To evaluate postural balance, displacement of the center of pressure (CoP) in will be estimated from the raw force plate data using the equation: CoP = Mx/Fz (medio-lateral) and CoP=My/Fz (anterior-posterior). CoP will be measured during upright standing with and without vision, on stable and unstable (foam pad) support surface. During some conditions, local muscle vibration on ankle and/or back muscles will be applied to evaluate the role of proprioception in postural control. Furthermore, in some conditions a repetitive ballistic arm movement will be asked to evaluate the effect of a internal perturbation on postural control. Root mean square values of the CoP displacements will be used for the analysis of postural stability measures and mean values will be calculated for the vibration trials to analyze the expected directional effect. A ratio of the CoP displacements of the ankle muscles vibration trial regard to the back muscles vibration trial will be calculated to determine the proprioceptive postural control strategy.