A "Straight" Spine Should Retain Its Natural Curves
Is the objective to straighten out the spine completely?
I ask only because again there are diagrams of the spine in yoga books with the centers where the spine has the characteristic elongated "stretched out S" shape.
Thanks for your clarification.
I'm very glad that you asked this question, because it's quite possible to go seriously wrong in pursuit of a straight spine - in asana practice, or in life generally.
When yogis say "straight spine" (synonym: neutral spine) they mean a spine in its natural curves: inward curve in the lumbar spine (around waist level), outward curve in the back of the chest, and inward curve in the neck. We want to have such a straight spine at all times - in asana practice, in meditation, in life generally - unless we are deliberately bending the spine one way or another.
However, a geometrically straight spine could be a nightmare. It would eliminate the "springy" cushioning effect that the S-shape provides, and would be very likely to lead to injury. Don't seek that. Rather, keep your natural curves.
That said, some people have developed unnatural curves in their spines from injuries, or years of poor posture, or jobs that reinforce poor posture. So sometimes it is advisable to correct an unnatural curve in the spine, but that should be done only under the guidance of a qualified health care provider (such as a chiropractor or physical therapist, perhaps supplemented by private work with a qualified yoga teacher).
Ask a question