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.
Have a professional fit you to your bike.
There are probably many ideas to solve knee pain as there 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