Interpretation of the Electromyogram

IN RENEWING electromyographic investigations of human muscles after an interval, I find that considerable differences of opinion have accumulated in regard to the interpretation of several electromyographic changes encountered in neurologic disorders. Since electromyography is in common use in some clinics and can provide useful diagnostic information, it may be of general interest to discuss critically some of the fundamental principles involved in the method. Early work on the action potentials of muscle has been reviewed by others,1and Pennybacker and I2previously discussed at length the relation of action potentials of single muscle fibers to those series of much larger rhythmic variations which are associated with the natural discharge of a single motor nerve cell. Physiologically, the "motor unit" is defined as a motor cell, its axon process and the group of muscle fibers which this one cell innervates. The rhythmic pattern of electrical changes (action potentials)