Silence of electromyographic (EMG) activity after an evoked muscle twitch has been studied extensively. However, different criteria have been used to determine the level at which the silence of muscle activity ends, and the purpose of this work was to develop an objective method able to determine the silent period even when data were acquired using different EMG instruments. The silent period evoked by transcranial magnetic stimulation was determined bilaterally From abductor pollicis brevis (APB) muscles in 11 subjects, from trapezius muscles in 9 subjects, and from sternocleidomastoid muscles in 11 subjects. AII subjects were healthy controls and gave their informed consent to participate in the study. Muscle activity was recorded via surface electrodes. Recordings from 10 stimuli were rectified, averaged, and plotted logarithmically by dividing the mean of the prestimulus EMG activity into the whole trace. Plotted in this way, the one-level represents the mean rectified EMG amplitude of the prestimulus activity. The end of the silent period was measured automatically as the moment at which Student's t test was no longer significantly different, when testing the window of mean prestimulus EMG activity with respect to a 4-ms window centered around the assumed end of the silent period (resolution 0.1 ms). The mean silent periods were 183.7 +/- 49.8 ms for APB, 194.2 +/- 28.8 ms for trapezius, and 194.8 +/- 73.6 ms for sternocleidomastoid muscles, measured from the M-response latency (mean latency: APB 19.7 +/- 1.9 ms, trapezius 7.8 +/- 0.6 ms. and sternocleidomastoid 6.7 +/- 0.8 ms). Computer-aided measurement proved to be a fast and objective tool able to standardize determination of the silent period.
