I've been reading Pavel Tsatsouline's book Relax into Stretch again and decided to grab a few quotes relevant to the discussion on back flexibility:
Do not abuse relaxed stretches and stay away from them altogether when it comes to your back. Soviet sports scientist L. P. Orlov warns: "While most large joints are stabilized by muscles and the ligaments do not affect their position, in the case of the spine it is the ligaments that play the important role of maintaining the normal spinal alignment. Insufficiency of the ligamentous apparatus makes it difficult to maintain the normal spinal curve with muscle tonus and tension alone. Weakening of the ligaments unavoidably leads to deformation of the spinal column."
Relaxed stretching develops flexibility without strength. This is unnatural. Normally your body does not allow a range of motion it cannot control. A graphic illustration of this is a medical condition known as the 'frozen shoulder'. If, after an injury, you do not use your shoulder for a long time, it will lose much of its range of motion. Under anesthesia though, the surgeon can turn the shoulder through three hundred sixty degrees without trouble.
When the patient wakes up and his muscles start working, the shoulder freezes again. The nervous system knows that the muscles are not strong enough to control the full range motion and will not let the shoulder's owner have it. Without proper rehab the problem keeps feeding on itself. Physical therapists know that muscles habitually kept in a shortened position lose their strength in the stretched position. Before you know it, the weakness-inflexibility vicious circle turns you into a piece of furniture!
The same situation, albeit less extreme, repeats itself with every Joe or Jane when they work out improperlyor are simply inathletic. Your muscles keep losing their strength in the lengthened positionif they ever had it to start withbecause your lifestyle always keeps them shortened. Physical therapists call this problem tight weakness. As strength goes south, so does flexibility. The muscles become even shorter which makes them ever weaker which makes them even short... ad nauseam.