Jarring is the reason most people want to return their rebounder. The ballistic impact is the force that hurts people’s knees. Inversion of the ankles is even more complained about than jarring. Both are bad but jarring is the biggest complaint with users over 200 pounds. A good rebounder takes out nearly 7/8ths of the ballistic impact so there is no jarring on the knees and back. If the deceleration is stopped to suddenly the spring stretch too much where it can’t go any further and this is when the jarring happens.
If using a multi purpose rebounder, the triple tiered springs have the lowest ballistic impact. If using a single purpose rebounder for lymphatic drainage, a high tensile low yield spring is the best. Those going for just soft springs may not get that pop at the bottom of the bounce that opens up the one way valves in the lymphatic vessels like they might want. If the springs are too bouncy and it gives too soft of a bounce, the jarring issue doesn’t exist but the inversion takes over which for some people can be just as bad. This can cause pressure on the knees and hips too. I have a 30 day return policy on every rebounder I carry. It is usually the people that buy a rebounder that is not my recommendation bases on the questionnaire that I have that people realize not at first but 2 to 3 weeks later their knees have pain. Or, their knees hurt from the mat inverting their feet.
Warning on buying the wrong rebounder for the lymphatic drainage. If someone gets a rebounder that the springs lose tensile strength over time and they don’t change the springs out every 2 years the tensile strength needed for the Health Bounce can disappear. If the spring gets weaker and it has a tensile strength of 180 Newtons and the person jumping puts a force of 200 Newtons, then the springs will yield too much and this is when spring hooks snap and the pop force that used to be there now becomes more of a throw motion at the acceleration of the bounce.