Coordination between posture and movement in a bimanual load lifting task: putative role of a medial frontal region including the supplementary motor area.

The aim of the present experimental series was to investigate the role of the medial frontal region including the supplementary motor area in the coordination between posture and movement in a bimanual load lifting task. The seated subject was instructed to maintain in a horizontal position one forearm (postural arm) which was loaded with a 1 kg weight. The unloading was performed either by the experimenter (imposed unloading) or by a voluntary movement of the other arm (voluntary unloading). In normal individuals, with the voluntary unloading, the movement control was accompanied by an anticipatory adjustment of the postural forearm flexor activity, which resulted in the maintenance of the forearm position despite the unloading. The anticipatory postural adjustments were impaired in 4 out of 5 patients with unilateral lesion of the SMA region; the defect was observed mainly when the postural forearm was contralateral to the lesion. No change in the anticipatory postural adjustment was observed in one patient with complete callosal section. This finding indicates that the coordination between the posture and movement in this task is not organized through callosal fibers linking the cortices on both sides but rather at a subcortical level. The anticipatory postural adjustments were abolished in two patients with spastic hemiparesis when the postural forearm was the spastic arm. It is suggested that the SMA region contralateral to the postural forearm, together with other premotor or motor areas, may select the circuits responsible for the phasic postural adjustments which are necessary to ensure postural maintenance, whereas the motor cortex contralateral to the voluntary movement controls both the movement and, via collaterals, the preselected circuits responsible for the associated postural adjustments.