Induced stabilization of center of mass decreases variability of center of pressure regardless of visual or tactile information.

Traditional theories claim that center of pressure (COP) is oscillating to minimize the center of mass (COM) movements, contrary to exploratory theories which propose that COP oscillates to increase sensory information flow from the environment. The aim of this work was to better understand the underlying postural control mechanisms, specifically the interplay of COP oscillations and sensory information flow on keeping the COM stable. Eighteen volunteers took part of the experiment divided into three parts based on sensory conditions: eyes opened, eyes closed and eyes closed with lightly touching a fixed object with one finger. Throughout each part the participants had to quietly stand for 335 s. In the middle of each part, we stabilized their COM for 105 s using a robotized waist-pull system. We recorded whole-body kinematics, COP oscillations, electromyographic activity of soleus and tibialis anterior muscles and the force applied by the finger during light touch conditions. The variability of COP significantly decreased when the COM was stabilized in all sensory conditions. The interaction between sensory condition and stabilization was also significant with different decline of COP variability between quiet standing and stabilization part in all three different sensory conditions. Ankle and knee angle variability decreased significantly while the hip angle variability did not. Our findings suggest that COP is not moving to explore the environment, but to attenuate oscillations of the COM. However, possible functional aspect of movement variability to keep the COM stable still remains.