Location of Electrodes in Surface EMG

Motor unit action potentials (MUAPs) from motoneurons are transmitted to muscles through end-plates and then propagated to the tendons. These bioelectrical signals are detected via electromyography (EMG), which is performed using electrodes. The electrodes used in EMG are primarily surface electrodes and inserted (wire or needle) electrodes, of which surface and wire electrodes are mainly used for kinesiological studies. Surface electrodes are widely used because of noninvasive attachment, painless usage, suitability for detecting muscle activation by generation of EMG signals and simplicity, although detection is usually limited in surface muscles. Surface EMG is a practical and noninvasive procedure that has potential usage in sports and rehabilitation medicine. The signal amplitude of surface EMG is analyzed to estimate the level of muscle contraction, while the frequency component is used to estimate performance of muscle activation. For example, a change in EMG signal amplitude is regarded as a change in the strength of muscle activation, and a shift of the surface EMG signal towards a lower mean frequency is correlated with decreasing muscle fiber conduction velocity due to muscle fatigue. However, the detected EMG signal amplitude and mean frequency are influenced by the location of surface electrodes, although the action potentials in a muscle are generated at the same time. For these reasons, the location of surface electrodes is very important for accurate evaluation of muscle activation. In this chapter, the propagation or conduction of action potentials is illustrated to understand the EMG signal recorded by surface electrodes. Proper electrode locations are suggested with theoretical and practical methods.