Stance does interfere with movement related brain potentials.

Movements of the upper limb are well known to be preceded by electromyographic activity in leg and trunk muscles known as postural preadjustment. Brain potentials preceding the actual commencement of voluntary displacement were also described. The relationship between the occurrence of these two electrophysiological phenomena was investigated by performing two types of movement in three different body positions. Healthy subjects were free standing, sitting and standing with backsupport. When fast forward arm elevation was executed, the preceding Bereitschaftspotential was higher in amplitude at 500 msec before EMG onset and alongside midline scalp leads during sitting. The amplitude at 150 msec, immediately preceding movement, did not show significant differences related to body positions and there was no contralateral dominance. When index finger flexion was performed, it did not yield differences in the Bereitschaftspotential between sitting and standing positions. These findings suggest the existence of two separate stages of the preparatory process and an influence of the muscle mass involved in the stance perturbation during voluntary movement.