My friend (see post
#1 ) in Australia tells me he just bought a Greenspeed Magnum XL. He says the seat is adjustable - as compared to the GTO he first tried. So thst may help with his lower back issues.
Hopefully he will persist with adjusting both the boom length, and gear selection to prevent his knee problems. I mentioned he should persist in riding tge trike spite of "not feeling like it today" and also to use lower gearing to make for faster pedalling and less stress on his knees.
Time will tell what happens with his mindset and efforts....
Interesting comment about the crank length and it's effect.