Changes in postural sway and its fractions in conditions of postural instability

We investigated changes in postural sway and its fractions associated with manipulations of the dimensions of the support area. Nine healthy adults stood as quietly as possible, with their eyes open, on a force plate as well as on 5 boards with reduced support area. The center of pressure (COP) trajectory was computed and decomposed into rambling (Rm) and trembling (Tr) trajectories. Sway components were quantified using RMS (root mean square) value, average velocity, and sway area. During standing on the force plate, the RMS was larger for the anterior-posterior (AP) sway components than for the mediolateral (ML) components. During standing on boards with reduced support area, sway increased in both directions. The increase was more pronounced when standing on boards with a smaller support area. Changes in the larger dimension of the support area also affected sway, but not as much as changes in the smaller dimension. ML instability had larger effects on indices of sway compared to AP instability. The average velocity of Rm was larger while the average velocity of Tr was smaller in the AP direction vs. the ML direction. The findings can be interpreted within the hypothesis of an active search function of postural sway. During standing on boards with reduced support area, increased sway may by itself lead to loss of balance. The findings also corroborate the hypothesis of Duarte and Zatsiorsky that Rm and Tr reveal different postural control mechanisms.     

Early postural adjustments in preparation to whole-body voluntary sway

We studied postural adjustments associated with a quick voluntary postural sway under two conditions, self-paced and simple reaction-time. Standing subjects were required to produce quick discrete shifts of the center of pressure (COP) forward. About 400-500 ms prior to the instructed COP shift, there were deviations of the COP in the opposite direction (backwards) accompanied by changes in the activation levels of several postural muscles. Under the reaction-time conditions, the timing of those early postural adjustments did not change (repeated measures MANOVA: p > 0.05) while its magnitude increased significantly (confirmed by repeated measures MANOVA: p < 0.05). These observations are opposite to those reported for anticipatory postural adjustments under simple reaction time conditions (a significant change in the timing without major changes in the magnitude). We conclude that there are two types of feed-forward postural adjustments. Early postural adjustments prepare the body for the planned action and/or expected perturbation. Some of these preparatory actions may be mechanically necessary. Later, anticipatory postural adjustments generate net forces and moments of force acting against those associated with the expected perturbation. Both types of adjustments fit well the referent configuration hypothesis, which offers a unified view on movement-posture control.     

Large postural fluctuations but unchanged postural sway dynamics during tiptoe standing compared to quiet standing

The purpose of this study was to detect the characteristics of center of pressure (COP) movement during tiptoe standing (TS) compared to quiet standing (QS). Eight healthy subjects were asked to perform QS and TS on a force platform. During standing, surface electromyograms (EMGs) were recorded from the soleus (SOL), flexor hallucis brevis (FHB), medial gastrocnemius (MG), lateral gastrocnemius (LG), and tibialis anterior (TA) muscles. The path length and rectangular area of the COP trajectory were significantly larger during TS than during QS. In contrast, irrespective of standing condition, the scaling coefficients in the short and long regions were above and below 0.5, respectively. The coherence spectrum between the COP and EMG from the SOL and FHB muscles was statistically significant during TS at frequencies up to 17 Hz, while that for the QS was only significant below 1 Hz. In conclusion, the control of COP movement during TS was similar to that during QS despite large COP fluctuations during TS. Our results suggest that unstable posture during TS is compensated for by the activities of the SOL and FHB muscles, which enhance postural control.     

Modeling human postural sway using an intermittent control and hemodynamic perturbations

Ground reaction force during human quiet stance is modulated synchronously with the cardiac cycle through hemodynamics [1]. This almost periodic hemodynamic force induces a small disturbance torque to the ankle joint, which is considered as a source of endogenous perturbation that induces postural sway. Here we consider postural sway dynamics of an inverted pendulum model with an intermittent control strategy, in comparison with the traditional continuous-time feedback controller. We examine whether each control model can exhibit human-like postural sway, characterized by its power law behavior at the low frequency band 0.1–0.7 Hz, when it is weakly perturbed by periodic and/or random forcing mimicking the hemodynamic perturbation. We show that the continuous control model with typical feedback gain parameters hardly exhibits the human-like sway pattern, in contrast with the intermittent control model. Further analyses suggest that deterministic, including chaotic, slow oscillations that characterize the intermittent control strategy, together with the small hemodynamic perturbation, could be a possible mechanism for generating the postural sway.     

Motor deficits in schizophrenia quantified by nonlinear analysis of postural sway

Motor dysfunction is a consistently reported but understudied aspect of schizophrenia. Postural sway area was examined in individuals with schizophrenia under four conditions with different amounts of visual and proprioceptive feedback: eyes open or closed and feet together or shoulder width apart. The nonlinear complexity of postural sway was assessed by detrended fluctuation analysis (DFA). The schizophrenia group (n = 27) exhibited greater sway area compared to controls (n = 37). Participants with schizophrenia showed increased sway area following the removal of visual input, while this pattern was absent in controls. Examination of DFA revealed decreased complexity of postural sway and abnormal changes in complexity upon removal of visual input in individuals with schizophrenia. Additionally, less complex postural sway was associated with increased symptom severity in participants with schizophrenia. Given the critical involvement of the cerebellum and related circuits in postural stability and sensorimotor integration, these results are consistent with growing evidence of motor, cerebellar, and sensory integration dysfunction in the disorder, and with theoretical models that implicate cerebellar deficits and more general disconnection of function in schizophrenia.     

Bi-lateral reactive force patterns in postural sway activity of normal subjects

Postural activity of normal subjects while standing still, as measured via the reactive foot-ground reactive forces, was investigated separately for each of the supporting legs, to provide detailed information on the individual activity of each side. Such information is not accessible if the overall reactive forces on the whole body are measured instead. Twenty-three normal adult subjects (average age 41 years) took part in the measuring tests, which were made on two collaterally installed force platforms. From the force traces obtained, the following parameters were determined: timings and amplitudes of the waveforms, separately for each foot and sequence of the force vectors on both feet and in relation to each other. Weight-bearing imbalance was defined in the vertical direction to express the difference between the average forces supported by each of the legs. Two parameters were defined by combining the force results obtained on each of the legs in the horizontal plane: sway total activity, representing the resultant of the added vectornorms in the anteroposterior and mediolateral directions, respectively; and asymmetry, representing the resultant of the subtracted above vector norms. The results indicated that, although the force traces were synchronous to each other, different vectorial force patterns were found, indicating different levels of stabilizing activities on each of the legs. This was also demonstrated by the fact that sway total activity was found considerably higher than the net reactive forces acting on the whole body during sway. Hence, in treating the external forces involved in the stabilization and regulation of human posture, the activity of each of the legs should be taken into account, rather than the resultant forces acting on the body as a whole. The obtained results may thus serve as input force vectors in a multi-segmental model of biped standing.This research was supported by the Technion V.P.R. Fund — M. Mandel Biomedical Engineering Research Fund     

Characteristics of postural sway in older adults standing on a soft surface

Postural responses to challenging situations were studied in older adults as they stood on a foam surface. The experiment was designed to assess the relative contributions made by visual and somatosensory information to the correction of postural sway. Twenty-four subjects, aged 56-83, stood for 20 s on a 1) firm or 2) foam surface with 1) the eyes open or 2) the eyes closed. Centre-of-pressure trajectories under the subjects' feet were measured by using a force platform. A repeated-measure two-way MANCOVA (two surfaces vs. two vision conditions) showed a significant main effect for the surface, but not for the vision. No covariate effect for age was found. Anterior-posterior sway increased in the subjects who were merely standing on the foam surface independent of the vision condition. Medial-lateral sway dramatically increased if the subjects stood on the foam surface with their eyes closed, but not if they stood with their eyes open. These results indicate that older adults rely more on visual information to correct mediolateral postural sway. It appears that the deterioration in visual acuity that occurs with aging may increase the risk of sideway falls, particularly in challenging situations, e.g., when standing on irregular or soft surfaces.     

The influence of stationary auditory fields on postural sway behaviour in man

Postural sway behaviour was investigated in 30 young subjects (15 male and 15 female) during 60 s of erect standing, under various combinations of auditory and visual input. Sway was assessed using a standard biomechanical measuring platform, the output of which led directly to an online computer from which the following parameters were determined: mean lateral and antero-posterior sway, velocity and radius of sway, length of the sway path and area within the sway profile. A marked difference in sway behaviour between the sexes was observed, with women showing increased magnitudes of some sway parameters. Postural sway was significantly increased in conditions without visual feedback. The presence of an auditory field tends to have a destabilising influence on sway behaviour, with both the direction of the sound source and the type of auditory input being important variables. Nevertheless there appears to be no interaction between the visual and the auditory environment in the control of posture.     

Model of two noise source dependent Ornstein-Uhlenbeck processes applied to postural sway

In this study the postural control system is modeled in terms of two counteracting bio-subsystems. Their activities are described by two Ornstein-Uhlenbeck processes with different-in-magnitude noise sources. The model is constructed as a sum of these processes, where in each of them the same noise source with opposite sign of the noise coefficients was introduced. Since the friction coefficients are also different for these processes, the delay of a crossover from ballistic to diffusive motion for one of the subsystems is greater than for its counterpart. It turns out that for smaller time intervals a superdiffusive behavior is observed, whereas, counteraction of subsystems is called into play for a larger time intervals, what for investigated range of data, is exhibited as a slow, subdiffusive behavior.     

Identification of the nonlinear state-space dynamics of the action-perception cycle for visually induced postural sway

Human subjects standing in a sinusoidally moving visual environment display postural sway with characteristic dynamical properties. We analyzed the spatiotemporal properties of this sway in an experiment in which the frequency of the visual motion was varied. We found a constant gain near 1, which implies that the sway motion matches the spatial parameters of the visual motion for a large range of frequencies. A linear dynamical model with constant parameters was compared quantitatively with the data. Its failure to describe correctly the spatiotemporal properties of the system led us to consider adaptive and nonlinear models. To differentiate between possible alternative structures we directly fitted nonlinear differential equations to the sway and visual motion trajectories on a trial-by-trial basis. We found that the eigenfrequency of the fitted model adapts strongly to the visual motion frequency. The damping coefficient decreases with increasing frequency. This indicates that the system destabilizes its postural state in the inertial frame. This leads to a faster internal dynamics which is capable of synchronizing posture with fast-moving visual environments. Using an algorithm which allows the identification of essentially nonlinear terms of the dynamics we found small nonlinear contributions. These nonlinearities are not consistent with a limit-cycle dynamics, accounting for the robustness of the amplitude of postural sway against frequency variations. We interpret our results in terms of active generation of postural sway specified by sensory information. We derive also a number of conclusions for a behavior-oriented analysis of the postural system.     

Prolonged occupational activity and 6-year changes in postural sway in elderly women

Objectives:

Prolonged occupational work such as farm work has been reported to adversely affect mobility in elderly women. The purpose of this study was to investigate possible relationships between prolonged occupational work and 6-year changes in postural sway in elderly women.

Methods:

Subjects were 392 women aged ≥69 years who participated in a 6-year follow-up examination of the Muramatsu Cohort Study. Handgrip strength and postural sway, measured as gravity-center velocity (cm/s), were evaluated at baseline and 6-year follow-up. Interviews were conducted to determine the time spent on moderate occupational activity (3-5 metabolic equivalents) such as farm work. Activity levels were defined as: 1, no-activity; 2, “short” (>0, ≤17.75 h/wk); and 3, “long” (≥17.75 h/wk).

Results:

At baseline, mean values for age, handgrip strength, and postural sway were 73.3 years (SD 3.7), 20.3 kg (SD 4.1), and 2.0 cm/s (SD 0.8), respectively, and 32.5% of participants engaged in occupational activity. The change in postural sway was significantly greater in the long-activity group (median, 35.0 h/wk) than the no-activity group (0.56 vs. 0.27 cm/s, P=0.021).

Conclusions:

Prolonged occupational work may be detrimental to the control of body balance. Accordingly, elderly individuals are not recommended to engage in prolonged occupational activity.     

The influence of moving auditory fields on postural sway behaviour in man.

Postural sway behaviour was assessed, using a standard biomechanical measuring platform, in 30 young subjects (15 men, 15 women) during 60 s of erect standing in various combinations of visual input and moving auditory fields. The sway parameters investigated were mean lateral, antero-posterior, radius and velocity of sway, the area within the sway profile and the length of the sway path. The findings support the view that moving auditory fields have a destabilising influence on postural sway behaviour, and suggest that under the appropriate conditions postural sway can be "driven" by the auditory environment.     

Shannon and Renyi entropies to classify effects of Mild Traumatic Brain Injury on postural sway

Background

Mild Traumatic Brain Injury (mTBI) has been identified as a major public and military health concern both in the United States and worldwide. Characterizing the effects of mTBI on postural sway could be an important tool for assessing recovery from the injury.

Methodology/Principal Findings

We assess postural sway by motion of the center of pressure (COP). Methods for data reduction include calculation of area of COP and fractal analysis of COP motion time courses. We found that fractal scaling appears applicable to sway power above about 0.5 Hz, thus fractal characterization is only quantifying the secondary effects (a small fraction of total power) in the sway time series, and is not effective in quantifying long-term effects of mTBI on postural sway. We also found that the area of COP sensitively depends on the length of data series over which the COP is obtained. These weaknesses motivated us to use instead Shannon and Renyi entropies to assess postural instability following mTBI. These entropy measures have a number of appealing properties, including capacity for determination of the optimal length of the time series for analysis and a new interpretation of the area of COP.

Conclusions

Entropy analysis can readily detect postural instability in athletes at least 10 days post-concussion so that it appears promising as a sensitive measure of effects of mTBI on postural sway.

Availability

The programs for analyses may be obtained from the authors.     

Low-frequency force steadiness practice in plantar flexor muscle reduces postural sway during quiet standing

The purpose of this study was to assess the effect of low-frequency force steadiness practice in the plantar flexor muscles on postural sway during quiet standing. Healthy young 21 men (21±1 yrs) were randomly assigned to a practice group (n=14) and a nonexercising control group (n=7). Practice groups were divided by frequency of practice: 7 participants practiced once a week, and the other 7 twice a week, for 4 weeks. Steadiness practice required practice group to 5 sets of 60-s contraction at levels corresponding to 10% and 20% maximal voluntary contraction (MVC) in the plantar flexor muscles. The 4-week-long practice period reduced the force fluctuations (assessed as the standard deviation (SD) of the outputted force during steady isometric plantar flexion) and postural sway (assessed as SD of the center of mass velocity during quiet standing). However, these practice effects were not significantly affected by the practice frequencies (1 vs. 2 sessions per week) examined in this study. Further, a linear regression analysis revealed the association between prepractice postural sway and the relative change in postural sway by the practice (r=-0.904) in the practice group. These results suggest that the steadiness practice in plantar flexor muscles improves postural stability during quiet standing, even though the practice is low-frequency (once a week) and low-intensity (within 20% MVC). These practice effects are dependent on prepractice postural stability. Further, the present results have provided the functional significance of force fluctuation in lower limb muscles.     

The spectral content of postural sway during quiet stance: Influences of age, vision and somatosensory inputs

Maintenance of human upright stance requires the acquisition and integration of sensory inputs. Conventional measures of sway have had success in identifying age- and some disease-related changes, but remain unable to address the complexities and dynamics associated with postural control. We investigated the effects of vision, surface compliance, age, and gender on the spectral content of center of pressure (COP) time series. Sixteen healthy young (age 18-24) and older participants (age 55-65) performed trials of quiet, upright stance under different vision (eyes open vs. closed) and surface (hard vs. compliant) conditions. Spectral analyses were conducted to describe COP mean normalized power in discretized bands. Effects of the two sensory modalities and age were distinct in the antero-posterior and medio-lateral directions, and a reorganization of spectral content was evident with increasing task difficulty (eyes open vs. closed and hard vs. compliant surface) and among older adults. These results indicate that vision and surface compliance are predominantly associated with responses from musculature associated with antero-posterior and medio-lateral directions of sway, respectively. Finally, distinguishing between the contributions of different afferent systems to the postural control system using the spectral content of sway bi-directionally may help in diagnosing individuals with balance impairments.     

Role of sequence and structure of the hendra fusion protein fusion Peptide in membrane fusion

Viral fusion proteins are intriguing molecular machines that undergo drastic conformational changes to facilitate virus-cell membrane fusion. During fusion a hydrophobic region of the protein, termed the fusion peptide (FP), is inserted into the target host cell membrane, with subsequent conformational changes culminating in membrane merger. Class I fusion proteins contain FPs between 20 and 30 amino acids in length that are highly conserved within viral families but not between. To examine the sequence dependence of the Hendra virus (HeV) fusion (F) protein FP, the first eight amino acids were mutated first as double, then single, alanine mutants. Mutation of highly conserved glycine residues resulted in inefficient F protein expression and processing, whereas substitution of valine residues resulted in hypofusogenic F proteins despite wild-type surface expression levels. Synthetic peptides corresponding to a portion of the HeV F FP were shown to adopt an α-helical secondary structure in dodecylphosphocholine micelles and small unilamellar vesicles using circular dichroism spectroscopy. Interestingly, peptides containing point mutations that promote lower levels of cell-cell fusion within the context of the whole F protein were less α-helical and induced less membrane disorder in model membranes. These data represent the first extensive structure-function relationship of any paramyxovirus FP and demonstrate that the HeV F FP and potentially other paramyxovirus FPs likely require an α-helical structure for efficient membrane disordering and fusion.     

Effect of six weeks of dura disc and mini-trampoline balance training on postural sway in athletes with functional ankle instability

Lateral ankle sprain (LAS) is one of the most common injuries incurred during sporting activities, and effective rehabilitation programs for this condition are challenging to develop. The purpose of this research was to compare the effect of 6 weeks of balance training on either a mini-trampoline or a dura disc on postural sway and to determine if the mini-trampoline or the dura disc is more effective in improving postural sway. Twenty subjects (11 men, 9 women) with a mean age of 25.4 +/- 4.2 years were randomly allocated into a control group, a dura disc training (DT) group, or a mini-trampoline (MT) group. Subjects completed 6 weeks of balance training. Postural sway was measured by subjects performing a single limb stance on a force plate. The disbursement of the center of pressure was obtained from the force plate in the medial-lateral and the anterior-posterior sway path and was subsequently used for pretest and posttest analysis. After the 6-week training intervention, there was a significant (p < 0.05) difference in postural sway between pre- and posttesting for both the MT (pretest = 56.8 +/- 20.5 mm, posttest = 33.3 +/- 8.5 mm) and DT (pretest = 41.3 +/- 2.6 mm, posttest = 27.2 +/- 4.8 mm) groups. There was no significant (p > 0.05) difference detected for improvements between the MT and DT groups. These results indicate that not only is the mini-trampoline an effective tool for improving balance after LAS, but it is equally as effective as the dura disc.     

Natural sway frequencies and damping ratios of trees: influence of crown structure

Natural frequency and damping ratio were measured for nine plantation-grown Douglas-fir (Pseudotsuga menziesii Mirb. Franco) trees from the Oregon Coast Range under different levels of crown removal. Natural frequency of trees, in both their unpruned and completely de-branched states, was linearly related to the ratio of diameter at breast height to total tree height squared (i.e., DBH/H²), as expected from the theory governing the oscillation of a cantilever beam. Pruning resulted in an increase in natural frequency; however, at least 80% of the crown mass needed to be removed before this increase was noticeable. A single equation was developed that enabled the natural frequency of a tree of given size and pruning intensity to be predicted. Damping ratios of unpruned trees varied considerably from 8% to almost critical, while those for completely de-branched trees ranged from 1% to 8%. Two different trends in damping ratio were observed during pruning. Some trees exhibited an increase in damping ratio with initial crown removal, followed by a sharp decrease when the uppermost portion of the crown was removed. Others showed little or no change in damping ratio followed by a sharp reduction upon removal of the uppermost portion of the crown. Damping was mainly due to aerodynamic drag and preventing interference with neighbouring trees had little effect. Theoretical analysis using the finite element method indicated that changes in natural frequency as a result of pruning are not due to changes in damping ratio, but rather changes in mass distribution. This analysis also suggested that treating branches as lumped masses rather than individual cantilevers attached to the main stem may not be appropriate.