*2.3.2. Signal rectification*

This procedure has the purpose to turn all the signal values integrative, submitting them to the cut of all negative values, that means, to delete the values that are under the baseline, or to turn all this negative values to positive adding the values, making them integrative. The second option is more recommended if the intention is to achieve the total muscle signal, if you cut off the negative part, half of the signal will be lost, so turning all of them positive is a more used and more interesting when it comes to final results. This procedure doesn't affect the signal noises like the filtration and the smoothing, which will be explained in sequence. However it is still recommended and made part of the studies involving this chapter, so it's important for the reader to know how we used and what it means.

This procedure is simple, and it can be easily understood by the figure above. Note that until the filtration moment the signal had both positive and negative side in the burst

Computational Intelligence in Electromyography Analysis – 102 A Perspective on Current Applications and Future Challenges

moments, taking the baseline as a zero mark, and once that the signal was rectified it became not only positive, but also increased the positive side size, that mean that we didn't exclude the negative part, we added it to the positive side. If the reader wants to know how the same signal without the adding of the negative values would be, you just have to take the filtrated signal and cut the down part, always considering the baseline as a zero mark. Look at the figure 3 under and try to make a qualitative analyze of the two methods.

Influence of Different Strategies of Treatment Muscle Contraction

and Relaxation Phases on EMG Signal Processing and Analysis During Cyclic Exercise 103

**Figure 4.** Burst and silence moments in an electromyography signal during a Wingate test in a cicloergometer. The break between the blue lines show a burst moment and the break between the red

The Burst moment is the muscle contraction moment, easily noticed by the sudden break in the baseline, and so, the Silence moment is when no contraction is occurring, so the signal stays at zero, or at least should stay, as said before in the treatment discussions. That's another important reason to maintain a baseline close to zero; it is easier to separate the onset and the end of the Burst from the Silence moment. When a signal is collected it's normal to treat it as an Entire signal, which means that it takes to account both Burst and Silence moments of the signal. The figure showed above is from the recto femoral muscle in a cyclic exercise in a cicloergometer during a Wingate test. As expected in this kind of exercise, it is found a lot of Burst and Silence moments, differing from isometric exercises for example, that would appear only a Burst moment, which would lose strength as time goes

The problem is the use of the entire signal, or only the Burst moments, taking into account that if the intention is to read only the muscle activity it can be assumed that should be used only the Burst moment once that a cyclic exercise will have a lot of Silence moments, and this could make the final results to become smaller, to decrease the meaning. Thus, to know if the Silence moment affects the final results in time and frequency domains variables is of great importance for the researchers that works especially with cyclic exercises. That is one

The term "time windows" is used to determine the size of the cuts that will be made in the EMG signal for further analyzes. The most normal is to use the 1 second window, and in case of short tasks it can easily be done once that the signal is short and it is easy to separate the total task time in 1 second parts. However, for longer tasks it can be really difficult for the researcher to separate a signal of 10 minutes in 600 windows of 1 second each for example. Thus, a study from [14] brought that to use a 5 second window and a 1 second in a cyclic exercise can provide the same result of muscle activity for further analyzes, providing for the EMG researchers an excitement about using the method in long tasks. To bring a better example, the figure 5 under shows us a signal and how it would be analyzed if it was

lines is the silence moment. Unification of Burst and Silence generates the full signal.

of the problems that will be further discussed on this chapter.

by for the fatigue process.

**2.5. Time windows** 

cut in one second windows.

**Figure 3. a**) Half-wave rectification: rectification excluding the negative part of signal amplitude; **b**) Full-wave rectification: aggregating the negative part (reducing the ripple) of EMG signal.

Notice that for a visual analysis excluding the negative parte can bring an error, it's hard to say that the three last bursts of the red signal are different from the three before it, but in the blue signal is much easier to assume that. Thus in the first one I could say that the muscle had reached his maximal power, while in the second I could not make the same affirmation.

#### *2.3.3. Signal smoothing*

The Smoothing and the filtration have some similar parameters, mostly because both have the intention of taking out the extremes, the parts that are considering noises. Smoothing creates a linear envelope in the signal, leaving only a center part of the signal. The mainly difference between the smoothing and the filtration, is that filtration take in account the muscle activation range, and the smoothing the signal obtained itself. If the filter is strong enough or considered really good, it can even make the smoothing unnecessary. However, it's recommended to use both, especially in cyclic dynamic contractions, that as we already saw, have a bigger chance to have noises interferences. Looking at the Figure again, the smoothed signal is also really easy to realize, it creates visually a much cleaner signal, creating almost a line, which means, it excludes the extremes, leaving only the signal that is considered the muscle activation signal.

#### **2.4. Burst and silence**

During the obtainment of the EMG signal we can separate two parts of it, the Silence and the Burst, as showed in the figure 4.

**Figure 4.** Burst and silence moments in an electromyography signal during a Wingate test in a cicloergometer. The break between the blue lines show a burst moment and the break between the red lines is the silence moment. Unification of Burst and Silence generates the full signal.

The Burst moment is the muscle contraction moment, easily noticed by the sudden break in the baseline, and so, the Silence moment is when no contraction is occurring, so the signal stays at zero, or at least should stay, as said before in the treatment discussions. That's another important reason to maintain a baseline close to zero; it is easier to separate the onset and the end of the Burst from the Silence moment. When a signal is collected it's normal to treat it as an Entire signal, which means that it takes to account both Burst and Silence moments of the signal. The figure showed above is from the recto femoral muscle in a cyclic exercise in a cicloergometer during a Wingate test. As expected in this kind of exercise, it is found a lot of Burst and Silence moments, differing from isometric exercises for example, that would appear only a Burst moment, which would lose strength as time goes by for the fatigue process.

The problem is the use of the entire signal, or only the Burst moments, taking into account that if the intention is to read only the muscle activity it can be assumed that should be used only the Burst moment once that a cyclic exercise will have a lot of Silence moments, and this could make the final results to become smaller, to decrease the meaning. Thus, to know if the Silence moment affects the final results in time and frequency domains variables is of great importance for the researchers that works especially with cyclic exercises. That is one of the problems that will be further discussed on this chapter.

#### **2.5. Time windows**

Computational Intelligence in Electromyography Analysis – 102 A Perspective on Current Applications and Future Challenges

affirmation.

*2.3.3. Signal smoothing* 

**2.4. Burst and silence** 

Burst, as showed in the figure 4.

considered the muscle activation signal.

moments, taking the baseline as a zero mark, and once that the signal was rectified it became not only positive, but also increased the positive side size, that mean that we didn't exclude the negative part, we added it to the positive side. If the reader wants to know how the same signal without the adding of the negative values would be, you just have to take the filtrated signal and cut the down part, always considering the baseline as a zero mark.

Look at the figure 3 under and try to make a qualitative analyze of the two methods.

**Figure 3. a**) Half-wave rectification: rectification excluding the negative part of signal amplitude; **b**)

Notice that for a visual analysis excluding the negative parte can bring an error, it's hard to say that the three last bursts of the red signal are different from the three before it, but in the blue signal is much easier to assume that. Thus in the first one I could say that the muscle had reached his maximal power, while in the second I could not make the same

The Smoothing and the filtration have some similar parameters, mostly because both have the intention of taking out the extremes, the parts that are considering noises. Smoothing creates a linear envelope in the signal, leaving only a center part of the signal. The mainly difference between the smoothing and the filtration, is that filtration take in account the muscle activation range, and the smoothing the signal obtained itself. If the filter is strong enough or considered really good, it can even make the smoothing unnecessary. However, it's recommended to use both, especially in cyclic dynamic contractions, that as we already saw, have a bigger chance to have noises interferences. Looking at the Figure again, the smoothed signal is also really easy to realize, it creates visually a much cleaner signal, creating almost a line, which means, it excludes the extremes, leaving only the signal that is

During the obtainment of the EMG signal we can separate two parts of it, the Silence and the

Full-wave rectification: aggregating the negative part (reducing the ripple) of EMG signal.

The term "time windows" is used to determine the size of the cuts that will be made in the EMG signal for further analyzes. The most normal is to use the 1 second window, and in case of short tasks it can easily be done once that the signal is short and it is easy to separate the total task time in 1 second parts. However, for longer tasks it can be really difficult for the researcher to separate a signal of 10 minutes in 600 windows of 1 second each for example. Thus, a study from [14] brought that to use a 5 second window and a 1 second in a cyclic exercise can provide the same result of muscle activity for further analyzes, providing for the EMG researchers an excitement about using the method in long tasks. To bring a better example, the figure 5 under shows us a signal and how it would be analyzed if it was cut in one second windows.

Influence of Different Strategies of Treatment Muscle Contraction

and Relaxation Phases on EMG Signal Processing and Analysis During Cyclic Exercise 105

understanding. The different intensities in severe domain were chosen with the intention to allow us to make affirmations including all domains. Each subject was tested in the same

**Figure 7.** Illustrative representation of the first study, involving signal treatments for RMS obtainment.

Initially it was realized the MWL with initial load in 100W and 20W of increments each minute until voluntary exhaustion, remain a cadence of 70 revolutions per minute (rpm). The MWL was preceded of a warm-up with a load of 50W, with a period of three minutes, follow by three minutes in rest. The MWL was defined as a higher work load maintained for 30 seconds at least, this was assumed so we could make sure to achieve the MWL and not

From the information obtained in the MWL, the subjects were oriented to realize three constant load tests (CLT) in different intensities, these being: submaximal (80%MWL), maximal (100%) and supramaximal (110%). Every test was realized in a cyclesimulator (Computrainer™, Racer Mate®, USA). The tests occur with at least 48 hours between then. The CLT was preceded of three minutes of warm-up with 50W, followed by three minutes of rest. After that the tests occur until exhaustion. The subjects were instructed to keep their cadence in 90 rpm, could not pedal less than that, and the test was interrupted when the subjects reported voluntary exhaustion or showed inability to keep the cadence stipulated

The EMG signal was obtained during all period of realization in CLT using an electromyography with 16 channels, model MP150™ (Biopac System®, USA) with a sampling rate of 2000 samples/second, in agreement with ISEK [15]. Before the beginning of each CLT, the subjects were submitted to asepsis and curettage. The electrodes used were active and bipolar, model TSD 150™ (BIOPAC Systems®, USA), with distance among electrodes fixed in two centimeters, putted above superficial muscles of quadriceps femoral of right leg: vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF), following

the standard of SENIAM [12], as showed by the white circles on the figure 8.

hour of day to minimize the effects of the circadian variations.

the peak load.

on the test. The verbal encouragement was used.

**Figure 5.** An EMG signal divided in 1 second time windows.

In the same signal, the next image has the cuts made in five seconds windows (Figure 6). The biggest importance about using a bigger window is not just because it would be hard for the researcher to divide the signal, but also because some routines that treat the signal don`t accept too much windows to process.

**Figure 6.** The same EMG signal of Figure 5, now divided in 5 seconds windows.
