Jitter of the stimulated motor axon.

Electrical microstimulation of motor axons in conjunction with single fiber EMG (SFEMG) is increasingly used to measure the jitter of the motor endplates. This study examines the jitter of the stimulation site on the axon when stimulus strength is at threshold. In the absence of spurious blocking, this was found to be 5 microseconds on the average. With intermittent blocking, however, a mean additional jitter of 40 microseconds was obtained. The latter is considered to result from changing propagation velocity in the muscle fiber due to irregularity of activation rate. In clinical jitter studies, inadvertent threshold stimulation can result in significant error when associated with intermittent blocking.