I used to have severe knee pain during and after a long trail ride. It was a combination of things. I had a saddle that the stirrup placement was too far back, my stirrups were too short and I needed to twist the stirrup leather. If your saddle is leather take a broom handle and place it through your stirrups to twist them into position then wet the leather and let it dry that way. When you put your saddle up put the broom handle back through the stirrups every time to help hold them in that twisted position. After awhile they will stay that way and it will cut down on the amount of pressure you will have on your knees. The length of your stirrup should be at your ankle with your legs hanging down.
I had an Abetta saddle that I tried everything with and never could get the knee pain to go away. Some saddles have the position of the stirrups in a bad place.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France