There are many reasons for recording muscle activity. Physical therapists record muscle action to document disabilities, to measure therapeutic progress, and to evaluate orthotic and prosthetic devices. The psychologist studying learning measures muscular activity in order to record the development of motor skills. The psychophysiologist also records muscle action, often when no movement occurs, as in the case of tension headaches or, more generally, to study the patterns of bodily reaction to stimulation, as in the startle response. One of the most significant uses of our muscles is to communicate emotional expressions. Another use of muscles is to help us move quickly, as when we are startled. Today, many psychophysiologists use the startle reflex as a way to measure emotionality. This chapter discusses the use of electromyography (EMG) to study muscles, the physiological basis of EMG, psychophysiological recording procedure, electrodes and electrode placement, typical recordings, common problems associated with EMG, analysis and quantification, and requirements for amplification of EMGs.
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