Motor nerve conduction studies record the compound muscle action potential (CMAP) — the summed response of all muscle fibers innervated by the nerve being tested. Measuring the CMAP’s latency (time from stimulus to response), amplitude (peak-to-peak voltage), and duration tells us how fast the nerve conducts, how many axons are still functional, and whether conduction is uniform or fragmented. Motor NCS forms the backbone of peripheral neuropathy evaluation, distinguishing axonal from demyelinating disease, identifying focal nerve injuries, and quantifying severity. This page covers the technique, normal values, abnormal patterns, and clinical interpretation of motor NCS.

Basic Setup and Method

Recording Electrodes

• Active (G1) electrode over the motor point (end-plate region) of the target muscle.
• Reference (G2) over the muscle’s distal tendon.
• Ground electrode between stimulator and recording electrodes.
• Skin preparation: clean with alcohol or abrasive gel; impedances <5 kΩ.

Stimulation

• Supramaximal stimulation: raise current until amplitude plateau, then 20–25% beyond.
• Stimulus duration 0.05–1 ms.
• Cathode placed distal to anode for orthodromic motor responses.
• Stimulator over the nerve trunk at standardized sites.

Filter Settings

• Low-frequency filter: 10 Hz (some labs 2 Hz).
• High-frequency filter: 10 kHz.
• Sweep speed: 2–5 ms/div.
• Sensitivity: 2–5 mV/div.

Measurements

Distal Latency (DL)

• Time from stimulus onset to onset of CMAP.
• Measured from stimulus at most distal site.
• Includes: nerve conduction time from stimulus to neuromuscular junction + synaptic delay + muscle activation.
• Reported in milliseconds.

CMAP Amplitude

• Peak-to-peak voltage of the negative deflection.
• Measured from baseline to most negative peak.
• Reflects the number of muscle fibers activated synchronously.
• Reported in millivolts (mV).
• Lower amplitude = fewer functional motor units OR conduction block.

CMAP Duration

• Time from onset to baseline crossing at end.
• Prolonged duration suggests temporal dispersion (conduction velocity variability among fibers).
• Reflects demyelination affecting different fibers differently.

Conduction Velocity (CV)

• Distance between stimulation sites / latency difference.
• For example, stimulating at wrist and elbow; CV = distance / (elbow latency − wrist latency).
• Reported in m/s.
• Reflects the speed of the fastest fibers.

F-Wave Latency (Discussed Separately)

• Late response after CMAP from antidromic activation of motor neurons.
• Tests proximal portion of nerve.
• Covered in dedicated page.

Normal Values (Approximate)

(Lab-specific norms vary; always reference your own laboratory’s values.)

Standard Tested Nerves and Sites

Upper Extremity

• Median motor: stim at wrist (just proximal to wrist crease), elbow (just lateral to brachial artery), record at abductor pollicis brevis (APB).
• Ulnar motor: stim at wrist (just proximal to wrist crease), below elbow, above elbow (over ulnar groove), record at abductor digiti minimi (ADM).
• Radial motor: stim at lateral arm, axilla, record at extensor digitorum communis (EDC) or extensor indicis proprius (EIP).

Lower Extremity

• Tibial motor: stim at ankle (just posterior to medial malleolus), popliteal fossa, record at abductor hallucis (AH) or extensor digitorum brevis (EDB).
• Peroneal motor: stim at ankle (just anterior to lateral malleolus), below fibular head, above fibular head, record at extensor digitorum brevis (EDB) or tibialis anterior (TA).

Demyelinating vs Axonal Patterns

Pure Demyelinating Pattern

• Prolonged distal latency.
• Slowed conduction velocity (often substantially below normal, e.g., 25–35 m/s).
• Prolonged F-wave latency.
• Conduction block (drop in amplitude with proximal vs distal stimulation).
• Temporal dispersion (broader CMAP with proximal stimulation).
• Amplitude relatively preserved early.
• Examples: GBS (acute), CIDP (chronic), CMT1 (hereditary).

Pure Axonal Pattern

• Reduced CMAP amplitude.
• Distal latency normal or mildly prolonged.
• Conduction velocity normal or near-normal.
• No conduction block or temporal dispersion.
• F-wave latency normal or slightly prolonged.
• Examples: diabetic neuropathy, alcoholic neuropathy, vasculitic neuropathy, chemotherapy-induced.

Mixed Patterns

• Most neuropathies have some of both.
• Diabetic neuropathy: predominantly axonal but with some demyelinating features.
• CIDP variants: predominantly demyelinating but with secondary axonal loss.
• Severe diabetic neuropathy: substantial axonal loss with secondary demyelinating features.

Conduction Block

Definition

• Drop in CMAP amplitude or area with proximal stimulation compared to distal stimulation.
• Standard threshold: >50% amplitude drop or >50% area drop (some labs use 30–40%).
• Must exclude temporal dispersion (broader pulse) — measure area, not just peak.

Causes

• Acute demyelination: GBS, CIDP.
• Focal nerve compression at sites of slowing: carpal tunnel, cubital tunnel.
• Acute focal nerve injury (block at the site of injury).
• Multifocal motor neuropathy (MMN): block in motor nerves only.

Implications

• Conduction block at a non-compressive site = acquired demyelinating disease.
• Reversible if demyelinating (with treatment of underlying cause).
• Irreversible if axonal damage at the site.

Temporal Dispersion

• The CMAP becomes broader (longer duration) with proximal stimulation.
• Different fibers conduct at different speeds → arrive at different times.
• Cause: demyelination affecting fibers heterogeneously.
• Threshold: >30% increase in duration with proximal stim, or 20% increase in area with normal amplitude.

Focal Nerve Compression Patterns

Carpal Tunnel Syndrome

• Median nerve compression at the wrist.
• Findings:
  • Prolonged median motor distal latency (most sensitive).
  • Prolonged median sensory distal latency.
  • Normal CMAP amplitude unless severe.
  • Normal forearm conduction velocity (compression at wrist, not forearm).
  • Comparison studies: median palm-to-wrist vs ulnar palm-to-wrist (if difference >0.3 ms, supports CTS).
• Severity grading (Padua, AAN criteria):
  • Mild: prolonged sensory latency only.
  • Moderate: prolonged motor + sensory latency.
  • Severe: + reduced CMAP amplitude or absent sensory response.

Ulnar Neuropathy at the Elbow

• Ulnar nerve compression at the cubital tunnel.
• Findings:
  • Slow conduction across the elbow (across-elbow CV reduced by ≥10 m/s compared to forearm).
  • Possible conduction block at the elbow.
  • Reduced CMAP amplitude with proximal stimulation.
  • Recording at ADM and first dorsal interosseous (FDI) can be combined.

Peroneal Neuropathy at the Fibular Head

• Peroneal nerve compression at the fibular head.
• Findings: across-fibular-head CV reduced; possible conduction block; reduced TA CMAP.

Tarsal Tunnel Syndrome

• Tibial nerve compression at medial ankle.
• Findings: prolonged distal latency, reduced amplitude, abnormal sensory branches.

Radiculopathy on Motor NCS

• Pure radiculopathy (with intact distal nerve) usually has normal motor NCS — the lesion is proximal to the dorsal root ganglion.
• Sensory NCS also normal (SNAPs preserved despite sensory radiculopathy — because the cell body is in the DRG, not the cord).
• EMG (needle examination) is the test that shows radiculopathy: denervation in muscles supplied by the affected nerve root.
• F-wave latency may be prolonged in severe radiculopathy.

Multifocal Motor Neuropathy (MMN)

• Motor-only demyelinating neuropathy.
• Conduction block at multiple non-compressive sites.
• Sensory NCS preserved (key distinguishing feature from CIDP).
• Often anti-GM1 antibody positive.
• Treatment: IVIG (mainstay).

Inching and Short-Segment Studies

• For focal nerve injuries, conduction is measured across short segments (1–2 cm).
• Slowing localized to a short segment confirms focal injury.
• Useful for accurate localization in cubital tunnel, peroneal neuropathy at fibular head.

Special Considerations

Temperature

• Cooler temperatures slow conduction (~1.5 m/s per °C drop in upper limb).
• Cold limbs produce false-positive demyelinating-appearing results.
• Always warm distal limb to 32°C before measuring.

Age

• Conduction velocity decreases gradually with age.
• Lab-specific norms should account for age.

Body Habitus

• Increased subcutaneous fat or large arms can affect electrode coupling and measurements.

Pacemakers and Cardiac Devices

• Stimulator generally safe to use; avoid placement directly over device.

Reporting

A motor NCS report should include:

• Indication.
• Specific nerves tested and recording sites.
• Measurements: distal latency, amplitude, conduction velocity, F-wave latency.
• Comparison to lab-specific normal values.
• Side-to-side comparison if relevant.
• Description of pattern (axonal, demyelinating, mixed, focal, multifocal).
• Interpretation and clinical correlation.

🔍 Did You Know?

The discovery that certain demyelinating neuropathies present with conduction block at specific motor nerves — particularly the wrist median, elbow ulnar, and forearm radial nerves — led to the recognition of multifocal motor neuropathy (MMN) as a distinct treatable disease that mimics ALS. Patients present with asymmetric, slowly progressive weakness — often in a single limb — without sensory symptoms, fasciculations, hyperreflexia, or upper motor neuron signs. The clinical picture can closely mimic early ALS, leading to wrong diagnosis with devastating implications. The key distinguishing finding is on motor NCS: conduction block at non-compressive sites in multiple nerves, with preserved sensory NCS (sensory fibers are typically spared in MMN). Anti-GM1 antibody is often positive. The clinical significance is profound because MMN responds dramatically to IVIG, while ALS does not — getting the diagnosis right means converting a progressive fatal disease into a treatable chronic one. For the electrodiagnostician, the lesson is that asymmetric weakness without sensory involvement warrants careful evaluation for conduction block — and an apparently normal nerve at one site doesn’t exclude block at another site. For practicing neurologists, the take-home is that any patient with progressive asymmetric weakness should have detailed motor NCS looking specifically for conduction block before accepting an ALS diagnosis. The same principle applies to CIDP — another treatable demyelinating disease often initially misdiagnosed as ALS or other progressive motor neuron disease.

Pitfalls and Pearls

• Setup matters: belly-tendon montage; cathode distal; impedance ≤5 kΩ; warm limb to 32°C.
• Supramaximal stimulation: required for reliable measurements; raise current until amplitude plateaus.
• Initial positive deflection: electrode off motor point; reposition.
• Distal latency: from stimulus onset to CMAP onset; includes synaptic delay.
• Conduction velocity: between stimulation sites; reflects fastest fibers.
• Pure demyelinating: prolonged latency, slow CV, conduction block, temporal dispersion; amplitude preserved.
• Pure axonal: reduced amplitude; normal CV; no block or dispersion.
• Conduction block: >50% amplitude drop with proximal stim at non-compressive site = acquired demyelinating.
• Temporal dispersion: >30% duration increase with proximal stim.
• CTS: prolonged median distal latency (sensitive); comparison studies if equivocal.
• Ulnar at elbow: across-elbow CV reduced ≥10 m/s vs forearm.
• MMN: motor conduction block + preserved sensory; anti-GM1 + IVIG-responsive; mimics ALS.
• Pure radiculopathy: normal NCS; needle EMG shows denervation.
• Cold limb: artifactual slowing; warm to 32°C.
• Lab-specific norms: textbook values don’t apply to individual labs.
• Side-to-side comparison: >50% amplitude asymmetry usually pathologic.
• Always pair motor NCS with EMG for complete peripheral nerve evaluation.

References

1. Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders: Clinical-Electrophysiologic-Ultrasound Correlations. 4th ed. Elsevier; 2021.
2. Kimura J. Electrodiagnosis in Diseases of Nerve and Muscle: Principles and Practice. 4th ed. Oxford University Press; 2013.
3. Aminoff MJ, ed. Aminoff’s Electrodiagnosis in Clinical Neurology. 6th ed. Elsevier; 2012.
4. Joint Task Force of the EFNS and the PNS. European Federation of Neurological Societies/Peripheral Nerve Society Guideline on management of chronic inflammatory demyelinating polyradiculoneuropathy: report of a joint task force—first revision. J Peripher Nerv Syst. 2010;15(1):1-9.
5. Padua L, LoMonaco M, Gregori B, Valente EM, Padua R, Tonali P. Neurophysiological classification and sensitivity in 500 carpal tunnel syndrome hands. Acta Neurol Scand. 1997;96(4):211-217.