The human spine is made of thirty-two separate vertebral segments that are separated by intervertebral discs made of collagen and ligaments. These discs are shock absorbers and allow a limited degree of flexibility and motion at each spinal segment. The cumulative effect allows a full range of movement around the axis of the spine, especially the neck (cervical) and lower back (lumbar spine).
Motion between each segment is limited by the tough outer disc ligaments and the joints that move (articulate) at each spinal level (the facet joint). Under each joint, just behind the disc, is a pair of nerve roots that exit the spinal canal. The exiting hole (foramina) that surrounds the nerve (disc in front, joints above and below) is relatively small and has little room for anything besides the exiting nerve.
Normal life stressors compounded by traumatic injuries to the spinal architecture cause degeneration in the discs and the joints of the spine. With age, injury, poor posture there is cumulative damage to the bone or joints of the spine:
As disc material slowly wears out, ligaments loosen and excess motion occurs at the joint.
The body naturally and necessarily thickens the ligaments that hold the bones together.
Over time, the thick ligaments tend to calcify, resulting in flecks of bone or bone spur formation.
As the central spinal canal and the foramina thicken their ligaments, compression of the nervous system causes clinical symptoms.
Degenerative changes to normal vital tissue begin in early adulthood, but usually this slow process does not present with nervous system compression until we are in our sixties or seventies. Factors that can accelerate the degenerative process and bone spur growth include:
Congenital or heredity
Life-style, including poor posture
Traumatic forces, especially sports related injuries and motor vehicle accidents
As always, to help avoid or minimize back pain it is generally advisable to stay well conditioned (both in terms of aerobics and strength) and to maintain good posture throughout one's life.