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IX congrès PMC

Abstract :

Matthieu CASTERAN, Pauline HILT, Thierry POZZO & Elizabeth THOMAS

An Analysis of the Centre of Mass Trajectories during a Whole Body Pointing Movement

Most previous studies of human motor control either focus on the postural equilibrium mechanisms or conversely on reaching movements of the focal module while restricting movement of the lower body part. Many of our actions however require the interaction of the two. The whole body pointing (WBP) task is one which involves these two elements. The aim of our study is to understand how the center of mass (CoM) is regulated during such movements. The increased height of the biped configuration would require a careful regulation of this variable for the maintenance of equilibrium while attaining the target. How does the nervous system control the horizontal and vertical aspects of the CoM trajectory?  Previous studies have shown that the nervous system reduces the number of degrees of freedom of a movement through the use of synergies. Would such a strategy of using common waveforms be utilized for controlling the anterio-posterior (AP) and vertical (V) displacements of the CoM? To answer these questions, we examined the CoM trajectories of individuals pointing to a target that was placed at a distance and height that corresponded to 15% of the subject height. Our first analyses reveal that the relationship of the control between these two variables, as measured by dAP (the speed of AP displacement) and dV (the speed of vertical displacement), is one that is nonlinear.  In particular, dV showed a low variability compared to dAP.  This raises the possibility that the movement is organized around the vertical rather than the horizontal dimension. A study of the two trajectories showed various stereotyped transition points. Among these were the moment at which dV reaches a minimum and secondly, the moment at which the changes in the vertical direction became greater than those in the anterior posterior direction. Future studies will involve verifying if the above observations hold for WBP movements requiring a greater AP displacement and how they are altered by ageing.

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