I often wonder about back pain with footstools.
I have to sit for 2 1/2 hours in concert when I play with my orchestra, and I never get back pain.
I use a footstool.
I teach a lot, and though there ARE people with weak backs, a lot of those who get back pain get it because they lift the left leg by tilting the pelvis. As soon as the spine is curved, the muscles supporting it are in constant tension and these are responsible for a lot of the pain and twinges.
I used to ride a pedal cycle a lot, and you could see the cyclists riding in front of you divide into two camps - those who twisted their spine and those who sat perfectly still and let the hip joints do the work. I can pretty much predict which group got exhausted first.
If you have back pain, humour me with this experiment...
Sit on a chair and pretend to pedal a cycle, lifting and lowering alternate legs.
Set up your camera to take a short video of you from the rear.
If you get back pain from a footstool and if you see your spine move at all doing this, I think we have the root cause. My spine doesn't move at all.
Nothing in this life is impossible except skiing through revolving doors