Involvement of the motor cortex in the bimanual unloading reaction: a transcranial magnetic stimulation study

The responses of the biceps brachii muscle of the upper arm to magnetic stimulation of the motor cortex during the postural pretuning and forearm unloading tasks were studied in humans. On active unloading, the amplitude of the evoked response decreased in parallel with a decrease in muscle activity. During stationary holding of the load, the muscle response changed in proportion to the load. When, on the background of stationary holding of the load, the other arm took on the same load, the amplitude of the evoked response in the biceps muscle of the arm holding the load decreased without any change in the muscle activity. Passive unloading was accompanied by similar changes in the response evoked by magnetic stimulation as seen with active unloading. The question of whether the decrease in muscle activity (postural pretuning) in active unloading may be associated with both direct corticospinal influences and influences mediated via subcortical structures is discussed.