Upright stance on a single vs double seesaw: are automatic and voluntary components similarly involved in balance...

The relevance of seesaw devices in postural evaluation and training is gathering growing evidence due to its sensory-motor specificity. Nonetheless, the physiological specificities resulting from the dissociation or not of the seesaws (single vs double) still need to be investigated, in particular by assessing the respective contribution of automatic and voluntary components in the postural control. A protocol based on attention disturbance through a dual-task paradigm was set to establish this contribution. The general assumption was that the larger the dual-task effects, the larger the voluntary component contribution. Based on the larger postural sway induced by the dissociated seesaws, it is expected that the larger dual-task effect occurs in that latter case. The subjects were required to stand with eyes closed on solid ground (SG), a single (SS) and a double (DS) seesaw device while mentally solving or not a navigation task. The movements of the seesaw, placed on a double force platform, were assessed through a frequency analysis of the resultant center-of-pressure displacements along both mediolateral and anteroposterior axes. A larger contribution (p < 0.05) of the voluntary component was observed when subjects were standing on the DS seesaw, especially along the mediolateral axis. This trend is thought to prepare for a step initiation and would be related to the threat encountered by the subjects. In contrast, performing a dual-task protocol does not affect the antero-posterior sway whatever the support conditions. These data give specific pointers for the relative cognitive demand in the postural strategies induced by the double seesaw device.