Electromyography and Nerve Conduction Studies

Electromyography (EMG) and nerve conduction studies (NCS) are the two complementary core techniques of electrodiagnostic medicine. Nerve conduction studies measure how electrical signals travel along peripheral nerves, while needle electromyography records the electrical activity generated within muscle. Performed together, they characterise the function of the motor unit and help localise disorders of nerve and muscle.

Definition

Electromyography and nerve conduction studies are electrodiagnostic techniques in which the electrical responses of peripheral nerves to stimulation (nerve conduction studies) and the spontaneous and voluntary electrical activity of muscle recorded with an intramuscular needle electrode (needle electromyography) are measured to assess the integrity of the motor unit.

Scope

This entry covers the principles of nerve conduction studies and needle electromyography: what each technique records, the main parameters interpreted, and how the two are combined to localise and characterise neuromuscular disorders. It is a reference and educational overview of method and is not a procedural manual or clinical guidance.

Core questions

• What do nerve conduction velocity, latency, and amplitude indicate about a peripheral nerve?
• How do abnormal spontaneous activity and motor unit potential changes on needle EMG point toward neurogenic or myopathic processes?
• How are NCS and EMG findings combined to localise a lesion within the motor unit?

Key concepts

• Compound muscle action potential (CMAP)
• Sensory nerve action potential (SNAP)
• Conduction velocity and distal latency
• Conduction block and temporal dispersion
• Spontaneous activity (fibrillation potentials, positive sharp waves)
• Motor unit potential morphology and recruitment
• Late responses (F-wave, H-reflex)

Mechanisms

In a nerve conduction study, a brief electrical stimulus depolarises a peripheral nerve and the resulting compound action potential is recorded over muscle (compound muscle action potential) or nerve (sensory nerve action potential); the latency, conduction velocity, and amplitude distinguish slowing or block from axon loss. Needle electromyography inserts a recording electrode into muscle and samples electrical activity at rest and during graded voluntary contraction. Abnormal spontaneous activity such as fibrillation potentials reflects muscle-fibre denervation or irritability, while the duration, amplitude, and recruitment of motor unit potentials shift in characteristic directions in neurogenic versus myopathic disorders. The two techniques are interpreted together against normative reference values to localise and characterise the disorder.

Clinical relevance

EMG and NCS extend the neuromuscular examination and provide the electrophysiologic basis for characterising nerve and muscle disorders. This description is educational; it explains what the techniques measure and does not provide protocols, reference cut-offs, or treatment recommendations.

Evidence & guidelines

Standardised terminology for these techniques is maintained in the AANEM glossary, and condition-specific practice parameters — for example for the electrodiagnosis of carpal tunnel syndrome — describe how studies should be selected and interpreted. Reference textbooks by Preston and Shapiro and by Kimura set out technique and normative interpretation in detail.

History

Nerve conduction studies and needle electromyography were developed and refined through the mid-twentieth century as clinical applications of neurophysiology, and were progressively standardised into the paired core methods of electrodiagnostic medicine, supported by professional glossaries and disease-specific practice parameters.

Key figures

• Jun Kimura
• David C. Preston
• Barbara E. Shapiro

Seminal works

• kimura-2013
• preston-shapiro-2013
• jablecki-2002-cts
• aanem-glossary-2015

Frequently asked questions

What is the difference between EMG and a nerve conduction study?

A nerve conduction study stimulates a nerve and records how its electrical signal travels, while needle electromyography records the electrical activity inside a muscle; the two are usually performed together as complementary parts of one electrodiagnostic examination.

Why are fibrillation potentials important on needle EMG?

Fibrillation potentials are a form of abnormal spontaneous activity that indicates muscle-fibre membrane instability, commonly from denervation, and contribute to distinguishing neurogenic from other processes when interpreted with the rest of the study.