Most people with sciatica describe the same thing: the back is not really the issue. There may be a dull ache in the lower back, but the pain that stops them sleeping, driving or sitting through a meeting runs down the back of the thigh, past the knee, and sometimes into the calf or foot. Some feel it as a deep burn. Others describe an electric shock, a tight band, or pins and needles that will not settle.

That pattern is not random. It follows the path of a specific nerve, and understanding that path explains a lot about why sciatica behaves the way it does.

Sciatica is a description, not a diagnosis

Strictly speaking, sciatica is a word for the symptom — pain travelling along the route of the sciatic nerve — rather than a condition in its own right. Two people can both be told they have sciatica while the underlying cause is completely different, and what helps one may do very little for the other.

The sciatic nerve is the thickest nerve in the body. It forms in the lower back from several nerve roots — L4, L5, S1, S2 and S3 — which leave the spine and merge into one large trunk. It then passes through the buttock, runs down the back of the thigh, and splits into two branches around the knee, continuing down the shin and into the foot.

Because that nerve carries sensation and muscle signals for so much of the leg, irritation anywhere along it can produce symptoms a long way from where the problem started.

Why the pain appears in your leg and not your back

This is the part that confuses people most. If a disc is pressing on a nerve root in the lower back, why is the pain in the calf?

The answer lies in how the nervous system is wired. A nerve root does not only serve the spot where it leaves the spine. It carries signals to and from a particular patch of skin and a particular set of muscles further down. The brain reads activity in that nerve as coming from the territory it supplies — so irritation at the root is felt as pain, tingling or numbness in the leg.

Clinicians call this radicular pain. Each nerve root has its own territory, which is one reason your symptoms can hint at the level involved:

• L4: pain may travel down the front of the thigh and inner shin, sometimes with changes to the knee reflex.
• L5: pain often runs down the outer thigh and shin towards the top of the foot and big toe, occasionally with weakness lifting the foot.
• S1: pain tends to run down the back of the calf to the outer foot and little toe, and the ankle reflex can be reduced.

Not everyone fits neatly into these patterns. The map is a guide, not a rule — but it explains why two people with "sciatica" can point to entirely different parts of their leg.

What is usually irritating the nerve

In younger adults, the most common culprit is a disc in the lower back that has bulged or herniated and is pressing on, or chemically irritating, a nearby nerve root. In older adults, narrowing of the spinal canal or of the holes the nerves pass through — spinal stenosis or foraminal narrowing — is more often responsible.

Other possibilities include:

• A slipped vertebra (spondylolisthesis) narrowing the exit route for a nerve.
• Inflammation around a nerve root, which can be as painful as direct pressure.
• Pregnancy, where changes in posture and ligament laxity add load to the lower back.
• More rarely, compression in the buttock itself as the nerve passes through the deep gluteal muscles.

Worth knowing: plenty of people have a bulging disc on a scan and no leg pain at all. What shows up on imaging matters far less than whether it matches your symptoms.

What sciatica usually feels like

A few features make nerve pain recognisable. It usually affects one leg. It tends to follow a line rather than sitting in one small spot. It often comes with altered sensation — numbness, tingling, or a feeling that the leg is heavy or not quite yours. Coughing, sneezing, straining or sitting for a long time can make it worse, because these raise pressure around the nerve roots.

Pain that stays purely in the lower back, or that spreads vaguely across both sides of the lower back and buttocks, is often something else — muscular, joint-related, or simply the back itself complaining.

How it is assessed

Most cases can be narrowed down in a consultation without any scan. A clinician will ask where the pain travels, what makes it worse, and whether there is numbness or weakness. They will check reflexes, muscle strength and sensation, and may use a straight leg raise test, which stretches the sciatic nerve and often reproduces the leg symptoms.

Scans are usually reserved for cases where symptoms are severe, not improving, or where something more serious needs ruling out. Imaging earlier than that can muddy the picture rather than clarify it.

What tends to help

For most people, sciatica eases as the irritated nerve calms down. That can take a few weeks, and it is normal for it to settle unevenly — better days followed by a bad one. What helps most in the meantime is unglamorous:

1. Keep moving gently. Short, regular walks usually work better than long stretches of rest, which stiffen the back and lower your tolerance for activity.
2. Change position often. If sitting sets it off, stand up every 20 to 30 minutes. If standing is worse, sit with support and your feet flat.
3. Use heat or cold on the lower back for comfort, whichever you find soothing.
4. Ask a pharmacist about pain relief. They can advise on what suits you and what may interact with other medicines.
5. See a physiotherapist if things are not improving. A tailored programme beats generic stretches found online, especially where there is weakness.

When to seek urgent help

Most sciatica is unpleasant but not dangerous. A small number of signs, however, need urgent attention. Seek emergency care the same day if you notice:

• Numbness around the saddle area — the inner thighs, genitals and buttocks.
• Difficulty starting to urinate, or loss of bladder or bowel control.
• Rapidly worsening weakness in one or both legs, or difficulty walking.
• Severe pain after a significant accident or fall, or back pain with fever or unexplained weight loss.

These are uncommon, but they need checking quickly. For everything else, the sensible approach is steady, gentle activity, a proper assessment, and patience while the nerve settles. If you are unsure, speak to your GP or a physiotherapist — that conversation will tell you far more than any symptom chart.

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