The sciatic nerve forms from the L4, L5, S1, S2 and S3 roots, leaves the pelvis through the greater sciatic foramen beneath piriformis, runs down the back of the thigh under the hamstrings, and divides above the knee into the tibial and common fibular nerves.
Knowing the anatomy is only half of it. The part that matters when your leg hurts is the map: which root produces pain in which patch of skin, and which muscle goes weak when that root is compressed. That map is what follows.
Where the nerve begins
The course, from spine to sole
The sciatic nerve is not a single root. It is the pooled output of five, gathered in the sacral plexus on the inner wall of the pelvis, and this is why the anatomy is worth understanding: a disc pressing on one of those five roots produces a narrow, specific pattern, not diffuse pain down the whole leg.
It is the largest peripheral nerve in the body, roughly the width of a little finger where it leaves the pelvis. It emerges through the greater sciatic foramen, a gap in the bony ring of the pelvis, and appears in the buttock deep to the gluteal muscles.
Piriformis sits directly across its path. In the great majority of people the nerve passes underneath the muscle, and this is the normal arrangement described in every anatomy text. In a minority the common fibular portion passes through the muscle belly itself, an anatomical variant that has become the basis of a great deal of speculation about piriformis syndrome.
Down the thigh
From the buttock the nerve runs vertically down the posterior thigh, in a plane deep to the hamstring muscles and superficial to adductor magnus. It sits roughly midway between the sitting bone and the outer bony prominence of the hip, and this is the region where a deep, hard chair edge presses on it.
Along the way it innervates the hamstring muscles that bend the knee, and part of adductor magnus. What it does not do in the thigh is supply skin. The sciatic nerve gives no cutaneous branches above the knee at all, which is a detail that surprises people: the skin on the back of your thigh is served by a separate nerve entirely.
That anatomical fact has a practical consequence. Numbness confined to the back of the thigh is not usually a sciatic nerve problem, whereas numbness in the foot very often is.
The division, and the two nerves it becomes
Above the knee, at the top of the hollow behind it, the nerve splits into two. The split can occur much higher, occasionally inside the pelvis, and surgeons operating in this region look for that variant because it changes where structures lie.
The tibial nerve continues straight down through the back of the calf. It supplies the muscles that point the foot down and push you off the ground, passes behind the inner ankle bone, and divides into the plantar nerves that supply the sole of the foot. Damage here weakens the calf raise and numbs the sole.
The common fibular nerve, still called the common peroneal by many British clinicians, wraps around the outside of the upper shin bone, sitting just under the skin at that point. It is the most commonly injured nerve in the lower limb precisely because it is so exposed there, and it divides into a superficial branch to the outer leg and the top of the foot, and a deep branch to the muscles that lift the foot.
Foot drop, an inability to lift the front of the foot so that it slaps or catches when walking, is the classic consequence of a common fibular nerve problem, whether that arises at the knee or from the L5 root much higher up.
The map: which root, which symptom
| Root | Where you feel it | What weakens | Reflex |
|---|---|---|---|
| L4 | Front of thigh, inner side of the shin, inner ankle | Straightening the knee, pulling the foot up and inwards | Knee jerk reduced |
| L5 | Outer shin, top of the foot, big toe | Lifting the big toe and the foot, heel walking | Usually unchanged |
| S1 | Back of the calf, outer border and sole of the foot | Pushing up on tiptoe, single leg calf raises | Ankle jerk reduced |
This is the part almost no anatomy page connects to the clinic. If your pain runs down the outer shin and across the top of the foot into the big toe, and lifting your foot is weak, the level is L5. If it runs down the back of the calf into the outer border and sole of the foot, and you cannot do ten calf raises on that leg, the level is S1.
Those two account for the large majority of nerve root problems in the lower back, because the L4 to L5 and L5 to S1 discs take the greatest mechanical load in the spine. They sit at the bottom of the lumbar curve, where the flexible spine meets the fixed pelvis, and they carry both the weight above and the leverage of every bend.
Testing the reflexes matters here too. A reduced ankle jerk supports S1, a reduced knee jerk supports L4, and L5 typically leaves both intact, which is why an L5 problem can be missed by someone who relies on reflexes alone.
Sciatica against pain that only feels like it
True sciatica against back pain that merely radiates
Referred pain from the back
- Aches across the buttock and upper thigh.
- Rarely travels past the knee.
- Vague and hard to draw a line around.
- No pins and needles, no numbness.
- Strength and reflexes are normal.
- Varies with position and with fatigue.
Nerve root pain, true sciatica
- Follows a narrow band down the leg.
- Usually passes the knee and reaches the foot.
- Sharp, electric, easy to trace with a finger.
- Pins and needles or numbness in a defined patch.
- A specific muscle or reflex may be weaker.
- Provoked by coughing, sneezing or straining.
Somatic referred pain, from the discs, facet joints or sacroiliac joints of the lower back, is far more common than true nerve root compression, and it is regularly labelled sciatica by patients and sometimes by clinicians. It aches across the buttock and the back of the thigh, it is hard to draw a boundary around, and it stops before the foot. In pregnancy that same spread across the back of the pelvis is usually pelvic girdle pain rather than a nerve root, and the exercises differ.
True radicular pain is different in character: narrow, electric, traceable with a fingertip, and accompanied by a sensory or motor sign in the territory of a single root. The straight leg raise, lifting the straight leg while you lie on your back, reproduces it between roughly 30 and 70 degrees.
Where along the route the nerve gets compressed
The pathway matters because compression can happen at several points along it, and the level determines the pattern. Four sites account for almost everything seen in NHS practice.
- At the disc, inside the spinal canal. Disc material bulges backwards and to one side, catching the root as it prepares to leave. The commonest cause by a wide margin, and the reason L5 and S1 dominate.
- In the lateral recess and the exit foramen. Bony narrowing rather than disc material, more typical with age. Symptoms are often worse standing and walking, and relieved by leaning forwards over a trolley.
- In the deep gluteal region. Below the pelvis, where the nerve passes the piriformis and the group of small rotator muscles. Uncommon, and over diagnosed.
- At the outer side of the knee. The common fibular nerve lies against bone just under the skin here, which is why crossing the legs for long periods, a tight plaster cast or prolonged squatting can produce a temporary foot drop with no back pain at all.
Imaging is not the first step for any of them. Scans of people with no leg pain whatsoever routinely show disc bulges, and a health service that scanned every case of sciatica would find a great many abnormalities that explain nothing. An MRI earns its place when the pain has not settled after six to twelve weeks, when there is progressive weakness, or when surgery is genuinely on the table. Everything in this article up to that point is done clinically.
Deep gluteal syndrome and the piriformis question
Piriformis syndrome, in which the muscle is said to compress the nerve, is one of the most confidently diagnosed conditions with the least agreed diagnostic criteria. There is no test that confirms it and no imaging finding that settles it.
The current term used in British specialist practice is deep gluteal syndrome, which acknowledges that the nerve can be irritated in that region by several structures: piriformis, the obturator internus and gemelli group, fibrous bands, or the hamstring origin at the sitting bone. Pain over the bony point on the side of the hip belongs to a different problem again, gluteal tendinopathy, where it is the tendon that is compressed rather than the nerve.
It is worth knowing about, and it is worth not reaching for first. When leg pain follows a root pattern, the lumbar spine is the overwhelmingly more likely source, and treating the buttock in that situation wastes months.
What actually helps, and what NICE advises against
Movement is the first line. Staying active, walking, changing position frequently and avoiding long unbroken periods of sitting all reduce symptoms more reliably than rest. NICE recommends considering a group exercise programme for people with low back pain with or without sciatica.
The same guidance is unusually direct about what not to do. Belts, corsets, foot supports and shoes with special soles are not to be used. Neither is traction, acupuncture or electrotherapy. That list covers a good proportion of what is sold to people with sciatica in this country.
Nerve gliding exercises, where the nerve is mobilised gently rather than stretched hard, suit an irritable nerve better than aggressive hamstring stretching, which frequently aggravates it. Pain relief is used to allow movement, not as the treatment in itself. Gliding, graded walking and the positions that settle an acute episode are the practical core of treatment.
Nerve root pain also has a genuine inflammatory component: material from a disc provokes a chemical reaction around the root, and part of the pain comes from that rather than from pressure alone. It is why symptoms can be severe with modest compression on a scan. That local reaction sits on top of whatever background inflammatory load you already carry, which is a separate measurement and a separate lever. Alongside movement and time, a bioavailable anti-inflammatory formula should be judged on how much active compound is absorbed, and anything aimed at sciatic pain treated as an addition to keeping moving rather than a replacement for it.
Red flags
- Numbness in the saddle region, or difficulty starting or controlling urination. Same day emergency assessment.
- Weakness affecting both legs, or weakness that is getting worse over days.
- A foot that drags or slaps when you walk, which is new.
- Sciatica with fever, night pain that is unrelenting, or unexplained weight loss.
- Sciatic pain following significant trauma, or in anyone with a history of cancer.
Frequently asked questions
What is the pathway of the sciatic nerve?
It is formed inside the pelvis from the nerve roots L4, L5, S1, S2 and S3, which merge in the sacral plexus. It leaves the pelvis through the greater sciatic foramen, passing under the piriformis muscle in the buttock, then runs down the back of the thigh beneath the hamstring muscles. Above the knee it divides into the tibial nerve and the common fibular nerve, which between them supply the whole of the lower leg and foot.
Where does it hurt when your sciatic nerve hurts?
In a band rather than an area. Typically from the buttock, down the back of the thigh, past the knee and into the calf or foot. Which part of the foot is the useful detail: the top of the foot and big toe point to the L5 root, the outer border and sole point to S1. Pain that stops at the knee is more often referred pain from the back than true nerve root compression.
What triggers a sciatica flare up?
Prolonged sitting, particularly in a slumped position or in a car, is the commonest trigger. Bending and lifting with a rounded back, a sudden increase in activity, and coughing or straining also provoke it, because they raise pressure around the nerve root. Cold and stress do not compress anything but they reliably lower the threshold at which the pain registers.
Can the piriformis muscle trap the sciatic nerve?
It can, but it is diagnosed far more often than it occurs. In most people the nerve passes below piriformis; in a minority part of it pierces the muscle, which is an anatomical variant rather than a disease. UK specialists increasingly use the broader term deep gluteal syndrome, because several structures in that region can irritate the nerve and the muscle is only one of them. A disc problem at L5 or S1 remains far more likely.
How do you release a trapped sciatic nerve?
Not by stretching hard into the pain. Gentle nerve gliding, where you move the nerve rather than pull on it, is better tolerated than static hamstring stretching, which frequently makes an irritated nerve worse. Keep moving through the day, walk, change position often, and avoid long spells of sitting. NICE advises against traction, belts, corsets, foot orthoses, acupuncture and electrotherapy for sciatica, so anything sold on that basis is not supported here.
How long does sciatica take to settle?
The majority improve substantially within six to twelve weeks, and the disc material that presses on the nerve often shrinks on its own over months. Pins and needles usually fade before the pain does. Persistent weakness is the symptom that changes the plan, because it is weakness rather than pain that prompts a surgical opinion in the NHS.
When is sciatica an emergency?
If you develop numbness around the saddle area between the legs, difficulty passing urine or a loss of bladder or bowel control, go to an emergency department the same day. That combination suggests cauda equina syndrome, where the bundle of nerve roots below the spinal cord is compressed, and delay causes permanent damage. Rapidly progressing weakness in both legs is treated with the same urgency.
What to take away
Draw the pain on your leg with a finger, and note exactly where it stops. Then walk ten steps on your heels, and do ten calf raises on each leg separately. Three minutes, no equipment.
Where the line ends and which of those two tests fails tells you the level, and that is the single most useful piece of information you can bring to a GP or a physiotherapist. It is more informative than most of what a scan will report.
Sources
Jensen R.K., Kongsted A., Kjaer P., Koes B. (2019). Diagnosis and treatment of sciatica. British Medical Journal, 367, l6273. DOI: 10.1136/bmj.l6273
Hartvigsen J., Hancock M.J., Kongsted A. et al. (2018). What low back pain is and why we need to pay attention. The Lancet, 391(10137), 2356-2367. DOI: 10.1016/S0140-6736(18)30480-X
Foster N.E., Anema J.R., Cherkin D. et al. (2018). Prevention and treatment of low back pain: evidence, challenges, and promising directions. The Lancet, 391(10137), 2368-2383. DOI: 10.1016/S0140-6736(18)30489-6
GBD 2021 Low Back Pain Collaborators (2023). Global, regional, and national burden of low back pain, 1990-2020, its attributable risk factors, and projections to 2050. The Lancet Rheumatology, 5(6), e316-e329. DOI: 10.1016/S2665-9913(23)00098-X
Robinson W.H., Lepus C.M., Wang Q. et al. (2016). Low-grade inflammation as a key mediator of the pathogenesis of osteoarthritis. Nature Reviews Rheumatology, 12(10), 580-592. DOI: 10.1038/nrrheum.2016.136
