The raw SEMG display is the oldest form of SEMG pres- entation. It presents an unprocessed, peak-to-peak os- cilloscopic display of the SEMG signal. As the MUAPs 0.5
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Figure 3–10 Power spectral density of the composite signal shown in Figure 3–9.
Figure 3–11 Power spectral analysis of an SEMG tracing from a normal contraction of latissimus dorsi. The upper panel shows the raw SEMG recording with the lines in the center representing the portion of the recording submitted for spectral analysis. The lower panel shows the spectral analysis of the SEMG signal.
Source:Copyright ©Clinical Resources, Inc.
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Figure 3–12 Surface EMG recording from cervical paraspinals during intense headache.
Source:Courtesy of Will Taylor, Portland, Oregon.
sum and reach the skin, the small SEMG potentials are amplified and their sinusoidal nature is presented as they oscillate between the positive and negative poles.
As Figure 3–13illustrates, the SEMG signal oscillates in both the positive and negative directions, and also varies in its thickness and height. The thickness of the tracing represents the amplitude or strength of the con- traction. The thicker the tracing, the stronger the SEMG signal and the stronger the contraction. In this example, the muscle goes from approximately 2 microvolts (peak to peak) at rest to approximately 200 microvolts (peak to peak) during the contraction. The unit of measurement for raw tracings is microvolts peak to peak (commonly referred to as pp), which represents the thickness of the tracing.
The advantage of the raw SEMG tracing is that it con- tains all of the information from the SEMG signal. None of it is processed out. One can readily see the various forms of artifact in the signal. These artifacts, which are explored in greater depth later in this chapter, include 60-cycle noise, ECG artifact, and movement artifact. In addition, raw SEMG tracings allow clinicians to see post-
movement irritability in a muscle that harbors a trigger point. Figure 3–14demonstrates this phenomenon in a recording taken from the upper trapezius muscle follow- ing abduction. Before the movement, the muscle is quiet. During the movement, the SEMG activity rises ap- propriately and becomes thicker. However, following the cessation of the movement, the SEMG activity level does not return to the resting baseline level. Not only does it remain thicker, but it contains hairlike elements in the left upper trapezius (LUT; top tracing) portion that extend above the majority of the tracing on a somewhat irregular basis. The right upper trapezius (RUT; second tracing) also does not return to the baseline levels, but it lacks the hairlike elements. The LUT site contains an ac- tive trigger point, and the RUT site does not. The lack of return to premovement baseline levels and the hairlike el- ements seen in the postrecruitment pattern may repre- sent a disturbance in the muscle spindle secondary to the presence of a trigger point. The assessment of trigger points is reviewed in Chapter 8.
The primary drawback of the raw SEMG display is that the additional information may make it more difficult
Figure 3–13 Raw SEMG tracing from the trapezius site during abduction and retraction. The signal goes from thin (low activity) to thick (high activity).
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stand, read, and interpret. Because the very nature of muscle activity is a random, staccotic firing of groups of muscle fibers, finding ways to reduce the variability of muscle activity makes it a little easier to understand. As noted previously, this consideration may be especially important when the SEMG instrument is used to train patients in how to control their muscle function. The simpler and easier the SEMG display is to understand, the better the training effects.
Although the processing of the signal may result in a variety of quantities (e.g., RMS, integral average), ini- tially they all begin with a common series of steps. The first step in the process entails rectifying the signal. That is, the portion of the signal that resides below the 0 point (the negative electrical potential) is made positive and artificially placed above the 0 crossing line, as illus- trated in Figure 3–15.
The next step entails smoothing out the signal in some way. This is frequently done mathematically, and is commonly referred to as digital filtering. For example, rather than displaying every point of the rectified signal, for the patient to interpret the signal. This complication
becomes relevant during biofeedback training sessions, in which the SEMG signal is presented to the patient as a means to guide the use of muscle. Teaching symmetry of movement, for example, may be easier if the two channels of SEMG are overlaid on top of each other so that the patient can see which muscle activity is higher or lower. In addition, when using a template to teach the patient a particular recruitment pattern, the processed signal is easier to use. An SEMG system that al- lows both processed and raw displays would be ideal.