The lumbar spine has five vertebrae, labeled L1 through L5, running from the bottom of your rib cage to the top of your sacrum. These are the largest vertebrae in your entire spine, built to handle the considerable load of your upper body weight.
Why Five Large Vertebrae
Your spine has 33 vertebrae in total, divided into regions that each serve a different purpose. The lumbar region sits at the bottom of the flexible portion of your spine, below the 12 thoracic vertebrae (mid-back) and 7 cervical vertebrae (neck). Because the lumbar vertebrae bear more weight than any other spinal segment, they’re noticeably thicker and wider than the vertebrae above them. Each one increases slightly in size from L1 down to L5, which is the largest of all.
The lumbar vertebrae have a kidney-shaped body when viewed from above, with short, sturdy bony projections at the back. Unlike thoracic vertebrae, they don’t attach to ribs, which is exactly why your lower back can bend and twist as freely as it does.
The Curve They Create
The five lumbar vertebrae form a forward curve called a lordosis. In most people, this curve measures around 60 degrees. That inward arch isn’t a flaw; it positions your center of gravity over your hips and lets your spine absorb shock efficiently when you walk, run, or jump. The lumbar lordosis is typically about 20 degrees greater than the outward curve of the thoracic spine above it, and the balance between these two curves is a major factor in comfortable, upright posture.
How Much Your Lower Back Moves
The lumbar spine handles a surprising amount of motion for a weight-bearing structure. You can bend sideways roughly 30 degrees in each direction, though that range changes significantly over a lifetime. Children between ages 2 and 13 can sidebend as much as 62 degrees to either side. By your mid-30s to late 40s, that drops to about 30 degrees. After 65, most people have around 20 degrees of lateral motion remaining.
Flexion (bending forward) and extension (arching backward) happen primarily at the L4-L5 and L5-S1 segments, which is one reason those two levels take the most mechanical stress over time.
What Runs Through Them
One detail that surprises many people: the spinal cord itself doesn’t extend through the entire lumbar spine. It ends in the upper lumbar area, typically around L1 or L2, and then fans out into a bundle of individual nerve roots called the cauda equina (Latin for “horse’s tail”). These nerve roots continue downward through the spinal canal, branching off at each lumbar level to serve your legs, bladder, bowels, and pelvic floor. This is why a lumbar injury can cause symptoms far from your back, including numbness, tingling, or weakness in your legs and feet.
Why L4-L5 and L5-S1 Cause the Most Problems
The lowest two lumbar discs are the most common sites for herniation. In one study of patients with lower back pain, combined herniation at both L4-L5 and L5-S1 was the most common pattern, affecting nearly 29% of cases. Isolated L4-L5 herniation accounted for about 17%, and isolated L5-S1 herniation for roughly 10%. This concentration of disc problems at the bottom of the lumbar spine makes sense biomechanically: L4-L5 and L5-S1 handle the greatest compressive and rotational forces of any spinal segment, especially during bending and lifting.
When Five Isn’t Exactly Five
Not everyone has precisely five lumbar vertebrae. About 19% of adults have what’s called a lumbosacral transitional vertebra, meaning the boundary between the lumbar spine and sacrum isn’t quite standard. The most common variation is sacralization, where L5 partially or fully fuses to the sacrum, effectively leaving you with four mobile lumbar vertebrae. This occurs in roughly 18% of symptomatic adults. The reverse, lumbarization, happens when the top segment of the sacrum separates and behaves like a sixth lumbar vertebra, though this is far less common at under 2%.
These variations often cause no symptoms at all and are discovered incidentally on imaging. But they can matter for surgical planning, since a surgeon needs to count vertebrae accurately to operate at the correct level. If you’ve ever been told your imaging looks “a little different” at the base of the spine, a transitional vertebra is one of the most likely explanations.