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
Research and Sources

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
Evidence and Limitations
Media and Accessibility
Research and Sources

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.

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

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]

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

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.

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

Evidence and Limitations

The 2024 IFCN chapter and 2025 MDS study-group review support interpreting the history, muscle recordings and premovement EEG together. Evidence comes mainly from small series and comparative studies, with differing recording and analysis methods. There is no single accuracy figure that applies to every person with jerks. [1][5]

In Beudel and colleagues’ 2018 retrospective comparison, a visually identified BP occurred in 14 of 29 functional-jerk cases and none of 16 cortical-myoclonus cases. Quantitative BP and beta analysis detected additional cases. The groups had already been classified using clinical and electrophysiological findings, so these results support specialist interpretation rather than establish accuracy in an unselected clinic population. [4]

BP can also precede motor tics. The 2012 comparison helps explain that overlap; its accuracy figures for a separate intentional wrist-extension task should not be applied to spontaneous jerks. The useful clinical question is which movement pattern the recording supports. A premovement signal does not establish conscious intention, and an absent signal does not exclude functional jerks. [1][3]

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

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
Preparation and Recording
Analysis and Interpretation
Explaining the Result
Evidence and Limitations
Media and Accessibility
Research and Sources

Research and Sources

Start with the IFCN chapter and MDS study-group review for clinical interpretation, then the primary comparisons for the evidence behind individual signal analyses.

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