Noise and artifact are functionally defined as anything contained in the SEMG signal that the practitioner does not want. One attribute of an SEMG amplifier is its sig- nal-to-noise ratio. The ideal amplifier has a very high signal-to-noise ratio—all signal and no noise. This sec- tion describes some of the primary examples of noise and artifact that pertain to the internal noise of the elec- tronic circuitry of the instrument. Noise from outside the instrument may also be a problem.
A common source of noise for the electromyographer is the ECG artifact. The ECG is very coherent and much larger than that of the muscles. It is clearly picked up on nearly all of the sites located on the torso. It is primarily seen on the left side of the body and, therefore, may lead to asymmetrical RMS values during rest. This artifact is clearly shown in both the raw and processed SEMG tracings from the left serratus anterior (LSer) in Figure 3–19. It is also commonly seen in the left lumbar area.
ECG artifact is not commonly seen in the extremities, except when wrist-to-wrist or ankle-to-ankle leads are used. This type of artifact can be minimized by placing the electrodes close together and using a 100- to 200-Hz band pass filter. Many practitioners, however, readily ac- cept the ECG artifact as a fact of life and merely educate their patients about this phenomenon. As a side benefit, one could use the ECG as an index of arousal by observ- ing changes in the patient’s pulse rate.
Movement artifactis seen as direct current (DC) shifts and/or massive deflections in the SEMG potentials of the raw SEMG recording. This occurs because the elec- trode slips around on the surface of the skin, generating an electrical potential of its own. The upper panel of Figure 3–19 shows this slippage in the raw SEMG for the left serratus anterior site as a distortion in the DC levels around which the SEMG signal oscillates. In this particu- lar instance, a momentary slippage of the electrode oc- curs as a function of skin distortion (stretch) as the patient moves his or her arms out of abduction and into shoulder elevation. When this tracing is then converted to the processed mode (Figure 3–19B), however, the movement artifact simply appears as an upward deflec- tion. With the process mode, it is difficult to differentiate the movement artifact from the real SEMG signal. Only
Figure 3–19 (A) Raw and (B) processed SEMG tracings with clear ECG and movement artifact. The ECG artifact is clearly seen as rhythmic deflections in the serratus anterior (LSer) for both the raw and processed tracings. The movement artifact is seen in the DC shifts in the LSer for the raw tracing. Note how it is difficult to discern the movement artifact in the LSer track of the processed EMG recording.
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such as running. During quiet sitting, however, it is highly unusual to see large recruitment patterns associ- ated with respiration in these muscles. The major excep- tions to this rule are patients with chronic obstructive pulmonary disease or patients who breathe in a para- muscles in Figure 3–21. Both sets of these muscles are
considered ancillary muscles of respiration, and they are invoked when the patient needs to raise the ribs to fa- cilitate breathing into the upper lobes of the lungs. This movement is very common in high-demand situations
Figure 3–20 (A) Raw SEMG tracing contaminated with 60-Hz noise. (B) Spectral analysis of the signal showing the harmonics of 60-Hz noise.
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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.