On the selection of concentric needle electromyogram motor unit action potentials: is the rise time criterion too restrictive?

Concentric needle electromyogram motor unit action potentials (MUAPs) were recorded from the biceps brachii muscle of normal subjects and in patients with neuromuscular diseases. Although the MUAPs had a crisp sound and appeared sharp, their rise time (RT), measured from the maximum negative peak to the preceding maximum positive peak before it, was often > 500 microseconds. All MUAPs with a RT < or = 500 microseconds were recorded from within the motor unit (MU) territory. MUAP recordings from outside the MU territory had a long RT but also low amplitude (< 50 microV) and/or a characteristic initial negative deflection. In the remaining recordings from within the MU territory, MUAP duration remained relatively constant while MUAP amplitude and RT varied inversely with each other. These MUAPs may be useful in electrodiagnosis but discarded due to their longer RT. We feel that while the current RT criterion for MUAP selection ensures that the electrode tip is within the MU territory, it is also too restrictive.