Anticipatory postural adjustment: the role of motor cortex in the natural and learned bimanual unloading.

Anticipatory postural adjustment (APA) during bimanual action is observed when participants hold an object in one hand and then lift that object with the other hand. The decrease in activity of a forearm flexor muscle prior to an active forearm unloading acts to stabilize the forearm position. Recent studies have investigated the influence of the corticospinal system on muscle activity during APA through transcranial magnetic stimulation. It was shown that at different times during APA, the amplitude of motor-evoked potentials in the forearm flexors decreased in conjunction with the decrease of muscle activity. If the unloading is triggered via an electromagnet by lifting an equal weight by the other arm, the anticipatory postural adjustment is learned through the repetition of unloading (three series of 20 trials). Using the transcranial magnetic stimulation, we examined changes in the motor-evoked potential in the forearm flexors before and after APA learning. Motor-evoked potential amplitude did not significantly change as forearm flexor activity decreased. The motor-evoked potential/background electromyogram ratio, however, increased in the final learning session in comparison to the initial learning session and stationary loading. The present results corroborate a hypothesis on the fundamental role of the motor cortex in the suppression of synergies that interfere with the execution of the new coordination in the process of motor learning.