Interaction of random electromyographic activity with averaged sensory evoked potentials

We averaged sensory nerve action potentials (SNAP) of the median nerve recorded at the wrist, forearm, and elbow with up to 6,000 trials in the presence of quantitated levels of background EMG activity in six normal volunteers. The SNAP could be recorded reproducibly with averaging when the EMG amplitude was up to 50 times the SNAP amplitude. EMG amplitudes of greater than 100 times the SNAP amplitude produced continuous variation in the averaged waveform that did not stabilize, probably because of the quasi-random, large, triphasic potentials that make up the EMG. Monitoring and reduction of background EMG activity can improve reliability of somatosensory evoked potential recording.