*3.2.2. Electrode material, size, montage and positioning*

ment. An electromyogram is the expression of the dynamic involvement of specific muscles within a determined range of that movement. The integrated EMG of that same pattern is the expression of its muscular intensity. However, intensity is not always related to force [12].

Mostly sEMG is used to investigate the activity of a series of muscles. The majority of scientists working in sports and occupational contexts measure EMG using surface electrodes [12,15]. Skeletal muscles do not always stay in the same place during complex dynamic movements and the entire muscle belly may not be fully under the skin, but covered by parts of other bellies or tendons and subcutaneous adipose tissue. It needs to be emphasized that the selection of muscles for EMG measurement requires careful consideration. Some of these choices can lead

Many factors may affect the quality of EMG signals; they can be divided into physiological,

Muscle tissue produce electrical potentials due to action potentials. With electrodes placed on surface or in muscle tissue, muscle action potentials can be determined. Several events must occur before contraction of muscle fibers. Central nervous system activity initiates a depola‐ rization in the motoneuron [59,60]. The depolarization is conducted along the motoneuron to the muscle fiber's motor end plate. At the endplate, a chemical substance is released that diffuses across the synaptic gap and causes a depolarization of the synaptic membrane. This phenomenon is called muscle action potential. The depolarization of the membrane spreads along the muscle fibers producing a depolarization wave that can be detected by recording

Preparation of the skin is essential to avoid artifacts and receive an appropriate signal. Before placing the electrodes on skin, it must be ensured that the skin is clean and dry. The skin must be cleaned by using gel, cream or alcohol and then it should be dried [61,62,25]. If necessary, shave excess body hair. Cleansing of the skin is useful to provide EMG recordings with low noise levels. Appropriate preparation of the skin assures the removal of body hair, oils and flaky skin layers and, consequently, reduces the impedance in the electrode-gel-skin interface. Shaving, wetting and rubbing with alcohol, acetone or ether, are often considered for the

Proper skin preparation and electrode positioning are essential elements in acquiring EMG measurements of high quality. Two key strategies govern electrode preparations (1) electrode contact must be stable (2) skin impedance must be minimized. While there are no general rules for skin preparations, the type of application and signal quality sought usually determines the extent of the skin preparation [43]. For example, given a targeted test condition if the movement is somewhat static or slow moving and only qualitative reading are desired, a simple alcohol swab around the area of interest is sufficient [43]. However, if dynamic conditions present risk of the introduction of movement artifacts like in walking, running or other planned accelerated

to erroneous registration, sometimes without being noticed by peer reviewers [12].

physical, and electrical types. Some factors can be controlled by the investigator [58].

**3.2. Origin of the EMG signal**

184 Electrodiagnosis in New Frontiers of Clinical Research

electrodes [60].

*3.2.1. Skin preparation*

cleansing of the skin [25].

Surface EMG is a helpful technique for the analysis of muscle activity. However, its efficacy is related to the correct electrode positioning, the adequate skin preparation and opportune recording instrumentation. In addition, it is mandatory to recognize artifacts which may alter EMG signals and choose a particular filtering procedure before any additional analysis [63].

Surface electrodes are usually made of silver/silver chloride (Ag/ AgCl), silver chloride (AgCl), silver (Ag) or gold (Au). Electrodes made of Ag/AgCl are often preferred over the others, as they are almost nonpolarizable electrodes, which mean that the electrode-skin impedance is a resistance and not a capacitance [25]. Therefore, the surface potential is less sensitive to relative movements between the electrode surface and the skin. Additionally, these electrodes provide a highly stable interface with the skin when electrolyte solution (for example gel) is interposed between the skin and the electrode [25]. Such a stable electrode-skin interface ensures high signal to noise ratios (for example the amplitude of EMGs exceeds fairly the noise amplitude), reduces the power line interference in bipolar derivations and attenuates the artifacts due to body movements [64,25]. The electrode should be placed between a motor point and the tendon insertion or between two motor points, and along the longitudinal midline of the muscle. The longitudinal axis of the electrode should be aligned parallel to the length of the muscle fibers. When an electrode is placed on the skin, the detection surface comes in contact with the electrolytes in the skin [65]. A chemical reaction takes place which requires some time to stabilize, typically in the order of a few seconds if the electrode is correctly designed. But, more importantly, the chemical reaction should remain stable during the recording session and should not change significantly if the electrical characteristics of the skin change from sweating or humidity changes. Given the high performance and small size of modern day electronics, it is possible to design active electrodes that satisfy the above requirements without requiring any abrasive skin preparation and removal of hair [65].

In localizing the site of detection of the electrode on the skin, a variety of approaches has been applied: (1) over the motor point; (2) equidistant from the motor point; (3) near the motor point; (4) on the mid-point of the muscle belly; (5) on the visual part of the muscle belly; (6) at standard distances of osteological reference points and (7) with no precision at all with respect to its placement [12].
