The fifth spinal movement, axial extension, is defined as a simultaneous reduction of both the primary and secondary curves of the spine (see figure 2.36). In other words,the cervical, thoracic, and lumbar curves are all reduced, and the result is that the overall length of the spine is increased.
Because the primary and secondary curves have a reciprocal relationship, which is expressed in the natural movements of flexion and extension,axial extension is ''unnatural'' in the sense that it bypasses this reciprocal relationship by reducing all three curves at once. In other words, extension generally doesn't happen all on its own; it usually requires conscious effort and training to accomplish.