I find the exact same thing for me. I am the same age as Kris, learned uni about the same time (though he stuck with it through the 90s and 00s’). My lower back is tight and gets sore easily. I too have narrowed it down to “lazy glutes”, as Kris puts it. I have a 10-15 min stretching routine every morning and after I ride that helps a lot.
I’m definately nowhere near as hardcore a rider as you, I do between 5-10 miles everyday though (on ungeared 29) With a good long ride of a weekend, so I’m probably going to start taking your points into account - My first fear when I started riding was that I’d become like the kid in that meme photo, where he’s got hulk-sized legs and a little twiggy torso/arms 
But now it sounds like there could be far worse consequences for those of us who’s only form of exercise is getting out of bed, typing and unicycling!
What sort of things do you guys do to maintain proper muscle balance and maximise your Uni performance? (I know this entire thread and a few others have been devoted to that last point, figured I’d ask again!) I’m a young, relatively new rider and would like to keep my riding at it’s prime until my (future) wife and kids stage an intervention!
I’ve been riding since I was 10… 25 years on the unicycle. I thank god and the universe every single day for not having injuries on my legs (knees, ankles, feet…).
For the past 10 years I’ve been doing cross training to prevent injuries, and it seems to work! Tons of exercises for the legs, and two full evaluations per year. It shows many interesting things. I’ll try to send you guys my last evaluation. It’s interesting. All the data is in portuguese, but it’s possible to understand. Basically it compares two evaluations, one made in january and the other a couple weeks ago. The pictures on the left are from january.
And I pay great attention to my position on the unicycle, like feet and knees angles, rip positioning. Thinking about the vetoers of strength during the ride.
My goal is to ride until I’m 70 or more…
RAB_Cafi Otta_1.pdf (1 MB)
Here is a thread I posted a few years ago. My idea is that if an average rider built their strength to near that of the best, their abilities will make a quick bump up.
http://unicyclist.com/forums/showthread.php?t=54545
I heard about this a long time ago, then applied it to skiing w/ nice results. (Mainly legs and core - multiple sets of squats @ twice your weight, I did 5 sets of 20 reps. I forget which & how much of the core exercises were recommended, what I do now I posted in my previous post)
At 59 years of age, just had my second heart stress test. The stress test was a treadmill workout bringing me up to my maximum heart rate. Just over 10 years ago at the end of the first test the doctor basically said, everything is ok, your just out of shape. That comment is what lead me back to riding the unicycle for exercise.
Looking at the latest results, the doctor comments were to repeat saying, excellent, and this is very good. Then he compared my results to my the results 10 years ago, said I improved considerably since my last test. I went 1 minute more than I did 10 years ago. Made this sound like a considerable improvement.
I can say that the improvement was completely due to unicycling since I started unicycling as a result of the first test. My routine has been to ride my KH36 usually three, sometimes six miles, 2 to 3 times a week on flat pavement. It’s a good workout for me.
Best of all, it’s still fun.
One of my greatest benefits from unicycling is the training of spinals deep support musculature which you can’t train such easily with any other sports. My old physiotherapist was really excited about the effect and from then, recomended unicycling to other patients with e.g. scoliosis, muscular imbalance or other issues with the dorsum.
Relating to pulse I can say, that I ride nearly the same pulse whether I use my bike or my uni.