**5. Biological amplifiers**

In signal acquisition, analyzable information is obtained by studying the physical quantities involved in the activation process. These physical quantities can be measured by sensors that convert them into electrical signals and then record them using a data acquisition system (Figure 3). Computers make data acquisition more efficient and reliable and have the advantage of combining data storage with analysis and processing capability [21].

Application of Surface Electromyography in the Dynamics of Human Movement 399

Gain is defined as the ratio between the voltage that enters and exits the amplifier. Gain should be selected to suit the characteristics of the experiment, the studied muscle, the electrode type and the use planned for the amplified signal. Considering that a sEMG signal has a maximum voluntary contraction amplitude not exceeding 5 mV peak-to-peak (Figure 6), the gain can be adjusted between 10 and 1000x. It is important to choose a gain that does not exceed at any stage the voltage expected from the system, or there will be a risk of either

Filters can be used to remove frequency components that do not belong to the signal or

The captured signal can be filtered by hardware or software. Signal-filtering hardware can be used in the amplification step, while signal filtering by means of software can be

When using surface electrodes to measure EMG signals, interference from various sources can be mixed with the EMG signal. Each type of interference has its own characteristics that must be understood in order to remove it during the measurement phase or during processing. The useful information in the sEMG signal, which is a sum of the waves of varying frequency, is located between 20 and 500 Hz [12]. The signal is reduced due to the filtering effect of tissue located between the muscle fibers and the active sensing surface. The band pass filter corresponds to the frequency between the low frequency (high pass) and high frequency (low pass) cut-offs. Specific frequencies can also be filtered out with what are

**6. Signal amplification** 

**Figure 5.** Appropriate gain range [3].

performed during processing.

called "notch filters" [5, 11, 3, 23].

components that are irrelevant for a given analysis.

**7. Signal filtering** 

losing part of information or damaging the system itself [1].

**Figure 3.** Diagram of a biological signal acquisition system [3].

Sensors and transducers are devices that convert physical quantities into electrical signals or current. Signal conditioners are electronic devices that modify the input signal in some way, whether by amplification, attenuation, filtering or isolation. The EMG signal, for example, enters at an amplitude of μV and must be amplified and filtered [3].

There are basically two techniques capturing an EMG signal: either monopolar or bipolar electrodes. In the monopolar configuration, only one electrode is placed on the skin over the muscle in question (Figure 4). This electrode detects the electrical potential relative to a reference electrode, which is placed in a location unaffected by the electrical activity generated by the analyzed muscle. In the bipolar configuration, two electrodes are used on the muscle as well as a reference (or ground) electrode placed in a neutral location (Figure 5). The human body is actually a good antenna for electromagnetic energy [3].

**Figure 4.** A) Schematic representation of a unipolar amplifier. B) Schematic representation of a bipolar amplifier [3].
