Interleg Coordination in Quiet Standing: Influence of Age and Visual Environment on Noise and Stability

Effects of footedness and stance asymmetry confirm an inter-leg metastable coordination dynamics of standing posture

We investigated the patterns of coordination between the left and right legs that support the task of maintaining an upright standing posture. We used cross-wavelet analyses to assess coordination between the centers of pressure under the left and right feet. We recruited participants with a lateralized functional preference for their right leg, and we manipulated whether these participants stood with symmetric/asymmetric stances and whether their eyes were open or closed. Our hypotheses were derived from the Haken-Kelso-Bunz (HKB) model of interlimb coordination dynamics. Consistent with HKB model predictions, we observed (1) coordination taking the form of metastable, transient epochs of stable phase relations, (2) preferences for in-phase and anti-phase coordination patterns, and (3) changes in pattern stability and phase leads associated with both stance asymmetry and right-side lateral preference. The form and stability of observed coordination patterns were mediated by the availability of visual information. Our findings confirm the existence of a metastable coordination dynamic associated with the task of maintaining upright stance. We discuss the implications of these findings in the context of evaluating the utility of the HKB model for understanding the functional organization of the posture system.

Dynamical Analysis of Standing Balance Control on Sloped Surfaces in Individuals with Lumbar Disc Herniation

The changes of balance control mechanism caused by lumbar disc herniation (LDH) has not been well understood. This study aimed to investigate the effects of LDH on the balance control during standing on sloped surfaces. Ten patients with LDH and 10 gender- and age-matched healthy subjects were instructed to stand quietly on a sloped surface at −5°, 0° or +5°, respectively. The trajectories of the center of pressure (COP) of each individual limb and the full-body were recorded. Cross recurrence quantification analysis (CRQA) was applied to assess the coordination of COP components at the anterior-posterior and medial-lateral directions. The patients with LDH presented magnified inter-limb load asymmetry and had more deterministic components in the COP coordination of the less-affected limb and the full-body than the healthy subjects. The LDH led to decreased dynamical degree of freedom and less flexibility in bidirectional controlling the center of mass simultaneously. The effects of sensorimotor deficits due to LDH could be more obviously exhibited as standing on a declined rather than an inclined surface. This study shed light on the effects of LDH on standing balance control and may facilitate to develop novel strategies for evaluation of LDH.

Coupling relations underlying the production of speech articulator movements and their invariance to speech rate

Since the seminal works of Bernstein (The coordination and regulation of movements. Pergamon Press, Oxford, 1967) several authors have supported the idea that, to produce a goal-oriented movement in general, and a movement of the organs responsible for the production of speech sounds in particular, individuals activate a set of coupling relations that coordinate the behavior of the elements of the motor system involved in the production of the target movement or sound. In order to characterize the configurations of the coupling relations underlying speech production articulator movements, we introduce an original method based on recurrence analysis. The method is validated through the analysis of simulated dynamical systems adapted to reproduce the features of speech gesture kinematics and it is applied to the analysis of speech articulator movements recorded in five German speakers during the production of labial and coronal plosive and fricative consonants at variable speech rates. We were able to show that the underlying coupling relations change systematically between labial and coronal consonants, but are not affected by speech rate, despite the presence of qualitative changes observed in the trajectory of the jaw at fast speech rate.

Evaluation of postural instability in stroke patient during quiet standing

This study was designed to investigate the postural instability in stroke survivals during quiet standing. Eleven stroke patients and 11 healthy volunteers (gender- and age-matched) were recruited for this experiment. The center of pressure (COP) signals from both planta in anterior-posterior (AP) and medial-lateral (ML) directions were recorded using a three-dimensional motion capture system (BTS Bioengineering Corp, Italy) when subjects stood on the two adjacent force platforms quietly with their eyes open (EO) for 30 s and with their eyes close (EC) for another 30 s. The standard deviation (SD) for the two limbs and the inter-limb cross correlation-coefficient (CC) of COP series were calculated under EO and EC conditions, respectively. Patients showed significant differences in SD of AP-COP between two sides under EO (p <; 0.05) and EC (p <; 0.001) conditions. The SD of the ML-COP on non-paretic limb of patients was higher than that of controls under EC (p <; 0.05). Lower CC values of AP-COP was found in patients versus controls under two vision conditions (p <; 0.05), and the values in two groups increased significantly after removal of vision information (p <; 0.05). Stroke led to an increased postural sway and decreased inter-limb coordination during quiet standing. This study may facilitate the evaluation of the standing instability in stroke survivors and may improve the strategies for rehabilitation.

Real-time visual feedback of COM and COP motion properties differentially modifies postural control structures

The experiment was setup to investigate the control of human quiet standing through the manipulation of augmented visual information feedback of selective properties of the motion of two primary variables in postural control: center of pressure (COP) and center of mass (COM). Five properties of feedback information were contrasted to a no feedback dual-task (watching a movie) control condition to determine the impact of visual real-time feedback on the coordination of the joint motions in postural control in both static and dynamic one-leg standing postures. The feedback information included 2D COP or COM position and macro variables derived from the COP and COM motions, namely virtual time-to-contact (VTC) and the COP-COM coupling. The findings in the static condition showed that the VTC and COP-COM coupling feedback conditions decreased postural motion more than the 2D COP or COM positional information. These variables also induced larger sway amplitudes in the dynamic condition showing a more progressive search strategy in exploring the stability limits. Canonical correlation analysis (CCA) found that COP-COM coupling contributed less than the other feedback variables to the redundancy of the system reflected in the common variance between joint motions and properties of sway motion. The COP-COM coupling had the lowest weighting of the motion properties to redundancy under the feedback conditions but overall the qualitative pattern of the joint motion structures was preserved within the respective static and dynamic balance conditions.

Inter-foot coordination dynamics of quiet standing postures

It has long been held that the net center of pressure (COP(NET)) is the controlling variable to human stance that indirectly represents postural sway. The formation of the COP(NET) trajectory emerges from an active control of transporting the body weight from one foot to the other and the self-organized coordination of the COP of each individual foot—properties that cannot be determined from the typical single force platform protocol. The findings of recent studies, with the application of the two-force platform paradigm, have revealed the coordination properties of the lower limbs in regulating COP(NET). In this article, we review these new findings and insights into the control of postural stability within the framework of a dynamic systems approach. The issues include: (1) the active asymmetrical body weight distribution and transportation process during both short- and long-term stances; (2) the spatial and temporal characteristics of the inter- and intra-foot COP coupling dynamics; (3) the influence of mechanical constraints (e.g., foot position, foot orientation, etc.) on the inter-foot and intra-foot COP coordination dynamics; and (4) the role of the specificity of task context to the functional asymmetry of the feet and its relation to footedness. The findings from foot coordination dynamics reveal subtle regulation of stability and instability in postural control that needs to be mapped to the coordination dynamics of the multi-link postural control system.