**2.2. The Motor Unit Action Potential (MUAP)**

up of development and performing corrections where necessary [5,12,13]. The electromyo‐ graphical analysis can determine muscle activation and fatigue and thus helps achieve

Muscle activation is a result of the effort of muscle but the relationship between EMG activity

Surface EMG in current sports studies also deals with determination and descriptions of the

Muscles are designed to exert force in order to move the body. Skeletal system and muscles are connected to each other by tendons. Combination of muscle and bone is brought about by the tendon intermeshing with the skeletal periosteum sheath. Tendons are the strong connec‐ tive tissue composed of three layers. And this extends the length of all the muscle and collagen protein. Epimysium, perimysium and endomysium are the connnective tissues forming each tendon. There are three types of muscle tissue in the body, they are smooth, cardiac and skeletal muscles [16,17]. Specific anatomical features that affect the length of muscle fiber, muscle fiber type and muscle compartments may differ between muscles. EMG signals may be affected by them and therefore EMG recordings and interpretation of them must take anatomical differ‐

Smooth muscle is found in the digestive tract, surrounds the blood vessels, airways and respiratory systems. Smooth muscle is innervated by the autonomic nervous system such as cardiac muscle and therefore we do not have voluntary control over its contractions [20,17].

Cardiac muscular system is located in the heart tissue and has striped appearance under light microscopy. The same striations are also found in skeletal muscle and indicate the presence of

Skeletal muscular system has the only muscle type that can be voluntarily contracted and skeletal muscle has active elements forming the movement. The human body consists of more than 600 muscles [21,17]. The functions of the muscular system are movement of blood and food within the body, the ability to stop the body moving, to store oxygen and nutrients such as glycogen for energy production and, through the energy production reactions, to produce

different proteins required for muscle contraction [21,17].

development of performance [9].

176 Electrodiagnosis in New Frontiers of Clinical Research

and effort is only qualitative [5].

muscle types [14,15].

**2. Muscular system**

ences into account [18,19].

**2.1. Muscle types**

*2.1.1. Smooth muscle*

*2.1.2. Cardiac muscle*

*2.1.3. Skeletal muscle*

Motor units are the functional assets of the neuromuscular system. Each motor unit consists of a single motoneuron and the muscle fibers supplied by its axonal branches [24,25]. Once a motoneuron discharges, action potentials are generated at its neuromuscular junctions and then propagate along all the muscle fibers, toward the tendon regions. The summation of these potentials is termed motor unit action potentials and is responsible for the muscle contraction [25]. MUAP is the sum of the extracellular potentials of muscle fiber action potentials of a motor unit [3]. The waveform is determined by the natural properties of the relationship between muscle fibers. [24,3]. The extracellularly recorded MUAP, recorded along the length of the muscle fibers and away from the endplate region, has a triphasic waveform. The initial positive deflection represents the action potential propagating towards the electrode. As the potential passes in front of the electrode the main positive-negative deflection is recorded. When the action potential propagates away from the electrode the potential returns to the baseline. Slight repositioning of the electrode causes major changes in the electrical profile of the same motor unit. Therefore, one motor unit can give rise to MUAPs of different morphology at different recording sites. If the electrode is placed immediately over the endplate area, the initial positive deflection will not be recorded and the potential will have a biphasic waveform with an initial negative deflection [3]. All the muscle fibers of a motor unit work in unison; that is, all are discharged nearly synchronously upon the arrival of a nerve impulse along the axon and through its terminal branches to the motor end plates. A MUAP is recorded by a needle electrode. The recorded motor unit action potential can be derived from action potentials of a small number of muscle fibers, a moderate number of muscle fibers, or a great majority of muscle fibers belonging to the motor unit [26].

## **2.3. Types of muscle action**

Among the variety of types of muscle action are the isometric, concentric and eccentric; all three forms occur during the actions seen in sport and exercise performance [27]. When there is no change in muscle length during muscle activation, the action is called isometric. Isometric action occurs when an athlete tries to leg-press a heavy load by flexing the quadriceps muscles, but cannot move the weight-stack in spite of a maximum effort. The muscle produces force, but it is insufficient to overcome the mass of the weight stack; hence, the overall muscle length does not shorten. Isometric muscle action occurs when the muscle contracts without moving, generating force while its length remains static. Isometric muscle actions are demonstrated in

studies provide unique quantitative information about neurological function in patients with a variety of neuromuscular disorders [31]. A nerve is stimulated at one or more sites along its course and the electrical response of the nerve is recorded. EMG testing involves evaluation of the electrical activity of a muscle and is one of the fundamental parts of the electrodiagnostic medical consultation. It is both an art and a science. It requires a thorough knowledge of the anatomy of the muscles being tested, machine settings and the neurophysiology behind the testing [2]. Obtaining the information produced by active muscle provides information about the activities of motor control centers [30,26]. This can be achieved invasively, by wires or needles inserted directly into the muscles, or noninvasively, by recording electrodes placed over the skin surface overlying the investigated muscles. The use of this latter modality is preferable in healthy voluntary sedentary subjects and in athletes, despite its limitations and drawbacks. To mention just a few of them, single-channel sEMG signals provide average information on the activity of many concurrently active motor units, the reproducibility of the results is often difficult, and standard recording procedures are still confined to few labora‐ tories, therefore limiting comparisons among results obtained by clinical researchers [26]. EMG signal recordings used for years in bio-engineering, occupational- and sports medicine, physiotherapy, sports biomechanics, and also eventually for trainers and coaches [19,32]. Since the end of the 1960s there has been a development in miniaturized telemetric devices for monitoring complex human movements remotely. Especially for kinesiological purposes, the telemetric devices have recently been changed from two-channel registrations to eight or morechannel systems [32]. EMG is a seductive muse because it provides easy access to physiological processes that cause the muscle to generate force, produce movement, and accomplish the countless functions that allow us to interact with the world around us. The current state of surface EMG is enigmatic. It provides many important and useful applications, but it has many limitations that must be understood, considered, and eventually removed so that the discipline is more scientifically based and less reliant on the art of use. To its detriment, EMG is too easy to use and consequently too easy to abuse [5]. Electromyographic recordings are performed with intramuscular needle electrodes. However, surface electrodes are used in the study of sports science. Most of the issues affecting this modality have already been covered. Electrodes are almost always sited along the body of the muscle in question, with locations one-third and two-thirds along the length being the norm, As mentioned earlier, small pre-amplifiers are often used in order to improve signal-to-noise ratios, especially since telemetry of signals is increasingly used in order to maintain ecologically valid movement patterns [30,33,1,34]. Once the signal is filtered and amplified, some form of rectification of the signal is usually applied. As with other indices, examination of the raw signal waveform is interesting but offers little in the way of empirically analyzable data. Accordingly, and since the signal is made up of both positive and negative potentials, signals may be rectified by either ignoring all negative signals or reversing their polarity so that all signals are positive. Further signal conditioning may involve totaling activity across a regular time base, resetting counters to zero in order to provide an integrated signal. Analysis may look at amplitude or, more rarely, frequency. Increasingly, however, signal patterns are compared across two or more conditions. Thus, investigators may contrast "at rest" with active patterns, or use an increase from baseline measure, or contrast signals obtained under different execution conditions such as variations

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**Figure 1.** When a skeletal muscle fiber is activated by a MUAP, a wave of electrical depolarization travels along the surface of the fiber (drawn by Sözen H.).

an attempt to lift an immovable object or an object that is too heavy to move. The muscle fibers contract in an attempt to move the weight, but the muscle does not shorten in overall length because the object is too heavy to move [28,23,29]. Concentric muscle action occurs when the muscle force exceeds the external resistance, resulting in joint movement as the muscle shortens. Concentric action occurs when a muscle is active and shortening; for example, during the biceps curl the biceps shortens and exerts enough force to lift the barbell [23,29]. Eccentric muscle action occurs when the external resistance exceeds the force supplied by the muscle, resulting in joint movement as the muscle lengthens; for example, when lowering the barbell the biceps exerts force to ensure the movement is controlled. This is often referred to as the negative portion of the repetition. Even though the fibers are lengthening, they are also in a state of contraction, permitting the weight to return to the starting position in a controlled manner. During an eccentric action, an activated muscle is forced to elongate while producing tension [27,23,29].
