Needle EMG: Insertional, Spontaneous, MUAP

Needle electromyography (EMG) records muscle electrical activity directly via a needle electrode inserted into the muscle. Unlike NCS, which tests nerve conduction, needle EMG tests the muscle and its motor units — and is sensitive to both the muscle (myopathy) and the motor neuron supplying it (neurogenic disease). EMG is the technique that distinguishes muscle from nerve disease, identifies acute vs chronic denervation, localizes nerve injury to specific roots or peripheral nerves, and characterizes motor unit morphology in disease. This page covers the EMG examination technique: insertional activity, spontaneous activity, and motor unit action potential (MUAP) analysis.

EMG Procedure

Equipment

  • Concentric needle electrode (most common): central recording wire surrounded by insulated outer cannula serving as reference.
  • Monopolar needle: less commonly used; surface reference electrode required.
  • Pre-amplifier near needle to reduce noise.
  • Amplifier with appropriate filters (10 Hz to 10 kHz).
  • Loudspeaker for auditory characterization (important for pattern recognition).
  • Display with adjustable sweep speed (5–10 ms/div standard).

Technique

  • Skin preparation; needle insertion at 90° to muscle.
  • Three phases evaluated:
    1. Insertional activity (needle movement).
    2. Spontaneous activity (needle still, muscle at rest).
    3. MUAP analysis (during voluntary activation).
  • Multiple sites (3–4) sampled per muscle.
  • Multiple muscles examined per limb to map distribution.

Phase 1: Insertional Activity

Normal

  • Brief burst of muscle action potentials during needle movement.
  • Duration: <300 ms after needle stops moving.
  • Resolves with needle motionless.

Increased Insertional Activity

  • Burst lasting >300 ms after needle stops.
  • Sustained or repetitive bursts.
  • Indicates muscle membrane irritability.
  • Causes:
    • Acute denervation (early; before fibrillations develop).
    • Myopathy with membrane instability.
    • Polymyositis, dermatomyositis.
    • Inflammatory myopathies.
    • Myotonia (specific waveform).

Decreased Insertional Activity

  • Brief or absent insertional burst.
  • Causes:
    • Severe fibrofatty replacement of muscle.
    • End-stage muscle disease.
    • Severe denervation atrophy.

Phase 2: Spontaneous Activity

Normal Resting Muscle

  • Electrically silent (no spontaneous activity).
  • “End-plate noise” (low-amplitude irregular noise) and “end-plate spikes” (small biphasic potentials) — both normal at the motor end-plate region; not spontaneous activity.

Fibrillation Potentials (Fibs)

  • Brief (1–5 ms), small (10–200 μV), regular, biphasic potentials with initial positive deflection.
  • Sound: stereotyped “popping” through speaker.
  • Cause: denervated muscle fibers spontaneously firing at their own pacemaker rate.
  • Indicates ACUTE neurogenic disease or active inflammatory myopathy.
  • Develop ~2–3 weeks after acute denervation; resolve as reinnervation occurs.

Positive Sharp Waves (PSWs)

  • Initial sharp positive deflection followed by slower negative phase.
  • Same etiology as fibrillations (denervated muscle fibers firing).
  • Often appear with fibs.
  • Same clinical significance as fibs.

Fasciculation Potentials

  • Spontaneous firing of an entire motor unit.
  • Larger amplitude than fibs (a whole motor unit, not single fiber).
  • Sound: “kerplunk” through speaker.
  • Causes:
    • Motor neuron disease (ALS) — coarse, irregular, low frequency.
    • Radiculopathy.
    • Benign fasciculations (no other features of denervation).
    • Cramp-fasciculation syndrome.
    • Hyperthyroidism.
    • Various metabolic states.
  • Single fasciculation is not pathologic; multiple in different distributions is suspicious.

Complex Repetitive Discharges (CRDs)

  • Trains of discharges firing at fixed frequency for variable duration.
  • Sounds like an engine starting.
  • Chronic neurogenic or chronic myopathic.
  • Common in CIDP, polymyositis, IBM, advanced chronic radiculopathy.

Myokymic Discharges

  • Rhythmic bursts of motor unit potentials.
  • Causes:
    • Multiple sclerosis (facial myokymia).
    • Radiation plexopathy.
    • Hyperthyroidism.
    • Brainstem lesions.
    • Various neuropathies.

Neuromyotonic Discharges

  • Very high frequency (150–250 Hz) bursts of motor unit potentials.
  • Acquired neuromyotonia (Isaacs syndrome): autoantibodies against K⁺ channels.
  • Crampi-fasciculation syndrome.

Myotonic Discharges

  • Stereotyped trains of muscle action potentials.
  • Wax and wane in amplitude and frequency.
  • “Dive-bomber” or “motorcycle revving” sound.
  • Causes:
    • Myotonic dystrophy type 1 and 2.
    • Myotonia congenita (Cl⁻ channel).
    • Paramyotonia congenita (Na⁺ channel).
    • Hyperkalemic periodic paralysis.
    • Acid maltase deficiency.

Phase 3: Motor Unit Action Potential (MUAP) Analysis

MUAP Basics

  • The MUAP is the summed potential of all muscle fibers innervated by a single motor neuron.
  • Recorded during minimal voluntary contraction.
  • Characterized by: amplitude, duration, polyphasia, and recruitment.

Normal MUAP

  • Amplitude: 500 μV – 3 mV.
  • Duration: 5–15 ms.
  • Phases: 2–4 (one direction change = one phase).
  • Stable from contraction to contraction.

Chronic Neurogenic MUAP

  • Increased amplitude (often 5–10 mV).
  • Increased duration (often >15 ms).
  • Polyphasic (>4 phases).
  • Cause: reinnervation. After axonal loss, surviving axons sprout and innervate denervated fibers, creating larger motor units.
  • Seen in:
    • Chronic radiculopathy.
    • Chronic peripheral nerve disease.
    • Motor neuron disease (ALS).
    • Post-poliomyelitis.

Acute Neurogenic MUAP

  • Normal-appearing MUAPs may still be present.
  • Reduced recruitment (firing of few motor units at high rates rather than many at low rates).
  • Best seen with mild contraction.

Myopathic MUAP

  • Small amplitude (often <500 μV).
  • Brief duration (<5 ms).
  • Polyphasic.
  • Cause: loss of muscle fibers within each motor unit; smaller summed potential.
  • Seen in:
    • Inflammatory myopathies.
    • Muscular dystrophies.
    • Metabolic myopathies.
    • End-stage neurogenic disease (severe denervation atrophy).

Recruitment Pattern

  • Normal: as effort increases, more motor units activated; firing rate of activated units increases.
  • Reduced recruitment: few motor units firing at high rates with mild effort (neurogenic).
  • Early recruitment: many motor units firing at low rates with mild effort (myopathic — recruiting more units to compensate for weaker individual units).
  • Distinguishes neurogenic from myopathic in cases with similar MUAP morphology.

Interpreting the EMG Pattern

Acute Neurogenic Pattern

  • Fibs + PSWs (denervation).
  • Normal MUAP morphology (if denervation recent).
  • Reduced recruitment.
  • Examples: acute radiculopathy, acute peripheral nerve injury, GBS (early).

Chronic Neurogenic Pattern

  • Fewer fibs/PSWs (or absent if chronic stable).
  • Large polyphasic MUAPs (reinnervation).
  • Reduced recruitment.
  • Examples: chronic radiculopathy, ALS, post-polio.

Acute Myopathic Pattern

  • Insertional activity may be increased (membrane instability).
  • Fibs/PSWs in active inflammatory myopathies.
  • Small polyphasic MUAPs.
  • Early recruitment.
  • Examples: polymyositis (active), inclusion body myositis.

Chronic Myopathic Pattern

  • Reduced insertional activity (fibrofatty replacement).
  • No or rare fibs.
  • Small polyphasic MUAPs.
  • Early recruitment.
  • Examples: muscular dystrophy, chronic polymyositis.

Mixed Patterns

  • Common in advanced disease.
  • Severe chronic neurogenic disease can develop myopathic MUAPs (severe fiber loss in remaining motor units).
  • Long-standing myopathy with reinnervation in some areas.

EMG Patterns in Specific Diseases

ALS (Motor Neuron Disease)

  • Diffuse fasciculations.
  • Fibs and PSWs in multiple regions (multiple cervical, lumbar, bulbar, thoracic regions per EFNS/El Escorial criteria).
  • Large polyphasic MUAPs.
  • Reduced recruitment.
  • Distribution affecting both upper and lower motor neurons; multifocal.

Radiculopathy

  • Fibs + PSWs in muscles supplied by affected root.
  • Paraspinal muscles often affected (key distinguishing feature from plexopathy).
  • Reinnervation pattern over months.

Peripheral Nerve Injury

  • Fibs + PSWs in muscles innervated by injured nerve distal to injury.
  • Paraspinal muscles spared (key distinguishing from radiculopathy).
  • Reinnervation if axonal regrowth.

Inflammatory Myopathy

  • Increased insertional activity.
  • Fibs/PSWs in active disease.
  • Small polyphasic MUAPs.
  • Early recruitment.
  • Distribution: proximal predominance (deltoid, biceps, quadriceps).

Muscular Dystrophy

  • Reduced insertional activity (fibrofatty replacement).
  • No fibs typically.
  • Small polyphasic MUAPs.
  • Early recruitment.
  • Distribution: proximal predominance.

Myasthenia Gravis

  • Resting MUAPs may be normal.
  • Reduced amplitude with sustained activation (decrementing response).
  • Repetitive nerve stimulation (RNS) more diagnostic than needle EMG.

Practical Considerations

Patient Comfort

  • Local anesthesia not typically used.
  • Brief discomfort expected.
  • Patient relaxation is key for proper interpretation.

Risks

  • Bleeding at needle site (avoid anticoagulants if possible; small risk in mild anticoagulation).
  • Bruising or hematoma.
  • Rarely: infection, pneumothorax (with paraspinal cervical examination), nerve injury.

Documentation

  • List every muscle examined.
  • For each muscle: insertional activity, spontaneous activity, MUAP morphology, recruitment.
  • Distribution pattern (which muscles affected, which spared).
  • Interpretation: pattern (neurogenic acute, chronic, myopathic acute, chronic, mixed); distribution; localization to root level, nerve, or generalized.

🔍 Did You Know?

The development of fibrillation potentials after acute nerve injury follows a predictable timeline that has profound clinical implications for the timing and interpretation of EMG. After acute nerve transection or severe axonal injury, the denervated muscle fibers begin to spontaneously fire — but not immediately. The latency to develop fibrillations is approximately 2–3 weeks, with most muscles showing them by day 14–21. This time lag explains a clinically important paradox: EMG of a patient with acute traumatic nerve injury can be completely normal at day 3 — because fibrillations haven’t yet developed. The same EMG performed at 3 weeks will show prominent fibrillations confirming axonal injury. Clinical implications: (1) early EMG is most useful for localizing focal nerve compression and characterizing demyelinating block, not for documenting acute axonal injury; (2) repeat EMG at 3 weeks after suspected nerve injury is often essential for prognosis; (3) the absence of fibrillations at 3+ weeks in muscles innervated by an injured nerve suggests neurapraxic injury (conduction block without axonal loss) — better prognosis; (4) presence of fibrillations confirms axonal injury but doesn’t establish severity. For practicing electrodiagnosticians, the lesson is that EMG timing matters as much as EMG findings, and patient management often requires serial studies. For neurologists evaluating acute nerve injuries, the take-home is to specifically request follow-up EMG at 3 weeks if initial study at <2 weeks shows no fibrillations.

Pitfalls and Pearls

  • Three phases of EMG: insertional, spontaneous (rest), MUAP analysis (contraction).
  • Insertional activity: increased = membrane irritability; decreased = severe atrophy.
  • Fibrillations + PSWs: denervated muscle fibers; develop 2–3 weeks after axonal injury.
  • Fibrillations resolve over time with reinnervation: can be absent in chronic stable neurogenic disease.
  • Fasciculations: whole motor unit firing; ALS, radiculopathy, benign, hyperthyroidism.
  • Myotonic discharges: wax/wane, dive-bomber sound; myotonic dystrophy, channelopathies.
  • Chronic neurogenic MUAP: large amplitude, long duration, polyphasic; reinnervation.
  • Myopathic MUAP: small amplitude, brief duration, polyphasic; early recruitment.
  • Recruitment: reduced = neurogenic; early = myopathic.
  • ALS pattern: fasciculations + fibs in multiple regions; large polyphasic MUAPs.
  • Radiculopathy: fibs in distribution of affected root including paraspinal.
  • Peripheral nerve injury: fibs in muscles distal to injury; paraspinal spared.
  • Inflammatory myopathy: increased insertional, fibs, small polyphasic MUAPs, early recruitment.
  • Muscular dystrophy: reduced insertional, no fibs, small polyphasic MUAPs.
  • Timing matters: EMG <2 weeks after acute injury may be normal; repeat at 3 weeks.
  • Listen to the speaker: each spontaneous activity has characteristic sound.
  • Examine multiple muscles: distribution pattern essential for localization.

References

  1. Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders. 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. Daube JR, Rubin DI. Needle electromyography. Muscle Nerve. 2009;39(2):244-270.
  5. Rubin DI. Needle electromyography: basic concepts and patterns of abnormalities. Neurol Clin. 2012;30(2):429-456.