Lower back pain is the single leading cause of disability worldwide, affecting an estimated 619 million people in 2020 alone. In roughly 70% of cases, the cause is a simple muscle strain or ligament sprain. The remaining cases involve structural changes in the spine, inflammatory conditions, or occasionally pain referred from organs elsewhere in the body. A precise anatomical diagnosis can only be identified in about 20% of cases, which means most lower back pain is categorized broadly as “mechanical” after more serious causes have been ruled out.
Muscle Strains and Ligament Sprains
The most common cause of lower back pain is an injury to the soft tissues that support the spine. A strain happens when muscles or tendons (the tissue connecting muscle to bone) get twisted, pulled, or torn. A sprain involves the ligaments, which are the bands of tissue connecting bones to each other at a joint. In practice, the two feel very similar and are often grouped together.
These injuries typically happen from a single moment of improper lifting, a sudden twist, or a fall. They can also develop gradually from repetitive movements that overwork the muscles over time. The good news is that most people with a lumbar strain or sprain improve within about two weeks. Staying gently active during recovery tends to produce better outcomes than strict bed rest.
Disc Problems
Between each pair of vertebrae sits a rubbery disc that acts as a shock absorber. These discs have a tough outer layer and a softer gel-like center. A herniated disc (sometimes called a slipped or ruptured disc) occurs when that soft inner material pushes through a tear in the outer layer. If the bulging material presses on a nearby nerve, it can cause not just back pain but also numbness, tingling, or weakness radiating down one leg. Disc herniations account for roughly 5% to 10% of mechanical back pain cases.
Degenerative disc disease is a separate, slower process. As you age, the discs naturally lose water content, shrink, and become less elastic. This gradual breakdown can lead to pain and instability in the spine, and it sometimes sets the stage for a herniation later on. Nearly everyone shows some degree of disc degeneration on imaging by middle age, though not everyone experiences pain from it.
Joint and Bone Changes
The spine contains small paired joints called facet joints that guide movement between vertebrae. Like any joint, they’re lined with cartilage that can wear down over time. When this cartilage thins, bones start rubbing against each other, and the body may respond by forming bone spurs or fluid-filled cysts. This process, called facet arthropathy, becomes increasingly common after age 45, though some degree of wear happens in everyone’s facet joints with age. It tends to cause a deep, localized ache in the lower back that worsens with prolonged standing or bending backward.
Lumbar spondylosis, a broader term for age-related spinal degeneration that includes disc and joint changes together, accounts for about 10% of mechanical back pain cases. Other structural causes include vertebral compression fractures (4% of cases), which are especially common in people with osteoporosis, and spondylolisthesis (3% to 4%), where one vertebra slips forward over the one below it.
Spinal Stenosis
Spinal stenosis is a narrowing of the spinal canal, the bony tunnel that houses the spinal cord and nerve roots. It accounts for about 3% of mechanical back pain and is most common in people over 60. The narrowing compresses the nerves that control sensation and movement in the lower body, causing pain, tingling, or cramping in the legs.
What makes spinal stenosis distinctive is how posture affects the pain. Standing upright and walking naturally narrow the spinal canal further, which increases pressure on the nerves and makes symptoms worse. Sitting down or leaning forward opens the canal slightly and provides relief. This is why people with stenosis often find they can ride a bike comfortably but struggle to walk the same distance. If you notice that back and leg pain consistently eases when you lean on a shopping cart or sit down, stenosis is a likely explanation.
Inflammatory Conditions
Not all back pain comes from wear and tear or injury. A smaller but important subset is caused by inflammation from the immune system itself. Ankylosing spondylitis is the most well-known example. It typically starts in the late teens or twenties and causes pain and stiffness that come on slowly, feel worst in the morning or wake you from sleep at night, and actually improve with movement rather than rest. This pattern is the opposite of mechanical back pain, which usually feels better after resting and worse with activity.
If your back pain has been present for more than three months, started before age 40, gradually worsened, and improves with exercise but not with rest, an inflammatory cause is worth investigating.
Pain Referred From Other Organs
Sometimes what feels like back pain actually originates in an organ elsewhere in the body. Kidney stones are a common example, though they’re frequently confused with musculoskeletal pain. Kidney stone pain typically hits beneath the ribs on one side, not low in the back near the waistline where most mechanical pain lives. It often wraps around the side toward the pelvis or groin. In men, the pain can extend into the scrotum. In women, it can feel like a menstrual cramp radiating to the labia.
A useful distinguishing feature: kidney stone pain does not shoot down the leg into the buttock or below. If your pain follows that path, a nerve issue in the spine is far more likely. Other organs that can refer pain to the lower back include the pancreas, the abdominal aorta, and the uterus or ovaries.
Why Some Back Pain Becomes Chronic
Most acute lower back pain resolves within a few weeks regardless of the cause. But for some people, pain persists well beyond the expected healing time. Research has identified several psychological and social factors that reliably predict whether someone will develop long-lasting pain and disability. These include believing that back pain is inherently dangerous or severely disabling, avoiding movement out of fear that it will cause more damage, withdrawing from social activities, low mood, and expecting that passive treatments like massage or medication will fix the problem rather than active participation in recovery.
These aren’t imaginary contributors. Fear of movement can lead to physical deconditioning, which genuinely makes the back more vulnerable and painful. Screening tools now classify patients into low, medium, and high risk categories based partly on these factors, because addressing them early can significantly change the trajectory of recovery.
When Imaging Is and Isn’t Helpful
Many people with back pain assume they need an MRI or X-ray to find the cause. The American College of Physicians has found strong evidence that routine imaging for lower back pain does not improve outcomes. Imaging is recommended only when there are severe or worsening neurological symptoms, suspicion of a serious underlying condition like cancer or infection, or when surgery or another invasive procedure is being considered.
One reason imaging can be misleading is that disc bulges, bone spurs, and other structural findings are extremely common in people with no back pain at all. An MRI might reveal something that looks alarming on paper but is actually an incidental, painless age-related change. This is why imaging results need to be interpreted alongside your actual symptoms, not in isolation.
Signs That Require Emergency Care
The vast majority of lower back pain is not dangerous, but a rare condition called cauda equina syndrome requires immediate treatment. It happens when the bundle of nerves at the base of the spinal cord becomes severely compressed. Warning signs include sudden difficulty urinating or having a bowel movement, loss of bladder or bowel control, numbness in the inner thighs, groin, or buttocks (sometimes called “saddle” numbness), and progressive weakness in one or both legs. If you develop any combination of these symptoms alongside back pain, go to the emergency room. Delayed treatment can result in permanent nerve damage.