There have been many discussions on message boards regarding knee pain. Thought I would provide what worked for me.
Being a member of the human race, a solution to all is difficult (e.g. one size does not fit all).
A little about me. I am 54 years old and own a recumbent trike, a Catrike Road.
I experienced knee pain with my DF, Weggie, as well with my Catrike this year.
I spent exorbitant monies on a SRAM and rotor cranks. on my Catrike. Knee pain still there, but much less.
Just bought knee savers for $46.00 and pain disappeared. There are three sizes. I bought the 30mm (biggest size) only cause I am 6'4" tall and everything should be bigger to fit? They essentially move your foot/leg further away from the pedal. Details are at www.kneesaver.net/
Another less expensive solution is increasing your RPMs, a higher number of revolutions. This is often called spinning at a higher rate. The easier exertion on the knee, the better. I use this process.
There are probably many ideas to solve knee pain as their are riders.
Recommend you start with the least expensive and move up from there.
Catrike Road #116.
"Cats just don't feel safe on a moving bicycle, no matter how much duct tape you use"--Author unknown