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Needle electromyography (EMG)
Needle EMG asks three questions: is there spontaneous activity at rest (active denervation), what do the motor units look like (large — chronic neurogenic; small — myopathic), and how are they recruited — and the lesion is localised by which muscles are abnormal and which are normal.
Why it matters in the exam
Insertional activity: increased in denervation and myotonia, and reduced when muscle has been replaced by fat or fibrous tissue. Spontaneous activity: fibrillations and positive sharp waves — signs of denervation, which appear only about two to three weeks after injury (earlier in proximal muscles), so EMG done too early may be normal; fasciculations — spontaneous discharge of a single motor unit, common in ALS but also normal; myotonic discharges that wax and wane in amplitude and frequency ("dive bomber") — myotonia (as in myotonic dystrophy); complex repetitive discharges that start and stop abruptly — a chronic process; myokymic discharges — typical of plexus damage after radiation.
Motor units (MUAP): in a chronic neurogenic process — large, long, polyphasic units (from reinnervation), with reduced recruitment (few units firing fast); in a myopathic process — small, short, polyphasic units, with early recruitment (many units for little force), as in Duchenne dystrophy. In acute neurogenic damage the units are still normal in shape, but recruitment is reduced. In conduction block alone (neurapraxia) there is reduced recruitment without fibrillations.
Localisation: fibrillations in the paraspinal muscles point to a root lesion (the posterior ramus leaves before the plexus). To tell L5 radiculopathy from peroneal nerve injury, test muscles supplied by L5 through other nerves — tibialis posterior (tibial nerve) and gluteus medius (superior gluteal nerve): they are abnormal in radiculopathy and normal in peroneal nerve injury; clinically, weak foot inversion points to L5. With suspected upper trunk plexus injury, also test the rhomboids and serratus anterior, whose nerves leave the roots before the trunk — they are normal in a trunk lesion and abnormal in root damage.
Key points
- Fibrillations — denervation; appear after two to three weeks.
- Neurogenic — large units, reduced recruitment.
- Myopathic — small units, early recruitment.
- Abnormal paraspinals — root.
- L5 versus peroneal — tibialis posterior and gluteus medius.
Summary table
| Finding | Meaning | Example |
|---|---|---|
| Fibrillations and positive sharp waves | Active denervation | Radiculopathy, nerve injury, ALS |
| Fasciculations | Spontaneous discharge of a motor unit | ALS, but also normal |
| Myotonic discharges | "Dive bomber" | Myotonic dystrophy |
| Large units with reduced recruitment | Chronic neurogenic | Old radiculopathy, ALS |
| Small units with early recruitment | Myopathic | Duchenne, myositis |
| Myokymic discharges | Post-radiation damage | Radiation plexopathy |
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Sources
- AANEM: Proper performance and interpretation of electrodiagnostic studies (position statement), 2014 — Needle electromyography
- Preston and Shapiro: Electromyography and Neuromuscular Disorders, 4th edition (2020) — Basic needle EMG: spontaneous activity and motor unit analysis
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.