Suppression of EMG activity by transcranial magnetic stimulation in human subjects during walking

Abstract

2. TMS of low intensity (below threshold for a motor-evoked potential, MEP) produced a suppression of ongoing EMG activity during walking. The average latency for this suppression was 40·0 ± 1.0 ms. At slightly higher intensities of stimulation there was a facilitation of the EMG activity with an average latency of 29.5 ± 1.0 ms. As the intensity of the stimulation was increased the facilitation increased in size and eventually a MEP was clear in individual sweeps.3. In three subjects TMS was replaced by electrical stimulation over the motor cortex. Just below MEP threshold there was a clear facilitation at short latency (~28 ms). As the intensity of the electrical stimulation was reduced the size of the facilitation decreased until it eventually disappeared. We did not observe a suppression of the EMG activity similar to that produced by TMS in any of the subjects.4. The present study demonstrates that motoneuronal activity during walking can be suppressed by activation of intracortical inhibitory circuits. This illustrates for the first time that activity in the motor cortex is directly involved in the control of the muscles during human walking.

Specific interpretation of augmented feedback changes motor performance and cortical processing

“…To avoid fatigue, a break of 3 min was given between the trials. To analyze whether TMS caused a facilitation or suppression in the FDI EMG, the rectified and then averaged 40 sweeps without stimulation (control EMG) were subtracted from 40 sweeps with stimulation (Davey et al 1994;Petersen et al 2001;Zuur et al 2010;Lauber et al 2012). The intensity of the TMS during the first trial was chosen to be high enough to evoke MEPs in the FDI (0.9 times motor threshold at rest).…”

“…After this block, subjects were asked to relax and the stimulation intensity was then gradually decreased before the next block started after a pause of 3 min. In this manner, the stimulus intensity was further decreased until a suppression of the EMG was visible without the presence of any facilitation (Petersen et al 2001;Zuur et al 2010;Lauber et al 2012). This adjustment served to reveal the maximal suppression by subthreshold TMS in all conditions.…”

Postural challenge affects motor cortical activity in young and old adults

“…Rest periods of 30 s were given between blocks and rest periods of 5 min between conditions. As in previous studies using subTMS (Davey et al, 1994;Petersen et al, 2001;Zuur et al, 2010), two young and five old adults were excluded because they did not exhibit suppression without a preceding facilitation in all conditions.…”

“…The subTMS method was first described by Davey et al (1994), and subsequent experiments using a variety of techniques supported the idea that the TMS-induced EMG suppression originates from activation of intracortical inhibitory circuits, reducing motor cortical output (Davey et al, 1994;Petersen et al, 2001;Di Lazzaro et al, 1998;Classen and Benecke, 1995). For example, transcranial electrical stimulation, thought to activate the axons of the corticospinal neurons directly, does not suppress ongoing EMG (Petersen et al, 2001).…”

“…For example, transcranial electrical stimulation, thought to activate the axons of the corticospinal neurons directly, does not suppress ongoing EMG (Petersen et al, 2001). Moreover, subTMS produces no recognizable descending volleys measured with high cervical, epidural electrodes (Di Lazzaro et al, 1998).…”