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ADEMG: automatic decomposition of the electromyogram
Kevin C McGill, PhD; Leslie J Dorfman, MD
Objective - The electromyogram (EMG) is the electrical signal recorded by needle or surface electrodes during a muscular contraction. Interpretation of the EMG is an important part of the clinical neurophysiological evaluation of patients with suspected neuromuscular disorders. Most electromyographers still make this interpretation in a subjective and qualitative manner, using an oscilloscope and a loudspeaker. However, the advent of computerized electrodiagnostic equipment is making quantitative EMG analysis more widespread. Quantitative methods are expected to provide greater objectivity, reproducibility, and diagnostic sensitivity in EMG examinations.
Approach - We have developed the ADEMG method (Automatic Decomposition Electromyography) for analyzing clinical EMGs. It is designed to be a convenient adjunct to the conventional needle EMG examination, analyzing EMG signals recorded during constant isometric contractions using a standard needle electrode and a conventional electromyograph. ADEMG identifies the individual discharges (motor-unit action potentials or MUAPs) that comprise the EMG signal and measures the properties of the MUAPs, which are known to be diagnostically significant (Figure 1). ADEMG allows rapid analysis of EMGs from contractions up to 30% of maximum force.
Figure 1. Schematic drawing of three motor units in a muscle, illustrating the summation of motor-unit action potential (MUAP) trains to form the EMG signal.
We have collected a database of normal MUAPs from several muscles in individuals of different ages, and have reported variations of MUAP properties with force, age, and electrode type. We have analyzed EMGs from patients with known neuromuscular disorders, and have reported changes in MUAP sizes, shapes, and firing rates. We have also used ADEMG to investigate MUAP behavior in fatigue and during tendon vibration, and have compared ADEMG analysis with turns analysis, another widely used quantitative method.
Status - We are currently developing a clinically practical method for estimating the number of motor units in a muscle. This is an area of increasing clinical interest for diagnosing and following the progression of disorders involving denervation or muscular atrophy. Our approach is to use ADEMG to speed up the collection of individual surface MUAPs for comparison with the compound surface potential evoked by electrical stimulation.
We are also trying to develop a more physiologically sound method for analyzing complex EMG signals (interference patterns). Our approach is to extract information about the number, size, and mean firing rate of the MUAPs from the histogram of intervals between the turns in the EMG signal.
Republished from the 1994 Rehabilitation R&D Center Progress Report. For
current information about this project, contact
Kevin C McGill.
