How your back is built
Your spine is made up of 30 bones stacked in a column. Each of these bones is called a vertebra. From the side, your vertebral column has a natural curve toward the back of your body as it passes through your upper chest, balanced by a similar curve toward the front through the lower vertebrae. Joints between the vertebrae in your neck enable you to tilt, rotate and flex your neck. Joints between the larger vertebrae in your lower back (lumbar spine) enable you to bend, turn and twist at your waist.
A tear in one of the spinal disks — rubbery cushions between the vertebrae — may cause back pain or play a role in the development of back pain. Spinal disks prevent the stacked vertebrae from rubbing against each other. The outer layer (annulus) of each disk is normally tough and pliable. The inner layer (nucleus) is soft.
A spinal disk can gradually develop aging-related changes that cause or contribute to back pain. There's less water within the annulus, so it's not as flexible, and it may develop tears. If it does, the nucleus may bulge out, press on a nerve and cause pain in your back as well as one or both legs.
Degenerative changes in the spine
Degenerative changes develop in the vertebrae as the spinal disks become flatter and less flexible. Without the cushioning that these disks normally provide, the joints (facets) between vertebrae press tightly against each other. This can cause back pain and stiffness. Your body may try to compensate for these changes by building new bone to support the area where loading pressure is increased.
As you age, the amount of calcium in your bones decreases. This lowers the density or mass of your bones, making them porous and brittle — a condition known as osteoporosis.
If you have osteoporosis, daily lifting and other routine activities can cause compression fractures — cracks, usually on the forward-facing sides of one or more vertebrae. Compression fractures often cause back pain, especially when they first develop.