5.2 Needle Electromyography & MUAP Analysis

ABPMR Physical Medicine and Rehabilitation Board Exam Study Guide Exam

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1 Chapter 1: Introduction & Exam Overview

1.1 ABPMR Part I Exam Overview & Format 1.2 Board Review Strategy & Content Blueprint Breakdown

2 Chapter 2: Musculoskeletal Medicine

2.1 Shoulder & Upper Extremity Diagnostics 2.2 Knee, Hip & Lower Extremity Conditions 2.3 Spine Pathology, Radiculopathy & Myelopathy 2.4 Inflammatory Arthritis, Tendinopathies & Concussion 2.5 Bone Health, Osteoporosis & Metabolic Bone Disease 2.6 Thoracic Outlet Syndrome & Peripheral Nerve Entrapment 2.7 Fracture Rehabilitation & Immobilization Complications 2.8 Overuse Tendinopathies & Regenerative Medicine 2.9 Gait Analysis & Abnormal Gait Patterns

3 Chapter 3: Spinal Cord Injury Rehabilitation

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4 Chapter 4: Stroke & Neuromuscular Rehabilitation

4.1 Stroke Vascular Syndromes & Acute Assessment 4.2 Stroke Motor Recovery & Hemiplegic Complications 4.3 Spasticity Assessment & Multimodal Management 4.4 ALS, GBS, CIDP & Neuromuscular Disease Rehab 4.5 Multiple Sclerosis & Demyelinating Disease Rehabilitation 4.6 Parkinsonism, Movement Disorders & Functional Neurologic Disorder

5 Chapter 5: Electrodiagnostics (EMG/NCS)

5.1 Nerve Conduction Studies Principles & Physics 5.2 Needle Electromyography & MUAP Analysis 5.3 Mononeuropathies & Radiculopathy Patterns 5.4 Neuromuscular Junction Disorders & Myopathies

6 Chapter 6: Traumatic Brain Injury

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7 Chapter 7: Chronic Pain & Interventional Spine

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8 Chapter 8: Amputee Rehabilitation & Prosthetics

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9 Chapter 9: Pediatric Rehabilitation

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10 Chapter 10: Prosthetics, Orthotics & Assistive Technology

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11 Chapter 11: Medical Rehabilitation & Outcomes

11.1 Dysphagia Assessment & Management 11.2 Neurogenic Bladder & Bowel Management 11.3 Pressure Injuries & Wound Management 11.4 Outcome Measures & Medical Complications 11.5 Cancer Rehabilitation & Oncologic Rehabilitation 11.6 Burns, Endocrine/Metabolic & Medical Complications Rehabilitation

12 Chapter 12: Cardiopulmonary Rehabilitation

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13 Chapter 13: Foundational Rehabilitation Concepts

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5.2 Needle Electromyography & MUAP Analysis

Key Takeaways Explain This Topic AI

Needle electromyography evaluates the motor unit (alpha motor neuron, axon, neuromuscular junction, and muscle fibers) to assess denervation and localize nerve lesions.

Denervation potentials (fibrillations and positive sharp waves) reflect muscle fiber sarcolemmal instability and appear 2 to 3 weeks following acute axonal injury.

Complex Repetitive Discharges (CRDs) exhibit abrupt onset/offset, uniform polyphasic morphology, and machine-like firing (10-100 Hz), characteristic of chronic radiculopathy and IBM.

Neurogenic MUAPs demonstrate long duration (>15 ms), high amplitude (>2-5 mV), and reduced recruitment, whereas myopathic MUAPs demonstrate short duration (<5 ms), low amplitude, and early excessive recruitment.

Last updated: July 2026

5.2 Needle Electromyography & MUAP Analysis
Principles of Needle Electromyography

Needle electromyography (EMG) evaluates the motor unit (single alpha motor neuron , its axon, neuromuscular junctions, and innervated muscle fibers). Needle EMG localizes nerve lesions (radiculopathy vs. plexopathy vs. peripheral nerve), establishes chronicity, assesses active denervation, and differentiates neurogenic from myopathic disorders.

Recording Needles & Safety

Concentric Needle : Central active wire inside a cannula reference electrode. Provides standardized, reproducible Motor Unit Action Potential (MUAP) measurements.

Monopolar Needle : Teflon-coated needle tip paired with a surface reference. Yields larger amplitudes and less pain.

Safety : Caution in patients on anticoagulation or with severe thrombocytopenia. High-risk muscles for pneumothorax include serratus anterior , diaphragm , rhomboids , and paraspinals . NEEDLE EMG EXAMINATION STAGES

+----------------------------------------------------------------+
| 1. Insertional Activity --> Mechanical insertion of needle |
| 2. Spontaneous Activity --> Assessment of resting muscle |
| 3. MUAP Morphology --> Minimal voluntary contraction |
| 4. Recruitment Pattern --> Maximal voluntary effort |
+----------------------------------------------------------------+
Insertional & Spontaneous Activity Analysis

In healthy muscle, needle insertion produces a brief burst lasting <300 ms, after which the muscle is electrically silent at rest.

Insertional Activity Variations

Increased Insertional Activity : Bursts extending >300 ms. Indicates membrane instability in active denervation, myopathies (polymyositis, myotonia), or acute injury.

Decreased Insertional Activity : Attenuated or absent bursts. Indicates replacement of muscle by fibrous tissue, severe atrophy, or periodic paralysis attacks.

Endplate Noise vs. Endplate Spikes (Normal Findings)

Direct needle placement in the neuromuscular endplate produces normal findings:

Endplate Noise : Low-amplitude (<50 µV), continuous negative sea-shell sound from spontaneous miniature endplate potentials (MEPPs).

Endplate Spikes : Irregular, biphasic potentials with initial negative deflection ("sputtering" sound) from single muscle fiber irritation.

Pathological Spontaneous Activity
Fibrillation Potentials & Positive Sharp Waves (PSWs)

Fibrillations and positive sharp waves indicate active muscle fiber denervation or sarcolemmal instability.

Pathophysiology : Loss of axonal innervation causes muscle fibers to upregulate extrajunctional acetylcholine receptors and develop spontaneous membrane oscillations.

Morphology : Fibrillations are brief (1–5 ms), low-amplitude (20–300 µV) spikes with an initial positive deflection. PSWs are sharp positive deflections followed by a slow negative phase. Both fire metronomically (0.5–10 Hz).

Timing Post-Injury : Fibrillations and PSWs appear 2 to 3 weeks following acute axonal injury (10–14 days in proximal paraspinals, 3–4 weeks in distal limbs).

Fasciculation Potentials

Involuntary discharges of an entire motor unit (from anterior horn cell or axon). Polyphasic or simple MUAP firing at irregular, slow intervals (0.1–2 Hz). Pathological in Amyotrophic Lateral Sclerosis (ALS) and radiculopathies; also occurs as a benign finding.

Myotonic Discharges

Spontaneous high-frequency trains of muscle fiber action potentials from channelopathies. Classic "dive-bomber" acoustic signature with waxing/waning amplitude (10 µV–1 mV) and frequency (20–150 Hz). Seen in Myotonic Dystrophy (DM1/DM2) and Myotonia Congenita.

Complex Repetitive Discharges (CRDs)

Originate from a pacemaker muscle fiber depolarizing adjacent fibers via ephaptic transmission . Abrupt onset/offset, uniform polyphasic waveform, rapid machine-like rhythm (10–100 Hz, "jackhammer" sound). Seen in chronic radiculopathy and Inclusion Body Myositis.

Spontaneous Potential Origin Firing Pattern Sound Signature Associated Pathology

Fibrillations / PSWs Single muscle fiber Regular, metronomic Ticking clock / rain on tin roof Active denervation (2-3 wks), polymyositis

Fasciculations Entire motor unit Irregular, erratic Popcorn popping ALS, radiculopathy, benign

Myotonic Discharges Muscle fiber membrane Waxing/waning frequency/amp Dive-bomber aircraft Myotonic dystrophy, channelopathies

CRDs Grouped muscle fibers Uniform, machine-like Motorcycle / jackhammer Chronic radiculopathy, IBM, SMA
Motor Unit Action Potential (MUAP) Analysis

During mild contraction, MUAPs are analyzed for duration, amplitude, and phase count. During maximal effort, recruitment is evaluated. MUAP MORPHOLOGY COMPARISON

Neurogenic MUAP Myopathic MUAP
Long Duration (>15 ms) Short Duration (<5 ms)
High Amplitude (>2-5 mV) Low Amplitude (<0.5 mV)
Polyphasic (>4 phases) Polyphasic (>4 phases)
Neurogenic MUAP & Recruitment Patterns

Axonal loss leads to collateral sprouting from surviving axons, expanding motor unit territory:

MUAP Morphology : Long duration (>15 ms), high amplitude (>2–5 mV), and polyphasic (>4 phases).

Recruitment Pattern : Reduced recruitment . Remaining active units fire at abnormally high frequencies (>20–30 Hz) to compensate.

Myopathic MUAP & Recruitment Patterns

Loss of muscle fibers per motor unit reduces force output per unit:

MUAP Morphology : Short duration (<5–8 ms), low amplitude (<0.5 mV), polyphasic.

Recruitment Pattern : Early / excessive recruitment . The CNS rapidly recruits an excessive number of small myopathic units for minimal effort.

Parameter Normal Chronic Neurogenic Primary Myopathic
MUAP Duration 5–15 ms Prolonged (>15 ms) Shortened (<5 ms)
MUAP Amplitude 0.5–2.0 mV High (>2.0–10.0 mV) Low (<0.5 mV)
Phases Biphasic / Triphasic (<4) Increased (>4 phases) Increased (>4 phases)
Recruitment Normal (5:1 ratio) Reduced (discrete, rapid firing) Early / Excessive (full screen at low effort)