REFERENCE · DIAGNOSTIC TECHNIQUE

EEG–EMG and Jerk-Locked Back-Averaging

Purpose and Suitability
Anatomy and Physiology
Preparation and Recording
Analysis and Interpretation
Explaining the Result
Evidence and Limitations
Media and Accessibility
Research and Sources

Purpose and Suitability

This specialist assessment records scalp EEG and surface EMG together to investigate repeated jerks. Back-averaging aligns the recordings to muscle onset and looks for preceding brain signals that may be hard to see in an ordinary trace. It can support a functional diagnosis or provide evidence for another kind of myoclonus. [1][2]

EEG (electroencephalography): Recording electrical activity from electrodes on the scalp. Surface EMG (electromyography): Recording muscle electrical activity using skin electrodes.

It is most useful when there are enough representative, clearly timed events. Sparse events, overlapping jerks or excessive recording noise may make analysis inconclusive. The question is the origin of the recorded movement, not whether the person is trying hard enough or how independently they function.

Purpose and Suitability
Anatomy and Physiology
Preparation and Recording
Analysis and Interpretation
Explaining the Result
Evidence and Limitations
Media and Accessibility
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Anatomy and Physiology

A muscle burst gives the team a timing marker for the jerk. Recording several muscles also shows their recruitment order: which activates first and how activity spreads. EEG adds information about activity in the cerebral cortex, the brain’s outer layer. A short spike close to muscle onset can support cortical myoclonus; a much slower premovement change is a different finding. [2]

Bereitschaftspotential (BP), or readiness potential: A slowly developing electrical change before a movement, detected by averaging suitable EEG segments.

Movement-preparation systems can contribute to involuntary functional jerks. The presence of a preparation-related signal does not demonstrate conscious intention. Proposed links with attention and motor preparation remain models rather than a complete explanation of an individual’s symptoms. [4]

Purpose and Suitability
Anatomy and Physiology
Preparation and Recording
Analysis and Interpretation
Explaining the Result
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Preparation and Recording

A clinical-neurophysiology team should explain the purpose, obtain consent and agree positioning, breaks and a stop signal. Ask about touch, light and sound sensitivity and discuss how electrodes and skin preparation will be tolerated. Surface recording is different from a needle-muscle examination.

The following source-based workflow is an educational outline, not a replacement for the laboratory’s acquisition protocol. [1][2]

Characterize the movement first. Confirm which events the study should capture, including the person’s description of typical jerks and possible competing diagnoses.
Choose recording sites. Place EEG electrodes using an appropriate montage and surface EMG over muscles that can identify the earliest onset and relevant spread. Record synchronized video when useful and consented.
Collect representative events. Label voluntary calibration movements separately. Monitor comfort and recording quality. Do not deliberately provoke a hazardous bout to achieve an event count.
Check the muscle traces. Identify a clear onset in the first involved muscle and document burst duration and recruitment. Exclude or separately analyze atypical events rather than averaging different movements indiscriminately.
Record adequacy. State how many usable events were obtained and why others were excluded. Too few events or poorly separable onsets can leave the test unanswered; they do not count as evidence against the person’s symptoms.

Additional stimulus testing is a separate clinical decision with its own rationale and consent. It is not a filming task or a home examination.

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Analysis and Interpretation

For accepted events, align EEG segments to the EMG onset, average them and inspect the preceding waveform alongside the raw recordings. The purpose is to reveal reproducible time-linked activity while reducing unrelated background noise. Technical settings must preserve the slow signals being sought; movement or eye artefacts need particular attention. [1][2]

Artefact: A recorded signal caused by movement, equipment or another source that can obscure or imitate the activity of interest.

Interpret three possibilities:

Supportive BP: A convincing slow premovement signal may support functional jerks against certain myoclonus comparators. It is not unique to functional jerks; some motor tics also have a BP.
No BP in an adequate study: Functional jerks remain possible. Integrate muscle patterns, clinical findings and other relevant investigations.
Inadequate or ambiguous recording: Explain what prevented interpretation and whether another approach would answer the clinical question. [1][3]

Jerk-linked cortical spikes, brief muscle bursts and other cortical-myoclonus findings require their own interpretation. Normal results on these investigations are not positive FND findings. Separate conclusions for distinct event types and possible coexisting disorders. [2][5]

Quantified BP and beta event-related desynchronisation are additional analyses discussed in the inventory. The latter measures a premovement decrease in certain EEG rhythms. Promising research does not make either a universal required test. [4][5]

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Anatomy and Physiology
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Explaining the Result

An example explanation is: “We aligned the brain recording with the start of your jerks. The pattern before them adds evidence for functional jerks. That is a finding about how movement is being organized; it does not mean you chose the movement.”

If the study is inconclusive, say why: “There were too few clear events to answer this question today.” That is different from saying the diagnosis has been ruled out.

These are suggested explanations, not research quotations. Discuss what the result changes in the plan and what questions remain. A waveform does not establish safe walking, sustained hand use or reduced assistance. Link those questions to everyday-function assessment.

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Evidence and Limitations

Primary comparisons: The 2012 cross-sectional series included 48 patients across functional jerks, Tourette syndrome and myoclonus, plus 25 healthy participants imitating movements. BP findings overlapped with motor tics. The abstract’s sensitivity/specificity figures for absence of BP during intentional wrist extension must not be relabelled as the accuracy of a BP preceding spontaneous jerks. Only the primary abstract/institutional record was retrieved for this study. [3]

Later study: Beudel and colleagues retrospectively compared 29 functional-jerk cases with 16 cortical-myoclonus cases. Visual BP was present in 14 functional cases and no cortical comparator. Combining quantitative BP and beta analysis improved detection within this dataset. Classification had already used clinical and electrophysiological information, including BP; thresholds were evaluated in the same cohort. Selection, incorporation of the sign into diagnosis, age differences and the limited comparator group constrain generalization. The full paper was checked. [4]

Incorporation bias: A test can appear more accurate when its result also helps define who has the diagnosis used as the reference.

Updated review: The 2025 MDS study-group narrative review describes mostly case-series and limited case-control evidence, inconsistent methods and technical constraints on averaging. It supports combined interpretation while identifying a need for stronger diagnostic validation. It is not a new prospective validation cohort. [5]

Targeted evidence review: September 28, 2026. The IFCN chapter, general myoclonus review, 2018 primary study and 2025 review were available in full. The 2012 full text and a fresh review of every other inventory source remain outstanding. No universal accuracy estimate is assigned to this outline.

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Anatomy and Physiology
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Explaining the Result
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Media and Accessibility

The preferred media is an annotated process diagram with four frames:

EEG electrodes on the scalp and EMG electrodes over a representative muscle.
Several spontaneous jerk events recorded over time.
Each EEG segment aligned to the EMG onset marker.
The averaged trace showing where a premovement potential may appear.

If a laboratory video is contributed, it should show electrode placement on a healthy volunteer or consenting patient and use an existing de-identified trace. Do not ask the participant to produce, imitate or endure repeated symptoms solely for the recording. State that only a qualified clinical-neurophysiology service can perform and interpret the analysis.

Include an accessible text description and transcript. Label illustrative traces as illustrative, distinguish a slow BP from a brief cortical spike, and retain the option to use the text without watching movement footage. A diagram or trace requires clinical review before publication.

Purpose and Suitability
Anatomy and Physiology
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Analysis and Interpretation
Explaining the Result
Evidence and Limitations
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Research and Sources

The sources distinguish clinical framing, laboratory methods, primary comparisons and newer evidence limitations. This page expands the featured combined assessment; the complete inventory is not a checklist of procedures everyone needs.

Citation	Figure	Full citation
[1]	—	Edwards MJ, Koens LH, Liepert J, Nonnekes J, Schwingenschuh P, van de Stouwe AMM, Morgante F. Clinical neurophysiology of functional motor disorders: IFCN Handbook Chapter. Clinical Neurophysiology Practice. 2024;9:69–77. FND-CIT-0022. https://doi.org/10.1016/j.cnp.2023.12.006
[2]	—	van der Veen S, Caviness JN, Dreissen YEM, et al. Myoclonus and other jerky movement disorders. Clinical neurophysiology practice. 2022;7:285-316. DOI. PMID: 36324989. FND-CIT-0151.
[3]	—	van der Salm SM, Tijssen MA, Koelman JH, van Rootselaar AF. The bereitschaftspotential in jerky movement disorders. Journal of neurology, neurosurgery, and psychiatry. 2012;83(12):1162-1167. DOI. PMID: 22952323. FND-CIT-0125.
[4]	—	Beudel M, Zutt R, Meppelink AM, et al. Improving neurophysiological biomarkers for functional myoclonic movements. Parkinsonism & related disorders. 2018;51:3-8. DOI. PMID: 29653908. FND-CIT-0122.
[5]	—	Latorre A, Ganos C, Hamada M, et al. Diagnostic utility of clinical neurophysiology in jerky movement disorders: a review from the MDS Clinical Neurophysiology Study Group. Movement Disorders Clinical Practice. 2025;12(3):272–284. DOI. Full text. FND-CIT-0226.

Purpose and Suitability
Anatomy and Physiology
Preparation and Recording
Analysis and Interpretation
Explaining the Result
Evidence and Limitations
Media and Accessibility
Research and Sources