13.1 Common, Deep, and Superficial Fibular (Peroneal) Studies
Key Takeaways
- Common fibular (peroneal) motor to extensor digitorum brevis (EDB) uses three cathode sites: anterior ankle, below the fibular head, and above the fibular head in the lateral popliteal fossa. A common teaching distal setup is about 8 cm from G1 to the ankle cathode; that figure is a lab teaching distance, not an unpublished official AAET required measurement.
- An EDB compound muscle action potential (CMAP) that is larger at the fibular head than at the ankle is the worksheet flag for an accessory deep fibular (peroneal) nerve traveling posterior to the lateral malleolus, not automatic conduction block.
- When EDB is atrophic, traumatized, or electrically silent, record tibialis anterior (TA) from the same below- and above-fibular-head sites so a deep/common fibular motor study is still possible. Ankle stimulation is not a TA motor point.
- High-intensity stimulation that drifts toward the midline popliteal fossa co-stimulates the tibial nerve, producing plantarflexion, a change in CMAP shape, and a falsely larger proximal response.
- Superficial fibular (peroneal) sensory studies record on the dorsolateral ankle or foot after anterolateral distal-leg stimulation, commonly taught at 12–14 cm, and sample the cutaneous territory of the dorsum rather than EDB motor axons.
13.1 Common, Deep, and Superficial Fibular (Peroneal) Studies
Quick Answer: The common fibular (peroneal) motor study usually records extensor digitorum brevis (EDB) and stimulates at the anterior ankle, below the fibular head, and above the fibular head in the lateral popliteal fossa. If EDB is wasted, record tibialis anterior (TA) from those same proximal sites. A larger EDB compound muscle action potential (CMAP) at the fibular head than at the ankle flags an accessory deep fibular nerve, not conduction block. The superficial fibular (peroneal) sensory study records on the dorsolateral ankle or foot. Distances below are common teaching distances used in many labs, not unpublished official AAET required measurements.
Independent OpenExamPrep teaching for outline item VI.A.2.d.1 is the practical montage: where G1 sits, where the cathode sits, and which artifacts mimic a lesion. This chapter maps lower-extremity neuroanatomy onto electrode placement. It does not work the fibular-head versus L5 localization case—that is the next chapter. You still need to know why a lab records TA instead of EDB.
Current anatomic name is fibular. Exam stems, older worksheets, and many stimulators still say peroneal. Treat common fibular = common peroneal, deep fibular = deep peroneal, and superficial fibular = superficial peroneal as the same nerves spoken in two dialects.
Course you are actually stimulating
The sciatic nerve usually divides in or just above the popliteal fossa into the tibial nerve (midline and deeper) and the common fibular (peroneal) nerve (lateral, hugging the biceps femoris tendon). The common fibular then winds around the fibular neck, the classic compression site. Distal to the neck it divides into:
- Deep fibular (deep peroneal) nerve: anterior compartment muscles (tibialis anterior, extensor hallucis longus, extensor digitorum longus, fibularis tertius) and then EDB on the dorsum of the foot, plus a cutaneous twig to the first dorsal web space.
- Superficial fibular (superficial peroneal) nerve: lateral compartment (fibularis/peroneus longus and brevis) and cutaneous supply to the dorsolateral ankle and most of the dorsum of the foot.
EDB is a small intrinsic muscle. It is convenient, distal, and easy to lose to local trauma, tight shoes, edema, or length-dependent axon loss. TA is a large proximal L4–L5 muscle still supplied by the deep fibular nerve. That pair is why both motor recordings exist. The next chapter uses them in localization. This section uses them so you still have a motor study when EDB is electrically silent.
Common fibular motor to EDB — montage
Position the patient supine, hip near neutral, knee slightly flexed if you need the popliteal fossa, ankle relaxed. Warm the limb. A common teaching skin-temperature target on the dorsum is at least 32 °C, because a cold foot prolongs distal latency and drops amplitude.
| Electrode | Placement (teaching montage) |
|---|---|
| G1 (active) | Belly of EDB on the dorsolateral foot. Palpate the muscle mound while the patient extends the toes. It sits lateral to the extensor hallucis longus (EHL) and extensor digitorum longus tendons, over the dorsolateral foot proximal to the fifth metatarsal region. |
| G2 (reference) | Dorsal fifth metatarsophalangeal (MTP) joint or distal fifth metatarsal, off the muscle belly. |
| Ground | Dorsum of the ankle or foot between the ankle cathode and G1. |
| Ankle cathode | Teaching distance about 8 cm proximal to G1, slightly lateral to the EHL tendon at the anterior ankle, where the deep fibular nerve lies under the extensor retinaculum. Anode proximal. |
| Below-fibular-head (BFH) cathode | Just distal and inferior to the fibular head/neck, in the palpable groove where the common fibular nerve is subcutaneous. |
| Above-fibular-head (AFH) cathode | Lateral popliteal fossa, medial to the biceps femoris tendon, teaching 8–12 cm proximal to the BFH site along the nerve path. Stay lateral. Midline popliteal stimuli hit tibial axons. |
Record a belly-tendon CMAP. Raise intensity to a supramaximal EDB response at each site without recruiting extra muscles. The expected distal movement is toe extension, not plantarflexion. An initial positivity at G1 usually means the active electrode is off the motor point—re-palpate EDB rather than turning the gain up and calling the trace axonal.
Across-fibular-head segment table
Conduction velocity is distance divided by the latency difference. The across-head segment is short, so a 1 cm tape error is a large velocity error. Label the numbers on your worksheet as teaching distances unless your own lab has published its protocol; they are not secret official exam measurements.
| Segment | Cathode | Recording | Teaching distance notes | What the numbers are for |
|---|---|---|---|---|
| Distal motor | Anterior ankle | EDB | Many labs use about 8 cm from G1 to the cathode | Distal motor latency and distal CMAP amplitude |
| Leg (BFH to ankle) | Below fibular head | EDB | Long segment; height-dependent, often in the mid-20s to mid-30s cm range in adults | Conduction velocity in the leg |
| Across fibular head (AFH to BFH) | Lateral popliteal versus below head | EDB | Keep about 8–12 cm between cathodes. Shorter than about 8 cm is a teaching red flag for unstable velocity | Focal slowing or conduction block at the fibular neck |
Measure along the estimated nerve path with the knee in the same slight flexion you used to stimulate. Do not mix a fully extended-knee tape measure with a flexed-knee stimulus. Do not take a chord through the fibular head as if the nerve traveled through bone.
Accessory deep fibular nerve — brief pitfall
An accessory deep fibular (accessory deep peroneal) nerve is an anatomic variant: motor fibers, usually traveling with the superficial fibular nerve, pass posterior to the lateral malleolus and innervate part of EDB, often the lateral belly. Anterior-ankle stimulation misses those axons. Proximal common-fibular stimulation includes them.
The worksheet flag is an EDB CMAP that is larger, or that grows a new negative component, at the fibular head compared with the ankle. That pattern is not automatically demyelinating block. Confirm by stimulating behind the lateral malleolus and recording EDB. A clear CMAP from that posterior site supports the variant. A later chapter covers anomaly reporting in more depth. Your job here is simpler: do not call a larger proximal EDB CMAP block until you have thought about the accessory deep fibular nerve.
Worked example: ankle EDB CMAP 2.0 mV, below-head CMAP 4.2 mV, no change in duration that looks like temporal dispersion. Before you write conduction block in reverse, place a cathode posterior to the lateral malleolus. If EDB fires, you have found the missing distal axons. If it does not, then re-check ankle G1 placement, ankle cathode position, and stimulus intensity before you invent a diagnosis.
Deep fibular motor to tibialis anterior
When EDB is atrophic, buried in edema, scarred from local trauma, or silent from length-dependent dying-back, a zero EDB study cannot tell you whether common and deep fibular axons still reach a more proximal muscle. Record TA.
| Electrode | TA teaching montage |
|---|---|
| G1 | TA muscle belly, about 8–10 cm distal to the tibial tubercle, lateral to the tibial crest |
| G2 | TA tendon at the ankle or over the medial malleolus |
| Stimuli | Below fibular head and above fibular head / lateral popliteal fossa — the same common-fibular sites used for EDB |
There is no useful ankle motor point for TA. Those deep-fibular axons have already entered the anterior compartment higher in the leg. Why TA versus EDB recording exists: EDB can be locally dead while TA still reports the common and deep fibular nerve across the fibular head. That is a protocol rescue, not a shortcut into the fibular-head versus L5 essay. Keep the TA study when EDB is uninterpretable so the next chapter still has a motor amplitude and an across-head segment to discuss.
Superficial fibular (peroneal) sensory
This sensory nerve action potential (SNAP) samples the cutaneous branch that supplies the dorsolateral ankle and foot. Most teaching labs record it antidromically. It is not a sural study and it is not an EDB motor study.
| Item | Teaching montage |
|---|---|
| G1 | Dorsolateral ankle, midway between the lateral malleolus and the TA tendon, or over the intermediate dorsal cutaneous nerve on the dorsum aimed toward the fourth toe |
| G2 | 3–4 cm distal, toward the fourth–fifth toe web |
| Cathode | Anterolateral distal leg, anterior to the fibula, teaching 12–14 cm proximal to G1 |
| Ground | Between cathode and G1 |
A small or absent SNAP is common with age, edema, prior ankle surgery, and length-dependent polyneuropathy. Compare side to side. Do not over-call a unilaterally small response on a cold, swollen foot. Stimulating too far medially wanders toward deep-fibular territory; too far posterior wanders toward sural. Palpate the fibula and stay anterior to it.
Technical pitfalls that wreck the study
Distance. Velocity is only as honest as the tape. Around the fibular head, measure the curved subcutaneous course. If AFH and BFH are 5 cm apart, a 5 mm placement difference is a 10 percent distance error. That error is how a normal across-head segment becomes a fake focal slowing on the worksheet.
Popliteal co-stimulation of tibial. The tibial nerve sits midline and deep in the popliteal fossa. A high-intensity, long-duration pulse from a supposedly lateral site that has drifted medially depolarizes tibial axons. The foot then plantarflexes, the CMAP changes shape, and the proximal amplitude may look larger than the distal EDB CMAP because extra foot muscles, including abductor hallucis, are now in the recording field. Fix the run: lower intensity, shorten pulse width, slide the cathode against the biceps femoris tendon, and watch the toes. Toe extension belongs with EDB and the deep fibular nerve. Plantarflexion means you bought tibial.
Stimulus spread at the ankle. Too high an anterior-ankle shock can leak toward tibial or superficial-fibular territories. Keep the cathode over the deep-fibular groove lateral to EHL and stop adding milliamperes once the EDB CMAP has plateaued.
Temperature, edema, and G1 placement. A cold or edematous EDB underestimates amplitude. G1 off the motor point gives an initial positivity and a small CMAP that is a placement error, not axon loss. Re-prep the skin rather than accepting a noisy 50 µV baseline as a sensory-range motor response.
Naming on the exam. If a stem says peroneal motor to EDB with stimulations at ankle, fibular neck, and lateral popliteal fossa, it is this study. If it says superficial peroneal sensory, it is the dorsolateral SNAP, not the sural.
Lab checks before you leave the fibular stations
Confirm four things on every common-fibular motor run: distal CMAP is recorded from a palpable EDB or you have switched to TA; AFH is lateral, not midline; the across-head tape is not tiny; and a larger proximal EDB CMAP triggered an accessory deep fibular check behind the lateral malleolus. Then add the superficial-fibular SNAP if the referral is dorsal-foot sensory change, polyneuropathy, or a suspected superficial-fibular cutaneous lesion that spares EDB. That sequence is protocol. Interpretation of a focal fibular-neck block versus an L5 root lesion is the next chapter.
On a common fibular (peroneal) motor study to EDB, the ankle CMAP is 1.8 mV and the below-fibular-head CMAP is 4.0 mV without a dispersed waveform. Which next technical step best evaluates the most important anatomic pitfall before calling conduction block?
EDB is wasted and electrically silent, but the laboratory still needs a deep and common fibular motor study that can include the across-fibular-head segment. Which montage is appropriate?
During a common fibular motor run, a high-intensity shock in the popliteal fossa makes the foot plantarflex and the proximal CMAP larger and different in shape from the distal EDB CMAP. What happened?