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Lateva ZC, McGill KC. Estimation of motor-unit properties by morphological analysis of motor-unit action potential waveforms. 11th Intl Congr Electromyogr Clin Neurophysiol, Prague, Czech Republic, 1999.
Computer simulations predict that motor-unit action potentials (MUAPs) recorded by monopolar needle electrodes have distinct features marking the four stages of electrical activity in the MU. First, the initiation of the action potential volley at the endplates is marked by the abrupt onset of the MUAP. Second, the propagation of the volley past the electrode is marked by the MUAP spike. Third, the termination of the volley at the muscle/tendon junction is marked by a distinct terminal wave. Finally, the slow repolarization of the muscle fibers is marked by a slow afterwave. The onset, terminal wave, and slow afterwave can have small amplitudes and have previously received little attention. We have developed a method for averaging MUAP waveforms from moderate EMG interference patterns that is sufficiently accurate to allow these features to be detected reliably. Our results show that these features contain useful information about MU anatomical and physiological properties. The MUAP onset reflects the synchronous activation of all the MU fibers, and its amplitude is related to the number of fibers. The latencies of the onset and terminal wave with respect to the MUAP spike are direct measures of the endplate-to-electrode and electrode-to-tendon propagation times, and thus provide an estimate of the relative location of the endplate and muscle/tendon junction with respect to the electrode. By analyzing multiple simultaneously recorded MUAPs we have found that neighboring MUs often have different spatial organizations which can be attributed to multiple endplate zones, intramuscular aponeurosis, or pinnation. Finally, the slow afterwave persists several milliseconds after the terminal wave, making clinical MUAP duration measurements similar for long and short fibered muscles. This type of morphological analysis of the MUAP promises to be informative not only for studying normal MU properties, but also for evaluating MU reorganization in disease.