L4-L5 vs. L5-S1 Disc Herniation: Different Levels, Different Symptoms

Why the Level of Herniation Matters

The two lowest lumbar discs — L4–L5 and L5–S1 — account for the overwhelming majority of lumbar disc herniations. Although they are adjacent, each level compresses a different nerve root, producing a distinct pattern of pain, weakness, and sensory change. Understanding these patterns helps patients and clinicians localise the problem accurately before imaging.

Anatomy: Which Nerve Root Is Affected

At L4–L5, a posterolateral herniation typically compresses the L5 nerve root, which exits below the L5 vertebra. At L5–S1, herniation compresses the S1 nerve root, exiting at the sacrum.

Each nerve root supplies a different region of the leg and foot:

The Achilles Reflex: A Key Clinical Differentiator

The ankle jerk (Achilles tendon reflex) tests the S1 nerve root. It is among the most reliable bedside indicators of L5–S1 disc herniation with nerve root compression. A reduced or absent Achilles reflex on the affected side, in combination with posterior leg pain and outer heel numbness, strongly suggests S1 root involvement.

The patellar reflex, testing L3–L4, is less commonly affected by L4–L5 herniation because the L5 root does not mediate this reflex. Weakness of big-toe extension (extensor hallucis longus) is the most sensitive L5 test — it is an L5-dominant muscle with minimal contribution from adjacent roots.

Clinical Differentiation: Foot Drop

A "foot drop" — inability to lift the front of the foot during walking — is a serious neurological finding that localises predominantly to the L5 root (L4–L5 disc). Patients compensate with an exaggerated hip flexion gait (steppage gait) to clear the ground. This finding warrants urgent assessment as it indicates significant nerve root compression and may not fully recover if decompression is delayed.

Structural Factors That Predispose Each Level

L4–L5 is the segment with the greatest range of motion in the lumbar spine, making it the highest-mobility and therefore highest-wear segment. It is most commonly affected in people who flex and extend repeatedly through work or sport.

L5–S1 sits at the lumbosacral junction — the transition from the mobile lumbar spine to the fixed sacrum. This junction bears high shear forces and is particularly vulnerable in people with a steep sacral base angle (high pelvic incidence) or significant spondylolysis (stress fractures in the posterior arch).

Structural Management by Level

L4–L5:
Extension-biased movement (McKenzie protocol) is effective for posterolateral L4–L5 herniations in the early phase. Adjustments targeting the L4–L5 segment restore posterior joint mobility. Hip flexor tightness is commonly associated and should be addressed.

L5–S1:
The lumbosacral angle and sacroiliac mechanics are central. SI joint dysfunction is frequently co-present with L5–S1 pathology and must be independently assessed and treated. Traction is often better tolerated at L5–S1 than at higher levels.

Accurate level identification — through clinical neurological testing rather than relying solely on MRI — enables a treatment plan tailored to the specific anatomy involved, improving outcomes and reducing unnecessary intervention at the wrong segment.