The spine is really important and in many ways is central to understanding yoga movement and posture.
The spine has five parts.
Starting from the top we have the neck or cervical spine made up of seven bones.
Below that comes the upper back (also called the thorax or thoracic spine) and it is made up of 12 bones.
Then comes the lower back (also called lumbar spine) and it is made up of five bones.
The lowest 2 parts of the back combine to form one fused block of bones: the sacrum (which sits between the hips) and the coccyx (tailbone).
Actually, not everyone has the same number of bones in their back. For instance, there are some people who have six or four bones in their lower back rather than the usual five.
More important than the basic divisions of the spine are the four curves of the spine, three of which can move and they can deepen or flatten with body movements. It's these top three curves (the neck, upper and lower back) with which we are particularly interested when we use yoga postures (āsana-s) to promote physical health.
You can see these curves when you look at someone's back. The upper back is convex towards you, while the neck and the lower back are concave.
The curves on the spine give it strength and help us move effectively. However, the shape of our spine deteriorates over time. As we grow older, the curves tend to deepen. Shoulders become rounded and stooped as the curve of the upper back (the thoracic spine) increases. This tendency can cause the head to tip back as it compensates for the change in the curve of the upper back, and this causes the curve of the cervical spine (the neck) to increase too, but in the opposite direction. And, if this wasn't enough, the curve of the lower back also increases and the belly bulges forward.
What we do for a living adds to the effect that aging has on us. If we are desk bound the curves of our spine will also deepen. Hours of regular gaming has a similar effect.
Why does this happen?
Several reasons, including the effects of gravity, loss of muscle tone and strength and the decreased use of some muscle groups. And when some experts say that sitting for hours each day has a deleterious effect on our health, similar in degree to smoking 20 cigarettes a day, well I can quite believe it.
Āsana practice, done in an appropriate way, can prevent or counteract the tendency of the spine to slump as we age. It can maintain or restore the optimum structure of the spine giving us good structural spinal alignment, strength and flexibility; improve functioning of various other body systems (e.g. breathing and digestion); and āsana practice can lead to a feeling of mental well being.
Forward-bending movements help flatten the curvature of the lumbar spine, although they also tend to increase the natural curve of the thorax.
Backward-bending movements have the opposite effect. They tend to flatten the thorax and deepen the lumbar.
Combining forward- and backward-bending movements in a carefully thought out sequence brings to the fore the benefits of āsana practice while minimizing the negatives.
Keeping the spine feeling long while practising āsana-s is helpful.
The breath can be an important part of yoga practice, particularly in the tradition of TKV Desikachar and his father T Krishnamacharya.
Inhalations that encourages the lifting and opening of the rib cage help to flatten the thorax.
Exhalations that use the abdominal muscles help support the lumbar spine.
I recommend something called 'locational breathing' my classes and in 1-1 sessions. This technique supports the spine while moving. It aims to encourage the inhale to start in the chest and move towards the belly. You should feel the chest gently filling up from the top of the chest and that filling up of the inhale progresses step-by-step towards the belly.
On the other hand if we want the exhale to help optimise the shape of the spine, it should start with a contraction of the lower abdominal muscles. The breath starts leaving the body from there. The chest is the last part of the body to 'empty'. This technique helps to support the way we can move while practising āsana-s.
The inhalation done as suggested here supports the opening of the chest and flattening the thorax, while the exhalation helps us to focus on the low back region, supporting and flattening it gently.
Not only does the breath interact positively with the body, it also influences the mind.
The mind, of course, is the real subject of Yoga.
TKV Desikachar, 1995, The Heart of Yoga
AG Mohan, 1993, Yoga for Body, Breath and Mind