13.2 Tibial Motor and Medial/Lateral Plantar Studies
Key Takeaways
- Tibial motor studies record abductor hallucis (AH). The distal cathode sits posterior to the medial malleolus at a common teaching distance of about 8 cm from G1, and the proximal cathode sits in the popliteal fossa.
- Popliteal tibial stimulation often needs high intensity because the nerve is deep; that same intensity can co-stimulate the common fibular (peroneal) nerve if the cathode wanders lateral.
- The H-reflex uses the same popliteal tibial nerve at submaximal intensity with a soleus recording. It is conceptually related, but it is not the supramaximal abductor hallucis CMAP.
- Medial and lateral plantar mixed or sensory studies—orthodromic from the sole to the ankle, or antidromic the other way—sample the tibial branches that traverse the tarsal tunnel. Numeric diagnostic criteria are a later chapter.
- Externally rotate the hip to expose the medial ankle, and treat a swollen ankle as an impedance problem: extra prep, higher current, and SNAP loss that can outrun true axon loss.
13.2 Tibial Motor and Medial/Lateral Plantar Studies
Quick Answer: Record the tibial motor compound muscle action potential (CMAP) from abductor hallucis (AH). Stimulate posterior to the medial malleolus (common teaching distance about 8 cm to G1) and in the popliteal fossa. Add medial and lateral plantar mixed or sensory studies, orthodromic from the sole to the ankle or antidromic the other way, when the referral is distal tibial sensory change or suspected tarsal tunnel. High popliteal currents can co-stimulate the common fibular (peroneal) nerve. The H-reflex shares the popliteal tibial site but is a different recording and intensity. Distances below are teaching distances, not unpublished official AAET required measurements.
Independent OpenExamPrep teaching for outline items VI.A.2.d.2.a–b is how the tibial nerve is actually shocked and recorded at the ankle and knee, and how its medial plantar and lateral plantar branches are sampled. Numeric tarsal-tunnel criteria belong in a later chapter. This section is the montage, the limb position, and the impedance problem of a swollen ankle.
Course you are actually stimulating
After the sciatic split, the tibial nerve travels in the midline popliteal fossa, then between the gastrocnemius heads and under the soleal arch into the posterior compartment. At the ankle it lies posterior to the medial malleolus in the tarsal tunnel, typically with the posterior tibial artery and the tendons of tibialis posterior, flexor digitorum longus, and flexor hallucis longus, under the flexor retinaculum. Distal to the tunnel it divides into the medial plantar nerve (to AH, flexor hallucis brevis, flexor digitorum brevis, and the medial sole skin) and the lateral plantar nerve (to abductor digiti minimi pedis, dorsal and plantar interossei, and the lateral sole skin), plus calcaneal sensory twigs that many routine worksheets do not record.
AH is the routine distal tibial muscle because you can palpate it on the medial foot, G1 is stable, and the ankle cathode sits on a short, repeatable segment. It is not a fibular muscle and it is not a surrogate for EDB.
Tibial motor to abductor hallucis — montage
Limb position matters more here than on the dorsum of the foot. Externally rotate the hip and flex the knee slightly so the medial malleolus and navicular face you (a gentle frog-leg). Dorsiflex the ankle just enough to flatten skin folds if edema allows. For the popliteal site, keep the knee slightly flexed; a locked-straight knee buries the nerve and tempts you into huge currents.
| Electrode | Placement (teaching montage) |
|---|---|
| G1 (active) | Abductor hallucis belly, commonly 1 cm posterior and 1 cm inferior to the navicular tuberosity, or over the most prominent AH bulge on the medial foot while the patient abducts the great toe. |
| G2 (reference) | Medial first metatarsophalangeal joint or medial proximal phalanx of the hallux, off the muscle. |
| Ground | Medial ankle or dorsum of the foot between the ankle cathode and G1. |
| Ankle cathode | Teaching distance about 8 cm proximal to G1, posterior to the medial malleolus, over the tibial nerve in the tarsal-tunnel region. Anode proximal. |
| Popliteal cathode | Midline popliteal fossa, slightly deep between the hamstring tendons. The tibial nerve is not the lateral biceps femoris groove used for common fibular studies. Anode proximal. |
Raise intensity to a supramaximal AH CMAP at the ankle first. Expected movement is great-toe abduction/plantarflexion, not toe extension or foot eversion. Then repeat in the popliteal fossa. The popliteal nerve is deep. You will often need higher milliamperes or a longer pulse width than at the ankle. That is expected. It is also the moment the study can go wrong.
High popliteal intensity and H-reflex overlap
Because the tibial nerve is deep at the knee, a weak shock underestimates the proximal CMAP and fakes an amplitude drop that looks like block. Because a strong shock spreads, a cathode that drifts laterally toward the biceps femoris tendon depolarizes the common fibular (peroneal) nerve. The foot then dorsiflexes or everts while AH is still on the screen, the waveform changes shape, and extra anterior-compartment volume contaminates the trace. Keep the popliteal cathode midline, watch the toes, and stop adding current once the AH CMAP has plateaued.
The H-reflex conceptually overlaps this station because it also stimulates the tibial nerve in the popliteal fossa. It is not the same study:
| Feature | Tibial motor to AH | H-reflex (conceptual) |
|---|---|---|
| Recording muscle | Abductor hallucis | Typically soleus (or gastrocnemius–soleus complex) |
| Intensity | Supramaximal M-wave | Submaximal; the H often shrinks as the M grows |
| What you are after | Distal latency, CMAP amplitude, knee-to-ankle velocity | A late spinal reflex, not an AH belly-tendon CMAP |
| Trap | Calling a late AH potential an H-reflex | Mixing H-reflex language into a routine AH motor worksheet |
If a late potential appears on an AH channel, think F-wave or technical artifact before you relabel the run as an H-reflex. H-reflex technique is its own station. Sharing a popliteal cathode site does not make the two traces interchangeable.
Medial and lateral plantar mixed and sensory studies
These studies sample the medial plantar and lateral plantar nerves after they leave the tarsal tunnel. Labs use mixed (nerve-to-nerve) or sensory montages, orthodromic (sole toward ankle) or antidromic (ankle toward sole). Orthodromic sole-to-ankle mixed studies are the common teaching default because the recorder sits in a quieter zone behind the medial malleolus and shock artifact is easier to manage than when you record on the sole.
| Study | Typical teaching montage | Why it is on the worksheet |
|---|---|---|
| Medial plantar mixed/sensory (orthodromic) | Stimulate the medial sole, often over the first metatarsal interspace or medial mid-arch. Record with a bar posterior to the medial malleolus. Common teaching distance about 14 cm. | Tests the medial plantar tibial branch used in distal tibial / tarsal-tunnel questions |
| Lateral plantar mixed/sensory (orthodromic) | Stimulate the lateral sole, often over the fourth metatarsal interspace or lateral midfoot. Record at the same ankle bar. Same teaching distance class as the medial plantar if the lab matches segments. | Tests the lateral plantar tibial branch; often smaller and harder than medial plantar |
| Antidromic plantar | Reverse the circuit: cathode at the ankle, recording electrodes on the sole. | Some labs prefer it; expect more shock artifact and more pain on the sole |
G1/G2 polarity on the ankle bar follows your lab’s convention (usually G1 closer to the incoming volley). The point for this exam topic is not a secret official millimeter. The point is: medial plantar and lateral plantar are separate tibial branches, they are mixed or sensory, and they are distal. A tibial motor CMAP to AH can still be present when plantar SNAPs are lost, because motor axons and a large muscle recording are more forgiving than a tiny mixed potential across edematous skin.
Uses for tarsal tunnel (criteria later): these are the studies that actually see the nerves inside and just beyond the flexor retinaculum. A later chapter will talk about latency comparisons, side-to-side amplitude, and how to avoid calling every painful foot tarsal tunnel. Here you only need to know which electrodes belong on that worksheet: AH motor plus medial and lateral plantar mixed/sensory, with the sural as a length-dependent control on many polyneuropathy panels—not as a tarsal-tunnel nerve.
Some laboratories also record a lateral plantar motor CMAP from abductor digiti minimi pedis. That is a useful extra motor channel, but it does not replace the mixed/sensory plantar pair the outline names in VI.A.2.d.2.b.
Swollen ankle impedance and other technical failures
A puffy, postoperative, or chronically edematous ankle is an impedance problem before it is a neuropathy problem. Dry, scaly, or lotioned skin under G1 or the ground raises impedance, widens the stimulus artifact, and can erase a plantar SNAP that is still anatomically intact. Alcohol, abrasive prep, fresh stickies, and a ground that is not floating on a bleb of edema are part of the protocol, not cosmetics.
High impedance forces higher stimulus current. Higher current spreads. At the ankle that spread can leak from the tibial groove toward the deep fibular nerve or toward cutaneous branches, so the AH CMAP is joined by unexpected toe extension. At the sole, a painful high-current orthodromic shock makes the patient withdraw and the electrodes lift. If the plantar responses vanish only on the swollen side, check temperature, prep, and distance before you write axon loss.
Other tibial-station traps:
- G1 off AH. Too dorsal or too distal sits on bone. The CMAP starts positive and looks tiny. Re-palpate the navicular landmark.
- Measuring the popliteal-to-ankle tape with the knee locked, then stimulating with the knee flexed. The nerve path length changes. Velocity becomes fiction.
- Calling an absent plantar SNAP a tarsal-tunnel proof in a patient whose sural is also absent. That pattern is length-dependent or age-related until a later criteria chapter says otherwise.
- Using the same popliteal intensity recipe as the common fibular study. Fibular is lateral and often more superficial at the neck. Tibial is midline and deeper. Copying the milliamperes from the last nerve is how you co-stimulate.
Worked example
A 64-year-old with ankle swelling and plantar numbness. AH CMAP is obtainable after extra skin prep, distal latency a bit long on a cold limb. Medial and lateral plantar mixed responses are absent on the swollen side. The sural is also absent. The popliteal tibial shock at high intensity everts the foot. Protocol reading: (1) re-warm and re-prep before trusting the long distal latency; (2) the plantar pair is the correct tarsal-tunnel sensory/mixed toolkit, but an absent sural means you cannot blame the flexor retinaculum yet; (3) drop popliteal intensity and move midline because eversion is fibular co-stimulation. Diagnostic cutoffs wait for the criteria chapter. The technologist’s job is a clean AH CMAP, honest distances, and plantar traces that failed for a named reason rather than for lotion and edema.
Lab checks before you leave the tibial stations
AH G1 is on the navicular landmark, not the dorsum. Ankle cathode is behind the medial malleolus, not on the anterior tibial pulse. Popliteal cathode is midline, toes plantarflex, and you did not relabel a submaximal soleus H as the AH M-wave. Plantar mixed/sensory electrodes are on the sole and medial ankle, distances marked as teaching distances if that is what they are, and a swollen ankle got prep, not just more milliamperes.
Which recording muscle and distal stimulus site describe the routine tibial motor study?
Why do medial and lateral plantar mixed or sensory studies appear on a tarsal-tunnel worksheet?
A high-intensity popliteal shock on a tibial-to-AH study makes the foot evert, and a late soleus potential is visible on another channel. Which technical reading is most accurate?