doxical fashion. When this artifact is noted, biofeedback training for the correct respiratory pattern is indicated.
Another artifact that may occasionally be encountered is caused by radio frequency (RF). Here, the signals from a local radio station are picked up by the antenna effect of the electrode leads and fed into the amplifiers. This phenomenon is fairly rare but does happen. It can be ob- served most dramatically on SEMG instruments with a raw SEMG audio option, when the radio can be heard on the speaker. Using only the visual display in a processed or raw mode, the practitioner would note large, sponta- neous changes in the SEMG signal that had nothing to do with the patient’s movements. If RF noise is suspected, it is necessary to move the SEMG instrument into another room or to the other side of the building. Moving the recording environment is the only solution to RF noise.
One other biological artifact is cross-talk, which occurs when the energy from a distant muscle reaches the elec- trodes placed over another muscle site. Although it is the bane of dynamic SEMG, it really does not matter for re- laxation-oriented protocols. A very clear example of cross- talk is seen in frontal SEMG recordings when the patient
clenches the teeth. An example may be seen in Figure 3–22, which illustrates the specificity of the frontal record- ing site during an eyebrow flash. Cross-talk occurs when the energy of the masseter and temporalis muscles that are recruited during a clench is picked up by the frontal leads.
Careful placement of closely spaced electrodes is the only hope for limiting the cross-talk artifact. It is impor- tant to recognize its presence and to monitor from those potentially offending distant muscles.
As an aid to the practitioner, a checklist is provided that lists all of the attributes of an SEMG instrument (Exhibit 3–1). This form can be used to organize all of the information available on an instrument.
ter so that practitioners can note their own instruments’
strengths and weaknesses or make an informed choice in an equipment purchase. Table 3–2reviews the differ- ent attributes of SEMG instruments.
Figure 3–22 Cross-talk from the masseter muscle (A) may clearly be seen in the frontal leads (B) during a jaw clench.
Table 3–2 The Specifications of Surface Electromyographic Instruments
Concept Desirable Range Comments
Input impedance 100 kilohm Most commercial machines have 1 megOhm input impedance or better. This is 1 gigaohm more than adequate for medical uses.
Common mode 70–180 dB This determines the ability of the SEMG amplifier to eliminate external noise rejection (CMR from the environment, such as noise from energy used to power lights and
or CMMR) computers. In general, the higher this value, the better.
Instrument noise 0.1–1.0 µV This represents the lowest level of SEMG an instrument can pick up. It is, in level essence, the noise level of the SEMG amplifier: The lower, the better. Most
commercially available instruments allow detection of 0.5 µV or higher.
Band pass filter Most of the SEMG energy resides between 20 and 300 Hz. The facial muscles width are exceptions; because they are closer to the surface, are smaller, and have a General case 20–1000 Hz high innervation ratio, they can be monitored up to 600 Hz. The band pass Relaxation training 100–200 Hz width also determines the nature of the noise that is let in. ECG artifact can Musculoskeletal 20–300 Hz be all but eliminated by using a 100–200 Hz filter. If the SEMG instrument
assessment and allows the user to set the band pass filter width, it provides more degrees of rehabilitation freedom. The filter width should be selected to fit the type of work. When Facial muscle 20–600 Hz working with musculoskeletal dysfunction or soft-tissue injury, where the
recordings muscle may have a fatigue component, the practitioner should make sure that the lower end of the band pass is around 20 Hz. On the other hand, the 100–200 Hz filter width should work fine with relaxation-based work.
(continues)
Table 3–2 (Continued)
Concept Desirable Range Comments
Range/gain Range represents the amplitude that can be monitored using a particular SEMG General case 0–1000 RMS µV instrument. For dynamic SEMG recordings, the general case of amplitude Dynamic 0–1000 RMS µV should be possible. If the SEMG instrument has a range of 0–500 µV and the
movement practitioner is studying events that exceed that amount, the amplifiers would Relaxation 0–100 RMS µV simply saturate at the top end of the scale and potentially valuable
information would be lost. For relaxation-oriented work, one would not expect to see amplitudes over 100 µV. It is best if the instrument allows the practitioner to select from several ranges so that the recording can be sensitive to what is being studied or treated.
Smoothing options 0.1–10 second time The smoothing option on an SEMG instrument may or may not be specified as constant a time constant. Usually it is necessary to turn a dial or increment a number on a computerized system. Sometimes this feature is referred to as a filter, which refers to digital filtering or smoothing.20It is highly desirable for instruments to allow the practitioner to choose how much smoothing or processing of the signal is done.
Visual displays The visual display is the practitioner’s link to the SEMG information. It should be easy to read.
Meter Linear versus Meters commonly come in two varieties: linear and logarithmic. With linear logarithmic meters, there is an equal spacing between each number. This is the most
common type of meter. Logarithmic meters give more space to the low end than to the high end of the scale. We have more precision in how we use our muscles at the low microvolt level than at the high microvolt level. The logarithmic meter makes more sense for relaxation work.
Digital Digital displays are highly desirable so that the practitioner can see the quantified SEMG as it changes from moment to moment. It is always nice to be able to control how frequently the digital value updates.
Computer Raw and processed Computer displays allow the practitioner to see time-series scrolls of the SEMG information. Instruments that allow the practitioner to choose between the raw and processed signals offer the greatest versatility. In general, raw SEMG provides greater diagnostic information, while processed visual displays are easier for the patient to understand.
Audio displays Raw The audio portion of the SEMG should be considered. A raw SEMG channel Raw SEMG feedback yields valuable diagnostic information to the trained ear. The audio feedback
audio features of an SEMG instrument are essential for retraining purposes. This Thresholds feedback may be in the form of an analog tone—one that simply varies in Binary versus pitch. Alternatively, it may come in the form of a binary tone that comes on
analog only when the patient is above or below a preset threshold. With today’s multimedia computer capabilities, it is possible to play music or complex tones or even have the computer talk to the patient.
Exhibit 3–1 Surface EMG Unit Worksheet for Physical Medicine Practitioners
Check User Priorities Criteria Unit Name/Model
____ Stand-alone (SA), personal computer
(PC), both (B) SA/PC/B SA/PC/B SA/PC/B SA/PC/B
____ Special computer requirements Power source/safety
____ 3-Prong plug, optical isolation yes/no/NA yes/no/NA yes/no/NA yes/no/NA
____ Battery type/life NA NA NA NA
Electrode compatibility
____ Any (A), manufacturer’s electrode A/M A/M A/M A/M
only (M)
____ Electrode system active/passive active/passive active/passive active/passive ____ Electrode distance fixed/modifiable fixed/modifiable fixed/modifiable fixed/modifiable Lead wires
____ Electrode connectors snap/alligator snap/alligator snap/alligator snap/alligator ____ Shielding, general quality sturdy/okay/flimsy sturdy/okay/flimsy sturdy/okay/flimsy sturdy/okay/flimsy ____ Quality of amplifier connectors sturdy/okay/flimsy sturdy/okay/flimsy sturdy/okay/flimsy sturdy/okay/flimsy ____ Ground lead integral/separate integral/separate integral/separate integral/separate ____ Lead length
Amplifier
____ Input impedance
____CMRR (common mode rejection ratio) ____ Total gain
Frequency filter
____ Band width limits/roll-off / / / /
____ 60-Hz notch filter/roll-off yes/no yes/no yes/no yes/no
____Sampling rate
____Time constant/smoothing range ____Microvolt quantification ____ RMS, integral average, other
____ Adequate noise suppression always/not always always/not always always/not always always/not always Visual display output
____ Display type video/crystal/LED video/crystal/LED video/crystal/LED video/crystal/LED
____ Display type linear/log/both linear/log/both linear/log/both linear/log/both
____ Optional raw display yes/no yes/no yes/no yes/no
____ Digital yes/no yes/no yes/no yes/no
____ Moving line, oscilloscope-like yes/no yes/no yes/no yes/no
____ Variable sweep speed yes/no yes/no yes/no yes/no
____ Bar graph yes/no yes/no yes/no yes/no
____ Motor templates yes/no yes/no yes/no yes/no
____ Games, others Audio output
____ Modulating pitch/click/other pitch/click/other pitch/click/other pitch/click/other
____ Optional raw audio yes/no yes/no yes/no yes/no
Audio yoking to signal
____ Above goal yes/no yes/no yes/no yes/no
____ Below goal yes/no yes/no yes/no yes/no
____ Between dual goals yes/no yes/no yes/no yes/no
(continues)
Exhibit 3–1 (Continued)
Check User Priorities Criteria Unit Name/Model
Sensitivity manual/auto only/ manual/auto only/ manual/auto only/ manual/auto only/
____ Adjustment manual & auto manual & auto manual & auto manual & auto
____ Adequate low-end response range yes/no yes/no yes/no yes/no
____ Adequate high-end response range yes/no yes/no yes/no yes/no
____ Choice of sensitivity ranges okay/limited okay/limited okay/limited okay/limited ____ Ease of adjustment easy/okay/difficult easy/okay/difficult easy/okay/difficult easy/okay/difficult
____ Scale offset available yes/no yes/no yes/no yes/no
Thresholds/goals
____ Easy to see easy/okay/difficult easy/okay/difficult easy/okay/difficult easy/okay/difficult ____ Ease of adjustment easy/okay/difficult easy/okay/difficult easy/okay/difficult easy/okay/difficult EMG channels
____ Number
____ Modular additions possible yes/no yes/no yes/no yes/no
____ Graphic print capability yes/no yes/no yes/no yes/no
Comments regarding print functions/report generation ____ EMG data storage capacity
____ Statistical management max/min/avg/rng max/min/avg/rng max/min/avg/rng max/min/avg/rng ratio/% diff ratio/% diff ratio/% diff ratio/% diff
____ Event counter yes/no yes/no yes/no yes/no
____ Frequency spectral analysis yes/no yes/no yes/no yes/no
____ Download to statistics package yes/no yes/no yes/no yes/no
Software: general
____ Overall ease of operation easy/okay/difficult easy/okay/difficult easy/okay/difficult easy/okay/difficult
____ Labeled/dedicated keys yes/no yes/no yes/no yes/no
____ Menu driven yes/no yes/no yes/no yes/no
____ Mouse drag and click yes/no yes/no yes/no yes/no
____ Ability to store patient files yes/no yes/no yes/no yes/no
____ Playback ability yes/no yes/no yes/no yes/no
____ Other special software features
____ Software upgradable yes/no yes/no yes/no yes/no
External relay for neuromuscular
electrical stimulation yes/no yes/no yes/no yes/no
____ Ease of EMG-NMES software easy/okay/difficult easy/okay/difficult easy/okay/difficult easy/okay/difficult Cost
____ Finance/lease arrangements ____ Warranty period
After-purchase services
____ Area representative yes/no yes/no yes/no yes/no
____ Educational support yes/no yes/no yes/no yes/no
____ Trade-ins for upgrades yes/no yes/no yes/no yes/no
____ Other
____ Other comments
Source: Courtesy of Movement Systems, Inc., Seattle, Washington.
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CHAPTER QUESTIONS
1. The tissue of the human body acts like:
a. a high-pass filter b. a low-pass filter c. a notch filter d. an amplifier 2. A differential amplifier:
a. amplifies the difference of two separate muscle sites
b. finds the difference between two muscles
c. amplifies everything that is common to the recording electrodes
d. amplifies everything that is unique to the recording electrodes
3. Which of the following would represent a band reject filter?
a. 100- to 200-Hz band pass filter b. 60-Hz notch filter
c. 35-Hz low-pass filter d. 20-Hz high-pass filter
4. Which of the following amplifiers is considered to have the best common mode rejection?
a. 70 dB b. 50 µv c. 140 dB d. 10 megOhm
5. How much larger should the impedance of the electrode to skin interface be than the input impedance of the amplifier?
a. 2 times greater b. 5 times greater c. 10 to 100 times greater
d. equal to the input impedance of the amplifier 6. How would the SEMG recordings of an obese individual
compare to the SEMG levels of a very thin individual?
a. They would be the same.
b. They would be higher.
c. They would be lower.
d. It doesn’t matter.
7. When monitoring the gluteus maximus and rectus femoris muscles during hip flexion and extension, how should the practitioner compare these two muscles?
a. Compare RMS microvolts only.
b. Compare only integral averages.
c. Compare only normalized recordings.
d. One cannot compare these two muscles.
8. Which filter typically eliminates or dramatically reduces any ECG artifact from SEMG recordings?
a. 25-Hz high-pass filter b. 60-Hz notch filter
c. 100- to 200-Hz band pass filter d. 35-Hz low-pass filter
9. During muscle fatigue, what typically happens to the median frequency of the SEMG power density?
a. It stays constant.
b. Its value falls.
c. Its value rises.
d. Its value rises in an inverse relation to SEMG amplitude.
10. Raw SEMG tracings are commonly quantified using which of the following methods?
a. peak to peak b. RMS
c. integral average d. raw
11. When examining the SEMG signal for interspersed rest, what is the criterion level at which rest is defined ( J&J M-501 benchmark)?
a. below 2 microvolts b. below 5 microvolts c. below 20 microvolts d. below 50 microvolts
12. The power spectral density curves used to examine the distribution of muscle energy across the various frequencies of the SEMG recording use which of the following methods of analysis?
a. FFT b. SEMG c. ABC
d. mean extrapolation
13. The energy spectrum of SEMG during work commonly resides within which of the following frequency bands?
a. 20–300 Hz b. 100–200 Hz c. 20–1000 Hz d. 20–40 Hz
14. In SEMG, RMS refers to:
a. random motor stimulation b. root mean square c. rotary motion study d. random motion stabilization
15. The ECG artifact is most commonly found where?
a. on the extremities b. on the torso
c. on the left aspect of the torso d. on the cephalic muscles 16. Smoothingis a term that refers to
a. digital filtering
b. abrading the skin for electrode placement c. the quality of a movement pattern during training d. relaxation training protocols
17. What can be said about SEMG amplitudes and force?
a. There is a one-to-one relationship.
b. There is a curvilinear relationship.
c. There is no relationship.
d. It actually doesn’t make sense to focus on this relationship because the issues are far too complex.
18. The impedance at each recording electrode should be:
a. low b. high
c. low and balanced d. high and balanced
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