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Electromyography

Electromyography

Electromyography (EMG) is an electrophysiological test that evaluates the type, location, and severity of neuromuscular diseases by inserting needle electrodes into muscles to record and analyze electrical activity at rest and during contraction.

2026-03-29

At a Glance

EMG is a key diagnostic tool performed alongside nerve conduction studies (NCS) to differentiate lesions of peripheral nerves, nerve roots, neuromuscular junctions, and muscles themselves. Abnormal spontaneous activity at rest (fibrillation potentials, positive sharp waves) indicates denervation, and motor unit potential (MUP) analysis during contraction distinguishes neurogenic from myopathic lesions [1]. It shows a sensitivity of approximately 50-70% and specificity of approximately 85-90% for diagnosing radiculopathy [2].

Definition and Overview

Electromyography (EMG) is a test that records and analyzes the electrical activity of muscles at rest and during various levels of contraction by inserting disposable concentric needle electrodes into the muscle.

Performed alongside nerve conduction studies (NCS), it constitutes an "electrodiagnostic study" and plays a central role in the differential diagnosis of peripheral nervous system and muscle diseases [1].

Test Procedure

Insertional and Spontaneous Activity

Electrical activity is observed during needle insertion and when the muscle is fully relaxed.

  • Insertional activity: Electrical activity generated when the needle is inserted or moved. Increased or decreased activity has pathological significance.
  • Normal: No electrical activity at rest (electrical silence)
  • Abnormal spontaneous potentials: Findings indicative of denervation

Types of abnormal spontaneous potentials include the following.

  • Fibrillation potential: Spontaneous contraction of individual denervated muscle fibers. Regular, biphasic waveform
  • Positive sharp wave: Same significance as fibrillation potentials. Characterized by an initial positive deflection
  • Myotonic discharge: "Dive bomber" sound. Characteristic of myotonia
  • Fasciculation potential: Spontaneous contraction of an entire motor unit. Significant in motor neuron disease

Voluntary Motor Unit Analysis

Motor unit potentials (MUPs) are analyzed as the patient progressively contracts the muscle [3].

Measured parameters include the following.
- MUP amplitude: Reflects the number of muscle fibers within a motor unit
- MUP duration: Reflects the territory of the motor unit
- MUP polyphasic rate: Increases with reinnervation or myopathy
- Recruitment pattern: The number of activated motor units and their firing frequency

Interpretation of Results

Neurogenic Pattern

When motor nerves are damaged, spontaneous potentials appear in denervated muscle fibers, and surviving motor neurons expand their motor units through reinnervation.

  • At rest: Fibrillation potentials, positive sharp waves (+)
  • MUP changes: Increased amplitude, increased duration, increased polyphasic rate (giant MUPs)
  • Recruitment pattern: Reduced recruitment -- fewer motor units are activated, with increased firing frequency of individual units
  • Representative conditions: Radiculopathy, peripheral nerve injury, motor neuron disease (ALS)

Myopathic Pattern

When muscle fibers themselves are damaged, the size of motor units decreases.

  • At rest: Mostly normal (spontaneous potentials possible in some inflammatory myopathies)
  • MUP changes: Decreased amplitude, decreased duration, increased polyphasic rate (small polyphasic MUPs)
  • Recruitment pattern: Early recruitment -- many small motor units are recruited even with mild contraction
  • Representative conditions: Polymyositis, dermatomyositis, muscular dystrophy, metabolic myopathy

Neuromuscular Junction Disorders

Evaluated by repetitive nerve stimulation (RNS) and single-fiber EMG (SFEMG). In myasthenia gravis, a characteristic decremental response of CMAP amplitude is observed with low-frequency (2-3 Hz) repetitive stimulation.

Clinical Applications

Radiculopathy

EMG plays a complementary role to imaging (MRI) in diagnosing radiculopathy [2].

  • Denervation findings are identified in the distribution of muscles innervated by the affected nerve root.
  • Denervation findings in the paraspinal muscles suggest that the lesion is proximal to the intervertebral foramen (at the nerve root level).
  • It is useful for distinguishing anatomical abnormalities seen on MRI from clinically significant nerve root compression.
  • Denervation findings do not fully develop until 3-4 weeks after symptom onset, so the timing of the test is important.

Motor Neuron Disease (ALS)

Widespread denervation and reinnervation findings are identified across multiple body regions (brainstem, cervical, thoracic, lumbosacral), while sensory nerve conduction studies are normal. Fasciculation potentials are commonly observed. Under the Awaji diagnostic criteria, EMG findings are recognized as objective evidence of clinical involvement.

Myopathy

In inflammatory myopathies such as polymyositis and dermatomyositis, spontaneous potentials and myopathic patterns are observed during the active phase. EMG is also used for selecting muscle biopsy sites.

Traumatic Nerve Injury

Following nerve injury, tracking the extent of denervation and progression of reinnervation helps determine the timing of surgical intervention.

Limitations of the Test

  • As an invasive test (needle insertion), it involves some patient discomfort.
  • The examiner's (neurologist's) experience and expertise affect result interpretation.
  • In the acute phase (within 1-2 weeks of symptom onset), denervation findings may not yet be present.
  • Testing may be limited for deep muscles or muscles that are difficult to access.
  • Patient cooperation (voluntary muscle contraction) is required, making the test limited in patients with altered consciousness or severe pain [4].

Pre-Test Precautions

  • No special fasting required
  • Can generally be performed while on anticoagulants (caution with deep muscle testing)
  • Do not apply moisturizer or lotion to the test area
  • Patients with pacemakers should inform the examiner before testing

Frequently Asked Questions

Since a thin needle is inserted into the muscle, there is some discomfort, but it is tolerable for most patients. The electrode used (approximately 0.3-0.5 mm in diameter) is much thinner than a standard injection needle, and the risk of bleeding or infection after the test is very low. The test typically takes approximately 30-60 minutes.

They are different tests, but are usually performed together. Nerve conduction studies (NCS) evaluate the electrical conduction of the nerves themselves, while EMG evaluates the electrical activity of muscles. Together they constitute an "electrodiagnostic study," enabling precise determination of the location and type of lesion.

Denervation is a condition in which the motor nerve supplying a muscle is damaged, cutting off the nerve supply to the muscle. It is identified by abnormal spontaneous potentials at rest (fibrillation potentials, positive sharp waves) and is seen in radiculopathy, peripheral nerve injury, motor neuron disease, and other conditions [3].

In most cases, the test can be performed. It can be done while taking aspirin or clopidogrel. With warfarin or NOAC use, there is a slightly higher risk of minor bleeding at the needle insertion site, but most muscles can be safely tested. Caution is needed when testing deep muscles.

EMG results are interpreted in real time, so findings can be explained immediately after the test or on the same day. The neurologist performing the test personally inserts the needle, analyzes the waveforms, and interprets the results in conjunction with clinical findings.

References

  1. [1] Preston DC, Shapiro BE (2020). "Electromyography and Neuromuscular Disorders: Clinical-Electrophysiologic-Ultrasound Correlations." Elsevier, 4th edition.
  2. [2] Dillingham TR, Lauder TD, Andary M, Kumar S, Pezzin LE, Stephens RT, Shannon S (2000). "Identifying lumbosacral radiculopathies: an optimal electromyographic screen." American Journal of Physical Medicine & Rehabilitation, 79: 496-503. DOI PubMed
  3. [3] Daube JR, Rubin DI (2009). "Needle electromyography." Muscle & Nerve, 39: 244-270. DOI PubMed
  4. [4] Stålberg E, van Dijk H, Falck B, Kimura J, Neuwirth C, Pitt M, Bromberg M (2019). "Standards for quantification of EMG and neurography." Clinical Neurophysiology, 130: 1688-1729. DOI PubMed
  5. [5] Cho SC, Ferrante MA, Levin KH, Harmon RL, So YT (2010). "Utility of electrodiagnostic testing in evaluating patients with lumbosacral radiculopathy: an evidence-based review." Muscle & Nerve, 42: 276-282. DOI PubMed

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