The longest nerve in your body, and the three places it gets pinched
You don't need to memorise every nerve root. You do need to know roughly where the sciatic nerve runs, what part of your skin and leg each root supplies, and the three points where it most commonly gets compressed. That tells you what pattern you have, and which routine helps.
Where the sciatic nerve comes from
The sciatic nerve is the largest nerve in the human body, about the diameter of an adult's little finger as it leaves the pelvis. It is formed from five spinal nerve roots: L4, L5, S1, S2, and S3. The roots exit your spine through small openings (foramina) at each vertebral level, then merge into the lumbosacral plexus inside your pelvis, before bundling together as one trunk that exits through the greater sciatic foramen at the back of the pelvis.
Once outside the pelvis, the nerve descends down the back of the thigh, deep to the hamstrings. Just above the back of the knee it splits into two branches, the tibial and common peroneal (fibular) nerves. The tibial runs down the calf to the sole of the foot; the common peroneal wraps around the outside of the knee to supply the front of the lower leg and the top of the foot.
The piriformis: a small muscle with outsized importance
The piriformis is a flat, pear-shaped muscle that runs from the front of your sacrum to the top of your femur (the greater trochanter, the bony bump on the side of your hip). Its day job is external rotation of the hip and stabilising the pelvis during walking.
The reason the piriformis matters for sciatica is anatomical proximity. In roughly 85 percent of people, the sciatic nerve passes directly UNDER the piriformis as both leave the pelvis. In the other roughly 15 percent (the anatomic variant), the sciatic nerve actually passes THROUGH part of the piriformis, splitting around or piercing the muscle. Either way, a tight or spasming piriformis can compress the sciatic nerve right at the point it leaves the pelvis, producing referred pain that runs down the leg.
The dermatome map (which root, which patch of skin)
A dermatome is the area of skin supplied by a single spinal nerve root. When a specific root is irritated (typically by a disc bulge), the resulting pain or numbness tends to follow that dermatome. This is the strongest clinical signal of which level is involved.
L4 root
Front of the thigh, inner shin, down to the inside of the foot. Weakness in knee extension (quadriceps). Reduced knee jerk reflex. Less commonly the cause of classic radiculopathy than L5 or S1.
L5 root
Outer thigh, outer shin, top of the foot, big toe. Weakness in lifting the foot (dorsiflexion) and the big toe. Foot drop, if severe, is an L5 sign. One of the two most common roots for disc herniation, usually at the L4-L5 level.
S1 root
Back of the thigh, back of the calf, outer side and sole of the foot, little toe. Weakness in pushing off (calf raise, plantarflexion). Reduced or absent ankle jerk reflex. The other of the two most common roots, usually at the L5-S1 disc.
S2-S3 roots
Back of the upper thigh, into the saddle area (the part of you that contacts a bicycle seat). Saddle anaesthesia (numbness here) is a cauda equina red flag and an A&E presentation, not a stretching problem.
The three most common compression sites
1. Lumbar disc herniation (most common)
A disc between two vertebrae bulges or herniates posterolaterally (back-and-to-the-side) and presses on the nerve root as it exits the spine. By far the most common cause, accounting for around 90 percent of true radicular sciatica in working-age adults. Almost always at L4-L5 (irritating the L5 root) or L5-S1 (irritating the S1 root).
2. Piriformis syndrome
The piriformis muscle compresses the sciatic nerve as both pass through the greater sciatic foramen. Common in runners (overworked external rotators), desk workers (chronically tightened from sitting), and people with pelvic asymmetry. Diagnosis is partly clinical (FAIR test, deep buttock tenderness) and partly by ruling out disc involvement.
3. Spinal stenosis
Narrowing of the central spinal canal or the foraminal openings, usually from age-related arthritic changes. Worse with standing and walking (extension narrows the canal further), better with sitting or leaning on a shopping trolley. More common over 60. The disc-bias McKenzie protocol typically aggravates stenosis; see a physiotherapist before self-treating.
Less common but important
Spondylolisthesis (vertebral slippage), cauda equina syndrome (medical emergency, see red flags), tumours, infections, and pregnancy-related compression. These are why the red-flag screen exists. The vast majority of sciatica is disc or piriformis driven, but the rare causes need different action.
Why the same anatomy produces different protocols
The nerve gets compressed in different places for different reasons, so the relief mechanism differs. Disc compression typically responds to positions that decompress the posterior disc and unload the nerve root (extension-bias). Piriformis compression typically responds to positions that lengthen the piriformis and reduce its tone (flexion-and-adduction). Stenosis responds to flexion (which opens the canal) and is typically made worse by extension.
That is why this site splits the routines along these lines, and why the disc-or-piriformis self-test is the entry-point for new visitors. Same nerve, different mechanism, different stretch.