Foot drop is a finding, not a diagnosis. The patient cannot lift the front of the foot, so the toes catch and the gait changes to compensate — but the weakness can originate at the nerve root, along the sciatic nerve, at the fibular head, or in the muscle itself, and the treatment for each is different.
Nerve conduction studies exist to answer exactly that question. Jim Lewis R. NCS.T, CNCT is direct about where to look first: “anytime you have a patient with foot drop, you wanna take a very close look at the fibular nerve, especially across the fibular head.”
This guide covers why that site is the first suspicion, how the study localises the lesion, what the findings say about severity and recovery, and the causes the fibular head does not explain. It sits under our clinical reference to electrodiagnostic testing.
Why the fibular head is the first place to look
The common peroneal nerve — one of the two terminal branches of the sciatic nerve — wraps around the neck of the fibula just below the knee, where it lies superficially against bone with very little soft tissue over it. That position makes it the most exposed nerve in the lower limb.
Dr. Gisele J. Girault describes it from the clinic side in the same terms — “the peroneal nerve is a very superficial nerve. It wraps around the fibula of the knee” — and lists where the injuries come from. It is commonly injured when the fibula is broken. It can be injured during knee surgery such as a knee replacement. And it can be injured by something as ordinary as immobilisation: a patient in a tight walking boot or a cast for a lower extremity injury.
Habitual leg crossing, prolonged squatting or kneeling, positioning during a long operation, and significant weight loss that removes the padding over the nerve do the same thing. The nerve supplies the ankle dorsiflexors, so a lesion at that point produces foot drop directly.
Lewis singles out the superficial peroneal sensory study as particularly valuable in this setting — “a very, very useful study, especially for things like peroneal neuropathy across the fibular head that you find with foot drop.”
Peroneal or fibular: the same nerve
Reports use both names, and patients who search the terms find apparently different nerves. Lewis explains the change, which is recent enough that both terms remain in circulation.
The problem was auditory. “If I say peroneal nerve, which many people do — peroneal nerve — then that sounds very close to perineal nerve, which of course is a very different nerve and a different part of the body.” Because of that confusion, he notes, the name was changed to fibular nerve in 1998.
Nothing about the anatomy changed. A report describing the common fibular nerve at the fibular head and one describing the common peroneal nerve at the fibular head describe the same finding.
How the study localises the lesion
The principle is the one Lewis applies to every entrapment: stimulate below and above the suspected site, so a segment that conducts normally on one side and abnormally on the other identifies where the problem is.
Across the fibular head, two patterns appear, and they mean different things.
Conduction block — focal loss of amplitude across the segment. Lewis describes exactly this on the fibular nerve: a block along the pathway at or above the fibular head, where amplitude drops across the lesion while the nerve below it responds normally. A conduction block localises with real confidence, because the change is confined to the segment being crossed.
Focal slowing — conduction velocity reduced across the segment without a drop in amplitude. In his framing this is the milder and more acute picture, because it reflects injury to myelin rather than loss of axons.
Where amplitude has also fallen, there is likely some axonal involvement. Lewis is careful not to let that reading run ahead of itself: it “doesn’t mean it’s more serious unless the amplitude drop continues on down” — the progressive loss that follows axonal injury, which he names as Wallerian degeneration.
What the pattern says about recovery
This is the question patients ask first, and it is the one the study is genuinely good at informing.
Demyelinating injury — slowing or block with preserved amplitudes distal to the lesion — carries the better outlook, because myelin remakes itself over weeks to a few months once the compression is relieved.
Axon loss is slower. Regrowth proceeds from the site of injury toward the muscle at a limited rate, so recovery is measured in many months and may be incomplete. The needle examination contributes here by showing whether the dorsiflexor muscles are denervated and whether reinnervation has begun.
Timing affects what can be seen. Needle findings after a nerve injury take weeks to appear, so a study performed very early can look reassuring in a patient with a genuine lesion — a limitation discussed in what a nerve conduction test diagnoses.
What the fibular head does not explain
Localisation matters most when the answer is somewhere else, and the common alternative is a nerve root.
L5 radiculopathy produces dorsiflexion weakness that can be indistinguishable from a fibular neuropathy on inspection. The separation comes from muscle selection on the needle study, and the logic is the one Lewis teaches throughout the course: sampling muscles that share a nerve root but reach it by different routes. His worked example is in the arm — “if we stick the needle in the deltoid muscle, we’re looking at the axillary nerve, and we’re looking at C5. Well, then if I move down to the biceps, I’m also looking at the C5 nerve root, but it’s a different place through the brachial plexus.” In the leg the same reasoning is applied to L5-supplied muscles lying outside the fibular nerve’s distribution: involvement there points to the root, while sparing points to the nerve.
Sciatic neuropathy can present as foot drop because the fibular division is more vulnerable than the tibial division within the sciatic nerve. Findings extending beyond the fibular nerve’s territory suggest a lesion higher up.
Generalised peripheral neuropathy produces bilateral, length-dependent findings rather than one weak foot — though it can coexist with, and mask, a focal compression.
Motor neuron disease produces widespread denervation across muscles supplied by different nerves and roots, a pattern no single compression can account for.
A whole other category sits outside the study’s reach entirely: central causes, including stroke and spinal cord lesions. Electrodiagnostic testing does not assess the brain or cord, and foot drop with upper motor neuron signs belongs in a different investigation.
Before concluding anything, check the technical factors
Lewis’s standing rule applies with particular force in the leg, where the nerve is long and the limb is cool.
Temperature first. A cold limb conducts slowly and mimics demyelination, and the lower leg is the coldest part of most patients. Then the distances, then which muscles were sampled.
The question he returns to is whether the result fits the patient. A finding that contradicts a clear clinical picture is a reason to interrogate the study, not to override the examination.
For clinicians
Studies across the fibular head depend on accurate stimulation sites and distance measurement over a curved segment, which is where the avoidable errors happen. Empire teaches nerve conduction and needle EMG technique hands-on across the pain management academy.
Learn blocks with your hands, not from a page
Empire’s Pain Management Training (THE Pain Show) is accredited for 25.25 AMA PRA Category 1 Credits™, jointly provided by AKH, Inc, and Empire Medical Training. For narrower peripheral work, Joint, Extremity and Non-Spinal Injection Training carries 6.75 credits for the complete in-person hybrid program, and Advanced Musculoskeletal Ultrasound Guided Injections builds the guidance skills above.
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