There have been many discussions on various message boards regarding knee pain. Thought I would provide summary and 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 new Catrike this year.
I spent exorbitant monies on a SRAM 81 speed and rotor cranks on my Catrike. Knee pain is still there, but much less.
Just bought knee savers for $46.00 and pain is again less. 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, cadence, 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.
Those who ride with shoes with cleats profess if cleats are all the way to back of shoe, then knee pain is less. I did this also.
Buy good shoes and pedals. Seek medical advice if no improvements.
Another inexpensive way is have a professional fit you to your bike. And, stretching prior to and after riding is an excellent way of reducing and preventing knee pain. Leg weight training; leg presses, extensions and hamstring work on the circuit machines can be very tough on knees. Plenty of stretching and some core exercise (without weights) should help the glutes and lower back, but you have to go slow and not overdo these either.
Doctor says is attributed to growth and told me to take some supplements that lubricate the joints and supposedly help them heal. The supplement was chondroitin+glucosamine with hyaluronic acid.
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