Comparison Of Ankle Torque Control Error In Healthy Older And Young Subjects During Quiet Standing

Abstract

It has been reported various times that spontaneous sway during quiet standing is larger in older (O) than in young (Y) healthy subjects. Though it is hypothesized that the larger body sway must imply balance inabilities in O, it has not been shown to date, how the larger sway is caused and how its properties can be concretized. PURPOSE To test the following hypotheses: 1) Due to the larger control error of the ankle torque in O defined as the distance from the center of pressure (COP) to the gravity line projection (GLP) of the center of mass (COM) of the body (COP-COM), the body sway is larger in O than in Y. 2) The defined control error is attributed to the neural regulation of balance. METHODS Fifteen O (73+6yrs) and eleven Y (29+8yrs) male subjects participated in this study. Each subject was requested to keep a quiet stance with his or her eyes open or closed. The modified 'zero-point-to-zero-point double integration technique' using a force platform was adopted to estimate the GLP path, which was applied to calculate COP-COM. The body acceleration (ACC), which is theoretically a proportional parameter to COP-COM, was calculated using the measured horizontal force. Crosscorrelation analysis was adopted to investigate the relation between ACC and the surface electromyogram from the right soleus. RESULTS The effect of age was significant in the standard deviations of COP-COM with a 2-way-ANOVA, which were also larger in O than in Y (EO 0.12 vs. 0.08, EC 0.20 vs. 0.12 cm, p=0.007), while the effect of eye condition was insignificant. ACC significantly correlated with muscle activity in both age groups (EO O r=.29, Y r=.32; EC O r=.33, Y r=.35, p<0.01 for all). CONCLUSIONS The larger control error of ankle torque causes the larger body sway in O compared to Y for both eye conditions. The result that ACC correlated with muscle activity suggests that the larger error, which is proportional to ACC, is generated neurally. Therefore, we conclude that neural inability to control the ankle torque accurately is a cause of the larger sway in O.