What Nerves Are Affected by L5 and S1?

The L5 and S1 nerve roots control much of what your lower leg and foot can do, from lifting your toes off the ground to pushing off when you walk. These two roots exit from the lowest segments of the lumbar and uppermost sacral spine, and together they supply motor power, sensation, and reflexes to distinct but overlapping territories in the leg. When either root is compressed or irritated, the symptoms follow a predictable pattern that clinicians use to pinpoint which level is involved. But the story is more nuanced than textbook diagrams suggest, with anatomical variations and overlapping pathways that can blur the lines.

Where L5 and S1 Sit in the Spine

The lumbar spine has five vertebrae, labeled L1 through L5 from top to bottom. Below L5 sits the sacrum, a fused triangular bone, and the first sacral segment is called S1. The nerve roots that exit at these levels are named for the vertebrae above them: the L5 root exits between the L5 vertebra and the sacrum, while the S1 root exits through an opening in the sacrum itself. Both roots are part of the cauda equina, the bundle of nerve fibers that trails below the spinal cord’s endpoint.

Anatomical studies of these intrathecal roots show that S1 is the largest nerve root in the entire lumbosacral region, with an average diameter of about 4.16 mm and roughly 12 to 13 rootlets feeding into it. The L5 root is also substantial, and its diameter is considerably larger than the upper lumbar roots. This size reflects the heavy workload both roots carry: they serve the muscles and skin of the lower leg, ankle, and foot.

What the L5 Nerve Root Controls

The L5 root is primarily responsible for the muscles that pull your foot and toes upward. If you dorsiflex your ankle (point your toes toward your shin), walk on your heels, or extend your big toe, you are relying heavily on L5. The key muscles it drives include the tibialis anterior (which lifts the foot), the extensor hallucis longus (which extends the big toe), and the extensor digitorum longus (which lifts the lesser toes). L5 also contributes to hip abduction, so weakness can sometimes show up as a subtle pelvic drop when you stand on one leg.

On the sensory side, the L5 dermatome covers the outer part of the lower leg, the top of the foot, and the web space between the big toe and second toe. When L5 is compressed, people often describe numbness or tingling across the top of their foot, sometimes extending up the outer shin.

L5 radiculopathy is one of the two most common causes of foot drop, a condition where you cannot lift the front part of your foot normally. A case report of L5 mononeuritis described a patient who presented with an antalgic gait and foot drop, with reduced power to 3/5 in dorsiflexion and big-toe extension, along with diminished sensation over the top of the foot. That clinical picture is the classic L5 signature.¹

What the S1 Nerve Root Controls

S1 does something close to the opposite of L5 in the ankle: it powers the muscles that push your foot downward. Plantarflexion (pointing your toes away from your body, as when you press a gas pedal or rise onto your toes) depends on S1. The gastrocnemius and soleus muscles in the calf are the main players, along with the peroneals on the outside of the lower leg that help with eversion (turning the sole of the foot outward).

The S1 dermatome covers the back of the calf, the outer edge of the foot, the sole, and the little toe. When S1 is irritated, pain and numbness tend to wrap around the outside and underside of the foot rather than the top. People frequently describe a burning or electric sensation running down the back of the leg into the heel.

S1 also has a distinctive reflex signature. The Achilles tendon reflex (ankle jerk) is primarily mediated by S1. A diminished or absent ankle jerk on one side is a reliable clinical sign pointing to S1 involvement. By contrast, L5 radiculopathy does not have an easily testable reflex in standard clinical practice. Researchers have investigated the medial hamstring reflex as a possible L5-level indicator, and one study found that the knee reflex correctly classified root level in about 86% of cases, the medial hamstring reflex in about 79%, and the ankle reflex in about 67%.² The absence of a clean, universally accepted L5 reflex is one reason clinicians rely more on motor testing and sensory mapping when trying to distinguish the two levels.

How L5 and S1 Feed Into the Sciatic Nerve

Both L5 and S1 contribute major fiber bundles to the sciatic nerve, the largest nerve in the body. The sciatic nerve forms in the pelvis from the L4, L5, S1, S2, and S3 roots, then travels through the buttock and down the back of the thigh. Near the knee, it splits into the common peroneal (fibular) nerve and the tibial nerve. The peroneal nerve carries most of the L5-derived fibers to the front and outside of the lower leg, while the tibial nerve carries most of the S1-derived fibers to the calf and sole.

This anatomy matters because a problem at the spine and a problem farther downstream can produce confusingly similar symptoms. A peroneal nerve injury at the knee, for example, can cause foot drop that looks almost identical to L5 radiculopathy. Likewise, a sciatic neuropathy in the buttock can mimic root-level compression. One study explored using peroneal nerve conduction studies to differentiate L5 radiculopathy from peroneal neuropathy at the fibular head and sciatic neuropathy, all three of which share foot drop as a common clinical feature.³ The distinction is clinically critical because treatment for a pinched nerve root in the spine is very different from treatment for a compressed nerve at the knee.

How Clinicians Tell L5 and S1 Apart

When you visit a doctor with leg pain, weakness, or numbness, a few specific tests help narrow down which root is the culprit. The examination breaks down roughly into three categories: motor strength testing, sensory mapping, and reflexes.

Motor testing: You’ll be asked to walk on your heels (L5) and on your toes (S1). Heel-walking weakness points to L5; calf weakness or an inability to rise onto your toes suggests S1. The big-toe extension test, where you push your big toe upward against resistance, is particularly sensitive for L5.
Sensory testing: Light touch and pinprick over the top of the foot (L5 territory) versus the outer edge and sole (S1 territory) help localize sensory loss. Numbness between the first and second toes is a strong L5 indicator.
Reflexes: A reduced ankle jerk points to S1. L5 has no standard, easily elicited reflex, which is why motor and sensory findings carry extra weight when L5 is suspected.

Electrodiagnostic testing with nerve conduction studies and needle electromyography can add another layer of precision. A study of 108 consecutive cases of lumbosacral radiculopathy from disc herniation found that the main clinical and electrophysiological differences between root levels involved deep reflexes and motor nerve conduction findings in the deep peroneal and tibial nerves.⁴ In practice, the peroneal nerve tends to show abnormalities in L5 radiculopathy, while the tibial nerve is more often affected with S1 involvement.

Common Causes of L5 and S1 Compression

Disc herniation is the most familiar cause. The L4-L5 disc (between the fourth and fifth lumbar vertebrae) most commonly compresses the L5 root, while the L5-S1 disc most commonly compresses the S1 root. A large central disc herniation can sometimes hit both roots, and a far-lateral or foraminal herniation can compress the root exiting at the same level rather than the one below, which makes imaging interpretation critical.

Foraminal stenosis at L5-S1 is another important and sometimes underrecognized cause. In this condition, the bony opening through which the nerve exits narrows enough to squeeze the root. A case series highlighted five patients with bilateral L5 radiculopathy caused purely by foraminal stenosis at L5-S1, noting that this pathology can be overlooked because imaging does not always clearly reveal foraminal narrowing unless three-dimensional reconstructions are used.⁵

Isthmic spondylolisthesis, where the L5 vertebra slips forward on S1 because of a stress fracture in a portion of the vertebra called the pars interarticularis, is another common culprit. The forward slip narrows the foramen and can directly irritate the exiting L5 root. One study noted that radicular pain in these patients is often attributed to the combination of spondylolisthesis and foraminal stenosis, and surgical decompression of the foramen can relieve the radiating leg symptoms.⁶ Other causes include degenerative facet joint hypertrophy, synovial cysts, and tumors, though these are less common.

When the Map Shifts: Anatomical Variations

The neat textbook picture of L5 controlling one zone and S1 controlling another breaks down in a meaningful percentage of people who have transitional vertebrae. A transitional vertebra is one that shares characteristics of two spinal regions: for instance, the lowest lumbar vertebra may be partially fused to the sacrum (a “sacralized L5”), or the top of the sacrum may be partially separated to resemble a lumbar vertebra (a “lumbarized S1”). These variants show up on imaging more often than you might expect.

In patients with a sacralized L5, the functions typically carried by the L5 root can shift upward. One study concluded that in these patients, the L4 nerve root takes over the usual role of the L5 nerve root.⁷ Similarly, in patients with a lumbarized S1, compression of the S1 root produced motor weakness patterns that looked like L5 radiculopathy in people with normal anatomy.⁸

This is why surgeons and radiologists carefully count vertebral segments and check for transitional anatomy before surgery. Operating at the “wrong” level because the numbering is ambiguous is a recognized risk. If your imaging report mentions a transitional vertebra, it does not mean something is wrong with you; it means your doctor needs to correlate the image findings with your symptoms more carefully than usual rather than relying on the standard nerve map alone.

Foot Drop and the L5-Versus-Peroneal Puzzle

Foot drop deserves its own discussion because it is one of the symptoms that most alarms patients, and figuring out where it is coming from dictates the treatment path. Both L5 radiculopathy and peroneal nerve injury are listed as the most common causes.⁹ A review of foot drop’s anatomical and electrodiagnostic localization noted that potential injury sites include the L5 root, the lumbosacral plexus, the sciatic nerve, and the common peroneal nerve.¹⁰

A few clinical clues help sort this out. With a peroneal nerve lesion at the knee, you tend to see weakness in the muscles that lift the foot and turn it outward (eversion), but the muscles that turn the foot inward (inversion via the tibialis posterior, which is supplied by the tibial nerve and the L5 root but not the peroneal nerve) are spared. In contrast, an L5 radiculopathy can weaken inversion along with dorsiflexion. So if your ankle is weak going up and inward, the problem is more likely at the root level. If it is weak going up and outward but strong going inward, the peroneal nerve is the likelier culprit. Electromyography can confirm the pattern by testing muscles innervated by L5 that are outside the peroneal nerve’s territory.

Complications After Surgery at L5-S1

The L5-S1 level is one of the most commonly operated segments in the spine. After surgery, whether it is a microdiscectomy, fusion, or decompression, the nerve roots can remain irritated or develop new problems. Epidural fibrosis, which is scar tissue that forms around the nerve roots after surgery, is a well-known complication. In severe cases, fibrosis can re-tether the nerve roots and recreate pain and weakness. One case study described a patient who underwent six revision surgeries for recurrent pain and weakness in the lower extremity caused by severe epidural fibrosis around the thecal sac and nerve roots, and noted that excision of the scar tissue alone did not prevent it from returning.¹¹

Failed back surgery syndrome, where pain persists or recurs after spinal surgery, frequently involves the L5 and S1 roots because this is where the mechanical demands on the spine are highest and where disc disease is most common. Patients who have ongoing symptoms after surgery deserve careful re-evaluation, including consideration of whether the nerve root is being compressed at a different location than the one addressed surgically, whether scar tissue is the issue, or whether peripheral nerve problems are contributing.

When Both Roots Are Involved at Once

Large disc herniations, severe spinal stenosis, or conditions like spondylolisthesis can compress both L5 and S1 simultaneously. When this happens, symptoms combine: you may see weakness in both dorsiflexion and plantarflexion, numbness across the top and bottom of the foot, and a diminished ankle jerk on top of the motor deficits. The piriformis muscle, deep in the buttock, can also sometimes irritate the sciatic nerve in a way that mimics dual-root involvement. One study that investigated chronic piriformis-related pain found that among patients with sciatica, electrodiagnostic testing revealed findings suggesting chronic L5 and S1 radiculopathy in a small number of cases.¹²

Simultaneous involvement of both roots also raises a red flag for cauda equina syndrome, especially if bowel or bladder function changes. The sacral roots, including S1 and the roots below it (S2-S4), carry the autonomic fibers that control the bladder and bowel sphincters. While L5 and S1 compression alone does not typically cause loss of bladder control, a large midline disc herniation at L5-S1 can compress the entire cauda equina and trigger this emergency. New-onset urinary retention or incontinence, combined with saddle numbness (loss of sensation around the groin and perineum), requires immediate medical evaluation.

Why the S1 Root Is Physically the Largest

If you have looked at anatomical diagrams, you may have wondered why S1 is so much thicker than the roots above and below it. Anatomical dissections have confirmed that the S1 root averages about 4.16 mm in diameter and carries roughly 12 to 13 rootlets, making it the largest root in the entire lumbosacral region by both measures. The L1 root, for comparison, averages about 0.80 mm with roughly 3 rootlets.¹³ Below S1, the roots rapidly thin out; the S5 root averages just 0.46 mm.

This matters clinically for a couple of reasons. The large size of S1 means it fills more of the available space in the spinal canal and foramen, which can make it more vulnerable to compression from even moderate disc bulges. It also means S1 carries a large number of both motor and sensory fibers, which is why S1 radiculopathy tends to produce pronounced symptoms: strong calf pain, noticeable weakness when pushing off while walking, and a clearly diminished ankle reflex. The physical bulk of the nerve root matches the clinical weight of its loss.

Piriformis Syndrome and Other Non-Spinal Mimics

Not all pain along the L5 or S1 pathway starts in the spine. The sciatic nerve passes through (or in some anatomical variants, directly through the belly of) the piriformis muscle in the buttock. When the piriformis spasms or becomes inflamed, it can compress the sciatic nerve and produce symptoms in the L5 and S1 distributions without any spinal pathology at all. This is often called piriformis syndrome, though its existence as a distinct entity has been debated for decades.

Other non-spinal causes include sacroiliac joint dysfunction, hip pathology (which can refer pain into the buttock and leg), and peripheral neuropathies like diabetic neuropathy affecting the peroneal or tibial nerves. The practical takeaway is that if imaging of the lumbar spine looks normal but you have classic L5 or S1 symptoms, the problem may be downstream along the nerve’s course. A thorough exam and sometimes electrodiagnostic testing can help sort out whether the spine, pelvis, or peripheral nerve is responsible.