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Background: Obesity and overweight are defined as abnormal or excessive fat accumulation and are associated with balance disorders. Objective: To assess the postural stability in a natural stance in overweight and obese women based on center of pressure (CoP) velocity in the anterior-posterior (AP) and medial-lateral (ML) directions. Methods: A total of 102 women categorized according to body mass index into normal weight, overweight and obese categories underwent a measurement of quiet standing with their eyes open (EO) and with their eyes closed (EC). Postural stability was assessed with a force platform. The mean CoP velocity was evaluated in both directions. Results: In the AP direction under EO conditions, obese women swayed significantly faster than normal weight women (1.01 cm/s and 0.80 cm/s). In the ML direction, a higher CoP velocity was observed in normal weight women than in obese women (0.52 cm/s and 0.41 cm/s). Under EC conditions in the AP direction, obese women swayed significantly faster than normal weight women (1.29 cm/s and 0.97 cm/s). In the ML direction, a higher CoP velocity was observed in normal weight women than in obese women (0.65 cm/s and 0.48 cm/s). Conclusions: Results suggest a negative impact of obesity on postural stability in the AP direction. In the ML direction, obese women were more stable than normal weight women, probably due to enlargement of the support base in a natural stance.

This study compares postural and trunk responses to translating platform perturbations of varied velocities and directions. A group of 18 young and physically active subjects were exposed to a set of postural perturbations at varied velocities (5, 10, 15, and 20 cm/s) and directions of platform movement (forward, backward, left-lateral, and right-lateral). The center of pressure (CoP) displacement measurement, in addition to the trunk motion (representing the center of mass (CoM) displacement), were both monitored. Results identified that the CoP displacement increased from slow to faster velocities of platform motion more widely in both anterior and posterior directions (50.4 % and 48.4 %) as compared to the CoM displacement (17.8 % and 14.9 %). However a greater increase in the peak CoM velocity (70.3 % and 69.6 %) and the peak CoM acceleration (60.5 % and 53.1 %) was observed. The values in the anterior and posterior direction only differed significantly at the highest velocity of platform motion (i.e. 20 cm/s). A similar tendency was observed in the medio-lateral direction, but there were no significant differences in any parameter in the left-lateral and right-lateral direction. The velocity of the platform motion highly correlated with peak velocity (r=0.92-0.97, P<0.01) and moderately with amplitude of trunk displacement (r=0.56-0.63, P<0.05). These findings indicate that the velocity of perturbation alters peak CoM velocity rather than the magnitude of CoM displacement. The effect of the direction of perturbations on the trunk response emerges only at a high velocity of platform motion, such that the peak CoM velocity and peak CoM acceleration are significantly greater in anterior than posterior direction.