Article

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

ABSTRACT

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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    • "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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    • "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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