As we get older our spines bend less, mostly because of the effects of gravity on the spinal discs, which begin to dehydrate and become compressed after 30, reducing the spaces between the facet joints in the vertebrae and limiting movement. Gravity and an upright human posture also causes some the spinal muscles found in children to be converted to more rigid and stable ligaments in adults.
Child-gymnasts and young yogis are capable of much more extreme backbending than adults, but this lessens over time, as flexibility gives way to stability. Backbending at all ages is inseparable from core strength and flexibility in the hips and shoulders, if the core or hips are weak, all extension force is directed into the spine, and children are just as vulnerable as adults to spinal injuries.
Back pain problems are much more complex than disc injuries, please refer to Lower Back Pain: Some Yoga- Related Causes for an outline of the factors that create lower back pain.
The spine is stabilised by the numerous small muscles that surround the vertebrae and the larger Erector Spinae muscles, as well a thick layer of connective tissue, called the Thoracolumbar fascia. The Transverse Abdominis and Oblique Abdominal muscles are attatched directly to the spine via the Thoracolumbar Fascia and when they contract during movements, the fascial network is pulled tight and this supports the vertebrae.
Teachers usually tell students to pull the belly button toward the spine to teach them to make these connections but in fact, although there are four different abdominal muscles, the abdominals are all connected to each other and function as a unit, the only difference between them is in the types of movements that they can create.
Focusing on isolating one muscle- the transverse Abdominis- will not necessarily improve spinal stability, nor alleviate back pain. Asanas which require core strength to perform, like arm-balancing, Navasana, or Tolasana are often better ways to learn to engage the abdominals, than crunch-style abdominal isolation exercise, which frequently causes neck and upper back problems.
Strengthening the body in lateral movement is often overlooked in yoga, most emphasis is on forward and backward movements, and it is the oblique abdominals that support backbending the most. Naturally, the back must also be strong and Salabhasana variations and Bhekasana work well.
Practicing Vasistasana, Parvritta Navasana, Jathara Parivatarasana, Parsva Bakasana, Astavakrasana, Anantasana, Mandalasana and twisting in Sirsasana and Salamba Sarvangasana can be the key to Kapotasana, Vrschikasana and drop backs, although strong, flexible hips are also crucial for backbending- the Psoas muscle needs to lengthen in the lower back, but is unable to do this if the Gluteus Maximus is weak. Eka Pada Urdhva Dhanurasana and Eka Pada Viparita Dandasana are useful here.
Reading sources: Ellenbecker, De Carlo, DeRosa, 2009, Effective Functional Progressions in Sport Rehabilitation Sharkey, 2008, The Concise Book of Neuromuscular Therapy