Some Factors Affecting the Reliability of Surface Electromyography

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Abstract
The use of surface electrodes for measuring the action potentials accompanying muscle contraction requires high-grain amplifiers having flat frequency response and high input impedance. Some polygraphs currently in use in "muscle tension" experiments fail to fulfill these requirements, having at high gain, output voltages which are a function of the frequency of the signal source at constant input voltage. A number of studies suggests that under certain circumstances this could be a significant source of error; further study of frequency variations of muscle potentials is required. Methods of measuring suitably amplified signals and the relationship of these measurements to muscle contraction are considered. The studies reviewed indicated the need for caution in comparing observations made on individual subjects and among different muscle groups.