Stiffness Control of Balance in Quiet Standing

Abstract

Our goal was to provide some insights into how the CNS controls and maintains an upright standing posture, which is an integral part of activities of daily living. Although researchers have used simple performance measures of maintenance of this posture quite effectively in clinical decision making, the mechanisms and control principles involved have not been clear. We propose a relatively simple control scheme for regulation of upright posture that provides almost instantaneous corrective response and reduces the operating demands on the CNS. The analytic model is derived and experimentally validated. A stiffness model was developed for quiet standing. The model assumes that muscles act as springs to cause the center-of-pressure (COP) to move in phase with the center-of-mass (COM) as the body sways about some desired position. In the sagittal plane this stiffness control exists at the ankle plantarflexors, in the frontal plane by the hip abductors/adductors. On the basis of observations that the COP-COM error signal continuously oscillates, it is evident that the inverted pendulum model is severely underdamped, approaching the undamped condition. The spectrum of this error signal is seen to match that of a tuned mass, spring, damper system, and a curve fit of this "tuned circuit" yields omega n the undamped natural frequency of the system. The effective stiffness of the system, Ke, is then estimated from Ke = I omega n2, and the damping B is estimated from B = BW X I, where BW is the bandwidth of the tuned response (in rad/s), and I is the moment of inertia of the body about the ankle joint. Ten adult subjects were assessed while standing quietly at three stance widths: 50% hip-to-hip distance, 100 and 150%. Subjects stood for 2 min in each position with eyes open; the 100% stance width was repeated with eyes closed. In all trials and in both planes, the COP oscillated virtually in phase (within 6 ms) with COM, which was predicted by a simple 0th order spring model. Sway amplitude decreased as stance width increased, and Ke increased with stance width. A stiffness model would predict sway to vary as Ke-0.5. The experimental results were close to this prediction: sway was proportional to Ke(-0.55). Reactive control of balance was not evident for several reasons. The visual system does not appear to contribute because no significant difference between eyes open and eyes closed results was found at 100% stance width. Vestibular (otolith) and joint proprioceptive reactive control were discounted because the necessary head accelerations, joint displacements, and velocities were well below reported thresholds. Besides, any reactive control would predict that COP would considerably lag (150-250 ms) behind the COM. Because the average COP was only 4 ms delayed behind the COM, reactive control was not evident; this small delay was accounted for by the damping in the tuned mechanical system.

Process stationarity and reliability of trunk postural stability

“…The filter cut off frequency was selected to assure sufficient bandwidth to represent torso dynamics; i.e. approximately ten times the natural frequency of the torso (Moorhouse and Granata 2005;Winter et al 1998). Center-of-pressure (CoP) was calculated by using recorded reaction forces and moments.…”

Revisão sobre posturografia baseada em plataforma de força para avaliação do equilíbrio

“…The first one is the kinematic method 5,14 , in which the positions of the body segments are evaluated in a certain instant, and the body's CG is determined through the use of these positions and the observation of the inertial parameters of the body, such as the CG position in each segment and its respective mass. The difficulties related to the use of the kinematic method are that the inertial parameters of the body segments present considerable errors ( from errors in the anthropometric models of the body) and the fact that this method is more complicated, as it requires the use of cinemetry (video cameras and software for calibration and coordinate reconstruction).…”

The influence of artificially increased hip and trunk stiffness on balance control in man

“…Studies on quiet stance and pitch-plane dynamic posturography fuelled theories about human balance control as an inverted pendulum about the ankle joint when body motion is small (Nashner and McCollum 1985;Fitzpatrick et al 1992Fitzpatrick et al , 1994Winter et al 1998;Gatev et al 1999;Jacobs 1997;Lauk et al 1999;Johansson and Magnusson 1991), or like a two-joint system with motion restricted to the hip and ankle joints (the "hip strategy") when body motion is larger (Horak and Nashner 1986;Kuo and Zajac 1993;Henry et al 1998a). For these models, motion at the knees or the lumbro-sacral joints is assumed to be minimal.…”