Most low back pain is triggered by some combination of overuse, muscle strain, and injury to the muscles, ligaments, and discs that support the spine. Many experts believe that over time muscle strain can lead to an overall imbalance in the spinal structure. This leads to a constant tension on the muscles, ligaments, bones, and discs, making the back more prone to injury or reinjury. The causes of pain in the low back, or lumbosacral region, tend to add on to one another. For example, after straining muscles, you are likely to walk or move in different ways to avoid pain or to use muscles that aren't sore. That can cause you to strain other muscles that don't usually move that way. The most common causes of low back pain are:

• Injury or overuse of muscles, ligaments, facet joints, and the sacroiliac joints.
• Pressure on nerve roots in the spinal canal. Nerve root compression can be caused by a herniated disc, often brought on by repeated vibration or motion (as during machine use or sport activity, or when lifting improperly), or by a sudden heavy strain or increased pressure to the lower back.
• Osteoarthritis (joint degeneration), which typically develops with age. When osteoarthritis affects the small facet joints in the spine, it can lead to back pain. Osteoarthritis in other joints, such as the hips, can cause you to limp or to change the way you walk. This can also lead to back pain.
• Spondylolysis and spondylolisthesis, vertebra defects that can allow a vertebra to slide over another when aggravated by certain activities.
• Spinal stenosis, or narrowing of the spinal canal, which typically develops with age.
• Fractures of the vertebrae caused by significant force, such as from an auto or bicycle accident, a direct blow to the spine, or compressing the spine by falling onto the buttocks or head.
• Spinal deformities, including curvature problems such as severe scoliosis or kyphosis.
• Compression fractures. Compression fractures are more common among postmenopausal women with osteoporosis, or in men or women after long-term corticosteroid use. In a person with osteoporosis, even a small amount of force put on the spine, as from a sneeze, may cause a compression fracture.

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The bones (vertebrae) that form the spine in your back are cushioned by small, spongy discs. When these discs are healthy, they act as shock absorbers for the spine and keep the spine flexible. But when a disc is damaged, it may bulge or break open. This is called a herniated disc. It may also be called a slipped or ruptured disc. You can have a herniated disc in any part of your spine. But most herniated discs affect the lower back (lumbar spine). Some happen in the neck (cervical spine) and, more rarely, in the upper back (thoracic spine). A herniated disc may be caused by:

• Wear and tear of the disc. As you age, your discs dry out and aren't as flexible.
• Injury to the spine. This may cause tiny tears or cracks in the hard outer layer of the disc. When this happens, the gel inside the disc can be forced out through the tears or cracks in the outer layer of the disc. This causes the disc to bulge, break open, or break into pieces.

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