Postural Sway Measures Research Articles

Unilateral lower limb muscle fatigue induces bilateral effects on undisturbed stance and muscle EMG activities

The study investigated the effects of an unilateral ankle muscle fatigue onto independent postural control parameters including the trajectories of the estimated resultant CoP (CoPres) and his components: the centre of gravity (CG) and CoP–CG trajectories. Nine healthy men realized series of 10 toe-lift immediately followed by 10 knee flexions until exhaustion with one (Ex) leg. Maximal isometric voluntary contractions, postural sway measures of each leg, and muscular activities of the ankle muscles were recorded before and immediately after the fatiguing exercise. As expected, the latter induced a decrease in maximal voluntary peak force associated with a greater variability of the relative contribution of each leg on the CoPres, enhanced all postural parameters of the non-exercised leg. A significant decreased of the tibialis anterior EMG activity for the Ex leg and an increased one for the NoEx leg. Finally, following unilateral fatigue, the body sway destabilisation seemed to occur only along the medio-lateral (ML) axis. The enhanced and greater variability of the variance along ML axis might be explained by the recourse at the loading–unloading strategy choice and suggests a central attempt to compensate for pain sensation.

Perceptions of Stability upon Standing from Working Postures Used in the Construction Industry

Falls on the same level or from heights are a significant cause of injuries and fatalities among the American workforce, particularly within the construction industry. Instability upon standing may be a contributing factor for many fall-related injuries and fatalities. This study explored self-reports of postural stability upon standing after maintaining static postures for varying durations. Thirty-three male participants completed three replications of the conditions created by four postures and three durations within posture for a total of 36 conditions. The results indicated that the working posture used to complete a task significantly affected postural stability upon standing (ANOVA, α=0.05), although individuals were not able to distinguish these changes. Postural stability ratings were affected more by the duration within posture (p=0.001). Small correlation values between the subjective ratings and the objective COP-based measures substantiated the finding that individuals were not sensitive to the changes in postural sway measures caused by the differing postures and may not be fully aware of the risk to balance encountered during transitions. Findings may lead to recommendations for redesign of tasks or tools to reduce the use of certain working postures (e.g. kneeling), particularly in high-risk environments such as construction.

Loss of otolith function with age is associated with increased postural sway measures

Loss of balance and increased fall risk is a common problem associated with aging. Changes in vestibular function occur with aging but the contribution of reduced vestibular otolith function to fall risk remains unknown. We examined a population of 151 healthy individuals (aged 21–93) for both balance (sway measures) and ocular counter-rolling (OCR) function. We assessed balance function with eyes open and closed on a firm surface, eyes open and closed on a foam surface and OCR during ±20 degree roll tilt at 0.005 Hz. Subjects demonstrated a significant age-related reduction in OCR and increase in postural sway. The effect of age on OCR was greater in females than males. The reduction in OCR was strongly correlated with the mediolateral measures of sway with eyes closed. This correlation was also present in the elderly group alone, suggesting that aging alone does not account for this effect. OCR decreased linearly with age and at a greater rate in females than males. This loss of vestibular otolith-ocular function is associated with increased mediolateral measures of sway which have been shown to be related to increased risk of falls. These data suggest a role for loss of otolith function in contributing to fall risk in the elderly. Further prospective, longitudinal studies are necessary to confirm these findings.

The effects of obesity and standing time on postural sway during prolonged quiet standing

This study examined the effects of obesity level, standing time and their interaction on postural sway during a prolonged quiet upright standing task. Ten extremely obese (BMI > 40 kg/m2) and 10 non-obese (18.5 kg/m2 < BMI < 24.9 kg/m2) participants performed quiet upright standing on a force plate for over 18 min. Eleven postural sway measures were computed for each 1-min time interval based on the centre-of-pressure data from the force plate. ANOVA and regression analyses showed that for all the 11 postural sway measures, the extremely obese group had higher postural sway than the non-obese at the beginning of the prolonged standing task and postural sway increased significantly faster for the extremely obese group than the non-obese over time. The results suggest that obesity may impair postural control and may be a risk factor of balance loss and falls, especially during prolonged physical work activities. The research findings are relevant to identifying and reducing risks of balance loss and falls in various workplace settings for a wide variety of workers.

The role of eye movement in upright postural control

Vision contributes to upright postural control by providing afferent feedback to the cerebellum. Vision is generally classified into central and peripheral vision. In measurements of postural sway, in which participants are required to maintain a stable upright posture while fixating on a visual target, non-retinal eye positional information due to the fixation is used as well as the retinal information from both visual fields. However, little is known about the role of non-visual eye positional information in postural control. This study examined the role of non-visual eye position information in upright postural control by comparing participants’ centre of pressure (COP) sway between two experimental conditions: (1) a space-fixed visual target condition (control), in which eye movement was not controlled, and (2) a head-fixed visual target condition (treatment), in which eye movement was inhibited. Using 12 university students, COP sway and electrooculograms (EOG) were measured under both conditions. In the space-fixed condition, participants maintain an upright posture while fixating on a visual target fixed on a screen 1 m in front of them. In the head-fixed condition, participants maintained an upright posture while gazing at a target moving in sync with their head sway on the screen. The COP was evaluated by path length, area, root mean square, velocity and position. Eye movements were evaluated by the mean eye movement angle. The mean eye movement angle was significantly larger in the vertical direction then in the horizontal direction in both experimental conditions and was also found to be larger in the space-fixed condition than in the head-fixed condition. No significant different was found in any COP parameter between both conditions. It was suggested that non-visual eye position information from the external eye muscles to the sensory perception system contributes little to postural stabilisation under the measurement conditions used in this study.

Effects of localized muscle fatigue on recovery from a postural perturbation without stepping

Several investigations have demonstrated that localized muscle fatigue (LMF) causes an increase in postural sway measures during quiet stance. Since many falls are likely the result of a postural perturbation, this study investigated the effects of LMF on balance recovery from sagittal plane postural perturbations. Thirty-two participants (16 young, 16 older) were tested. Postural perturbations were administered with ballistic pendulums before and after exercises to fatigue the lumbar extensors and plantar flexors. Measures of balance recovery were based on the center of pressure (COP) and center of mass (COM) trajectories and the maximum perturbation that could be withstood without stepping. A covariate analysis that included initial conditions at the time of the perturbation was also performed. The results demonstrated changes in the COM trajectory that were consistent with an LMF-induced decrement in the ability to recover from the perturbations without stepping. Interpretation of the COP trajectory was presented in light of the COM and indicated a modified postural control strategy following LMF.

Orthostatic hypotension, balance and falls in Parkinson's disease

Orthostatic hypotension (OH) is a common feature in Parkinson's disease (PD). As the control of balance and gait is already affected by PD per se, OH may further predispose patients to falls and accidents. The study was conducted to evaluate the clinical correlates of OH and its association with mobility and balance in PD. From a total population of 205,000 inhabitants, 120 PD patients were included in the study. Medical data including history of recent falls were collected, and patients were clinically examined using the orthostatic test, the Timed Up & Go test, walking speed, and the quantitative measurement of postural sway. Sixty-three (52.5%) patients had OH in the orthostatic test. Twenty-five (39.5%) patients with and 16 (28.1%) patients without OH (P = 0.614) had fallen during the past 3 months. Patients with OH had significantly increased postural sway in standing compared with patients without OH. However, OH was not associated with mobility or walking speed. The current results support the concept that the control of body balance and OH may be closely linked.

Mobility and balance in Parkinson’s disease: a population‐based study

To assess the clinical correlates of mobility and balance, and to identify the risk factors for falls in Parkinson's disease (PD). One-hundred and nineteen PD patients underwent clinical examination and tests for mobility and balance using the Timed Up & Go (TUG) test, walking speed, and the measurement of postural sway. The fallers (35% of the subjects) performed significantly worse in the TUG test than the non-fallers, and they also had a slower walking speed (P = 0.037 and P = 0.006, respectively). The total Unified Parkinson's Disease Rating Scale (UPDRS) score and age were positively associated with the TUG-test score. The severity of the disease and the use of walking aids correlated negatively with the walking speed, whereas the use of dopamine agonists was positively associated with the walking speed. The UPDRS total score [odds ratio (OR) 1.04, 95% confidence intervals (CI) 1.01-1.07] and increased postural sway (OR 1.25, 95% CI 1.02-1.54) were independent risk factors for falling in PD. Advanced age and severity of the disease are related to impaired mobility and balance in PD patients. The severity of the disease and increased postural sway seem to be the most important independent risk factors for falling in PD.

Postural sway following prolonged exposure to an inclined surface

Fatal and non-fatal falls from elevation have been documented to be a significant issue. Historically, roofing work has been ranked among the highest in incidents of falls. Recent literature has indicated that exposure to inclined surfaces (roof) has a detrimental effect on postural stability. The purpose of this preliminary study was to determine if postural instability was found to increase following extended exposure to an inclined surface. Twenty-three participants were pre/post tested under four postural stability conditions while varying duration exposure (0.0–2.0 h) to an inclined surface. Dependent variable postural sway measures were calculated from center of pressure data derived from a portable force plate. The results indicate significant differences between pre and post-exposure measures. Specifically, the results imply that an individual is less stable directly after performing tasks on an inclined surface. These findings contribute to the literature and serve as a basis for further research into work rest cycles for those individuals who routinely work on inclined surfaces.

Direction-Induced Effects of Visually Guided Weight-Shifting Training on Standing Balance in the Elderly

Background: Controversial findings exist in the literature with respect to the efficacy of visually guided weight-shifting (WS) training as a means of improving balance in healthy older adults. Objective: The purpose of this study was to investigate the impact of two direction-specific, visually guided WS training protocols on standing balance of healthy elderly women. Methods: Forty-eight community-dwelling elderly women, all free of any neurological or musculoskeletal impairment, were randomly assigned into: a group that practiced WS in the anterior/posterior direction (A/P group, n = 19), a group that practiced WS in the medio/lateral direction (M/L group, n = 15) and a control group (n = 14). Participants performed 12 training sessions of visually guided WS (3 sessions a week for 25 minutes per session). Static balance was measured before and after training in normal (bipedal) quiet stance (NQS) and sharpened-Romberg stance (SRS) by recording center of pressure (CoP) variations and angular segment kinematics. Results: In NQS, neither of the two training protocols had a significant impact on postural sway measures, although a significant decrease in interlimb asymmetry of CoP displacement was noted for the A/P group. In SRS, A/P training induced a significant reduction of CoP displacement, lower limb pitch and upper trunk roll rotation. Conclusion: The results of the study stress the importance of using direction-specific WS tasks in balance training, particularly in the A/P direction, in order to improve control of static balance in elderly women.

Older Women with Age‐Related Macular Degeneration Have a Greater Risk of Falls: A Physiological Profile Assessment Study

To determine whether older women with exudative age-related macular degeneration (AMD) are at greater risk of falls. Cross-sectional study. A hospital-based ophthalmology clinic in Vancouver, Canada. One hundred fifteen older (aged > or = 70) community-dwelling women with exudative AMD (AMD cohort) and two control groups: 54 community-dwelling women without exudative AMD drawn from the same community (non-AMD cohort) and 341 community-dwelling Australian women (Australian normative cohort). Participants were assessed for falls risk using the short-form Physiological Profile Assessment (PPA), which provides a fall risk index score and subcomponent measures of vision, proprioception, strength, reaction time, and postural sway. The mean fall risk index score in the AMD cohort (3.20) was significantly greater than that of the non-AMD cohort (1.21; P<.001), and fall risk scores increased with age to a greater extent in the AMD cohort. The higher fall risk scores in the AMD cohort resulted from significantly worse performance on each PPA test, not just the test of vision. The AMD cohort also performed worse than the Australian normative cohort in tests of vision, reaction time, and postural sway. Older women with AMD have impaired balance, slow visual reaction times, and poor vision, which in combination result in a significantly greater risk of falls than population norms. These deficits are clearly indicated in the physiological falls profile for the group. Strategies to enhance balance may be particularly beneficial to prevent falls in this group.

Reliability of COP-based postural sway measures and age-related differences

The objectives of this study were to assess the within-day and between-day reliability of several center of pressure (COP)-based measures of postural sway and identify whether there were age-related differences in reliability. Thirty-two healthy individuals (16 younger and 16 older) participated. COP was recorded during quiet upright stance on 4 different days, and a variety of measures determined: mean velocity, median power frequency, RMS distance, sway area, and two fractal measures derived from Hurst rescaled range analysis ( H R/S) and detrended fluctuation analysis (DFA). Intraclass correlation coefficient (ICC) and standard error of measurement (SEM) were used to quantify reliability. Mean velocity was the most reliable measure. DFA exponents had relatively better reliability than H R/S exponents. In general, within-day reliability was better than between-day. In comparison with younger participants, older participants exhibited better relative reliability (ICC) for all COP measures and comparable absolute reliability (SEM) except for mean velocity and sway area. These results may be useful in guiding the future selection and interpretation of COP-based measures.

Balance (perceived and actual) and preferred stance width during pregnancy

Background Pregnant women often remark that their balance degrades during pregnancy; however, it appears that no studies have documented the gravida’s perception of her balance nor measured direction-specific changes in balance throughout pregnancy or after delivery. Methods Thirty women, fifteen pregnant and fifteen non-pregnant controls, were tested monthly and through 6-month postpartum. For each session, perceived degradation in sense of balance, laboratory-based balance measures, stance width, and the number of falls since the previous session were recorded. Laboratory-based balance measures, quantified by direction-specific measures of postural sway, were computed from ten 30 s quiet-standing trials on a stationary force platform. Repeated-measures analysis of variance, paired t-tests, and Pearson correlations were use to examine group and time effects. Findings For the pregnant group, perceived balance degradation and stance width were highly correlated ( r = 0.94). Both increased during pregnancy ( P ⩽ 0.016 ) and dropped to near-control levels after delivery ( P ⩽ 0.004 ). Compared to the control group, pregnant subjects displayed increased sway, especially in the anterior–posterior and radial directions ( P ⩽ 0.039 ). Anterior–posterior sway measures strongly correlated with perceived balance (0.82 > r > 0.72) and also decreased significantly between the third trimester and postpartum ( P ⩽ 0.029 ). Interestingly, medial–lateral balance measures varied little during pregnancy, but increased after delivery. Contrary to recent work suggesting fall rates of 25%, only 13% of our subjects ( n = 2) fell during pregnancy. Interpretation Perceived degradation in balance during pregnancy was strongly related to increasing postural sway instability in the anterior–posterior direction. Lateral stability was maintained during pregnancy and likely accomplished by increasing stance width.

Direct parameterization of postural stability during quiet upright stance: Effects of age and altered sensory conditions

The purpose of this study was to determine the relationship between measures of local dynamic stability (LDS) during upright stance and both descriptive measures of postural sway and a scaling index ( α) derived from detrended fluctuation analysis. Center of pressure (COP) time series were obtained from healthy participants (16 young and 16 older) during upright quiet stance. Vision and somatosensation were altered by eye closure and standing on a compliant surface, respectively. A non-linear time-series analysis method was used to compute three LDS parameters from the COP data: A which was defined as the COP excursion range in state space, and τ S and τ L which were defined as the divergence rates over short- and long-term timescales, respectively. LDS parameters, descriptive COP measures, and α had generally consistent sensitivities to age and/or altered sensory conditions. Age x sensory condition interactions, however, had distinct effects on LDS parameters compared to the other COP-based measures. Older individuals exhibited faster divergence rates while having similar magnitudes of A, compared to young individuals. These results suggest that older individuals stiffen the musculoskeletal system via increased muscle activity, perhaps as an age-related postural adaptation. In addition, correlations between LDS parameters and other COP measures were relatively small ( r 2⩽0.29). Hence, LDS parameters ( A, τ S and τ L) provide distinct information on postural control and stability, supplementing other COP-based measures.

Quantitative measurement of postural sway in mouse models of human neurodegenerative disease

Detection of motor dysfunction in genetic mouse models of neurodegenerative disease requires reproducible, standardized and sensitive behavioral assays. We have utilized a center of pressure (CoP) assay in mice to quantify postural sway produced by genetic mutations that affect motor control centers of the brain. As a positive control for postural instability, wild type mice were injected with harmaline, a tremorigenic agent, and the average areas of the 95% confidence ellipse, which measures 95% of the CoP trajectory values recorded in a single trial, were measured. Ellipse area significantly increased in mice treated with increasing doses of harmaline and returned to control values after recovery. We also examined postural sway in mice expressing mutations that mimic frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17) (T-279, P301L or P301L–nitric oxide synthase 2 (NOS2) −/− mice) and that demonstrate motor symptoms. These mice were then compared with a mouse model of Alzheimer’s disease (APPSwDI mice) that demonstrates cognitive, but not motor deficits. T-279 and P301L-NOS2 −/− mice demonstrated a significant increase in CoP ellipse area compared with appropriate wild type control mice or to mice expressing the P301L mutation alone. In contrast, postural instability was significantly reduced in APPSwDI mice that have cognitive deficits but do not have associated motor deficits. The CoP assay provides a simple, sensitive and quantitative tool to detect motor deficits resulting from postural abnormalities in mice and may be useful in understanding the underlying mechanisms of disease.

Assessment of postural instability in patients with Parkinson’s disease

Postural instability is one of the most disabling features of idiopathic Parkinson's disease (PD). In this study, we focused on postural instability as the main factor predisposing parkinsonians to falls. For this purpose, changes in sway characteristics during quiet stance due to visual feedback exclusion were studied. We searched for postural sway measures that could be potential discriminators for an increased fall risk. A group of 110 subjects: 55 parkinsonians (Hoehn and Yahr: 1-3), and 55 age-matched healthy volunteers participated in the experiment. Their spontaneous sway characteristics while standing quiet with eyes open and eyes closed were analyzed. We found that an increased mediolateral sway and sway area while standing with eyes closed are characteristic of parkinsonian postural instability and may serve to quantify well a tendency to fall. These sway indices significantly correlated with disease severity rated both by the Hoehn and Yahr scale as well as by the Motor Section of the UPDRS. A forward shift of a mean COP position in parkinsonians which reflects their flexed posture was also significantly greater to compare with the elderly subjects and exhibited a high sensitivity to visual conditions. Both groups of postural sway abnormalities identified here may be used as accessible and reliable measures which allow for quantitative assessment of postural instability in Parkinson's disease.

Effect of Orthoses on Postural Stability in Asymptomatic Subjects With Rearfoot Malalignment During a 6-Week Acclimation Period

Mattacola CG, Dwyer MK, Miller AK, Uhl TL, McCrory JL, Malone TR. Effect of orthoses on postural stability in asymptomatic subjects with rearfoot malalignment during a 6-week acclimation period. Objective To determine the effect of custom-fitted orthoses on postural sway over a 6-week acclimation period. Design Repeated-measures analysis of variance on postural sway measures with factors being group (control, malaligned), time (initial, 2wk, 4wk, 6wk postintervention), and condition (with orthoses, without orthoses). For single-limb stance, side (right, left) was analyzed to determine bilateral differences. Setting Biodynamics laboratory. Participants Twenty-one subjects, 11 asymptomatic with rearfoot malalignment and 10 asymptomatic with normal rearfoot alignment. Interventions Orthoses were prescribed and worn for 6 weeks. Balance testing was performed on 4 different dates with each subject tested in both orthotic conditions. Postural control was measured with three 10-second eyes-closed trials for single-limb stance, one 20-second eyes-closed bilateral stance with the platform moving, and one 20-second eyes-open bilateral stance with the platform and surroundings moving. Main Outcome Measures Sway velocity (in deg/s) for single-limb stance and equilibrium score for bilateral stance. Results Postural sway measures were significantly decreased during single-limb testing with orthoses versus without orthoses, regardless of group. The orthotic intervention significantly improved bilateral stance equilibrium score in the malaligned group at weeks 2, 4, and 6 when compared with measures at the initial week. Equilibrium score of the malaligned group with orthoses at initial week was significantly lower (worse) than the control group with orthoses at initial week; however, these results were not repeated during measurements taken at weeks 2, 4, or 6. Conclusions The application of orthoses decreased sway velocity for single-limb stance, improving postural stability regardless of group when visual feedback was removed. During bilateral stance, postural stability was initially worse for the malaligned group with and without orthoses when compared with the control group; however, improvements were seen by week 2 and continued throughout the remainder of testing. Clinically, the application of orthoses appears to improve postural control in people with rearfoot malalignment, particularly when vision is removed.

Influence of fatigue time and level on increases in postural sway

The purpose of this study was to investigate the influence of fatigue time and fatigue level on the increases in postural sway during quiet standing. Centre of pressure-based measures of postural sway were collected both before and after fatiguing participants using three different fatigue levels and two different fatigue times. Results showed increasing fatigue time increased sway velocity and sway area, and increasing fatigue level increased sway velocity. Fatigue time effects are important to consider when applying laboratory-based findings to the field given that the fatigue time can differ substantially between the two. Fatigue level effects imply a dose – response relationship between localized muscle fatigue and risk of falling that can have important implications in work/rest cycle scheduling for occupations at risk of injurious falls.

Reliability of Cop-Based Postural Sway Measures

Measures of human sway during upright standing are frequently used as indirect measures of the postural control system. The objective of this study was to assess the reliability of several center-of-pressure-based sway measures. Test-retest reliability was determined both within-days (under different visual and surface conditions) and between-days (5 different days). Intraclass Correlation Coefficient (ICC), Standard Error of Measurement (SEM), Minimal Metrically Detectable Change (MMDC), and Coefficient of Variation (CV) were used as reliability indices. Mean velocity was found to be the most reliable measure among those evaluated. Overall, between-day reliability indices were better than within-day reliability. When subjects were standing on a compliant surface, several measures exhibited better reliability under ‘eyes open’ condition. These results can be useful in guiding the selection of postural sway measures and appropriate number of replications to improve sway assessment.

Light Finger Touch on the Upper Legs Reduces Postural Sway during Quasi-Static Standing

The purpose of this study was to test whether a light finger touch on one's own body (upper legs) reduces postural sway. Ten healthy males participated. In the first part of the study, the participants stood upright with their eyes closed on a force platform while ground reaction force data were collected. Two conditions differing in the placement of the arms and fingers were tested. In the no-touch condition, the participants kept their hands in loose fists. In the finger-touch condition, the participants lightly touched the lateral sides of the upper legs with all fingers. Postural sway measures were calculated from the ground reaction force data. In the second part of the study, the participants stood upright on a pneumatic balance disk while ground reaction force data were collected. Experimental and measurement protocols were identical to those used in the first part of the study. The results showed that light finger touch on the upper legs significantly reduced postural sway on the balance disk up to approximately 7%. The data from this study suggest that decreased postural sway due to finger contact may improve balance control during other standing tasks.