Asymmetry of recurrent dynamics as a function of postural stance

Adolescents running in conventional running shoes have lower vertical instantaneous loading rates but greater asymmetry than running barefoot or in partial-minimal shoes

Footwear may moderate the transiently heightened asymmetry in lower limb loading associated with peak growth in adolescence during running. This repeated-measures study compared the magnitude and symmetry of peak vertical ground reaction force and instantaneous loading rates (VILRs) in adolescents during barefoot and shod running. Ten adolescents (age, 10.6 ± 1.7 years) ran at self-selected speed (1.7 ± 0.3 m/s) on an instrumented treadmill under three counter-balanced conditions; barefoot and shod with partial-minimal and conventional running shoes. All participants were within one year of their estimated peak height velocity based on sex-specific regression equations. Foot-strike patterns, peak vertical ground reaction force and VILRs were recorded during 20 seconds of steady-state running. Symmetry of ground reaction forces was assessed using the symmetry index. Repeated-measures ANOVAs were used to compare conditions (α=.05). Adolescents used a rearfoot foot-strike pattern during barefoot and shod running. Use of conventional shoes resulted in a lower VILR (P < .05, dz = 0.9), but higher VILR asymmetry (P < .05) than running barefoot (dz = 1.5) or in partial-minimal shoes (dz = 1.6). Conventional running shoes result in a lower VILR than running unshod or in partial-minimal shoes but may have the unintended consequence of increasing VILR asymmetry. The findings may have implications for performance, musculoskeletal development and injury in adolescents.

Lower Limb Ground Reaction Force and Center of Pressure Asymmetry During Bodyweight Squats

Background

Performance asymmetries between the lower limbs have been reported across a variety of variables and for numerous motor tasks including double leg squats. Additionally, the degree of symmetry is often used as a recovery metric during rehabilitation programs.

Hypothesis/Purpose

The purpose of this investigation was to examine leg asymmetry during a bodyweight double leg squat task and assess the effects of squat speed in a physically active population.

Study Design

Cross-over Study Design.

Methods

Eighteen healthy individuals completed two sets of 20 squats at two tempos (preferred tempo and 60 bpm) while ground reaction force and center of pressure data were recorded using dual force plates. Peak vertical ground reaction force, force impulse, and center of pressure (COP) standard deviation in the anterior-posterior (AP) and mediolateral (ML) direction were calculated and analyzed to identify any differences between legs, tempo, and as a function of repetitions. Significance was set at ρ ≤ .05.

Results

The subjects exhibited greater ground reaction forces during the self-paced tempo compared to the metronome-paced tempo (F 1,79 = 14.48, p < .001) with the preferred leg generating larger values than the non-preferred leg during the self-paced condition. There was also a significant tempo x leg interaction for force impulse (F 1,79 = 5.927, p = 0.015). A greater amount of COP variability was found in the preferred leg compared to the non-preferred leg in both the AP (F 1,79 = 30.147, p < 0.001) and ML (F 1,79 = 41.204, p < 0.001) directions.

Conclusions

These findings highlight the importance of considering multiple levels of analysis when assessing lower limb symmetry as separate variables may provide differential evidence for asymmetry. Practically, these results emphasize the need for coaches and practitioners to consider different degrees of lower limb asymmetries that may impact the development and design of strength and rehabilitation programs.

Level of Evidence

3.

Multiple roles of active stiffness in upright balance and multidirectional sway

Both passive and active mechanisms are necessary to explain small amplitude forward-backward (FB) voluntary swaying. Parallel and symmetric leg inverted pendulum models with stiffness control are a simple way to replicate FB swaying during quiet stance. However, it has been more difficult to model lateral left-right (LR) voluntary swaying involving the dual mechanisms of hip loading-unloading and ankle pressure distribution. To assess these factors, we had subjects perform small amplitude FB and LR sways and circular rotation. We experimentally identified three parameters that characterized their two-dimensional stiffnesses: AP stiffness (K_S^AP), and lateral stiffness (K_S^ML), at the ankles and a parameter we refer to as the engagement-disengagement rate (K_ED) of the legs. We performed simulations with our engaged leg model (Bakshi A, DiZio P, Lackner JR. J Neurophysiol 121: 2042-2060, 2019; Bakshi A, DiZio P, Lackner JR. J Neurophysiol 121: 2028-2041, 2019) to test its predictions about the limits of balance stability during sway in the three test conditions. Comparing the model's predictions with the experimental data, we found that K_S^AP has a task-dependent dual role in upright balance and is crucial to prevent falling; K_S^ML helps overcome viscous drags but is not instrumental to stability; K_ED has a key role in stability and is dependent on the biomechanical geometry of the body, which is invariant across balance tasks. These findings provide new insights into balance control that have important clinical implications for falling, especially for patients who are unable to use a hip strategy during balance control.NEW & NOTEWORTHY Our previously published Engaged Leg Model here shows how stiffness plays complex multicausal roles in balance. In one role, it is crucial to stability, with task contingent influences over balance. In another, it overcomes viscous drag. Task-dependent stiffness alone does not explain stable balance; geometrical, invariant aspects of body biomechanics also matter. Our model is fully applicable to clinical balance pathologies involving asymmetries in movement and balance control.

Center-of-pressure dynamics of upright standing as a function of sloped surfaces and vision

Upright postural control system exhibits dynamic behavior to produce flexible adaptations to a variety of internal and external perturbations. Understanding the range of postural adaptability in healthy individuals can index the overall state of the system and needs to be defined over various environmental and task constraints. The purpose of the current investigation was to understand the role of vision and support surface angle on the multiple time scales of control that maintain upright posture. Thirteen young, healthy adults performed quiet standing tasks on flat, inclined and declined support surfaces with either eyes open or closed. The variability of the anterior-posterior center of pressure (COP) trajectory was analyzed using linear (COP_length) and non-linear (multiscale entropy - MSE) approaches to index postural dynamics. Sway magnitude - COP_length - was greater in both sloped conditions compared to the flat support surface standing and with the removal of vision. Increased irregularity was revealed during the sloped conditions compared to flat surface standing with additional increases of COP complexity when vision was removed. Overall, a similar range of postural adaptability was revealed for both the singular and combined sensory manipulations suggesting limits to the degree of change of COP dynamics.

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.

Rapid adaptation to Coriolis force perturbations of voluntary body sway

Studying adaptation to Coriolis perturbations of arm movements has advanced our understanding of motor control and learning. We have now applied this paradigm to two-dimensional postural sway. We measured how subjects (n = 8) standing at the center of a fully enclosed rotating room who made voluntary anterior-posterior swaying movements adapted to the Coriolis perturbations generated by their sway. Subjects underwent four voluntary sway trials prerotation, 20 per-rotation at 10 rpm counterclockwise, and 10 postrotation. Each trial lasted 20 s, and subjects were permitted normal vision. Their voluntary sway during rotation generated Coriolis forces that initially induced rightward deviations of their forward sway paths and leftward deviations of their backward sway. Sagittal plane sway was gradually restored over per-rotation trials, and a mirror image aftereffect occurred in postrotation trials. Dual force plate data analysis showed that subjects learned to counter the Coriolis accelerations during rotation by executing a bimodal torque pattern that was asymmetric across legs and contingent on forward vs. backward movement. The experience-dependent acquisition and washout of this compensation indicate that an internal, feedforward model underlies the leg-asymmetric bimodal torque compensation, contingent on forward vs. backward movement. The learned torque asymmetry we observed for forward vs. backward sway is not consistent with parallel two-leg models of postural control. NEW & NOTEWORTHY This paper describes adaptation to Coriolis force perturbations of voluntary sway in a rotating environment. During counterclockwise rotation, sway paths are deviated clockwise, but full restoration of fore-aft sway is regained in minutes. Negative aftereffects are briefly present postrotation. Current parallel leg models of postural control cannot account for these findings, which show that postural control, like arm movement control, can adapt rapidly and completely to the Coriolis forces generated in artificial gravity environments.

Recurrence dynamics reveals differential control strategies to maintain balance on sloped surfaces

BACKGROUND

Studies on postural control have primarily focused on the maintenance of balance in quiet upright standing on flat horizontal support surfaces that can reveal only a subset of the potential postural stability/instability configurations in everyday contexts.

OBJECTIVES

Here we investigated the nature of dynamical properties of postural coordination in an upright standing task as a function of the systematic scaling of seven support surface angles, +20°, +10° dorsiflexion (+), 0 °Flat, -10°, -20°, -30°, -35° plantarflexion (-), mounted on a force plate.

METHODS

The center of pressure (CoP) and virtual time-to-contact (VTC) were analyzed to examine the spatial and spatio-temporal aspects of postural coordination dynamics, respectively. Recurrence quantification analysis (RQA) was used to characterize the dynamic postural control strategies as a function of slope surface angle.

RESULTS

The recurrence findings showed that on a flat surface the postural CoP dynamic are recurrent with a largely deterministic process and higher Shannon entropy compared to elevated slope angles in dorsiflexion and plantarflexion. There were asymmetrical patterns between similar slope angles for dorsiflexion and plantarflexion postures. The recurrence measures revealed that VTC operates on a higher embedding dimension than that of CoP.

SIGNIFICANCE

VTC showed an enhanced sensitivity to detection of postural instability in relation to the stability boundary that was magnified on the flat surface but progressively reduced over larger surface angles for both the dorsiflexion and plantarflexion postures.

Orderliness of Visual Stimulus Motion Mediates Sensorimotor Coordination

We explored the coupling of gaze and postural sway to the motion of a visual stimulus, to further understand sensorimotor coordination. Visual stimuli consisted of a horizontally oscillating red dot, moving with periodic (sine), chaotic, or aperiodic (brown noise) temporal structure. Cross Recurrence Quantification Analysis (cRQA) was used to investigate the coupling between each measured signal with the time series of the visual stimulus position. The cRQA parameter of percent determinism indicated similar strength of coupling of gaze with either periodic or chaotic motion structures, yet weaker coupling to aperiodic stimulus motion. The cRQA parameter of Maxline indicated a particular affinity toward chaotic motion. Analysis of postural coupling supports the idea that the complex periodicity of body sway affords interactivity with non-simple environmental dynamics. These results collectively strengthen the argument that chaos is an invariant and beneficial feature of biological motion, a feature which may be critical for immediate and robust coordination of the self with the environment and other environmental agents.

Scaling oscillatory platform frequency reveals recurrence of intermittent postural attractor states

The study of postural control has been dominated by experiments on the maintenance of quiet upright standing balance on flat stationary support surfaces that reveal only limited modes of potential configurations of balance stability/instability. Here we examine the self-organization properties of postural coordination as revealed in a dynamic balance task with a moving platform. We scaled a control parameter (platform frequency) to investigate the evolving nature of the coupled oscillator dynamics between center of mass (CoM) and platform. Recurrent map measures were used to reveal whether episodic postural control strategies exist that can be scaled by systematically changing the magnitude of platform motion. The findings showed that at higher platform frequencies (1.2 Hz), the CoM-Platform coupling was less deterministic than lower platform frequencies and evolved to intermittent postural control strategies that oscillated between periodic-chaotic transitions to maintain upright postural balance. Collectively, the recurrence map measures indicated that quasi-static postural attractor states were progressively emerging to the changing task constraints of platform frequency in the maintenance of postural stability. It appears that several dynamic modes of intermittent coupling in postural control can interchangeably co-exist and are expressed as a function of the control parameter of platform frequency.

Static postural sway of women with and without fibromyalgia syndrome: A cross-sectional study

BACKGROUND

There is a frequent complaint about balance problems among fibromyalgia syndrome patients; however, there are not enough studies that have shown static postural sway of women with fibromyalgia syndrome. This study aimed to compare static postural sway of women with and without fibromyalgia syndrome.

METHODS

This is a cross-sectional study in which twenty-nine women with fibromyalgia syndrome and 20 without took part. A posturography evaluation was performed in six different situations (bipedal, right tandem and left tandem, with eyes opened and closed), and questionnaires for clinical depression symptoms, clinical anxiety symptoms, sleep quality, and Visual Analogue Scales for Pain and Fatigue were applied. Mann-Whitney U test was used to check differences among groups; Wilcoxon matched-pair test was used to check differences intragroup; Cohen d coefficient was used to measure effect sizes and Pearson Correlation Coefficient was used for correlations among variables. Level of significance adopted was 5%.

FINDINGS

Women with fibromyalgia syndrome have presented worse postural sway than women without fibromyalgia syndrome in all situations (P<0.05), and worse scores in all questionnaires (P<0.05). In the eyes closed situations, women with fibromyalgia syndrome presented worse postural sway than women without in the same conditions.

INTERPRETATION

Women with fibromyalgia syndrome have worse performance in the static posture test, more prominent in reduced support bases with eyes closed. Pain, fatigue, depression and anxiety may have directly influenced postural sway in fibromyalgia syndrome patients.

Laterality of Stance during Optic Flow Stimulation in Male and Female Young Adults

During self-motion, the spatial and temporal properties of the optic flow input directly influence the body sway. Men and women have anatomical and biomechanical differences that influence the postural control during visual stimulation. Given that recent findings suggest a peculiar role of each leg in the postural control of the two genders, we investigated whether the body sway during optic flow perturbances is lateralized and whether anteroposterior and mediolateral components of specific center of pressure (COP) parameters of the right and left legs differ, reexamining a previous experiment (Raffi et al. (2014)) performed with two, side-by-side, force plates. Experiments were performed on 24 right-handed and right-footed young subjects. We analyzed five measures related to the COP of each foot and global data: anteroposterior and mediolateral range of oscillation, anteroposterior and mediolateral COP velocity, and sway area. Results showed that men consistently had larger COP parameters than women. The values of the COP parameters were correlated between the two feet only in the mediolateral axis of women. These findings suggest that optic flow stimulation causes asymmetry in postural balance and different lateralization of postural controls in men and women.

Degradation of postural control with aging

Aging negatively impacts the ability to maintain postural stability due to degraded control systems. The entropic half-life, a non-linear variable that quantifies the transition of sample entropy with increasing time scales, quantifies the time that elapses before old positional information no longer influences, or is no longer related to, the control mechanisms that regulate the movement at the current center of pressure location. The entropic half-life provides a more representative and comprehendible way of detecting changes in complexity using measurement units of time. The purpose of this study was to determine the effects of aging on the magnitude and temporal structure of the center of pressure movement during quiet single-limb stance. Center of pressure data of 24 older and 24 younger subjects were analyzed. The complexity of the temporal structure of the center of pressure signal was quantified by calculating the entropic half-life of the center of pressure in the medio-lateral and anterior–posterior directions. The magnitude of movement was quantified using excursion of the center of pressure in the medio-lateral and anterior–posterior directions, the path length, and the 95% ellipse area of the center of pressure. The older subjects demonstrated a significantly shorter entropic half-life for the center of pressure in the anterior–posterior direction ( p < 0.001), longer excursions of the center of pressure in the medio-lateral ( p < 0.001) and anterior–posterior ( p = 0.001) directions, increased center of pressure path lengths ( p < 0.001), and increased 95% ellipse areas of the center of pressure ( p < 0.001). The results from this study showed that even though older subjects demonstrated more frequent postural adjustments (shorter entropic half-life), this did not help to reduce the magnitude of movement of their center of pressure during quiet stance, thus indicating an impaired peripheral and/or central neuromuscular control mechanism.

Postural sway in single-limb and bilateral quiet standing after unilateral total knee arthroplasty

AIM

To investigate whether total knee arthroplasty (TKA) was associated with stability in single-limb stance and whether reduced stability in single-limb stance was associated with increased postural sway in bilateral quiet standing.

METHODS

3D kinematics for center of mass was used to assess postural sway in 23 subjects with TKA and 23 controls. Tests included bilateral quiet standing with and without vision and on a compliant surface, and single-limb stance.

RESULTS

30% of the subjects in the TKA group were unable to maintain single-limb stance for 20s on any leg. Of the 70% in the TKA group able to stand on one leg, mean sway velocity in the medio-lateral direction was marginally higher for the prosthetic side (p=.02), but no differences were found between the TKA and the control group in single-limb stance. Performance in bilateral quiet standing was similar in TKA-subjects, able as well as unable to stand on one leg, and controls. Reduced quadriceps strength in the contralateral leg, higher BMI, and older age predicted failure to maintain single-limb stance.

CONCLUSION

In subjects able to stand on one leg, performance was considered comparable between the prosthetic and contralateral side and between groups. Inability to stand on one leg did not affect postural sway in bilateral quiet standing. The results suggest that inability to maintain single-limb stance is explained by reduced physical capacity rather than the knee condition in itself. The present study emphasizes the importance of physical activity to improve strength and functional capacity.

Postural instability detection: aging and the complexity of spatial-temporal distributional patterns for virtually contacting the stability boundary in human stance

Falls among the older population can severely restrict their functional mobility and even cause death. Therefore, it is crucial to understand the mechanisms and conditions that cause falls, for which it is important to develop a predictive model of falls. One critical quantity for postural instability detection and prediction is the instantaneous stability of quiet upright stance based on motion data. However, well-established measures in the field of motor control that quantify overall postural stability using center-of-pressure (COP) or center-of-mass (COM) fluctuations are inadequate predictors of instantaneous stability. For this reason, 2D COP/COM virtual-time-to-contact (VTC) is investigated to detect the postural stability deficits of healthy older people compared to young adults. VTC predicts the temporal safety margin to the functional stability boundary ( =  limits of the region of feasible COP or COM displacement) and, therefore, provides an index of the risk of losing postural stability. The spatial directions with increased instability were also determined using quantities of VTC that have not previously been considered. Further, Lempel-Ziv-Complexity (LZC), a measure suitable for on-line monitoring of stability/instability, was applied to explore the temporal structure or complexity of VTC and the predictability of future postural instability based on previous behavior. These features were examined as a function of age, vision and different load weighting on the legs. The primary findings showed that for old adults the stability boundary was contracted and VTC reduced. Furthermore, the complexity decreased with aging and the direction with highest postural instability also changed in aging compared to the young adults. The findings reveal the sensitivity of the time dependent properties of 2D VTC to the detection of postural instability in aging, availability of visual information and postural stance and potential applicability as a predictive model of postural instability during upright stance.

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.