I drilled extra holes in my cranks, and tapped threads in them (9/16"x20tpi), so now they are 145mm long.
Why would I want to do this? Well, my python uses 20inch wheels, so normally I would need uncommonly large front chainrings (60T). When cutting the crank arm, the pedaling frequency rises. And when the pedaling frequency rises, less tooths are needed on the front chain ring.
If we assume that instead of the cadence it is the speed that the legs that is the constant factor, then the optimal cadence would be proportional to the crank length.
With normal length cranks (e.g. 178mm) a 60T chain ring on a 20" recumbent would be necesary. With 145mm crankset, I can get it down to 49T (145/178*60T). Which means I can use normal sized chain rings.
Another reason to shorten the cranks is the range of movement of the joints gets smaller. This I have heard is good for your joints (hips, knees or ankles).
In the following article Effect of Pedal Crankarm Length on Cycling Duration in a Recumbent Position it seems to be no problem to reduce the pedal arm length to even 145mms.
I have been riding with shorter cranks for some time and yes, they do help in "upgearing". I'd like to use even smaller cranks (120mm), but then I would need to change the recumbent first --didn't make the seat-BB distance adjustable. Anyway, I my view shortening the cranks to upgear is a viable solution.
But I do not think this helps against knee problems. At least not in my case, because I switch between different crank sizes all the time. When switching from a large crank to a smaller, I need a concious effort to turn in a lighter and smooth way (remember it is not only the circle that is smaller, but also the cadence is higher). And when I forget I get an overstrained the knee. On the other hand switching from small to large crankset seems to be no problem at all (and may be it is even good for the knee).