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Surface
Electromyography (sEMG) is the electrophysiological technique
for quantifying motor activity (muscle contraction) in specific
muscle groups as determined by electrode placement. A muscle
contraction involves stimulation of a group of fibers controlled
by the motor neuron. This combination of motor neuron and
group of muscle fibers is known as the motor unit. The number
of fibers controlled by the motorneuron correlates highly
with the function of the muscle: large muscle fiber to motor
neuron ratios are found in muscles controlling gross movement,
while smaller muscle fiber to motor neuron ratios are found
in muscle groups responsible for finer movements, as in the
hand.
sEMG is currently utilized by researchers and medical practitioners
alike, including physical therapists, medical doctors, chiropractors,
osteopaths and psychologists. Ergonomics researchers are currently
using sEMG in validating the potential of certain input devices
such as keyboard and mice for causing Carpal Tunnel Syndrome.
Clinical applications include neuromuscular re-education,
disability evaluation, evaluation of various movement disorders
and documentation of soft tissue injury.
Currently, the two areas of greatest interest are the measurement
of muscles during movement (attached electrode dynamic evaluations)
and spectral analysis of sEMG during isolated isometric contractions.
Both show great promise in evaluating low back pain. Specifically,
the Flexion-Relaxation Response utilizing the attached electrode
dynamic technique of measurement has been evaluated thoroughly
and found to be quite robust. Additionally, we at PBI have
designed the static sEMG, with its high quality torso graphs
and pre-post comparison tests, to be a very powerful patient
education tool.
History of sEMG
Galvani in 1791 observed the relationship between muscle contraction
and electricity when he depolarized the muscles of a frog's
leg by using metal rods. Galvani's initial findings were reproduced
by many, but not until the galvanometer was made more sensitive
by Nobili was it proven that muscles generate electrical currents.
In 1849, Dubois-Reymond performed an experiment where he immersed
his fingers into a saline solution and was able to detect
electrical activity as he contracted his hands and arms. He
also was able to show the important role skin resistance played
in the measurement of Surface EMG when he observed that the
electrical impedance (resistance) of his skin reduced the
ability of the galvanometer toe detect the tiny currents generated
by the muscles.
One of the most significant developments in the history of
the SEMG was the development of the Cathode Ray Tube. While
the galvanometer only provided a meter (similar to a speedometer)
which registered deflections proportional to muscle contraction,
the Cathode Ray Tube was able to display the actual SEMG signal
itself. Gasser and Erlanger used the CRT to display the Action
Potential which lead to a Nobel Prize for them in 1944.
Baines was the first electrical engineer to get involved in
the development of SEMG apparatus, and argued that proper
technical considerations must be taken into account when performing
SEMG testing.
sEMG testing was limited to a small number of researchers
until the advent of the electrically very stable Silver/Silver
Chloride Electrode during the early sixties.
Silver/Silver Chloride Electrodes to this day are a requirement
for SEMG testing. They provide the ability to measure motor
activity from the skin without the polarization inherent in
electrodes made of single elements such as stainless steel.
Stainless steel polarizes, allowing one electrode to become
negatively charged while another electrode becomes positively
charged generated a "false" SEMG signal. Due to
the balance of electrons with the Silver/Silver Chloride arrangement,
this polarization does not occur.
Since the development of this electrode, and the development
of more sophisticated "noise immune" electronics
(ones that are not affected by electrical interference such
as lights refrigerators etc.), Surface EMG has made its way
into clinical and research applications throughout the medical
and research communities.
Static Scanning and Dynamic Evaluations
In the end of 1989, the use of a technique called Static Scanning
SEMG proliferated in the United States and Canada. Unlike
the well accepted technique of attaching electrodes to the
skin and measuring motor activity over time (dynamic SEMG),
with Static SEMG, the operator held a device with post-type
electrodes against the skin while a quick momentary measure
of muscle activity was recorded. This application was the
result of newer electronics which allowed measurement through
the skin without concern for the skin resistance that had
previously made this type of measurement impossible.
Which Is Better? Static Scanning or Attached Electrode
Dynamic sEMG?
With Scanning Mode, the SEMG device is held momentarily against
the skin as readings are taken (much like a stethoscope) while
the patient is in a standing or seated fixed position. With
Dynamic Mode, EKG electrodes (or a similar type) are physically
attached to the skin and readings taken over time as movement
is performed. The resultant graph shows muscle activity over
time.
The comparison between static and dynamic testing is like
comparing a photograph with a videotape, or X-Ray to video
fluoroscopy. One provides some information but does not include
movement. The other, dynamic, is a functional, kinetic test.
Can you imagine viewing the nightly news as photographs instead
of video? Obviously dynamic evaluations provide greater information
and are therefore more valuable.
What
Is The Problem With Static Scanning sEMG?
| 1. |
False
negatives may occur when patients become muscularly "accustomed"
to their condition. In dynamic testing, the abnormalities
appear under load. Dynamic testing is always more likely
to show the condition over static testing. |
| 2. |
False
positives can occur due to vigorous exercise. Dynamic
testing is not as affected by this variable. |
| 3. |
Poor
electrode/skin contact can affect results. Dynamic testing
relies on attached electrodes, removing this problem. |
Keep in mind that both static and dynamic sEMG are valuable
in their own respective ways. As a general rule, static sEMG
is a highly effective screening and educational tool, whereas
dynamic sEMG is more useful when hard diagnostic data is required
for PI or Workers Compensation claims.
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