Foxy lady I have arthritis in my right knee and I suggest you warm up your joints first and do a low impact on this. My arthritis doesn't like me to sit long I have to walk to exercise my knee.
When I first used it my knee was sore for a couple of days but once I warmed up the muscle and use lightly and my knee is getting use to this workout. My advice is take it easy while jumping but do a 5 - 10 minute warm up marching, walking, knee lifts and lunges. Good question.
My answer is unique to my situation, but I have some knee problems and I develop pain in my knee if I run for very long periods. I have not had any pain from using the trampoline. As a matter of fact, recently I noticed that my knee was feeling weak before I jumpped on the the trampoline and when I finished, it felt like it had tightened up (improved, not hurt) from using the trampoline.
I would recommend that you ask your doctor before starting to use it though.
Also, I don't jump for height, but stay low and use my arms to twist, giving me a lighter full body workout.
This does not cause me any problems with my knees, you can jump very gingerly or do heal lifts with very little stress to the hips and knees.
I have knee problems, but I adjust my impact. In other words, I don't really jump up and down, I do twists, dance in place, leg slides, arm raises etc. It definitely gets my heart rate up!
I laid floors for numerous years and officiated basketball for years. This for me is the perfect exercise items as it does not bother my hips or knee's. I try to walk also however when walking my hips sometimes starts aching and creates some discomfort.
This will kill your knees!