Characteristics of the silent period after transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) of human cortex during voluntary muscle contraction produces a transient period of inhibition (i.e., silent period, SP) in the electromyographic (EMG) activity. The duration of the SP in relation to the level of muscle force (10%, 50% and 100% of maximum voluntary contraction) as well as possible cumulative effects of sequential TMS on the SP were studied. Methodologic problems were encountered in defining the SP and thus the duration of both an absolute (complete EMG silence) and relative (return of uninterrupted EMG activity) SP was measured. In all subjects, shortening of the SP duration occurred in relation to an increase in force when the criterion for absolute SP was used. Conversely, the relative SP duration suggested a trend toward prolongation with increasing force of contraction. No cumulative effects of TMS were observed on the absolute SP duration, whereas two subjects showed a cumulative effect of TMS on the relative SP. We conclude that the effect of muscle force and sequential TMS on the SP duration is dependent on the methods used to measure the SP. It is therefore essential to agree on methodology before SP measurements are clinically useful.