TMS-responses during anticipatory postural adjustment in bimanual unloading in humans.

Transcranial magnetic brain stimulation (TMS) was used to assess the influence of the corticospinal system on motor output during forearm unloading in humans. Unloading was obtained either "passively" by the experimenter, or "actively" with the subjects' own contralateral arm. Anticipatory postural adjustments consisted of changes in the activity of a forearm flexor muscle prior to active unloading of the limb and acted to stabilize the forearm position. Motor evoked potentials (MEPs) were recorded in the forearm flexor at different times during active and passive unloading, static forearm loading, and during lifting of an equivalent weight by the contralateral arm while the ipsilateral forearm was statically loaded and held stationary. In active unloading, MEP amplitude decreased with the decrease of muscle activity. Passive unloading resulted in a similar decrease of MEP as with active unloading. During stationary forearm loading, the change in MEP corresponded to the degree of loading. If during static loading the contralateral arm has lifted a separate, equivalent weight, the amplitude of MEP decreased. A possible role of direct corticospinal volley and the motor command mediated by subcortical structures in anticipatory postural adjustments is discussed.