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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    • "In contrast with findings in other pathologies (Cavanaugh et al., 2006; Donker et al., 2008; Roerdink et al., 2006; Schmit et al., 2006), subjects with current LBP in the present study showed a more irregular sway pattern with a higher frequency content, when standing on foam. The subjects in these other studies had pathologies that contain larger threats to postural balance than LBP does, such as CVA and Parkinson disease. "
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