These two publications demonstrate the in vivo helical axis of motion for the cervical spine:

https://www.ncbi.nlm.nih.gov/pubmed/23429677
https://www.ncbi.nlm.nih.gov/pubmed/25814180

And this one for the lumbar spine:

https://www.ncbi.nlm.nih.gov/pubmed/28062121

William Anderst, PhD
Assistant Professor
Director, Biodynamics Lab
Department of Orthopaedic Surgery
University of Pittsburgh
anderst@pitt.edu
http://www.bdl.pitt.edu