A good rebounder should help loosen up the joints, NOT jar them. If someone already has knee or back problem, I would avoid the rebounders that don't take out at least 80 to 90% of the ballistic impact. The extra ballistic impact of a rebounder that jars you people don't always feel it at first but after months it can start to impact the joints. I believe the real issue behind joint pain is heavy metals, until they are removed there will probably always be joint problems. If you can't do the Health bounce comfortably on a rebounder, then this will probably never be done.
Out of all the rebounders that people buy who have joint problems, the Needak and Cellerciser are the easiest on the joints. They both have a different feel. The Cellerciser is slightly firmer but it doesn't invert the ankles (pronation of the ankles is practically non existent on the Cellerciser.
The Needak feels softer but it has more deceleration and for people with bad ankles.
The Lymphaciser, forget it, I do not recommend it for someone with joint issues. Way to firm. The Reboundair I have the same issue with. Basically, if someone has weak or painful joints, you don't want a short spring.
Once the lactic acid is removed from the muscles, the body can become tolerant of nearly any position its in. The muscles become looser and there is less pull on the bones holding them that pull them out of alignment.
Diffusing the toxins out of the body is probably the root of the problems that are causing joint pain.
Compare the top Lymphatic Rebounders
New Zealand Lymphaciser VS CELLERCISER VS JUMPSPORT