engleski

The overview of application of surface electromyography in clinical practice

The most frequent applications of surface EMG are determining the activation timing of muscles, establishing the force/EMG signal relationship and using the EMG signal as the fatigue index. For determining the activation timing of muscles, it does not matter if the contraction is isometric or anisometric. It is only relevant to determine if any segment of the muscle in the vicinity of the electrode is active. This is effectively accomplished by determining if the EMG signal originates from the muscle of interest. The issue of crosstalk from other adjacent muscles is important in this case because the amplitude of the signal being analyzed is low and near the noise level. To eliminate the cross talk, the double differential method can be used or an approximation, where a crosstalk signal will always have a lower frequency spectrum because it originates further away and will be subject to additional low-pass filtering due to spatial filtering. The delay between the EMG signal and the force is a variable that depends on several factors, including the fiber-type composition of the muscle, the firing rate dynamics of the muscle, and the viscoelastic properties of the muscle and tendon tissues (including their length.) The force-EMG signal relationship has become very important since the method began its evolution towards quantification. If a quantitative relationship between the EMG signal and force is being determined, then the contraction must be isometric. When a motor unit is recruited, it contributes a quantum of force to the muscle contraction; however, the contribution to the EMG signal amplitude is dependent on the proximity of the detection surfaces of the electrode to the nearest fibers of the recruited motor unit. Thus, the vector representing the incremental increase may increase or decrease the instantaneous slope of the force-EMG signal relationship. A newly recruited motor unit will increase its firing rate as the force demand increases. The tetanization force increases rapidly as a function of the increasing firing rate, whereas, the contribution to the amplitude of the EMG signal increases less rapidly. Thus, as in the previous case, the vector representing the incremental increase may increase or decrease the instantaneous slope of the relationship depending on the firing rate value with respect to its dynamic range. The use of EMG signal as a fatigue index is very simple. It is based on the spectral modification which occurs in the EMG signal during sustained contractions. The muscle fatigue index is represented by the median frequency of the spectrum. Combining these applications with nerve conduction velocities and reflexologic studies covers most of the neuromuscular diseases, although the signal can be interpreted only if we are aware of what we are dealing with and how to overcome the limitations of the method itself. The method is painless, well-suited for children and sensitive patients and decreases the risk of infection for both patient and examiner.

surface electromyography; clinical practice

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