Intermittent control models of human standing: similarities and differences

Two architectures of intermittent control are compared and contrasted in the context of the single inverted pendulum model often used for describing standing in humans. The architectures are similar insofar as they use periods of open-loop control punctuated by switching events when crossing a switching surface to keep the system state trajectories close to trajectories leading to equilibrium. The architectures differ in two significant ways. Firstly, in one case, the open-loop control trajectory is generated by a system-matched hold, and in the other case, the open-loop control signal is zero. Secondly, prediction is used in one case but not the other. The former difference is examined in this paper. The zero control alternative leads to periodic oscillations associated with limit cycles; whereas the system-matched control alternative gives trajectories (including homoclinic orbits) which contain the equilibrium point and do not have oscillatory behaviour. Despite this difference in behaviour, it is further shown that behaviour can appear similar when either the system is perturbed by additive noise or the system-matched trajectory generation is perturbed. The purpose of the research is to come to a common approach for understanding the theoretical properties of the two alternatives with the twin aims of choosing which provides the best explanation of current experimental data (which may not, by itself, distinguish beween the two alternatives) and suggesting future experiments to distinguish between the two alternatives

Dynamics of Simple Balancing Models with State Dependent Switching Control

Time-delayed control in a balancing problem may be a nonsmooth function for a variety of reasons. In this paper we study a simple model of the control of an inverted pendulum by either a connected movable cart or an applied torque for which the control is turned off when the pendulum is located within certain regions of phase space. Without applying a small angle approximation for deviations about the vertical position, we see structurally stable periodic orbits which may be attracting or repelling. Due to the nonsmooth nature of the control, these periodic orbits are born in various discontinuity-induced bifurcations. Also we show that a coincidence of switching events can produce complicated periodic and aperiodic solutions.Comment: 36 pages, 12 figure

Control of posture with FES systems

One of the major obstacles in restoration of functional FES supported standing in paraplegia is the lack of knowledge of a suitable control strategy. The main issue is how to integrate the purposeful actions of the non-paralysed upper body when interacting with the environment while standing, and the actions of the artificial FES control system supporting the paralyzed lower extremities. In this paper we provide a review of our approach to solving this question, which focuses on three inter-related areas: investigations of the basic mechanisms of functional postural responses in neurologically intact subjects; re-training of the residual sensory-motor activities of the upper body in paralyzed individuals; and development of closed-loop FES control systems for support of the paralyzed joints

Postural instability via a loss of intermittent control in elderly and patients with Parkinson's disease: a model-based and data-driven approach

Postural instability is one of the major symptoms of Parkinson's disease. Here, we assimilated a model of intermittent delay feedback control during quiet standing into postural sway data from healthy young and elderly individuals as well as patients with Parkinson's disease to elucidate the possible mechanisms of instability. Specifically, we estimated the joint probability distribution of a set of parameters in the model using the Bayesian parameter inference such that the model with the inferred parameters can best-fit sway data for each individual. It was expected that the parameter values for three populations would distribute differently in the parameter space depending on their balance capability. Because the intermittent control model is parameterized by a parameter associated with the degree of intermittency in the control, it can represent not only the intermittent model but also the traditional continuous control model with no intermittency. We showed that the inferred parameter values for the three groups of individuals are classified into two major groups in the parameter space: one represents the intermittent control mostly for healthy people and patients with mild postural symptoms and the other the continuous control mostly for some elderly and patients with severe postural symptoms. The results of this study may be interpreted by postulating that increased postural instability in most Parkinson's patients and some elderly persons might be characterized as a dynamical disease

Disruption of right posterior parietal cortex by continuous Theta Burst Stimulation alters the control of body balance in quiet stance

Control of body balance relies on the integration of multiple sensory modalities. Lightly touching an earth-fixed reference augments the control of body sway. We aimed to advance the understanding of cortical integration of an afferent signal from light fingertip contact (LT) for the stabilisation of standing body balance. Assuming that right-hemisphere Posterior Parietal Cortex (rPPC) is involved in the integration and processing of touch for postural control, we expected that disrupting rPPC would attenuate any effects of light touch. Eleven healthy right-handed young adults received continuous Theta Burst Stimulation over the left- and right-hemisphere PPC with sham stimulation as an additional control. Before and after stimulation, sway of the blindfolded participants was assessed in Tandem-Romberg stance with and without haptic contact. We analysed sway in terms of the variability of Centre-of-Pressure (CoP) rate of change as well as Detrended Fluctuation Analysis of CoP position. Light touch decreased sway variability in both directions but showed direction-specific changes in its dynamic complexity: a positive increase in complexity in the mediolateral direction coincided with a reduction in the anteroposterior direction. rPPC disruption affected the control of body sway in two ways: first, it led to an overall decrease in sway variability irrespective of the presence of LT; second, it reduced the complexity of sway with LT at the contralateral, non-dominant hand. We speculate that rPPC is involved in the active exploration of the postural stability state, with utilisation of LT for this purpose if available, by normally inhibiting mechanisms of postural stiffness regulation

Wavelet-frequency analysis for the detection of discontinuities in switched system models of human balance

This paper is concerned with detecting the presence of switching behavior in experimentally obtained posturographic data sets by means of a novel algorithm that is based on a combination of wavelet analysis and Hilbert transform. As a test-bed for the algorithm, we first use a switched model of human balance control during quiet standing with known switching behavior in four distinct configurations. We obtain a time-frequency representation of a signal generated by our model system. We are then able to detect manifestations of discontinuities (switchings) in the signal as spiking behavior. The frequency of switchings, measured by means of our algorithm and detected in our models systems, agrees with the frequency of spiking behavior found in the experimentally obtained posturographic data

The Relationship Between Intermittent Limit Cycles and Postural Instability Associated with Parkinson’s Disease

Background: Many disease-specifc factors such as muscular weakness, increased muscle stiffness, varying postural strategies, and changes in postural refexes have been shown to lead to postural instability and fall risk in people with Parkinson’s disease (PD). Recently, analytical techniques, inspired by the dynamical systems perspective on movement control and coordination, have been used to examine the mechanisms underlying the dynamics of postural declines and the emergence of postural instabilities in people with PD. Methods: A wavelet-based technique was used to identify limit cycle oscillations (LCOs) in the anterior–posterior (AP) postural sway of people with mild PD (n = 10) compared to age-matched controls (n = 10). Participants stood on a foam and on a rigid surface while completing a dual task (speaking). Results: There was no signifcant difference in the root mean square of center of pressure between groups. Three out of 10 participants with PD demonstrated LCOs on the foam surface, while none in the control group demonstrated LCOs. An inverted pendulum model of bipedal stance was used to demonstrate that LCOs occur due to disease-specifc changes associated with PD: time-delay and neuromuscular feedback gain. Conclusion: Overall, the LCO analysis and mathematical model appear to capture the subtle postural instabilities associated with mild PD. In addition, these fndings provide insights into the mechanisms that lead to the emergence of unstable posture in patients with PD