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Figure 4–1 Types of SEMG electrodes.
Source:(Top left) Courtesy of Vermed, Inc.; (top right) Reproduced with permission of Myotronics, Inc.; (center left) Courtesy of Vermed, Inc.; (center center) Courtesy of Thought Technology Ltd.; (center right) Courtesy of Thought Technology Ltd.;
(bottom) Courtesy of BIOPAC Systems, Inc.
electrodes placed on the low back need to adhere very well to the skin because the area is subject to extreme skin distortions and stretching during dynamic move- ments. However, when recording focuses on the facial muscles, some patients cannot tolerate an extremely ad- hesive pad. Hydrogel electrodes (discussed later in this section) provide a very gentle adhesive to be used on pa- tients with sensitive skin.
Direct contact electrodes are ideal for quiet SEMG recordings (i.e., relaxation-based therapies). Because they reside directly on the skin, they are more subject to movement artifact (see Chapter 3) and, therefore, are not recommended for recording of dynamic move- ments. Activity of the body may create movement of the electrode itself upon the skin’s surface, creating a direct current (DC) offset potential or movement arti- fact. Such artifacts commonly occur during vigorous movement, but may be present even during soft, quiet movements.
Floating electrodesare recommended for the record- ing of dynamic movements. They are created in such a way that the electrode is housed inside a cup, elevated above the skin by a millimeter or so. The cup is then filled with an electrolytic medium. An electrode paste or gel provides the bridge between the electrode and the skin. This substance potentiates the biological signal from the skin to the electrode and provides a cushion that absorbs the movement of the electrode (housing) on the skin’s surface.
Floating electrodes may take more time to prepare and are usually a little more expensive than direct con- tact electrodes. While the electrolytic cushion reduces the movement artifact, it may not totally eliminate it. In addition, the use of pastes and gels may lead to a bridg- ing of electrode paste between two active electrodes—
even when electrodes are placed as much as 2 cm apart. One can reduce the probability of bridging by ap- plying only a light pressure to the cup electrodes when they are applied to the skin. Firm pressure, however, may accidentally force the paste in one cell over to the ad- joining cell. When such bridging occurs, the impedance at the electrode site is radically reduced or short-cir- cuited. The biological energy seen by the SEMG amplifiers is significantly reduced, yielding a digital reading that is atypically low for the site and that does not change in a predictable way as a function of vigorous muscle con- traction. If the practitioner is monitoring multiple sites, the SEMG readings for the site with bridging will likely ap- pear quite low in comparison to the other sites. It is nec- essary to remove the bridged electrode, abrade the skin
again, and replace the electrode before a high-quality recording may be obtained. Some patients may be aller- gic or sensitive to the electrode paste or gel. Patients with such sensitivities should be offered hydrogel elec- trodes.
Hydrogel electrodeswere initially developed for pro- viding electrical stimulation to the skin (i.e., transcuta- neous electrical nerve stimulation [TENS]) and have since been used to record biological potentials. They commonly consist of a silver chloride disk electrode covered with a dry, sticky layer of gel as large as the usual adhesive collar. Sometimes the electrode consists of a foil with a tab. Hydrogel has a water and acidity content that is similar to the skin and, therefore, does not adversely affect the skin. Although this type of elec- trode allows the electrical potentials from the muscles to flow into the amplifiers, it has a higher impedance than the direct contact electrode or floating electrode;
consequently, it may be a bit noisier. In addition, the ad- hesion to the skin is lighter, allowing for easy removal of the electrode. Hydrogel electrodes may be moved from one electrode site to another, although this practice is not recommended. In a sense, these electrodes are re- placeable. The lightness of the adhesion also means that the electrode may come loose when perspiration occurs or during movements that place a strain on the lead at- tached to the electrode. To ensure good adhesion, hy- drogel electrodes typically have a large surface area.
They are probably best used for relaxation or other forms of quiet training and are the ideal solution for pa- tients with sensitive skin.
Electrodes for recording from the perineum (pelvic floor) have also been developed. These vaginal and rectal electrodesmay have the dumbbell shape seen in Figure 4–2. Two or three electrodes are located on the middle part of the electrode. They are inserted into the rectum or vaginal barrel and allow recording from the perianal muscles or the muscles of the pelvic floor. They are somewhat intrusive and, therefore, may not be appro- priate for children. In addition, sterilization of the elec- trode may be an issue. Therefore, it is wise to use a single-user type of electrode, so that the patient can clean and keep the sensor for repeated uses, but the same sensor is not used on multiple patients.
Ribbon electrodesare relatively new and consist of a silver ink electrode array and lead wires printed onto a thin sheet of Mylar and then covered by a thin foam pad with “cells” that define the electrode size and allow for electrode paste or gel. Such an electrode is shown in Figure 4–3. These electrodes are ideal for recording
Figure 4–2 Vaginal recording electrode.
Source: Courtesy of SRS Medical.
Figure 4–3 Ribbon electrode designed for specific monitoring of frontalis and corrugator muscles of the face.
Source: Courtesy of Patient Comfort.
from predetermined sets of muscles, such as those on the face. They are currently being used to monitor the fa- cial displays of emotion from the muscle activity of the frontalis and corrugator, or the zygomaticus and orbicu- laris oculi during surgical procedures.2In addition, the flat pad allows for recordings when the patient is in a face-down position. This kind of recording would be im- possible with pellet types of electrodes. Ribbon elec- trodes will certainly be extended to other applications in the future.