Back extensions are crucial to lower back development. Here you can find instructions about building the lower back for power. Without it you won't be able to grow much muscle anywhere and your condition will remain weak. Check it out.

Here's the intro to it:

"Your options for back extensions depends on the equipment available, of course. The most important are the Roman chair, a gate belt, and barbell weights. Using this routine I rehabilitated a compressed LL2 (lower lumbar 2) vertebrate over a period of about 3 months. At the end of 6 months I was free squatting 365lbs. and leg pressing 700lbs."

He claims this is the key that allowed him to enter into the hard work necessary to build radical strength. Try it.