Do equilibrium constraints modulate postural reaction when viewing imbalance?

Action observation and action execution are tightly coupled on a neurophysiological and a behavioral level, such that visually perceiving an action can contaminate simultaneous and subsequent action execution. More specifically, observing a model in postural disequilibrium was shown to induce an increase in observers' body sway. Here we reciprocally questioned the role of observers' motor system in the contagion process by comparing participants' body sway when watching displays of antero-posterior vs. lateral imbalance. Indeed, during upright standing, biomechanical constraints differ along the antero-posterior (A-P) and medio-lateral (M-L) axes; hence an impact of observers' postural constraints on the contagion response would result in different reactions to both types of stimuli. In response to the displays, we recorded greater area of center of pressure (CoP) displacement when watching forward/backward compared to left/right imbalance. In addition, after normalizing A-P and M-L CoP displacements by a control condition (fixation cross), A-P CoP path length when viewing forward imbalance tended to be higher than M-L CoP path length when viewing imbalance to the left or right. These results indicate that postural contagion is promoted when the display is compatible with observers' motor stabilization strategy which is mainly oriented along the A-P axis. In terms of clinical application, this study brings new indications for adaptation of observational training devices in rehabilitation programs.