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1 Dept of Life Sciences, The University of Tokyo, Tokyo, Japan; Rehabilitation Engineering Laboratory, Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, Ontario, Canada
2 Rehabilitation Engineering Laboratory, Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, Ontario, Canada
3 Dept of Motor Dysfunction, National Rehabilitation Center for the Disabled, Tokorozawa, Japan
4 Dept of Life Sciences, The University of Tokyo, Tokyo, Japan
* To whom correspondence should be addressed. E-mail: masani{at}idaten.c.u-tokyo.ac.jp.
In literature, it has been suggested that the central nervous system (CNS) anticipates spontaneous change in body position during quiet stance, and continuously modulates ankle extensor muscle activity to compensate for the change. The purpose of this study was to investigate whether velocity feedback contributes in modulating ankle extensors activities in an anticipatory fashion, facilitating effective control of quiet stance. Both theoretical analysis and experiments were carried out to investigate to what extent velocity feedback contributes to controlling quiet stance. The experiments were carried out with sixteen healthy subjects who were asked to stand quietly with their eyes open or closed. During the experiments, the center of pressure (COP) displacement (COPdis), the center of mass (COM) displacement (COMdis) and COM velocity (COMvel) in the anteroposterior direction were measured. Rectified electromyograms (EMGs) were used to measure muscle activity in the right soleus muscle, the medial gastrocnemius muscle and the lateral gastrocnemius muscle. The simulations were performed using an inverted pendulum model that described the anteroposterior kinematics and dynamics of quiet stance. In the simulations, an assumption was made that the COMdis of the body would be regulated using a proportional-derivative (PD) controller. Two different PD controllers were evaluated in these simulations: 1) a controller with the high derivative/velocity gain (HDG), and 2) a controller with the low derivative/velocity gain (LDG). Cross-correlation analysis was applied to investigate the relationships between time series obtained in experiments: a) COMdis and EMGs, and b) COMvel and EMGs. Identical cross-correlation analysis was applied to investigate the relationships between time series obtained in simulations: c) COMdis and ankle torque, and d) COMvel and ankle torque. The results of these analyses showed that the COMdis was positively correlated with all three EMGs and that the EMGs temporally preceded the COMdis. These findings agree with the previously published studies in which it was shown that the lateral gastrocnemius muscle is actively modulated in anticipation of the body's COM position change. The COMvel and all three EMGs were also correlated and the cross-correlation function (CCF) had two peaks; one which was positive and the other one which was negative. The positive peaks were statistically significant, unlike the negative ones; they were larger than the negative peaks, and their time shifts were much shorter compared to the time shifts of the negative peaks. When these results were compared to the CCF results obtained for simulated time series, it was discovered that the cross-correlation results for the HDG controller closely matched cross-correlation results for the experimental time series. On the other hand, the simulation result obtained for LDG controller did not match the experimental results. These findings suggest that the actual postural control system during quiet stance adopts a control strategy that relies notably on velocity information, and that such a controller can modulate muscle activity in anticipatory manner without using a feed-forward mechanism.
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