SURFACE ELECTROMYOGRAPHY
DETECTION AND RECORDING
Contents
General
When detecting and recording the EMG signal, there are two main issues
of concern that influence the fidelity of the signal. The first is the
signal-to-noise ratio. That is, the ratio of the energy in the EMG signal
to the energy in the noise signal. In general, noise is defined as electrical
signals that are not part of the "wanted" EMG signal. The other
is the distortion of the signal, meaning that the relative contribution
of any frequency component in the EMG signal should not be altered.
Characteristics of the EMG signal
It is well established that the amplitude of the EMG signal is stochastic
(random) in nature and can be reasonably represented by a Gausian distribution
function. The amplitude of the signal can range from 0 to 6 mV (peak-to-peak)
or 0 to 1.5 mV (rms). The usable energy of the signal is limited to the
0 to 500 Hz frequency range, with the dominant energy being in the 50-150
Hz range. Usable signals are those with energy above the electrical noise
level. An example of the frequency spectrum of the EMG signal is presented
in Figure 1.
Figure 1 - Frequency spectrum of the EMG signal detected from
the Tibialis Anterior muscle during a constant force isometric contraction
at 50% of voluntary maximum.
Characteristics of the electrical noise
The noise may emanate from various sources such as:
- Inherent noise in the electronics components in the detection and
recording equipment - All electronics equipment generates electrical
noise. This noise has frequency components that range from 0 Hz to several
thousand Hz. This noise cannot be eliminated; it can only be reduced by
using high quality electronic components, intelligent circuit design and
construction techniques.
- Ambient noise - This noise originates from sources of electromagnetic
radiation, such as radio and television transmission, electrical power
wires, light bulbs, fluorescent lamps, etc. In fact, any electromagnetic
device generates and may contribute noise. The surfaces of our bodies are
constantly inundated with electric-magnetic radiation and it is virtually
impossible to avoid exposure to it on the surface of the earth. The dominant
concern for the ambient noise arises from the 60 Hz (or 50 Hz) radiation
from power sources. The ambient noise signal may have an amplitude that
is one to three orders of magnitude greater than the EMG signal.
- Motion artifacts - There are two main sources of motion artifact:
one from the interface between the detection surface of the electrode and
the skin, the other from movement of the cable connecting the electrode
to the amplifier. Both of these sources can be essentially reduced by proper
design of the electronics circuitry. The electrical signals of both noise
sources have most of their energy in the frequency range from 0 to 20 Hz.
- Inherent instability of the signal - The amplitude of the EMG
signal is quasi-random in nature. The frequency components between 0 and
20 Hz are particularly unstable because they are affected by the quasi-random
nature of the firing rate of the motor units which, in most conditions,
fire in this frequency region. Because of the unstable nature of these
components of the signal, it is advisable to consider them as unwanted
noise and remove them from the signal.
How can the fidelity of the EMG signal be maximized?
It is desirable to obtain an EMG signal that contains the maximum amount
of information from the EMG signal and the minimum amount of contamination
from electrical noise. Thus, the maximization of the signal-to-noise ratio
should be done with minimal distortion to the EMG signal. Therefore, it
is important that any detecting and recording device process the signal
linearly. In particular, the signal should not be clipped, that is, the
peaks should not be distorted and no unnecessary filtering should be performed.
Because the power line radiation (50 or 60 Hz) is a dominant source
of electrical noise, it is tempting to design devices that have a notch-filter
at this frequency. Theoretically, this type of filter would only remove
the unwanted power line frequency, however, practical implementations also
remove portions of the adjacent frequency components. Because the dominant
energy of the EMG signal is located in the 50-100 Hz range, the use of
notch filters is not advisable when there are alternative methods of dealing
with the power line radiation.
Electrical characteristics of the electrode unit and amplifiers
The design of the electrode unit is the most critical aspect of the
electronics apparatus which will be used to obtain the signal. The fidelity
of the EMG signal detected by the electrode influences all subsequent treatment
of the signal. It is very difficult (almost impossible) to improve the
fidelity and signal-to-noise ratio of the signal beyond this point. Therefore,
it is important to devise an electrode unit that provides minimal distortion
and highest signal-to-noise ratio. The following characteristics are important
for achieving this requirement.
- Differential amplification - To eliminate the potentially much
greater noise signal from power line sources, a differential detecting
configuration is employed. The differential amplification technique is
shown schematically in Figure 2. The premise is simple. The signal is detected
at two sites, electronics circuitry subtracts the two signals and then
amplifies the difference. As a result, any signal that is "common"
to both detection sites will be removed and signals that are different
at the two sites will have a "differential" that will be amplified.
Any signal that originates far away from the detection sites will appear
as a common signal, whereas signals in the immediate vicinity of the detection
surfaces will be different and consequently will be amplified. Thus, relatively
distant power lines noise signals will be removed and relatively local
EMG signals will be amplified. This explanation requires the availability
of a highly accurate "subtractor". In practice, even with
the wondrous electronics of today, it is very difficult to subtract signals
perfectly. The accuracy with which the differential amplifier can subtract
the signals is measured by the Common Mode Rejection Ratio (CMRR). A perfect
subtractor would have a CMRR of infinity. A CMRR of 32,000 or 90 dB is
generally sufficient to suppress extraneous electrical noises. Current
technology allows for a CMRR of 120 dB, but there are at least three reasons
for not pushing the CMRR to the limit: 1) Such devices are expensive. 2)
They are difficult to maintain electrically stable, and 3) the extraneous
noise signals may not arrive at the two detection surfaces in phase, and
hence they are not common mode signals in the absolute sense.
Figure 2 - A schematic of the differential amplifier configuration.
The EMG signal is represented by 'm' and the noise signals by 'n'.
- Input impedance - The source impedance at the junction of the
skin and detection surface may range from several thousand ohms to several
megohms for dry skin. To prevent attenuation and distortion of the detected
signal due to the effects of input loading, the input impedance of the
differential amplifier should be as large as possible, without causing
ancillary complications to the workings of the differential amplifier.
Present day electronics devices easily provide input impedances of the
order of 10 gigohms in parallel with 5 picofarads. In addition to the magnitude
of the input impedance, the balance between the impedances of the
two detection sites is also of great importance. This consideration requires
careful circuit design.
- Active electrode design - The requirement for a high input impedance
introduces a problem known as capacitance coupling at the input of the
differential amplifier. A small capacitance between the wires leading to
the input of the differential amplifier and the power line will introduce
a power line noise signal into the amplifier. This phenomenon is similar
to that which causes a television signal strength to increase when one
places ones hand near the antenna input, but does not touch it. The solution
is to place the differential amplifier as close as possible to the detection
surfaces of the electrode. This solution has become known as the "active
electrode". One other advantage of this configuration is that the
output impedance of the differential amplifier can be made to be very low,
on the order of 10 ohms. Therefore, any movement of the cable from the
output of the electrode will not generate significant or even notable noise
signals in the cable which feeds into the subsequent amplifier.
- Filtering - Even with the above considerations, the EMG signal
will be contaminated by some noise. The signal-to-noise ratio can be increased
by judicious filtering between 20 - 500 Hz with a roll-off of 12 dB/octave.
(Strict design characteristics could consider 400 Hz as the upper bandwidth
cut-off. The 500 Hz value allows for a safety margin in the design
of thecircuitry.) This filtering is generally accomplished at the amplifier
stage located outside the active electrode.
- Electrode stability - When an electrode is placed on the skin,
the detection surfaces come in contact with the electrolytes in the skin.
A chemical reaction takes place which requires some time to stabilize,
typically in the order of a few seconds if the electrode is correctly designed.
But, more importantly, the chemical reaction should remain stable during
the recording session and should not change significantly if the electrical
characteristics of the skin change from sweating or humidity changes.
- Preferred method of use - Given the high performance and small
size of modern day electronics, it is possible to design active electrodes
that satisfy the above requirements without requiring any abrasive skin
preparation and removal of hair.
Electrode geometry
Throughout the history of electromyography, the shape and the layout
of the detection surface of the electrode have not received much attention.
Most likely because past users of electromyography have been interested
only in the qualitative aspects of the EMG signal. The advent of new processing
techniques for extracting quantitative information from the EMG signal
requires greater focus on the configuration of the electrode. Two main
points need to be considered.
Figure 3 - Schematic representation of bar and circular configurations
for electrodes. Note that for equivalent areas of the detection surfaces,
the bar configuration intersects more fibers. For the specified dimensions,
the ratio of intersected fibers is 2.8.
The distance between the detection surfaces - This inter detection-surface
distance affects the bandwidth and amplitude of the EMG signal; a smaller
distance shifts the bandwidth to higher frequencies and lowers the amplitude
of the signal. For this reason, the distance should be fixed so that quantitative
comparisons between recordings can be made within and among muscles, and
among individuals. Preferably, the detection surfaces should be mounted
on a fixed platform. The other consideration in determining the inter detection-surface
distance is the overall dimensions of the electrode. It should be such
that the electrode can be conveniently used on small muscles, such as those
in the hand, as well as on larger muscles. It is not necessary to separate
the two detection surfaces by a large space to obtain a representative
sample of the EMG signal from a muscle. The fibers of a specific motor
unit are distributed over a considerably large volume of the muscle (in
cats this volume is typically one-third of the muscle). Larger inter detection-surfaces
distances obviously result in physically larger electrodes. This presents
considerable disadvantages when detecting from relatively small muscles,
where the issue of crosstalk becomes important. Crosstalk refers to the
contamination of an EMG signal by other signals emanating from adjacent
muscles. We have found that an inter detection-surface of 1.0 cm provides
a configuration that detects representative electrical activity of the
muscle during a contraction. Limiting the distance to 1.0 cm is also convenient
for detecting signals from small muscles. Although it could be argued that
the inter detection-surface distance could be made smaller and still detect
a representative signal, the practical consideration of impedance alterations
of the skin due to sweating and humidity changes preclude the use of small
distances. This is because the inputs to the differential preamplifiers
on the electrode may become partially electrically shorted.
- The size and shape of the detection surfaces - The larger the
size of the detection surfaces, the greater the amplitude of the signal
that will be detected and the lesser the electrical noise that will be
generated at the skin-detection surface interface. However, larger surface
areas imply larger electrodes. Thus, the preferred design would maximize
the number of muscle fibers that a detection surface would cover, minimize
the electrical noise generated at the skin interface and minimize the physical
size of the detection surface. These are conflicting requirements which
require a compromise. A reasonable solution can be obtained by considering
the shape of the detection surface. Historically, detection surfaces have
been designed as circular. To my knowledge, this shape has been chosen
by Platonic thinking. For the past 15 years we have been using a "bar"
shape which presents the following advantages. For any area value, the
bar configuration intersects more fibers if l > 0.785 w ( l=
length and w = width of the bar). For a bar length of 10 mm and
a width of 1 mm, the bar configuration will detect approximately 2.80 times
more muscle fibers. Therefore, the bar configuration provides a signal
representative of more motor units in the muscle and consequently a greater
signal amplitude. Also, because the total surface area of the detection
surfaces of both configurations is the same, the electrical characteristics
will be essentially the same, while the overall size of the electrode is
reduced. See Figure 3 for a descriptive schematic. Again, heuristic has
shown that a bar 1.0 cm in length and 1 or 2 mm in width provides an acceptable
practical compromise.
Where should the electrode be placed on the surface of the muscle?
- Location and orientation of the electrode - The electrode should
be placed between a motor point and the tendon insertion or between two
motor points, and along the longitudinal midline of the muscle. The longitudinal
axis of the electrode (which passes through both detection surfaces) should
be aligned parallel to the length of the muscle fibers. Figure 4 provides
a schematic representation of the preferred electrode location.
Figure 4 - The preferred electrode location is between the
motor point (or innervation zone) and the tendinous insertion, with the
detection surfaces arranged so that they intersect as many muscle fibers
as possible.
- NOT on or near the tendon of the muscle - As the muscle fibers
approach the fibers of the tendon, the muscle fibers become thinner and
fewer in number, reducing the amplitude of the EMG signal. Also in this
region the physical dimension of the muscle is considerably reduced rendering
it difficult to properly locate the electrode, and making the detection
of the signal susceptible to crosstalk because of the likely proximity
of agonistic muscles.
- NOT on the motor point - During the past one-half century it
has been taught that for the purpose of detecting a surface EMG signal
the electrode should be located on a motor point of the muscle. The motor
point is that point on the muscle where the introduction of minimal electrical
current causes a perceptible twitch of the surface muscle fibers. This
point usually, but not always, corresponds to that part of the innervationzone
in the muscle having the greatest neural density, depending on the anisotropy
of the muscle in this region. Presumably, the motor points have been used
as landmarks because they were identifiable and provided a fixed anatomical
landmark. Unfortunately from the point of view of signal stability, a
motor point provides the worst location for detecting an EMG signal.
In the region of a motor point, the action potentials travel caudally and
rostrally along the muscle fibers, thus the positive and negative phases
of the action potentials (detected by the differential configuration) will
add and subtract with minor phase differences causing the resulting EMG
signal to have higher frequency components. In the time domain, the signal
appears as more jagged and with more sharp peaks. The loss of stability
occurs from the fact that a minor displacement (0.1 mm) will affect in
an unpredictable fashion the amount of change in the frequency characteristics
of the signal.
A note of caution about the motor points and innervation zones. Most
muscles have multiple innervation zones throughout the muscle. They can
be identified by applying electrical stimulation to the skin above the
surface of the muscle or by other more technically complicated surface
mapping techniques. If neither procedure is convenient, then place the
electrode in the middle of the muscle between the origin and insertion
point.
- NOT at the outside edges of the muscle - In this region, the
electrode is susceptible to detecting crosstalk signals from adjacent muscles.
It is good practice to avoid this situation. For some applications, crosstalk
signals may be undesirable.
- Orientation of the electrode with respect to the muscle fibers -
The longitudinal axis of the electrode (which passes through both detection
surfaces) should be aligned parallel to the length of the muscle fibers.
When so arranged, both detection surfaces will intersect most of the same
muscle fibers. Hence, the spectral characteristics of the EMG signal will
reflect the properties of a fixed set of muscle fibers in the region of
the electrode. Also, the frequency spectrum of the EMG signal will be independent
of any trigonometric factor that would provide an erroneous estimate of
the conduction velocity. The resultant value of the conduction velocity
affects the EMG signal by altering the temporal characteristics of the
EMG signal, and consequently its frequency spectrum.
Where should the reference electrode be placed?
The reference electrode (at times called the ground electrode) is necessary
for providing a common reference to the differential input of the preamplifier
in the electrode. For this purpose, the reference electrode should be placed
as far away as possible and on electrically neutral tissue (say over abony
prominence). Often this arrangement is inconvenient because the separation
of the detecting electrode and reference electrode leads requires two wires
between the electrodes and the amplifier.
It is imperative that the reference electrode make very good electrical
contact with the skin. For this reason, the electrode should be large (2
cm x 2 cm). If smaller, the material must be highly conductive and should
have strong adhesive properties that will secure it to the skin with considerable
mechanical stability. Electrically conductive gels are particularly good
for this purpose. Often, power line interference noise may be reduced an
deliminated by judicious placement of the ground electrode.
Electrical safety concerns
The failure of any electrical instrumentation making direct or indirect
galvanic contact with the skin can cause a potentially harmful fault current
to pass through the skin of the subject. This concern is less relevant
in devices that are powered exclusively by low voltage (3-15 V) batteries.
To ensure safety, the subject should be electrically isolated from any
electrical connection (to the power line or ground) associated with the
power source. This isolation is generally achieved in one of two ways:either
through the use of optical isolators or through the use of isolation transformers.
Both approaches are satisfactory, but both require careful consideration
for not distorting the EMG signal. This is especially true when a transformer
is used.
This isolation provides the added benefit of reducing the amount of
radiated power line noise at the electrode detection surfaces.
How should the EMG signal be processed?
For several decades it has been commonly accepted that the preferred
manner for processing the EMG signal was to calculate the Integrated Rectified
signal. This was done by rectifying (rendering the signal to have excursions
of one polarity) the EMG signal, integrating the signal over a specified
interval of time and subsequently forming a time series of the integrated
values. This approach became widespread because it was possible to make
these calculations somewhat accurately and inexpensively with the limited
electronics technology of earlier decades. The advances made in electronics
devices during the past two decades have made it possible to conveniently
and accurately calculate the root-mean-squared (rms) value and the average
rectified (avr) value of the EMG signal. The avr value is similar to the
integrated rectified value, if the calculations are made correctly and
accurately. Both these variables provide a measure of the area under the
signal but do not have a specific physical meaning. On the other hand,
the rms value is a measure of the power of the signal, thus it has a clear
physical meaning. For this reason, the rms value is preferred for most
applications.
Applications of the EMG signal
Currently there are three common applications of the EMG signal. They
are:
- To determine the activation timing of the muscle; that is, when the
excitation to the muscle begins and ends
- To estimate the force produced by the muscle.
- To obtain an index of the rate at which a muscle fatigues through the
analysis of the frequency spectrum of the signal.
In the not so distant future, we can expect applications in the assessment
of neurological diseases which affect the fiber typing or the fiber cross-sectional
area of the muscle.
The relationship between the force produced by the muscle and the amplitude
of the EMG signal requires further description. During the past five decades,
the scientific literature has promulgated an apparent controversy on this
issue. Some reports describe a relatively linear relationship, whereas
others describe a relative non-linear relationship, with the amplitude
of the EMG signal increasing greater than the force. In fact, both positions
are correct and the controversy is artificial. It is now known that in
small muscles where the firing rate of the motor units has a greater dynamic
range and motor unit recruitment is limited to the lower end of the force
range, the relationship is relatively linear. Whereas, in larger muscles
where motor unit recruitment continues into the upper end of the force
range and the firing rate has a lower dynamic range, the relationship is
relatively non-linear.
Copyright (c) 1996 by Delsys Inc. All rightsreserved.
This page was last modified on May 10, 1995
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