Which Nerve Is Pinched? L2 to S1 Radiculopathy

What it is

Radiculopathy means a nerve root is pinched or irritated where it leaves the spine. In the low back there are five roots that matter most, named for the bones they pass: L2, L3, L4, L5, and S1 (L for lumbar, S for sacral). Each root goes to its own patch of skin, its own set of muscles, and in some cases its own reflex. So the pattern of your symptoms is a map. Where the pain travels, where the skin feels numb, which movement is weak, and which reflex is dull all point to one root, and usually to one disc or one narrowed opening.

This guide walks through the five levels from top to bottom. The two lowest, L5 and S1, account for about nine in ten cases, because the discs between L4 and L5 and between L5 and S1 do the most work and wear the most.

How a level gets its name

The roots exit the spine one below the other. A disc herniation at the L4–L5 level most often pinches the L5 root, which is on its way down to leave at the next level. A herniation that pushes far to the side, or bone spurs that narrow the opening where the root leaves, catch the root one number higher. This is why I say "an L5 radiculopathy from the L4–L5 disc": the first part is the nerve, the second is the cause. Your MRI has to match your symptoms before I trust it.

L2: the upper thigh

• Pain: the front of the upper thigh and the groin. People sometimes think it is a hip problem.
• Numbness: a band across the front of the upper thigh, just below the crease.
• Weakness: lifting the knee toward the chest (hip flexion). Climbing stairs or getting the leg into a car feels heavy.
• Reflex: none that I can check reliably.

L2 problems are uncommon, and when I see one I look carefully for other causes, including the hip joint and, in older adults, a stress fracture in the pelvis.

L3: the front of the thigh to the knee

• Pain: the front of the thigh, often reaching the inside of the knee. It can be mistaken for knee arthritis.
• Numbness: the front and inner thigh, and the inside of the knee.
• Weakness: straightening the knee (the quadriceps) and pulling the knees together. The leg may buckle on stairs or when stepping off a curb.
• Reflex: the knee-jerk (patellar) reflex may be dulled.

L4: the inner shin

• Pain: from the front of the thigh across the knee and down the inside of the shin toward the inner ankle.
• Numbness: the inside of the lower leg and the inner ankle, sometimes the inner edge of the foot.
• Weakness: pulling the foot up at the ankle (dorsiflexion) and straightening the knee. Walking on the heels is hard on that side.
• Reflex: the knee-jerk reflex is often dulled or absent.

L5: the outer leg and the top of the foot

This is the most common level.

• Pain: the buttock, the outside of the thigh, the outside of the calf, and across the top of the foot to the big toe.
• Numbness: the top of the foot, the web between the big toe and the second toe, and the outer calf.
• Weakness: lifting the big toe, pulling the foot up at the ankle, and turning the foot outward. When it is severe, the foot slaps or drags when you walk (a foot drop). The hip muscles on that side can weaken too, so the pelvis dips when you stand on that leg.
• Reflex: there is no reliable reflex for L5, which is one reason the exam leans on strength here.

S1: the back of the leg and the bottom of the foot

• Pain: the buttock, the back of the thigh, the calf, the heel, and the outer edge and bottom of the foot to the little toe. This is the pattern most people mean when they say sciatica.
• Numbness: the outer foot, the little toe, the heel, and the sole.
• Weakness: pushing the foot down (plantarflexion), as in standing on tiptoe or pushing off when you walk. Rising onto the toes on one leg ten times is my usual test. The buttock muscles that push the hip back can weaken too.
• Reflex: the ankle-jerk (Achilles) reflex is often dulled or absent.

A quick summary

Root	Pain travels to	Numbness	Weakness	Reflex
L2	Groin, upper front of thigh	Upper front of thigh	Lifting the knee	None
L3	Front of thigh to inner knee	Front and inner thigh, inner knee	Straightening the knee	Knee-jerk (sometimes)
L4	Inner shin to inner ankle	Inner lower leg, inner ankle	Pulling the foot up, straightening the knee	Knee-jerk
L5	Outer thigh and calf, top of foot, big toe	Top of foot, big toe web, outer calf	Lifting the big toe, foot drop, hip dip	None
S1	Back of thigh, calf, heel, outer foot	Outer foot, little toe, sole	Pushing the foot down, tiptoe	Ankle-jerk

How we diagnose it

Your description of where the pain goes is the first and most useful clue. The exam adds strength testing muscle by muscle, a check of the skin with a light touch and a pinprick, the reflexes, and a straight-leg raise or a femoral stretch test, which tensions the lower roots or the upper ones respectively. An MRI confirms the cause and the level. When the MRI shows several things and the picture is unclear, a nerve test (EMG) or a targeted numbing injection around one root can tell me which one is really responsible.

How we treat it

The level changes where the symptoms are, not how most radiculopathy is treated. Most disc-related cases settle on their own: roughly 8 or 9 in 10 people improve without surgery, often within six to twelve weeks. Staying active, a short course of anti-inflammatory medicine, physical therapy, and an epidural steroid injection for stubborn pain are the usual steps. I have laid those out in my home program for sciatica.

Two things do depend on the level. Weakness in a muscle that matters for walking, especially a foot drop (L5) or a quadriceps that buckles (L3 or L4), pushes me toward earlier surgery, because a nerve that is losing power is one I do not want to wait on. And the kind of surgery depends on where the root is caught: a herniated disc comes out through a small incision (a microdiscectomy or an endoscopic discectomy), while a root caught in a narrowed opening needs that opening widened (a foraminotomy). In nearly every case, the nerve can be freed without fusing the level.

The science: for the curious

The map of skin patches is called a dermatome map, and it has been drawn by careful study of people with a single injured root. The muscle map is the myotome. They overlap at the edges, because every patch of skin and every muscle gets a little supply from the neighboring roots, which is why the exam uses the combination rather than any single sign. The reflexes are the most objective part: a dull ankle-jerk on one side is hard to fake and hard to miss, and it points to S1 before the MRI has been ordered.

When to call

New or worsening weakness, a foot that is starting to drop, or numbness that is spreading all need a visit soon. New trouble controlling your bladder or bowels, or numbness in the groin and inner thighs, is an emergency: go to the nearest emergency room or call 911.

Learn more

OrthoInfo, Sciatica: orthoinfo.aaos.org (opens in a new tab)

OrthoInfo, Herniated Disk in the Lower Back: orthoinfo.aaos.org (opens in a new tab)

My guides to lumbar radiculopathy, lumbar foraminal stenosis, and the home program for sciatica.

This handout is general education about a common condition and does not replace the personalized advice we discuss in clinic. If anything here does not match what we talked about, what we talked about wins: please ask me.