Macroelectromyography in reinnervation

Macroelectromyography is used to study the electrical activity of the entire motor unit. The amplitude of the motor unit action potential (MUP), recorded from a bared cannula, is dependent on the number and size of muscle fibers in the motor unit. In most situations of reinnervation the local fiber density (FD) as recorded with single fiber EMG, and the total motor unit size as indicated by the macro-EMG amplitudes, are increased, that is, denser motor units with a total increase in their number of fibers. In some situations, increased FD, but normal macro-MUPs are seen, which may reflect localized reinnervation in one area of the motor unit and a dropout of fibers in another area without on overall increase in number of muscle fibers in the motor unit. Rarely, a combination of increased macro-MUP amplitude and normal FD are seen, which may indicate a dropout of small motor units. By combining SFEMG and macro-EMG, new information is obtained concerning the reinnervating motor unit.