FREQUENTLY ASKED QUESTIONS ABOUT
ELECTROMYOGRAPHY (EMG)
Gary Kamen, PhD, FACSM
University of Massachusetts at Amherst
What equipment is typically used?
What does the EMG signal actually indicate?
How does one view the EMG signal?
What are some of the applications for which EMG might be
used?
Are there any problems in interpreting the EMG signal?
What are the major EMG variables?
Where can I go for more information?
EMG stands for electromyography. It is the study of muscle electrical
signals. EMG is sometimes referred
to as myoelectric activity.
EMG is measured using similar techniques
to that used for measuring EKG, EEG or other electrophysiological signals. Electrodes are placed on the skin
overlying the muscle.
Alternatively, wire or needle electrodes are used and these can be
placed directly in the muscle.
Similar to other electrophysiological
signals, EMG signals are small and need to be amplified by an amplifier
designed to measure physiological signals. These amplifiers include a differential amplifier circuit,
and frequently include some filtering and other signal processing features.
When EMG is acquired from electrodes
mounted directly on the skin, the signal is a composite of all the muscle fiber
action potentials occurring in the muscle(s) underlying the skin. These action potentials occur at
somewhat random intervals so at any one moment, the EMG signal may be either
positive or negative voltage.
Individual muscle fiber action potentials are sometimes acquired using
wire or needle electrodes placed directly in the muscle.
The signal can be displayed directly on
an oscilloscope, stored on a device like a computer hard disk. If the signal is stored digitally,
software is needed to retrieve it and display it on a monitor or in hardcopy
format.
There are many, many applications for the
use of EMG. EMG is used clinically
for the diagnosis of neurological and neuromuscular problems. It is used diagnostically by gait
laboratories and by clinicians trained in the use of biofeedback or ergonomic
assessment. EMG is also used in
many types of research laboratories, including those involved in biomechanics,
motor control, neuromuscular physiology, movement disorders, postural control,
physical therapy, and many others.
Yes, many. The signal is susceptible to numerous technical
problems. These include signal
interference like hum, signal acquisition problems like clipping or baseline
drift, skin artifacts, signal processing errors, and many other kinds of interpretation
problems. There are many excellent
sources that describe some of the problems and pitfalls in the interpretation
of the EMG signal.
First, the signal is picked up at the
electrode and amplified.
Typically, a differential amplifier is used as a first stage
amplifier. Additional
amplification stages may follow.
Before being displayed or stored, the signal can be processed to eliminate
low-frequency or high-frequency noise, or other possible artifacts. Frequently, the user is interested in
the amplitude of the signal.
Consequently, the signal is frequently rectified and averaged in some
format to indicate EMG amplitude.
However, there are many types of EMG analysis schemes.
The EMG signal is typically described
using a variable related to the size or amplitude of the signal. Rectified, averaged EMG, integrated
EMG, and linear envelope displays are all ways to display the amplitude of the
EMG signal. Frequency analysis
comprises the second category of analysis for the EMG signal, and there are
many ways to conduct frequency analysis, including analysis of zero crossings,
spectral analysis, numerous time-frequency algorithms, and many other
techniques.
If your institution has
electronic subscriptions to these journals, you can follow the links to go to
the journal home page and downloads current articles:
Journal
of Electromyography and Kinesiology
Electromyography and Clinical
Neurophysiology
and many other journals in biomedical
engineering, biomechanics, motor control, ergonomics and other fields.
Selected topics in surface electromyography for use in
the occupational setting: expert perspectives.
U.S. Department of Health and Human Services. Public Health Service.
Centers for Disease Control.
National Institute for Occupational Safety and Health, 1992. Publication No 91-100.
Cram, J. Introduction
to Surface Electromyography: Chronic Musculoskeletal Pain. Pro-Ed Publishers, 1997
Kamen, G. and G.E. Caldwell. Physiology and interpretation of the electromyogram. Journal of Clinical Neurophysiology, 13:366-384, 1996.
Kumar, S (Ed).
Electromyography in Ergonomics.
Taylor & Francis, 1996.
Leis, A.A. Atlas
of Electromyography. Oxford,
2000.
Loeb, G.E.
and C. Gans. Electromyography
for Experimentalists. The
University of Chicago Press.
Chicago, 1986.
Oh, S.J. Clinical
Electromyography.
Lippincott, 2002.
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