This is the second reason why most people return their rebounder. (Jarring is the first reason). If the mat cones in too much or there is not enough sweet spot in the middle, the feet pronate inward and this over time really can cause damage to the ankles and knees. This is also called edge roll pressure that make the feet turn inwards. The best rebounders allow the user to jump with the feet flat and not at an angle. The best way to test this is to wear socks and see how your feet slide towards the center of the mat. The longer the lateral displacement the more I would stay away from that rebounder. The single purpose rebounders dedicated to the Health Bounce should never invert the ankles while rebounding. The Lymphaciser, Health Circulator, and Lympholine will never invert as the rebounder doesn’t have a deep enough bounce to do it anyway.
As far as multipurpose rebounders, the Cellerciser does not invert the ankles like the tapered and barrel spring rebounder models. I have videos showing how the feet invert on different brands for those that watch my videos. People should not have to supinate their feet (bouncing on the outer edge of the feet) to top the foot from pronating inwards. Nor should someone have to jump on the mat’s outer edge to prevent inversion in the middle of the mat. The proper rebounder allows for a bounce that is always shoulder width apart. Jumping at an angle is not good over time. Note: this is different than the gravitational pull towards the center of the mat that all circular rebounders have. It is the degree of pull that I take into consideration when rebounding.