While I agree that some people may have instability in this region, I don't agree that it's caused by AS....it would be a separate issue, I would find it odd though that there's an absence of stiffness in that region because that is where AS symptoms usually begin, in the SI Joints (sacroilitis is the hallmark symptom of early AS) so if there was an absence of stiffness and immobility in the lumbar region, AS would be ruled out until there is radiographic evidence to support a DX of AS....
