Lumbosacral spine anatomy

Understanding the Anatomy of the Lower Back

The lumbosacral spine, commonly referred to as the lower back, is the lowest portion of the spinal column. It includes the lumbar spine (L1–L5) and the sacrum, which connects the spine to the pelvis. Together, these structures support the weight of the upper body, protect the spinal nerves, and allow movement needed for standing, walking, lifting, and bending.

Because the lumbosacral spine bears much of the body’s weight and is involved in nearly every daily activity, it is one of the most common areas affected by back pain and degenerative spine conditions.

Quick Facts

  • The lumbar spine consists of five vertebrae (L1–L5).
  • The sacrum connects the spine to the pelvis through the sacroiliac (SI) joints.
  • The lumbosacral spine supports most of the body’s weight.
  • Nerves in the lower back control sensation and movement in the hips, legs, feet, and bladder.
  • Healthy discs, joints, muscles, and ligaments work together to provide strength, stability, and flexibility.

What Is the Lumbosacral Spine?

The lumbosacral spine is made up of two connected regions:

  • The Lumbar Spine (L1–L5): The mobile portion of the lower back that provides strength and flexibility.
  • The Sacrum: A triangular bone formed by five fused vertebrae that connects the spine to the pelvis.

Together, these structures provide support for the body while protecting the spinal nerves that travel to the lower extremities.

The Parts of the Lumbosacral Spine

Lumbar Vertebrae

The lumbar spine contains five large vertebrae, labeled L1 through L5.

These vertebrae:

  • Support the majority of body weight
  • Protect the spinal canal
  • Allow bending and twisting
  • Serve as attachment points for muscles and ligaments

Because they bear significant mechanical stress, the lumbar vertebrae are commonly affected by arthritis and degenerative changes.

The Sacrum

The sacrum is located below the lumbar spine and above the tailbone (coccyx).

It:

  • Connects the spine to the pelvis
  • Transfers body weight into the hips and legs
  • Forms the sacroiliac (SI) joints with the pelvic bones
  • Provides stability during standing and walking

Intervertebral Discs

Between each lumbar vertebra is an intervertebral disc.

These discs:

  • Act as shock absorbers
  • Maintain space between vertebrae
  • Allow smooth movement
  • Help distribute forces placed on the spine

Over time, discs may lose water content and become more susceptible to bulging, herniation, or degeneration.

Facet Joints

Each vertebra connects through facet joints located at the back of the spine.

Facet joints:

  • Guide spinal movement
  • Prevent excessive motion
  • Provide stability
  • May develop arthritis with aging

Spinal Cord and Cauda Equina

The spinal cord ends near the L1–L2 level.

Below this point, the spinal canal contains a bundle of nerves called the cauda equina, which resembles a horse’s tail.

These nerves control:

  • Leg movement
  • Sensation in the lower extremities
  • Bladder function
  • Bowel function
  • Sexual function

Compression of the cauda equina is a medical emergency requiring immediate evaluation.

Lumbar and Sacral Nerves

Nerve roots exit the spine through openings called neural foramina.

These nerves provide:

  • Strength to the hips, legs, ankles, and feet
  • Sensation throughout the lower extremities
  • Reflexes
  • Control of bladder and bowel function

When compressed, these nerves may cause:

  • Sciatica
  • Leg pain
  • Numbness
  • Tingling
  • Weakness

Muscles and Ligaments

Strong muscles and ligaments stabilize the lower back and pelvis.

These structures:

  • Support posture
  • Protect the spine during movement
  • Assist with lifting and walking
  • Help maintain spinal alignment

Muscle strain remains one of the most common causes of lower back pain.

How the Lumbosacral Spine Moves

The lumbar spine provides significant mobility while maintaining stability.

It allows:

  • Forward bending (flexion)
  • Backward bending (extension)
  • Side bending (lateral flexion)
  • Limited rotation (twisting)

The sacrum provides a stable foundation while transferring forces between the spine and pelvis during movement.

What to learn next…

Now that you understand the lumbosacral anatomy, you may want to explore the following topicws:

Signs and symptoms

Diagnoses

  • Lumbar Disc Conditions
  • Lumbar Facet Joint Conditions
  • Lumbar Instability
  • Lumbar Spinal Stenosis & Nerve Compression
  • Lumbar Fractures
  • Lumbar Deformity
  • Lumbar Tumors
  • Lumbar Infections
  • Sacroiliac Joint Disorders
  • Congenital Lumbar Disorders

Treatments