*3.1.1. The limitations of sEMG*

**•** Ergonomics Design **•** Product Certification

182 Electrodiagnosis in New Frontiers of Clinical Research

**•** Movement Analysis

**•** Sports Rehabilitation

**•** Athletes Strength Training

**•** Motion analysis [41,42,43]

ment of the pedal revolution.

Although the noninvasive nature of surface EMG makes this technique ideal for clinical use and research, EMG data can be variable, which raises questions about the reliability of this technique [44]. Repeatability of EMG data is established for many isometric exercises but less is known about the reliability of this method of analysis during dynamic exercise, particularly ballistic movements [45,46,44]. Most studies assessing EMG reliability of data in dynamic movements have examined slow, controlled tasks, such as resistance training exercises or gait. Therefore, evaluation of the reliability of EMG during ballistic tasks is essential to determine the viability of this methodology for clinical and research applications [47,48-49-50,44]. Surface EMG, sometimes called kinesiological electromyography, is the electromyographic analysis that makes it possible to obtain an electrical signal from a muscle in a moving body [41]. It has to be added, by way of clarification, that according to this definition its use is limited to those actions that involve a dynamic movement. Nevertheless, it is also applicable to the study of

The visual systems employed for motion analysis of cycling, even though scientific and accurate, can only indicate the apparent movements. It is often necessary to know how the movements are actually performed against a resistive load. For this reason electro‐ myographic techniques are employed in conjunction with biomechanical analyses [51]. EMG is generally used to indicate which muscle groups are active during a given seg‐

Surface electrodes are usually attached to the muscle groups to be studied. The action poten‐ tials generated are recorded during the pedaling action, thereby allowing the researcher to gain a more complete insight into the muscles employed and the extent of their involvement while pedaling [22]. Within EMG, a particular specialty has been developed wherein the aim is to use EMG for the study of muscular function and co-ordination. This area of research is usually called kinesiological EMG [52,47]. The general aims of kinesiological EMG are to analyze the function and co-ordination of muscles in different movements and postures, in healthy subjects as well as in the disabled, in skilled actions as well as during training, in humans as well as in animals, under laboratory conditions as well as during daily or vacational activities [52]. This is usually used by a combination of EMG, kinesiological and biomechanical measurement techniques [52,47]. Because there are over 600 skeletal muscles in the human body and both irregular and complex involvement of the muscles may occur in neuromuscular

static actions that require a muscular effort of a postural type [41].

Sports Science **•** Biomechanics

> Because of the characteristics of electrodes used, sEMG enables us to study different muscles at the same time, without any inconvenience to the individual, with the advantage that the majority of sEMG equipment can accommodate different inputs simultaneously [55,41]. It also allows greater reproducibility of the traces obtained in different recordings. In addition, the recording obtained is more representative of the muscle as a whole rather than of a particular area. Nevertheless, as already discussed, obtaining traces that provide less information regarding the characteristics of the MUAPs is a limitation in those cases where this particular type of examination is of specific interest [41].

> Another limitation is the fact that in some dynamic actions there can be displacement and modification of the volume of the muscle being analyzed. A change in the relative position of the muscle in relation to the electrode means that the same spatial relationship is not main‐ tained between them, which affect the intensity of the signal that is recorded. Because of this, the best conditions for carrying out an sEMG, depending on the use and application required, are those that are similar to those needed for an isometric type of study [5,56,57,11].

> The majority of activities in sport and occupational settings involve complex movement pat‐ terns often complicated by external forces, impacts and the equipment used during the move‐

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].

movements, a thorough preparation is required. Some EMG systems have built in impedance checking circuit that sends an imperceptible burst of current through the electrodes and controlled measurements are correlated to a known impedance level to indicate the quality of

Surface Electromyography in Sports and Exercise

http://dx.doi.org/10.5772/56167

185

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

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

EMG enables us to record muscular activity, and it is often advisable to carry out a synchron‐ ized cinematic measurement at the same time. In this way, the two types of data can be

the electrode contacts [43].

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

any abrasive skin preparation and removal of hair [65].

placement [12].

**4. Analysis of a movement**

contrasted and it is possible to establish:

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

Many factors may affect the quality of EMG signals; they can be divided into physiological, physical, and electrical types. Some factors can be controlled by the investigator [58].
