**2. Brief review and basic methodological considerations**

This brief review presents a few basic concepts related to electrical muscle activity recorded with electrodes over the skin. Methodological aspects that might influence the interpretation of experimental results are discussed. Further details concerning these basic aspects can be found elsewhere [6, 7].

## **2.1. Some important definitions**

In the preceding section we have referred to EMG as the electrical muscle activity recorded with surface electrodes. This electrical activity is the result of the depolarization of a number of muscle fibers. A group of muscle fibers innervated by the same spinal cord MN is called a muscle unit while the MN and the muscle unit it innervates is the motor unit (MU). During voluntary muscle activation, the number of recruited muscle fibers contributing to the EMG depends on the net excitatory drive from the brain and peripheral sensory afferents arriving onto the spinal MNs. During a mild voluntary contraction only a small fraction of MUs is recruited. As the excitatory command is increased, two distinct mechanisms take place: the MUs previously recruited increase their firing rate (rate coding) and new MUs with higher firing threshold are recruited (population coding). These basics may be found in many references, such as [8].

The activity of a single MU is easily recorded with needle electrodes inserted into the muscle (the most common is the concentric needle electrode). However, during low-intensity con‐ traction, surface electrodes can also record activity of superficial MUs [9] as can be seen in the upper trace in Figure 1. For increased levels of voluntary contraction additional MUs are recruited (see middle trace in Figure 1). For high-intensity contraction the EMG recording tends to show a filled random pattern due to the superposition of a greater number of MU action potentials (MUAPs) known as interference pattern (bottom trace in Figure 1). Thus, the interference pattern of the EMG is associated with the asynchronous firing of different MUs. When the conditioning of an interference pattern EMG is used to infer spinal cord processes, the experimental control of the level of activity is crucial. When the strength of descending commands changes, different populations of MNs and interneurons (INs) are recruited, leading to different conditioned EMG responses.

**Figure 1.** Surface EMG recordings from soleus (SO) muscle during a weak contraction (unpublished data). Upper pan‐ el: EMG recording during a very weak contraction in which only one MU is recruited. Middle panel: when the subject was told to slightly increase the voluntary contraction, the MU previously recruited increased its firing rate (see the green arrows) and other MUs with different firing rates were recruited (red and blue arrows). The letter "S" indicates a sum of at least two distinct MUAPs. Lower panel: Interference pattern of EMG.

#### **2.2. Acquisition**

notion that it is only a relay station, or a pathway that simply transfers information from the

Despite the relative limitations of non-invasive techniques employed in humans, it is currently possible to establish a parallel between the findings from animal preparations (such as cat) and experiments in humans (e.g., [2, 3]). In addition to the use of animal models as aids for understanding human data, another source of information comes from new multi-scale computer simulators of neuronal circuitry and muscle control [4, 5]. Moreover, with the development of these simulators, supported by anatomical and biophysical data from animal experiments, it is also possible to reinforce hypotheses formulated to explain experimental

The aim of the present chapter is to provide some conceptual and methodological background for researchers and clinicians who intend to use EMG to study human spinal cord neuro‐ physiology. Here we will discuss different methods frequently used in the study of human neurophysiology based on surface EMG. These will be illustrated by results from both experimental studies and simulations performed in a multi-scale model of the spinal cord and leg muscles. Additionally, a brief account will be given of some processing techniques of surface EMG that are used to quantify spinal cord excitability and effects of inhibitory pathways. The methods explored in the chapter have been used in both healthy subjects and

This brief review presents a few basic concepts related to electrical muscle activity recorded with electrodes over the skin. Methodological aspects that might influence the interpretation of experimental results are discussed. Further details concerning these basic aspects can be

In the preceding section we have referred to EMG as the electrical muscle activity recorded with surface electrodes. This electrical activity is the result of the depolarization of a number of muscle fibers. A group of muscle fibers innervated by the same spinal cord MN is called a muscle unit while the MN and the muscle unit it innervates is the motor unit (MU). During voluntary muscle activation, the number of recruited muscle fibers contributing to the EMG depends on the net excitatory drive from the brain and peripheral sensory afferents arriving onto the spinal MNs. During a mild voluntary contraction only a small fraction of MUs is recruited. As the excitatory command is increased, two distinct mechanisms take place: the MUs previously recruited increase their firing rate (rate coding) and new MUs with higher firing threshold are recruited (population coding). These basics may be found in many

The activity of a single MU is easily recorded with needle electrodes inserted into the muscle (the most common is the concentric needle electrode). However, during low-intensity con‐

brain to the muscle fibers.

58 Electrodiagnosis in New Frontiers of Clinical Research

found elsewhere [6, 7].

references, such as [8].

**2.1. Some important definitions**

results obtained from humans (e.g., [5]).

patients with a variety of neuromuscular disorders.

**2. Brief review and basic methodological considerations**

Some technical aspects need to be considered for an accurate recording of the EMG signal. Here we are going to briefly discuss filtering, sampling rate and electrode positioning.

The spectral composition of a signal has implications on the choice of the band-pass filter cutoff frequencies used before the analog-to-digital conversion as well as for the selection of a suitable sampling frequency (SF). Figure 2 shows the power spectrum of an EMG recorded with surface (upper panel) and needle (lower panel) electrodes during a sustained contraction. It is interesting to note the dramatic difference in the spectra of both recordings (note the different calibrations of the abscissa). For the surface EMG, band-pass filter cutoff frequencies from 10Hz to 500Hz would be appropriate and a SF of at least 1kHz would be used; however, in the second example, the 500Hz cutoff frequency (see the red area in the lower panel of Figure 2) would be clearly inappropriate due to the significant contributions of high-frequency spectral components of the signal. Therefore, when using needle EMG, the high-frequency cutoff should be higher, e.g., 5kHz-10kHz and sampling done at 20kHz-40kHz.

generated by depolarization of the muscle fibers travel through the connective tissues, fat, vessels, skin (all of which comprises the volume conductor), reaching the region underneath the electrodes. The volume conductor has the property of a low-pass filter [12] and the signals reach the electrodes placed over the skin with a slower time course and decreased amplitude. On the other hand, a needle electrode is much closer to the source of the electrical activity than a surface electrode and hence it does not suffer the low-pass filtering and amplitude attenua‐ tion caused by volume conduction [6, 12]. This explains why the needle EMG signals have better signal-to-noise ratios and why their power spectra have components at higher frequen‐

Experimental and Simulated EMG Responses in the Study of the Human Spinal Cord

http://dx.doi.org/10.5772/54870

61

The main advantage of invasive techniques such as needle or wire EMG is its high selec‐ tivity (one or very few MUs can be recorded with a high signal-to-noise ratio). Converse‐ ly, this may be a disadvantage when the purpose is to evaluate a larger number of MUs to obtain a more comprehensive view of muscle activation. In this case, surface EMG is more indicated. The main shortcomings of surface EMG are that (1) not all muscles are superficial and (2) the possibility of interference from nearby muscles' electrical activities on the EMG signal recorded from the desired muscle. These recorded interferences are attenuated or perhaps distorted versions of the electrical activities from the nearby mus‐ cles and are known as crosstalk [13, 14]. The crosstalk effect can sometimes be mini‐

The distance between electrodes is a key factor to increase or decrease the relative selectivity of the EMG recording. Figure 3 shows an example in which the EMG activity is recorded with an array of three electrodes. When the potential difference is calculated between the more distant pair of electrodes (E1 – E3) the recording is less selective than when the electrodes are

**Figure 3.** Surface EMG showing the effect of the distance between electrodes (unpublished data). Upper panel: EMG recorded with a distance of 2.5cm between electrodes (E1-E3). The lower panel shows the same recording with inter-

cies (see the lower panel in Figure 2).

mized by a careful placement of the electrodes.

closer to each other (E1 – E2).

electrode distance of 1cm (E1-E2).

**Figure 2.** Power spectra of EMG signals from the SO muscle recorded with surface (upper panel) and needle (lower panel) electrodes (unpublished data). They show the frequency content (in Hz) of each signal. The green marks in the abscissa (small vertical lines) comprise the frequencies of the band-pass filter used for surface EMG. The corresponding frequency ranges are indicated in red (upper panel) and in blue (lower panel) for surface and needle recordings, re‐ spectively. It is clear in the lower panel that if we used the same frequency range of surface EMG for needle EMG a considerable amount of information would be lost (see the red area delimited by the green marks).

The choice of a suitable frequency range for the band-pass filter to be applied to the surface EMG signal needs to be done with caution according to the objectives of the study. A wrong choice of filter parameters may cause information loss and misleading interpretations of the results. For instance, if the focus is to investigate slow variations of the surface EMG signal during stepping or gait (e.g., EMG envelope), a frequency band of 10-300Hz could be adequate [10]. However, using the same recording technique to evaluate reflex components (e.g., Hreflex), the high cutoff frequency should be raised to 1kHz (with a SF of at least 2kHz) for better reproduction of the phasic EMG signal generated [10].

Generally, in surface EMG, the electrodes are located on the skin above the belly of the muscle of interest, in a region between the tendon and the innervation zone [11]. The electric currents generated by depolarization of the muscle fibers travel through the connective tissues, fat, vessels, skin (all of which comprises the volume conductor), reaching the region underneath the electrodes. The volume conductor has the property of a low-pass filter [12] and the signals reach the electrodes placed over the skin with a slower time course and decreased amplitude. On the other hand, a needle electrode is much closer to the source of the electrical activity than a surface electrode and hence it does not suffer the low-pass filtering and amplitude attenua‐ tion caused by volume conduction [6, 12]. This explains why the needle EMG signals have better signal-to-noise ratios and why their power spectra have components at higher frequen‐ cies (see the lower panel in Figure 2).

sampling frequency (SF). Figure 2 shows the power spectrum of an EMG recorded with surface (upper panel) and needle (lower panel) electrodes during a sustained contraction. It is interesting to note the dramatic difference in the spectra of both recordings (note the different calibrations of the abscissa). For the surface EMG, band-pass filter cutoff frequencies from 10Hz to 500Hz would be appropriate and a SF of at least 1kHz would be used; however, in the second example, the 500Hz cutoff frequency (see the red area in the lower panel of Figure 2) would be clearly inappropriate due to the significant contributions of high-frequency spectral components of the signal. Therefore, when using needle EMG, the high-frequency cutoff

**Figure 2.** Power spectra of EMG signals from the SO muscle recorded with surface (upper panel) and needle (lower panel) electrodes (unpublished data). They show the frequency content (in Hz) of each signal. The green marks in the abscissa (small vertical lines) comprise the frequencies of the band-pass filter used for surface EMG. The corresponding frequency ranges are indicated in red (upper panel) and in blue (lower panel) for surface and needle recordings, re‐ spectively. It is clear in the lower panel that if we used the same frequency range of surface EMG for needle EMG a

The choice of a suitable frequency range for the band-pass filter to be applied to the surface EMG signal needs to be done with caution according to the objectives of the study. A wrong choice of filter parameters may cause information loss and misleading interpretations of the results. For instance, if the focus is to investigate slow variations of the surface EMG signal during stepping or gait (e.g., EMG envelope), a frequency band of 10-300Hz could be adequate [10]. However, using the same recording technique to evaluate reflex components (e.g., Hreflex), the high cutoff frequency should be raised to 1kHz (with a SF of at least 2kHz) for better

Generally, in surface EMG, the electrodes are located on the skin above the belly of the muscle of interest, in a region between the tendon and the innervation zone [11]. The electric currents

considerable amount of information would be lost (see the red area delimited by the green marks).

reproduction of the phasic EMG signal generated [10].

should be higher, e.g., 5kHz-10kHz and sampling done at 20kHz-40kHz.

60 Electrodiagnosis in New Frontiers of Clinical Research

The main advantage of invasive techniques such as needle or wire EMG is its high selec‐ tivity (one or very few MUs can be recorded with a high signal-to-noise ratio). Converse‐ ly, this may be a disadvantage when the purpose is to evaluate a larger number of MUs to obtain a more comprehensive view of muscle activation. In this case, surface EMG is more indicated. The main shortcomings of surface EMG are that (1) not all muscles are superficial and (2) the possibility of interference from nearby muscles' electrical activities on the EMG signal recorded from the desired muscle. These recorded interferences are attenuated or perhaps distorted versions of the electrical activities from the nearby mus‐ cles and are known as crosstalk [13, 14]. The crosstalk effect can sometimes be mini‐ mized by a careful placement of the electrodes.

The distance between electrodes is a key factor to increase or decrease the relative selectivity of the EMG recording. Figure 3 shows an example in which the EMG activity is recorded with an array of three electrodes. When the potential difference is calculated between the more distant pair of electrodes (E1 – E3) the recording is less selective than when the electrodes are closer to each other (E1 – E2).

**Figure 3.** Surface EMG showing the effect of the distance between electrodes (unpublished data). Upper panel: EMG recorded with a distance of 2.5cm between electrodes (E1-E3). The lower panel shows the same recording with interelectrode distance of 1cm (E1-E2).
