Motor unit action potential (MUAP) morphology and needle electromyography (EMG) position

The position of the EMG needle influences the morphology of the recorded MUAP. To properly assess MUAP parameters, the major spike must be as steep as possible, indicating the proximity of the needle to the motor unit. Note that needle electrode position E3 has the shortest major spike rise time and is the preferable position in which to assess the MUAP. Also note that although MUAP amplitude changes markedly with needle position (compare position E1 with E3), duration is relatively unaffected. (From D Dimitru, JA DeLisa. AAEM minimonograph #10: volume conduction. Muscle Nerve 1991;14:605. Reprinted by permission of Wiley.)