Electromyographic bursting following the cortical silent period induced by transcranial magnetic stimulation

In subjects performing voluntary background contraction, transcranial magnetic stimulation (TMS) induces an interruption of electromyographic (EMG) activity known as the silent period (SP). This is thought to be mediated through the action of inhibitory cortical neurons, in particular involving γ-aminobutyric acid type B (GABA(B)) receptors. In some studies of the SP, a post-SP increase in EMG activity has been reported but not described in detail. In the present study we have sought to determine the presence and persistence of late EMG bursting associated with the return of voluntary drive after the SP, and to characterize the relationship to background contraction level, stimulus intensity, and SP duration. TMS was delivered at 3 levels of intensity (120, 140 and 160% of active motor threshold) and during 3 levels of voluntary contraction of the first dorsal interosseous muscle (10, 30 and 50% of maximum contraction) in a pseudo-randomized order in 11 healthy participants. The SP was followed by a brief (~60 ms) burst of EMG up to 290±42% of the pre-stimulus EMG level. Both SP duration and the amplitude of the EMG burst increased with TMS intensity (p<0.001). Burst amplitude correlated with SP duration (r2=0.750; p=0.003). We conclude that post-SP EMG bursting is a quantifiable phenomenon that depends on the strength of TMS and the duration of the SP. This bursting may correspond with the post inhibitory period of disinhibition that has recently been identified in human motor cortex.