The Lumbar spine, unlike the lower thoracic spine, moves very little and should not be used during back extension movements, because the vertebrae or discs can be damaged. Any muscular imbalances between the hips, legs and lower back cause pain and restriction in the lumbar spine, and can ultimately result in injury.
Hamstring muscles that are overactive cause the lower back muscles to tighten up, jamming the lumbar facet joints. An overactive Iliopsoas will do the same thing, which is why we are cautioned against sit-up exercises.
When we go to doctors with these injuries, they tell us that back extension is the cause and that it should be avoided. Back extension is a large part of yoga and many people can do it safely, if their spines are healthy to begin with, they use correct technique and, most important, maintain a good balance of strength in their bodies. Although extension is the actual cause of most lumbar injury, underlying factors can be inflexibility of the hips or thoracic spine.
Correct back extension occurs in the lower Thoracic spine. Tightness in the Thoracic spine can occur if abdominal strength is greater than spinal muscle strength and can usually be corrected with Salabhasana. Exercises that are similar to Salabhasana are prescribed in back rehabilitation and are usually safe for a healthy back. Backbending postures like Dhanurasana,
Kapotasana and Vrschikasana require great spinal flexibility but if the hips are tight, the Lumbar spine is hyperextended, causing pain or injury. Hip tightness is often a product of an imbalance between leg and core strength. Strong, tight leg muscles inhibit hip muscles, making them weak and inflexible.
Forced back extension, using straps and walls as aids is sometimes taught in classes and is extremely risky- the spine does not naturally bend beyond its ability to support itself and forcing it to bend is an invitation to injury. Back flexibility can be acquired with patience, never with the use of force. Hip flexibility and a balance between abdominal and back strength are a sane way to create a flexible back.
Oblique abdominal strength is often overlooked in yoga, a lot of posture sequencing ignores lateral movements and yet including more of these can be the key to unlocking a stiff, sore back. Postures include Parsva Bakasana, Astavakrasana, Vasistasana, Viswamitrasana, Mandalasana, and twisting movements in Sirsasana, Salamba Sarvangasana and Navasana. Strong oblique muscles are the main supporter of backbends in yogic practise.
Reading Sources: De Franca, 1996, Pelvic Locomotor Dysfunction Brunnstrom’s Clinical Kinesiology, 5th edition 1996 Ellenbecker, De Carlo, DeRosa, 2009, Effective Functional Progressions in Sport Rehabilitation