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Turns-amplitude analysis at different sampling frequencies
S A Jørgensen1, A Fuglsang-Frederiksen
1Department of Clinical Neurophysiology, Hvidovre Hospital, University of Copenhagen, Denmark.
Abstract:
In the EMG interference pattern the number of potential reversals of more than 100 microV (i.e., turns) measured may be low if a low sampling frequency is used. This might be more pronounced in a myopathic muscle with small short potentials than in a control muscle, resulting in a decreased diagnostic yield. The influence of sampling frequencies from 6 to 200 kHz on the turns, mean amplitude and ratio of turns to mean amplitude analysed on the interference pattern at a force of 30% of maximum was examined at 10 sites in each of 5 control muscles, 5 myopathic muscles and 5 neurogenic muscles (i.e., 150 sites in all). In this small group of muscles low sampling frequency did not tend to reduce the diagnostic yield of the measurements. Proper reproducibility was obtained with sampling frequencies down to 17 kHz. However, due to systematic errors, measurements obtained with sampling frequencies below 25 kHz require comparable control values or compensations for errors. As time intervals between turns (not studied here) have a skew distribution with many small time intervals a sampling frequency of 50 kHz or higher is probably required.
Insights
Low sampling frequencies in electromyography (EMG) may not reduce diagnostic yield but require careful calibration. Optimal frequencies ensure accurate measurement of muscle electrical activity for reliable diagnosis.
Area of Science:
- Neurology
- Biomedical Engineering
- Physiology
Background:
- Electromyography (EMG) interference pattern analysis is crucial for diagnosing neuromuscular disorders.
- Low sampling frequencies can potentially underestimate motor unit potential features, affecting diagnostic accuracy.
- Understanding the impact of sampling frequency on EMG parameters is essential for reliable clinical interpretation.
Purpose of the Study:
- To investigate the influence of varying sampling frequencies (6-200 kHz) on EMG interference pattern parameters.
- To assess the diagnostic yield and reproducibility of EMG measurements at different sampling rates.
- To determine optimal sampling frequencies for accurate EMG analysis in control, myopathic, and neurogenic muscles.
Main Methods:
- Examined turns, mean amplitude, and turns-to-mean amplitude ratio in EMG interference patterns.
- Tested sampling frequencies ranging from 6 kHz to 200 kHz.
- Analyzed data from 150 muscle sites across control, myopathic, and neurogenic muscle groups at 30% maximum force.
Main Results:
- Low sampling frequencies (down to 17 kHz) showed good reproducibility.
- Diagnostic yield was not significantly reduced by lower sampling frequencies in this study cohort.
- Measurements below 25 kHz necessitate comparable control data or error compensation due to systematic errors.
Conclusions:
- Sampling frequencies as low as 17 kHz can yield reproducible EMG results.
- Careful calibration and consideration of systematic errors are vital for EMG data acquired below 25 kHz.
- A sampling frequency of 50 kHz or higher is likely necessary for accurate analysis of inter-turn time intervals.