Foot drop: telling L5 radiculopathy from common fibular neuropathy in clinic

A foot that won't lift is a sign, not a diagnosis

Foot drop usually shows up in the gait before anyone names it: the toe catches on the step, the foot slaps down after heel strike, or the knee lifts higher than it should so the foot can clear the floor. Sometimes the person has back and leg pain and the cause seems obvious. Sometimes there's no back pain at all, just a foot that has started to trip them up.

Weakness of ankle dorsiflexion can come from a problem anywhere along the pathway to tibialis anterior: the L5 nerve root, the lumbosacral plexus, the sciatic nerve, the common fibular (peroneal) nerve, or the brain and spinal cord. A 2026 narrative review on localising foot drop describes it as "a clinical sign rather than a single diagnosis", and a 2019 interdisciplinary review in Deutsches Ärzteblatt International names L5 radiculopathy and peroneal nerve injury as the most common causes. Fibular neuropathy is also the most frequent mononeuropathy in the lower limb.

Those two common causes are the ones most easily confused, and they lead to very different conversations with the patient and the GP. This article covers the anatomy that separates them, the handful of extra tests that do most of the work, what the evidence says about hip abduction as a discriminator, the mimics worth keeping in mind, and when a new foot drop needs an urgent referral rather than a rehab plan. It's written for chiropractors, physiotherapists, osteopaths and sports therapists who assess lower limb weakness in clinic.

The anatomy that does the sorting

The common fibular nerve carries fibres that come mainly from L4 to S1. It runs down the lateral side of the knee and becomes superficial as it wraps around the neck of the fibula, which is why it is so vulnerable to pressure and injury there. Just below that point it divides:

• The deep fibular nerve supplies the main ankle and toe dorsiflexors (tibialis anterior, extensor hallucis longus, extensor digitorum longus) and gives sensation to the first dorsal web space.
• The superficial fibular nerve supplies the main evertors (fibularis longus and brevis) and gives sensation to the distal anterolateral leg and most of the dorsum of the foot.

The L5 root feeds those same dorsiflexors, but it also supplies muscles that the common fibular nerve never reaches. Two of them are especially useful in clinic:

• Tibialis posterior, the main ankle invertor, is L5-predominant but is supplied by the tibial nerve.
• Gluteus medius, the main hip abductor, has a substantial L5 contribution but is supplied by the superior gluteal nerve.

So if dorsiflexion is weak and inversion or hip abduction is weak as well, the lesion is probably above the knee, and the L5 root becomes the leading suspect. If dorsiflexion and eversion are weak but inversion, plantarflexion and hip abduction are strong, a lesion at the fibular head fits better.

Test five things, not one

The 2026 localisation review recommends that strength testing goes beyond ankle dorsiflexion to include toe extension, ankle eversion, ankle inversion, plantarflexion and hip abduction. In practice that adds a couple of minutes to a lower limb neuro exam you'd be doing anyway.

Ankle dorsiflexion and great toe extension. Weak in both L5 radiculopathy and common fibular neuropathy, so on their own they confirm the problem without locating it. Grade them, because the grade matters for prognosis and for the referral letter.

Ankle eversion. Weak in common fibular neuropathy because fibularis longus and brevis are supplied by the superficial branch. It can also be weak with an L5 lesion, so eversion weakness by itself doesn't settle the question.

Ankle inversion. This is one of the most useful tests. Tibialis posterior is L5-predominant but tibial nerve-supplied, so preserved inversion supports an isolated common fibular lesion, while weak inversion raises concern about something more proximal, particularly L5 radiculopathy or a plexus lesion. The review's caveat matters: preserved inversion supports a fibular-head lesion but doesn't prove it.

Plantarflexion and the Achilles reflex. Usually spared in an isolated common fibular neuropathy. Weakness or a reduced reflex points to S1 involvement or a sciatic nerve lesion that affects the tibial division as well.

Hip abduction. Weakness supports an L5-level process because gluteus medius sits outside the fibular nerve's territory. Compare sides.

What the evidence says about hip abduction

The clearest data on a single bedside discriminator come from a prospective diagnostic study published in Spine in 2013 by Jeon and colleagues. They examined 61 consecutive patients whose tibialis anterior strength was below MRC grade 3, including hip abductor power. Final diagnoses, based on clinical information, imaging and, in some cases, electrophysiology, were common fibular neuropathy in 28 people, lumbar radiculopathy in 21, lumbosacral plexopathy in 9 and a sciatic nerve disorder in 3.

Hip abductor weakness was present in 85.7% of the people with lumbar radiculopathy and in 3.6% of those with peroneal neuropathy. For telling those two groups apart, the test had a sensitivity of 85.7%, a specificity of 96.4%, a positive predictive value of 94.7% and a negative predictive value of 90%.

Those figures are encouraging, but read them with the study in mind. It came from a single orthopaedic surgery department, the patients had marked weakness to start with, and electrophysiology was used only in some cases. Predictive values also depend on how common each cause is in the people you see. Treat a weak gluteus medius as a strong pointer towards the L5 root, and a normal one as useful but not definitive. The 2026 review says the same thing: hip abduction weakness supports an L5 process but isn't independently diagnostic.

Sensation, Tinel's sign and the story

Sensory findings help, but they're the weakest part of the exam for localisation because nerve and dermatome territories overlap.

• Common fibular neuropathy tends to affect the anterolateral lower leg and the dorsum of the foot and spare the sole.
• L5 radicular symptoms can involve the lateral leg and the dorsum of the foot towards the big toe, but dermatomal patterns vary a lot between people.
• A first dorsal web space change on its own points to the deep fibular branch.
• A positive Tinel's sign at the fibular head supports local irritation of the nerve, but the review is clear that it isn't enough to establish where the lesion is.

Back pain, radiating leg pain and positive neural tension tests add weight to a radicular cause. Their absence makes a peripheral cause more likely but doesn't rule out a root lesion, because radiculopathy can be painless.

The history often helps most. In an Italian multicentre study of 69 people with peroneal mononeuropathy (Aprile and colleagues, 2005), most cases had an identifiable predisposing factor. Prolonged posture accounted for 23.1%, surgery for 20.3%, weight loss for 14.5%, trauma for 11.6%, being bedridden for 7.3% and compression from a cast for 5.8%. In 16% no cause was found. Men outnumbered women about four to one, and 68% of the group walked with difficulty. The surgical cases weren't only operations near the knee; thoracic and abdominal surgery featured too.

The NHS foot drop page lists similar everyday triggers: crossing the legs, kneeling or squatting for long periods, hip or knee replacement surgery and long periods without moving. So ask about:

• recent rapid weight loss, for any reason;
• habitual leg crossing, prolonged squatting or kneeling, for example at work or during a DIY project;
• a recent operation, hospital stay, plaster cast or tight brace;
• knee or fibular head trauma;
• diabetes or other conditions linked with peripheral neuropathy.

Rapid weight loss is worth asking about specifically now that more people are taking GLP-1 medicines. A 2026 report in Annals of Plastic Surgery described two people without diabetes who developed acute foot drop after losing 14% and 18% of their body weight over three to six months on semaglutide and tirzepatide. That's two cases, so it tells us the problem can happen, not how often. Loss of the fat that cushions the nerve at the fibular head is the proposed mechanism, the same one described after weight loss from other causes.

The mimics worth keeping in mind

Two other peripheral lesions can look a lot like the common pair.

A sciatic nerve lesion that mainly affects the fibular division. The fibular and tibial fibres run in separate bundles within the sciatic nerve, and the fibular bundle is often the more vulnerable. The 2026 review cites a series of 100 people with sciatic neuropathy in which the peroneal division was more severely affected than the tibial division in 64%. That means a lesion high in the thigh or buttock, after hip surgery, an injection injury, prolonged compression or pelvic trauma, can look at first like a problem at the knee. A history pointing above the knee is a reason to say so in your referral.

Lumbosacral plexopathy. Weakness and sensory change across several roots and nerves that can't be explained by one root or one nerve suggests the plexus. Pelvic or retroperitoneal malignancy, haematoma, radiation, surgery and trauma are among the causes to consider, so an unexplained multi-nerve pattern needs medical assessment.

Then there are the causes outside the peripheral nerves. Stroke, multiple sclerosis and other brain or spinal cord problems can cause weakness that mainly affects dorsiflexion. Upper motor neuron signs, such as brisk reflexes, spasticity or an upgoing plantar response, should take you out of the peripheral framework altogether. Weakness that's on both sides, slowly progressive or accompanied by broader neurological change raises other possibilities, including generalised or inherited neuropathies such as Charcot-Marie-Tooth disease and motor neurone disease, which the NHS also lists among the less common causes.

When it can't wait

Foot drop is a motor deficit, and motor deficits change the timetable.

For foot drop caused by lumbar disc disease, time seems to matter. A 2025 systematic review with an individual patient data meta-analysis in the European Spine Journal (Baig Mirza and colleagues) looked at people with unilateral foot drop from lumbar disc herniation who had single-level decompression surgery. Younger age, a shorter duration of symptoms, less severe weakness before surgery and early improvement after surgery were linked with better motor recovery. People who had surgery within six weeks of symptom onset were up to six times more likely to show greater recovery at final follow-up, and people with some preserved strength (MRC grade 2 or 3) before surgery did better than those with none.

A 2026 scoping review of 22 studies (1,882 patients) in the same journal adds an important caveat. Most of the studies were retrospective, definitions of foot drop and recovery varied, and none used electrodiagnostic testing to confirm the deficit. Shorter symptom duration and better strength before surgery were the most consistent predictors, but the authors rated the certainty as low.

So the evidence doesn't give a precise deadline, but it does give a direction. A new or worsening foot drop needs a prompt medical or spinal opinion rather than a wait-and-see trial of treatment. Escalate faster when:

• the weakness is new, severe or getting worse between visits;
• both legs are affected;
• there are upper motor neuron signs, or other neurological change in the face, arm or speech;
• there are any cauda equina symptoms.

The NHS sciatica page tells people to go to A&E or call 999 if they have sciatica on both sides, weakness or numbness in both legs that's severe or getting worse, numbness around the genitals or anus, or new problems starting, stopping or controlling the bladder or bowel. Those are emergencies in any patient with back and leg symptoms, with or without foot drop.

What the tests add, and what to put in the letter

Nerve conduction studies and electromyography are how foot drop is localised with confidence. Sensory nerve responses are usually preserved in radiculopathy, because the lesion sits before the dorsal root ganglion, and are often reduced when the lesion is in the peripheral nerve. A normal sensory response doesn't exclude a fibular nerve lesion, though. Needle testing of the short head of biceps femoris, the one thigh muscle supplied by the fibular division, helps separate a sciatic lesion from one at the fibular head. Ultrasound is good at looking at the nerve around the fibular head, and MRI or MR neurography is better for deeper or more proximal problems. These decisions sit with the medical team, but the quality of your examination shapes how quickly the right test is ordered.

A useful referral or GP letter includes:

• when the weakness started, how it started, and whether it's changing;
• MRC grades for dorsiflexion, great toe extension, eversion, inversion, plantarflexion and hip abduction, on both sides;
• reflexes, any upper motor neuron signs and the sensory pattern;
• back and leg pain, neural tension findings and any Tinel's sign at the fibular head;
• relevant history, such as weight loss, prolonged postures, recent surgery or immobility, trauma and diabetes;
• your working localisation, written as a hypothesis, and why you think it.

Looking after the foot while the cause is found

Management depends on the cause, and for a compressive fibular neuropathy the 2026 review lists removing the source of pressure, changing contributing activities or positions, physical therapy and an ankle-foot orthosis where needed for safe walking. The NHS adds that people with foot drop are at higher risk of tripping and falling, and suggests well-fitting supportive shoes, walking aids where needed and clearing trip hazards at home. The NHS also mentions electrical stimulation devices, especially after a stroke or with multiple sclerosis.

While the cause is being worked out, there is plenty that's in scope for us:

• advise the person to stop crossing their legs and avoid long periods of squatting or kneeling if a fibular cause is suspected;
• check footwear and talk through falls risk, especially on stairs and at night;
• discuss whether an ankle-foot orthosis might help them walk safely, and who can provide one;
• keep the rest of the limb moving and strong within comfort;
• re-test and record strength at each visit, because a change in grade is useful information for the medical team.

What's out of scope is just as important. Don't let a rehab plan delay a referral, and be careful not to promise recovery. Some foot drop recovers well, some partly, and some is permanent.

Key points

1. Foot drop is a sign. L5 radiculopathy and common fibular neuropathy are the most common causes, and they're easy to confuse.
2. Test inversion, eversion, plantarflexion and hip abduction as well as dorsiflexion. Weak inversion or hip abduction points above the knee, especially to L5.
3. In one prospective study, hip abductor weakness separated L5 radiculopathy from peroneal neuropathy with 85.7% sensitivity and 96.4% specificity. Treat it as a strong pointer, not a diagnosis.
4. Sensation and Tinel's sign support localisation but can't establish it on their own.
5. Ask about rapid weight loss, leg crossing, squatting, recent surgery or immobility, casts and knee trauma.
6. Look for upper motor neuron signs and bilateral or progressive weakness, which take you beyond a single nerve or root.
7. New or worsening foot drop needs a prompt medical or spinal referral. Bilateral symptoms or cauda equina symptoms are an emergency.
8. Document grades on both sides and write your localisation as a reasoned hypothesis.

Neurological assessment and clinical reasoning come up across ManualCPD's accredited online courses. ManualCPD offers on-demand CPD for healthcare professionals, mainly chiropractors, physiotherapists and osteopaths, as well as sports therapists, with lessons broken into manageable chunks you can work through between patients, at lunch or after clinic. Every ManualCPD course is accredited by The CPD Group (Provider 788442) and issues a CPD certificate immediately after completion.