AvatarPatrick Thomas
On the second video it shows how to scale it when on a couch. Your knee does not need to be bent all the way.
Are you able to pass the prone heel to butt test and/or the kneeling heel to butt test?
Are you able to do split squats?
Start smashing your quads. The mushball is perfect as you can determine the firmness it is. It can be less as you start and more firm as you are making progress.