# Support Vector Machine-Based EMG Signal Classification Techniques: A Review
## 2 EMG Signals
### Signal Acquisition  
Due to the acquisition process, it is necessary to make a signal pre-processing step before performing the sample segmentation. This pre-processing stage consists of applying different classes of filters, such as a notch filter to eliminate the power line noise. The noise generated by the skin layers is at frequencies above 500 Hz [1; 2; 8; 9; 10; 11; 12] and at those below 10 Hz [1; 9; 11; 12; 14; 18; 20; 22]. Some authors consider that there is noise also in higher frequencies, and thus they use filters that cancel up to 20 Hz [2; 6; 8; 13; 15; 16; 17; 19; 21; 23; 24]. Nevertheless, in [7], the authors suppressed frequencies between 90 and 250 Hz and, in [25], the authors removed frequencies lower than 5 Hz and higher than 375 Hz. Table 1 summarizes the band-pass frequency allowed by each study.  
\begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Reference**} & \multicolumn{4}{c}{**Hz**} \\  & **10–500** & **20–450** & **20–500** & **25–500** \\ \hline
[1] & X & & & \\
[9] & X & & & \\
[11] & X & & & \\
[26] & & & & \(\chi\) \\
[23] & & X & & \\
[12] & X & & & \\
[13] & & & X & \\
[2] & & & & \(\chi\) \\
[14] & X & & & \\
[15] & & & X & \\
[16] & & X & & \\
[24] & & & X & \\
[17] & & & & \(\chi\) \\
[18] & X & & & \\
[19] & & & X & \\
[6] & & & & \\
[20] & X & & & \\
[21] & & & X & \\
[8] & & & X & \\ \hline \end{tabular}
\end{table}
Table 1: Reported band-pass frequency.  
Another issue found in the study of myoelectric signals is the frequency at which EMG signals should be sampled--a high frequency could give excess noise, and a lower one could lose a vast amount of information. The sampled frequency most commonly used is 1 kHz [19; 10; 12; 13; 18; 19; 27; 28; 29]. Other authors (e.g., [2; 11; 22; 26; 30; 31]) use a higher frequency of 1.5, 2, 3, 4, or 10 kHz. In addition, some authors use lower frequencies, such as 500 Hz [15; 32]. Table 2 exhibits a recap of the sampling frequencies used by different studies.  
Generally, for signal classification, more than one signal is required, because every movement is originated from different parts of the muscle and depends on a number of different muscles; therefore, the use of different channels helps to extract as much information as possible from the action(s) performed by the muscle(s). Among the various studies that have been done, it is common to work with four [19; 13; 23; 29; 38; 39], six [19; 40; 41], or eight [2; 7; 11; 22; 30] channels for the acquisition of the signal; some research papers even work with a smaller number of channels [26; 42]. Table 3 depicts an abridgement of the number of channels used by different studies and Table 4 summarizes the electrode type used and the place of electrode placement body. On the other hand, [PERSON] et al. [33] used a device that includes potentiometers, accelerometers, gyroscopes and force sensors and [PERSON] et al. [43] only used potentiometers to perform their calculations.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Reference** & **Electrode Type and Body Region** \\ \hline
[5; 14; 16; 18; 19; 37; 39; 40; 48; 49; 50] & Monopolar and Upper limb \\
[2; 26; 51] & Monopolar and Lower limb \\
[36; 47] & Monopolar and Facial muscles \\
[4; 6; 9; 10; 11; 15; 20; 22; 24; 27; 29; 31; 32; 33; 34; 35; 44; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61  
The separation between electrode locations is also vital. Most authors recommend a 20 mm distance; however, other authors differ from this opinion since their studies have yielded different results. There are those who indicate that the optimal distance is 10 mm [12] while others point out the optimal distance as 40 mm [55]. At the other extreme, certain authors remark that, when there is more than one channel available to read EMG signals, using a longitudinal channel as well as a transversal one is recommendable.