Q: When I do backbending postures I often end up with pain in my lower back.
Is there something I can do to prevent this?

A: Backbends are one of the most challenging asana groups. They require the spine to be in a position(called extension) that is rarely otherwise included in our limited repertoire of habitual daily movements. Understanding the anatomy of the lumbar spine and sacrum can add a dimension of "intelligence" to backbends that can prevent injury. If you already have an existing lower back problem, doing backbends without proper alignment can make it worse.
The lumbar spine contains the lowest five vertebrae . They are the largest, heaviest and thickest because they bear the weight of the entire upper body. Every vertebra has a spinous process ( the bumps you can see and feel on the spine) and each lumbar spinous is thick and hatchet-shaped. L5, the lowest vertebra, forms a joint with the top of the sacrum –the triangular bone commonly called the tailbone. This joint is particularly vulnerable to injury.
To maximize the benefit and minimize potential discomfort in backbends, the pelvis and lumbar spine must be positioned in a way that lengthens the lower spine and opens the groins. To prevent compression in the lumbar spine, the pivot point in the backbend must be the bottom or apex of the sacrum, not the base. Many yogis mistakenly pivot from the base which compresses the vulnerable joint between the the sacrum and L5 and can cause discomfort by pinching muscles and ligaments in the lumbar area.
To experiment with these concepts, first warm up the spine with Catcow: on all fours, arch the back and tip the tail up to the ceiling on each inhalation; on each exhalation, round the back and tuck the tail and head under. Repeat slowly and with awareness for 2 or 3 minutes.
Bow Pose is a good one in which to practice lengthening the lumbar spine and groins in backbends:
1. Lie on your stomach with arms overhead. Reach back with your right hand to grasp the right foot.
2. Inhale and press your pubic bone gently into the floor. This encourages you to center the pivot point of the arch in the backbend at the apex of the sacrum. As you inhale, lift your upper body and right thigh off the floor. Keep the back of the neck long. Exhale and return to the floor. Continue for several breaths then repeat on the left side.
3. To contrast the results, do several movements without first pressing the pubic bone into the floor. Notice the increased compression in the lower back.
4. If you are able, do the full pose by lifting both thighs and the upper body off the floor as you inhale and anchor the pubic bone into the floor. If you feel any compression in the lowback, practice one side at a time until you learn to lengthen the spine and pivot from the sacral apex.
5. Do several more Catcows as a counterpose.
Take this concept into more complex poses such as Camel and Wheel. The more you can lengthen the lumbar spine and maintain space between the vertebrae by lowering the pivot point of the backbend to the sacral apex, the less the compression and discomfort. Subtle energies can then flow more freely and you will receive maximum benefit from the energizing and cleansing backbends.