Effects of Cognitive Interference and Type of Support Surface on Postural Stability: A Biomechanical Analysis

Balance is essential for human daily activities. Standing on an unstable platform requires continuous effort of the neuro-musculoskeletal system. Cognitive interference and support surface perturbation may cause loss of balance. The aim of this study is to evaluate the ability of stability provision of individuals while standing in different levels of postural and cognitive difficulty. To this end, twelve healthy young women were participated in six levels (three levels of support surface × two levels of cognitive intereference). Three levels support surface were standing on a firm surface, unstable platform surface with and without spring support. Two levels of attentional cognitive involvements were considered with or without questions by presenting on a curtain and asking to response by a yes/no joystick. Motion analysis was used to measure joint angles by capturing body movements in the sagittal plane by a high-speed camera and active markers. To quantitatively investigate the stability, two linear (pathlength, root mean square) and two nonlinear (approximate entropy, fractal dimension) metrtics were calculated. Results showed that the ankle mechanism plays a more prominent role in keeping balance than the knee and hip joint mechanisms. Merely the approximate entropy indicated significant differences between the postural difficulty levels. Also, the mediocre level of support surface perturbation (spring-supported unstable platform) revealed multi-joint collaboration between the mechanisms. The inconsistence between postural and cognitive difficulty levels might vanish the role of cognitive questions in the present study. Therefore, considering consistent postural and cognitive tasks may highlight the effects of cognitive involvements on standing.