I chose my Navaro Fusion partly because it's got a semi-upright sitting position which is easier on my back - I've got two herniated discs, one cervical and one lumbar. I can't do drop bars because leaning over and looking up while riding is horrible for my neck. The jury is still out on whether or not the Fusion is going to work for the long term. I still have to maintain a somewhat forward-leaning posture, and I've noticed a slight increase in pain and numbness in my arms.
But enough about me, my question is: Upright sitting bikes like the Trek Navigator are supposed to be better for people with bad backs. However, I've watched some videos of people right bikes like that, and I noticed that they really have to shift their pelvis from side to side when pedaling. I'm worried this type of movement would be bad for my lumbar. In fact, upright sitting bikes in general seem like they put all the pressure on your tailbone (and lower spine) without enough assistance from your upper body.
Can anybody with bad back share their experiences trying to find the right bike for the sitting position? The Trek Navigator seems to be popular for people who are getting back into biking later in life, or people who want a bike to just toddle along with their kids to the park. Anybody use the Trek Navigator for a daily commute of more than 5 miles?