Back bends open the chest and heart and counteract the tendency to round in the upper back and thrust the head forward, which our life in front of computers is encouraging. Breathing is improved and enhancement of wellbeing follows.
Contracting into the muscles of the back strengthens the muscles, but also helps those muscles to know how to relax. When the brain experiences the sensation of the contraction and the subsequent release it learns how to distinguish between tension and relaxation, and so the muscle can relax better after the back bend, which often gives relief to back pain.
Back bends involve bending the spine, and some parts of the spine love to bend backwards while others do not. It is important to balance the bending across the whole spine, encouraging mobility in the less mobile parts and stabilising the more mobile parts, so force is evened out over the whole curve. The result is a sense of wholeness in which energy flows effortlessly along the length of the spine.
The bones of the spine are divided into sections and are numbered from the top down. The neck (cervical spine) has seven bones, numbered 1 to 7 from the top down, C1 to C7. Below that is the thoracic spine, which is where the ribs attach. The thoracic has 12 bones, topmost is T1, counting down to T12. The lower back is called the lumbar spine and has five bones, L1 to L5 counting down. The sacrum is the triangular bone that is a bridge between the two halves of the pelvis. It is really five bones, but they fuse together, still we number them S1 to S5. Below that we have the four bones of the tailbone (coccygeal vertebrae), also fused. Between each vertebra is a gel cushion, the intevertebral disc. We can name them by their two adjacent vertebrae, such as C2/C3 which is the disc between C2 and C3.
So the combination between the natural curve and the angle of the spinous processes creates areas of the spine that are more mobile and areas that are less mobile. The two areas of the spine that have secondary curves, the neck and the lower spine, are quite mobile in extension and flexion. The thoracic spine, which has a primary curve is great at flexion, bending forwards, but not so good at all at extension.
The danger therefore is that we make all the movement in the two mobile areas and this can place stress on these areas. There is particular weakness at the places of transition between a mobile part with a less mobile part, so C5/T1, T12/L1 and L5/S1 are places where injury more often occurs. If we keep bending sharply into the same area it is a bit like taking a metal coat hanger and bending it back and forwards on the same place repeatedly. Eventually it breaks.
Consider these two pictures of camel pose (utsrasana).
To my eye the picture on the left is taking more of the bend in the lumbar, and the neck is also taken back to its limit, whereas the woman in the picture on the right is making the curve more even, stabilising the lower back and neck and encouraging more mobility in the thoracic.
In the following silhouettes of cobra pose (bhujangasana) you may have a sense of an energetic blockage created by the sharp extension in the neck and lower back in the one on the left compared to a freer flow of energy in the one on the right which contains the extension in the neck and lower back and opens the chest to mobilise the thoracic.
As you come into your back bend start by lengthening the spine which helps to mobilise the thoracic, lifting the sternum. Drawing the throat back will engage support for the spine from the entire digestive tract, helping to stabilise it.
And do choose versions of the back bend that are appropriate for your body. For example use props for the hands to reach to in camel pose or keep them in the lower back for additional support and stabilisation.
After your back bending practice, counter pose in child's pose, balasana.