riding 27" today - the rocking is still present
the discomform is a kind of pressure / pinching sensation in the lower back, mostly left lower back
I think it’s at the part of the back where you bend it when you lean to do a turn (turn left, in my case), or when you are riding on a sidewalk which is tilted right
And after I get off the uni, I often instinctively do a side bend exercise - to the opposite, right side.
From all of this I suspect I may ride it incorrectly - always bending to the left, w/o even noticing it. And, may be, because the spinal column is always bend, it is hurt by normally tolerable vertical forces. And my back coped with this for 2 years until it became too much.
If this is what it is, I have no idea how could this happen. Even if all my routes are tilted to the same side, most of the trips is two-ways. But now somehow I should understand that I bend even on absolutely untilted floor? I will have to check this somehow.
May be something happened to my unicycle that I started tilting while riding it. I will try to switch to my older one and look if there’s any difference.