Dynamical structure of center-of-pressure trajectories in patients recovering from stroke

Faculty of Human Movement Sciences, Institute for Fundamental and Clinical Human Movement Sciences, Vrije Universiteit, Van der Boechorststraat 9, 1081 BT, Amsterdam, The Netherlands.
Experimental Brain Research (Impact Factor: 2.04). 10/2006; 174(2):256-69. DOI: 10.1007/s00221-006-0441-7
Source: PubMed


In a recent study, De Haart et al. (Arch Phys Med Rehabil 85:886-895, 2004) investigated the recovery of balance in stroke patients using traditional analyses of center-of-pressure (COP) trajectories to assess the effects of health status, rehabilitation, and task conditions like standing with eyes open or closed and standing while performing a cognitive dual task. To unravel the underlying control processes, we reanalyzed these data in terms of stochastic dynamics using more advanced analyses. Dimensionality, local stability, regularity, and scaling behavior of COP trajectories were determined and compared with shuffled and phase-randomized surrogate data. The presence of long-range correlations discarded the possibility that the COP trajectories were purely random. Compared to the healthy controls, the COP trajectories of the stroke patients were characterized by increased dimensionality and instability, but greater regularity in the frontal plane. These findings were taken to imply that the stroke patients actively (i.e., cognitively) coped with the stroke-induced impairment of posture, as reflected in the increased regularity and decreased local stability, by recruiting additional control processes (i.e., more degrees of freedom) and/or by tightening the present control structure while releasing non-essential degrees of freedom from postural control. In the course of rehabilitation, dimensionality stayed fairly constant, whereas local stability increased and regularity decreased. The progressively less regular COP trajectories were interpreted to indicate a reduction of cognitive involvement in postural control as recovery from stroke progressed. Consistent with this interpretation, the dual task condition resulted in less regular COP trajectories of greater dimensionality, reflecting a task-related decrease of active, cognitive contributions to postural control. In comparison with conventional posturography, our results show a clear surplus value of dynamical measures in studying postural control.

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    • "[Obj2] To test the hypothesis that EnHL and ˛ reveal the presence of a non-random, deterministic structure in the COP signal that is captured in the phase of the COP signal. This will be tested by studying changes in EnHL and ˛ caused by phase randomizations in a surrogate analysis [4]. If EnHL and ˛ change significantly as a result of surrogate realizations, then EnHL represents an entropic measure for the complexity of the phase of the original signal. "
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    ABSTRACT: The reshape scale (RS) method computes the transition of Sample Entropy (SEn) from low to large values as the scale is increased. At the largest scale, SEn asymptotically converges to the maximum SEn evaluated using the average of SEn for the fully randomly re-ordered realizations of the original signal. The entropic half-life (EnHL) characterizes such a transition. The EnHL is the scale representing the midpoint of the transition and yields a measure for the temporal degradation of the regularity in a signal. In postural balance studies, the EnHL of the center of pressure (COP) signal can be interpreted as the time elapsed before the old sensory states is no longer utilized by the postural control system to adjust the current COP position. Other equally sensitive measures of regularity, such as de-trended fluctuation analysis (DFA), can be interpreted in the same way; however, it results in a dimensionless measure of regularity and complexity. The primary objective of this study was to experimentally demonstrate the correlation between the scaling exponent α calculated using DFA and the inverse of EnHL. When the COP signal was studied in the Fourier domain, a non-random structure was observed in the phase of the COP signal, which might be related to the neuromuscular characteristics of the postural control system. The second objective of this paper was to demonstrate that the EnHL and α both retain information contained in the phase of the Fourier transformed signal. It is shown in this paper that the EnHL and α are both sensitive to non-random structures of the phase of the Fourier transformed signal. Contrary to α, which is a dimensionless number, the EnHL measures the regularity and complexity of the signal in units of time. Therefore, it was concluded that the EnHL provides a more physically interpretable and intuitively understandable measure of the properties of the control of the COP signal.
    Full-text · Article · Feb 2016 · Biomedical Signal Processing and Control
    • "The COP offers a direct measure of mechanical stability in the sense that a COP position too close to the border of the base of support indicates an instability that must be corrected in order to prevent a fall. Furthermore, the characteristics of the COP motion provide information about the neuro-muscular control, particularly in cases of neuro-muscular deficits, for example, cerebral palsy (Donker et al., 2008; Rose et al., 2002), stroke (Corriveau et al., 2004; Roerdink et al., 2006), concussion (Cavanaugh et al., 2005; Cavanaugh et al., 2006; Rubin et al., 1995), or frailty (Lipsitz, 2002) and fall risk (Maki et al., 1994) in the elderly. "
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    ABSTRACT: Human upright posture is maintained by postural movements, which can be quantified by "principal movements" (PMs) obtained through a principal component analysis (PCA) of kinematic marker data. The current study expands the concept of "principal movements" in analogy to Newton's mechanics by defining "principal position" (PP), "principal velocity" (PV), and "principal acceleration" (PA) and demonstrates that a linear combination of PPs and PAs determines the center of pressure (COP) variance in upright standing. Twenty-one subjects equipped with 27-markers distributed over all body segments stood on a force plate while their postural movements were recorded using a standard motion tracking system. A PCA calculated on normalized and weighted posture vectors yielded the PPs and their time derivatives, the PVs and PAs. COP variance explained by the PPs and PAs was obtained through a regression analysis. The first 15 PMs quantified 99.3% of the postural variance and explained 99.60%±0.22% (mean±SD) of the anterior-posterior and 98.82±0.74% of the lateral COP variance in the 21 subjects. Calculation of the PMs thus provides a data-driven definition of variables that simultaneously quantify the state of the postural system (PPs and PVs) and the activity of the neuro-muscular controller (PAs). Since the definition of PPs and PAs is consistent with Newton's mechanics, these variables facilitate studying how mechanical variables, such as the COP motion, are governed by the postural control system.
    No preview · Article · Dec 2015 · Journal of Biomechanics
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    • "Patients with severe motor impairments of the paretic lower limb seem to use this strategy more systematically than do less severely impaired patients [60]. The CoP could shift towards the non-paretic side during standing to minimize instability due to unilateral lower limb impairment [60] [59] [24]. Sway is greater when loading the paretic than non-paretic lower limb during quiet standing [62]. "
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    ABSTRACT: After stroke, the causes of balance disorders include motor disorders, sensory loss, perceptual deficits and altered spatial cognition. This review focuses on motor strategies for postural control after stroke. Weight-bearing asymmetry, smaller surface of stability, increased sway, body tilting and sometimes pushing syndrome are observed. Weakness and sensory impairments account only for some of these disturbances; altered postural reactions and anticipated postural adjustments as well as abnormal synergistic muscular activation play an important part. These disorders are often linked to cognitive impairments (visuospatial analysis, perception of verticality, use of sensory information, attention, etc.), which explain the preeminent disorders of postural control seen with right rather than left-hemisphere lesions. Most of the motor changes are due to an impaired central nervous system but some could be considered adaptive behaviors. These changes have consequences for rehabilitation and need further studies for building customized programs based on the motor comportment of a given patient.
    Full-text · Article · Nov 2015 · Neurophysiologie Clinique/Clinical Neurophysiology
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