EMG signals can be recorded using needle, fine wire, or surface electrodes, and each electrode type has distinct advantages and disadvantages. First, it is important to consider the purpose of EMG recording. Will the EMG data be used for biofeedback, for research, or for diagnostic purposes? Next, it is important to consider the availability of appropriate equipment, the skill level of the examiner, and what certification is mandated by the jurisdiction when using more invasive techniques. Finally, and perhaps most importantly, specific factors to consider include the patient’s acceptance of the electrode (internal vs. external), age, anatomy (size), sexual experience, as well as the possibility of the recording electrode itself inducing muscle activation (e.g., due to pain or fear). A thorough understanding of the strengths and limitations of each type of electrode, summarized in Table 12.1, will guide this decision.

Needle electrodes are small and highly specific, picking up only that electrical activity propagating within approximately 0.5 cm² of the needle tip [89]. They are mainly used by neurologists and physiatrists in the clinical EMG evaluation of MUP properties to assist with diagnosis of neuropathy or myopathy [90]. Needle electrodes can, however, be used to study activation in muscles that are small and/or situated deep beneath the skin. They can provide a clear indication of the timing of motor unit activation, demonstrating the exact time that the first motor unit near the needle tip becomes active, but are not very useful when determining the state of activation of the muscle as a whole. The presence of a needle within a muscle during contraction can lead to pain inhibition or muscle spasm, thus influencing the validity of the outcomes. In many jurisdictions, needle electrodes can only be inserted by a health professional with specific certification in EMG.

In PFM research, needle electrodes have been used in key neurophysiologic studies by Podnar et al. [91–96], who investigated MUP morphology and firing rates primarily in the EAS to elucidate the pathophysiology of fecal incontinence and constipation, and by Shafik and colleagues [5, 36, 57, 58, 60, 97–99], who investigated neurophysiologic links between the various PFMs during examinations of sexual function and dysfunction.

Fine wire electrodes can be particularly useful when studying small muscles and/or muscles that are located deep beneath the skin surface, and should be considered when electrodes placed on the skin surface are likely to receive electrical contributions from other nearby muscles, a phenomenon known as cross talk . Often they are inserted as pairs, where both wires are threaded into a single needle cannula. The wires are hooked at their ends such that when the needle is inserted into the muscle and then withdrawn, the hooks hold the wires in situ. The fine wires are insulated except for their tips, which register EMG signals within the local muscle region where they are located. As with needle electrodes, fine wire electrodes can pick up single motor unit activity and are very useful when studying the onset of muscle activation because the rise in EMG during activation is more abrupt than it is with surface electrodes, however, as with needle electrodes, they may not record the global level of activation within a muscle [100, 101], particularly when studying larger muscles. The major advantage of fine wire electrodes over needle electrodes in terms of studying muscle activation amplitude and/or timing is that fine wire electrodes are virtually painless once the needle cannula is removed. In some patients with an overactive pelvic floor where pain may prevent the use of intravaginal or intra-anal probes or a pain response may be triggered by the use of probes, fine wire electrodes may be the preferred choice. They might also prove highly useful in the study of dyssynergic voiding, where there is a need to determine accurate timing information.

Despite the fact that fine wires are an ideal electrode type for recording EMG from the small PFMs that lie deep within the pelvis where potential for recording cross talk is high [102], fine wire electrodes have not commonly been used when studying the PFMs. Binnie et al. [103] reported high correlations between fine wire EMG recorded from the EAS and sEMG recorded intra-anally using longitudinally oriented electrodes on either side of an anal probe. However, their sample size was small (n = 8) and the reliability analysis was insufficient to support this conclusion. Stafford et al. [104] recently described a technique for using fine wire electrodes to study the activation of the bulbocavernosus and puborectalis in men, and Auchincloss and McLean [105] used fine wire electrodes to determine that there was no measurable effect of having an electrode probe located in the vagina when women performed voluntary PFM contractions (n = 12). As with needle electrodes, the insertion of fine wire electrodes requires technical skill, certification, and specialized instrumentation (refer to Sect. 12.3.3), which may explain their limited use. For any fine-wire insertions, and especially for the PFMs, t he availability of an ultrasound system to validate the location of insertion is highly advantageous.

Surface electrodes are by far the most common method of recording EMG activity as they are noninvasive and provide a global signal that reflects summed action potential activity from a large portion of the active muscle fibers within the muscle. However, surface electrodes are only useful for recording EMG activity from muscles that are located close to the skin surface; otherwise, the resulting EMG signals may contain cross talk which confounds study results by overestimating the EMG activity within a single muscle.

As noted in Sect. 12.2, surface “patch” electrodes are commonly used to study PFM dyssynergia, particularly during clinical urodynamic and defecation studies. These adhesive electrodes are normally placed perianally and are assumed to reflect the activation of the levator ani group of muscles. This approach is, however, highly nonspecific, as the electrodes are located far from the desired signal source, except in the case of the EAS [103]. In general, electrodes mounted on intravaginal or intra-anal probes are preferable; however, there are situations in which intravaginal or intra-anal probes are inappropriate, for example, in pediatric populations, in accordance with certain sociocultural norms, or in women or men with severe pain with penetrative activities.

When recording EMG from the levator ani, surface electrodes are most often mounted on a vaginal or anal probe to record activity in close anatomic proximity of the pubovaginal and pubococcygeal muscles. Intravaginal and intra-anal surface electrodes have been used to evaluate the tonic and phasic activation of the levator ani in women and men with vulvovaginal and pelvic pain, respectively. Some of the discrepancies in the literature described in Sect. 12.2 may be explained by differences in electrode type, size, and configuration.

Although the same principles hold true for needle and fine wire electrodes [106], electrode configuration errors are by far most common when surface electrodes are used to record EMG signals, and thus our discussion of electrode configuration will focus on sEMG.

Most modern EMG systems use differential amplifiers, meaning that they subtract (take the difference between) activity seen at two “active” electrodes before the data are recorded. If both electrodes are placed over the same muscle, since MUP s propagate along the muscle, they are “seen” by one electrode before they are “seen” by the other; therefore, signals do not pass by both electrodes at the same time and are not subtracted out of the resultant signal. Signals that are common to both electrodes, for example, electrical noise, are subtracted away. Noise sources, such as interference from other electrical equipment in the area, are further minimized by using a ground electrode placed on the skin over a bony prominence where physiological activity is expected to be minimal, which removes noise that is common to all channels (termed common mode rejection). In order for this system to work appropriately, the two active electrodes should be oriented along the line of action of the muscle fibers. The levator ani muscle group has distinct origins, insertions, and innervations on each side of the pelvis; therefore, its activation should be recorded using intravaginal probes that make distinct and differential recordings from two active electrodes on each side [107, 108]. In comparison, because the EAS has circumferential fibers, intra-anal probes with one bar on each side of the probe may be appropriate for recording its activation. Results by Binnie et al. [103] support this principle, where they found that electrodes oriented longitudinally on either side of an intra-anal probe recorded higher voluntary and reflex activation amplitudes from the EAS than circumferential electrodes.

In addition, large, widely spaced electrodes will record signals that originate from sources much deeper or farther away than small and closely spaced electrodes [109], and will therefore record higher EMG activation amplitudes, representing activity from a larger motor unit pool, whether those motor units are generated by the muscle of interest or not. The larger amplitude signals, having traveled some distance through the tissues to arrive at the electrodes, undergo a “volume conductor” effect [109], meaning that their peaks will not be as sharp and their timing information will be less precise. Similarly, if an intravaginal or intra-anal probe is tilted, higher EMG amplitudes can be expected on the side of the probe that more closely approximates the PFMs. Hence, researchers and clinicians alike must keep in mind that depending on PFM tone, vaginal/anal laxity, and individual anatomical differences, the distance between the PFMs and the recording electrodes can change within a session, introducing bias in the signals.

Larger, more widely spaced electrodes are most useful when large, superficial muscles are investigated. They are advantageous in terms of the reliability of the resulting EMG signals since they record contributions from a larger proportion of the whole motor unit pool, and they are less susceptible to differences in motor unit recruitment between trials or between sessions [56, 108, 109]. Unfortunately, most commercially available intravaginal and intra-anal probes use electrode surfaces that are larger than the contact area of the PFMs or the EAS , thus increasing the likelihood of cross talk contamination. The impact of electrode size and spacing is evident in recent EMG data published by Auchincloss and McLean [56]. Using identical signal processing techniques, EMG amplitudes reported for maximum voluntary contractions (MVC) of the PFMs ranged from 45 to 50 μV when acquired by the FemiScan™ probe and from 120 to 140 μV when acquired from the same sample on the same day but by larger, further separated Periform^® electrodes. According to our research, even though the electrodes are likely too long, the FemiScan™ is the only currently available commercial intravaginal probe that records EMG activation from the deep muscles of the pelvic floor with an appropriate electrode configuration [107].

Other appropriate intravaginal surface electrodes have been reported in the literature; however, none appear to be commercially available at this point. As early as 1985, Lose et al. [110, 111] used paired electrodes (surface area 38.5 mm each) mounted on a sponge that was inserted into the vagina over the lateral vaginal wall to record levator ani activity. Stafford et al. [112] described an intra-urethral electrode to investigate EMG activation of the striated urethral sphincter in which four electrodes were located around a catheter, and suction was used to hold the urethral walls still against the catheter tip, and have used this electrode to study urethral sphincter function in healthy men [104]. Keshwani and McLean [102, 108] developed an intravaginal differential suction electrode with small, closely spaced electrode surfaces that are less prone to cross talk and, because it is held to the vaginal wall using suction, it is less prone to movement with respect to the vaginal wall causing motion artifact (e.g., artifact is noise contamination from nearby muscles that falsely increases EMG amplitudes) in the resulting EMG signals [102]. Voorham-van der Zalm et al. [113] have also reported a vaginal and anal probe design that incorporates small electrodes. However, their electrode configuration is monopolar, rather than differential, with activity under each small electrode referenced to a surface electrode placed on the skin surface over the pelvis. This probe may prove to be quite useful for the study of different clinical populations [114]; however, it requires further psychometric assessment at this time, since the electrode configuration has potential for recording high levels of crosstalk and noise interference. Readers who desire more in depth information and a critical appraisal of various commercially available probes can refer to a review published in 2015 by Keshwani and McLean [107].

With conventional EMG electrodes and systems [115], it is recommended that users perform a proper skin preparation before adhering surface electrodes to the skin surface over the muscle(s) of interest. This process involves cleaning and gently abrading the skin surface and rubbing conductive paste into the skin in order to reduce the impedance between the skin and the electrode surfaces to minimize the loss of signal amplitude. This and other formerly essential criteria for the electrode skin interface such as recessing electrodes and using conductive paste between the electrode and the skin surface have been relaxed in recent years as modern amplifiers have improved substantially [116]. That said, attention should be paid when placing electrodes on the perineum: The skin should be hairless, clean, and dry, and the electrodes should be securely adhered to the skin surface using adhesive or suction. If one of the electrodes lifts even slightly during the EMG evaluation, the recorded sEMG signals are no longer valid.

Despite following recommended guidelines, the electrode–tissue interface is still a large potential source for noise contamination in sEMG signals when the electrodes move with respect to the tissue surface from which they are recording signals. This is particularly a problem with intravaginal and intra-anal electrodes that do not generally adhere to the vaginal wall. The resultant motion artifact (Fig. 12.2) can be a major problem especially given that the lifting action of the levator ani naturally causes migration of the electrodes relative to the underlying muscles. Motion artifact can also be caused by motion of the leads attaching the electrodes to the amplifiers [117]. Keshwani and McLean [102] investigated motion artifact contamination in EMG data recoded from the PFMs during coughing and found that close to 30 % of files recorded using the FemiScan™ probe were contaminated with motion artifact, whereas only 14 % of files were contaminated when suction was used to hold a differential electrode securely to the vaginal wall.