Postural Stability in Young Adults with Down Syndrome in Challenging Conditions

To evaluate postural control and performance in subjects with Down syndrome (SwDS), we measured postural sway (COP) in quiet stance in four 20-second tests: with eyes open or closed and on hard or foam surface. Ten SwDS and eleven healthy subjects participated, aged 29.8 (4.8) and 28.4 (3.9), respectively. The time-series recorded with the sampling rate of 100 Hz were used to evaluate postural performance (COP amplitude and mean velocity) and strategies (COP frequency, fractal dimension and entropy). There were no intergroup differences in the amplitude except the stance on foam pad with eyes open when SwDS had larger sway. The COP velocity and frequency were larger in SwDS than controls in all trials on foam pad. During stances on the foam pad SwDS increased fractal dimension showing higher complexity of their equilibrium system, while controls decreased sample entropy exhibiting more conscious control of posture in comparison to the stances on hard support surface. This indicated that each group used entirely different adjustments of postural strategies to the somatosensory challenge. It is proposed that the inferior postural control of SwDS results mainly from insufficient experience in dealing with unpredictable postural stimuli and deficit in motor learning.     

Effects of two passive back-support exoskeletons on postural balance during quiet stance and functional limits of stability

While occupational back-support exoskeletons (BSEs) are considered as potential workplace interventions, BSE use may compromise postural control. Thus, we investigated the effects of passive BSEs on postural balance during quiet upright stance and functional limits of stability. Twenty healthy adults completed trials of quiet upright stance with differing levels of difficulty (bipedal and unipedal stance; each with eyes open and closed), and executed maximal voluntary leans. Trials were done while wearing two different BSEs (SuitX™, Laevo™) and in a control (no-BSE) condition. BSE use significantly increased center-of-pressure (COP) median frequency and mean velocity during bipedal stance. In unipedal stance, using the Laevo™ was associated with a significant improvement in postural balance, especially among males, as indicated by smaller COP displacement and sway area, and a longer time to contact the stability boundary. BSE use may affect postural balance, through translation of the human + BSE center-of-mass, restricted motion, and added supportive torques. Furthermore, larger effects of BSEs on postural balance were evident among males. Future work should further investigate the gender-specificity of BSE effects on postural balance and consider the effects of BSEs on dynamic stability.     

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.     

The Degrees of Freedom Problem in Human Standing Posture: Collective and Component Dynamics

The experiment was setup to investigate the coordination and control of the degrees of freedom (DFs) of human standing posture with particular reference to the identification of the collective and component variables. Subjects stood in 3 postural tasks: feet side by side, single left foot quiet stance and single left foot stance with body rocking at the ankle joint in the sagittal plane. All three postural tasks showed very high coherence (∼1) of center of pressure (COP) - center of mass (COM) in the low frequency range. The ankle and hip coherence was mid range (∼.5) with the tasks having different ankle/hip compensatory cophase patterns. The findings support the view that the in-phase relation of the low frequency components of the COP-COM dynamic is the collective variable in the postural tasks investigated. The motions of the individual joints (ankle, knee, hip, neck) and couplings of pair wise joint synergies (e.g., ankle-hip) provide a supporting cooperative role to the preservation of the collective variable in maintaining the COM within the stability region of the base of support (BOS) and minimizing the amount of body motion consistent with the task constraint.     

Time-to-boundary measures of postural control during single leg quiet standing

A novel approach to quantifying postural stability in single leg stance is assessment of time-to-boundary (TTB) of center of pressure (COP) excursions. TTB measures estimate the time required for the COP to reach the boundary of the base of support if it were to continue on its instantaneous trajectory and velocity, thus quantifying the spatiotemporal characteristics of postural control. Our purposes were to examine: (a) the intrasession reliability of TTB and traditional COP-based measures of postural control, and (b) the correlations between these measures. Twenty-four young women completed three 10-second trials of single-limb quiet standing on each limb. Traditional measures included mean velocity, standard deviation, and range of mediolateral (ML) and anterior-posterior (AP) COP excursions. TTB variables were the absolute minimum, mean of minimum samples, and standard deviation of minimum samples in the ML and AP directions. The intrasession reliability of TTB measures was comparable to traditional COP based measures. Correlations between TTB and traditional COP based measures were weaker than those within each category of measures, indicating that TTB measures capture different aspects of postural control than traditional measures. TTB measures provide a unique method of assessing spatiotemporal characteristics of postural control during single limb stance.     

Predicting the dynamic postural control response from quiet-stance behavior in elderly adults

Human postural sway, as measured by fluctuations of the center of pressure (COP) under the feet of a quietly standing individual, can be characterized as a stochastic process. The fluctuation-dissipation theorem (FDT) provides a linear relationship between the fluctuations of a quasi-static, stochastic system to the same system's relaxation to equilibrium following a perturbation. We applied a similar linear relationship, based on the FDT, to the human postural control system to explore whether anterior-posterior (AP) fluctuations of the COP during quiet stance can be used to predict the AP response of the postural control system to a weak posteriorly directed mechanical perturbation (tug or pull at the waist). We tested 10 healthy elderly (mean age of 69yr) and 10 healthy young (mean age of 25yr) adult subjects. We found that this linear relationship was applicable to the postural control system of all 10 young and eight of the 10 elderly adult subjects. These results suggest that it is possible to predict an individual's dynamic response to a mild perturbation using quiet-stance data, regardless of age. The existence of this FDT-based linear relationship with respect to the human postural control system suggests that, for a given individual, the postural control system may use the same control mechanisms during quiet stance and mild-perturbation conditions, regardless of age.     

Alterations in evertor/invertor muscle activation and center of pressure trajectory in participants with functional ankle instability

Participants with ankle instability demonstrate more foot inversion during the stance phase of gait than able-bodied subjects. Invertor excitation, coupled with evertor inhibition may contribute to this potentially injurious position. The purpose of this experiment was to examine evertor/invertor muscle activation and foot COP trajectory during walking in participants with functional ankle instability (FI). Twelve subjects were identified with FI and matched to healthy controls. Tibialis anterior (TA) and peroneus longus (PL) electromyography (EMG), as well as COP, were recorded during walking. Functional analyses were used to detect differences between FI and control subjects with respect to normalized EMG and COP trajectory during walking. Relative to matched controls, COP trajectory was more laterally deviated in the FI group from 20% to 90% of the stance phase. TA activation was greater in the FI group from 15% to 30% and 45% to 70% of stance. PL activation was greater in the FI group at initial heel contact and toe off and trended lower from 20% to 40% of stance in the FI group. Altered motor strategies appear to contribute to COP deviations in FI participants and may increase the susceptibility to repeated ankle inversion injury.     

Visual and proprioceptive contributions to postural control of upright stance in unilateral vestibulopathy

Preserving upright stance requires central integration of the sensory systems and appropriate motor output from the neuromuscular system to keep the centre of pressure (COP) within the base of support. Unilateral peripheral vestibular disorder (UPVD) causes diminished stance stability. The aim of this study was to determine the limits of stability and to examine the contribution of multiple sensory systems to upright standing in UPVD patients and healthy subjects. We hypothesized that closure of the eyes and Achilles tendon vibration during upright stance will augment the postural sway in UPVD patients more than in healthy subjects. Seventeen UPVD patients and 17 healthy subjects performed six tasks on a force plate: forwards and backwards leaning, to determine limits of stability, and upright standing with and without Achilles tendon vibration, each with eyes open and closed (with blackout glasses). The COP displacement of the patients was significantly greater in the vibration tasks than the controls and came closer to the posterior base of support boundary than the controls in all tasks. Achilles tendon vibration led to a distinctly more backward sway in both subject groups. Five of the patients could not complete the eyes closed with vibration task. Due to the greater reduction in stance stability when the proprioceptive, compared with the visual, sensory system was disturbed, we suggest that proprioception may be more important for maintaining upright stance than vision. UPVD patients, in particular, showed more difficulty in controlling postural stability in the posterior direction with visual and proprioceptive sensory disturbance.     

The influence of gender and somatotype on single-leg upright standing postural stability in children

The purpose of this study was to investigate the influence of gender and somatotypes on single-leg upright standing postural stability in children. A total of 709 healthy children from different schools were recruited to measure the anthropometric somatotypes and the mean radius of center of pressure (COP) on a force platform with their eyes open and eyes closed. The results were that (a) girls revealed significantly smaller mean radius of COP distribution than boys, both in the eyes open and eyes closed conditions, and (b) the mesomorphic, muscular children had significantly smaller mean radius of COP distribution than the endomorphic, fatty children and the ectomorphic, linear children during the eyes closed condition. The explanation for gender differences might be due to the larger body weight in boys. The explanation for somatotype differences might be due to the significantly lower body height and higher portion of muscular profile in the mesomorphic children.     

The use of continuous spectral analysis for the assessment of postural stability changes after sports-related concussion

Impaired postural stability is associated with a variety of pathologies including sports-related concussion (SRC). Quantification of centre of pressure (COP) movement is the most common focus of instrumented assessment. Frequency-domain COP analyses have focused primarily on summary measures or pre-defined frequency bands but continuous analysis may provide novel and complementary insight into pathological control mechanisms. Our aims were (i) to compare post-SRC COP trajectory changes identified using clinician scores (Modified Balance Error Scoring System (M-BESS)), time-domain COP variables and continuous frequency spectral comparison; and (ii) to characterise frequency spectra changes. Male rugby players aged 15–19 years (n = 135) completed a pre-season baseline assessment comprising vision-obscured double-leg, single-leg and tandem stances on a force platform. Participants diagnosed with SRC during the season (n = 15) underwent repeat testing (median 4 days post-SRC; IQR 2.5–6.5). Baseline and post-SRC COP trajectories were compared using common time-domain COP variables, M-BESS scores and continuous frequency spectra. Post-SRC changes were identified using all three approaches. Spectral analysis revealed the largest effect size (Cliff’s delta 0.39) and was the only method to identify differences in all three stances and in double-leg stance. All post-SRC increases in spectral content were in the anteroposterior direction; all decreases were in the mediolateral direction. Changes were localised to higher frequencies (1.7–8 Hz) except for double-leg stance anteroposterior direction, for which increases were observed throughout the analysed range. Our findings suggest that this method of spectral comparison may provide a more responsive and meaningful measure of postural stability changes after SRC than other commonly-used variables.     

Effects of localized and general fatigue on static and dynamic postural control in male team handball athletes

In team sports, sensorimotor impairments resulting from previous injuries or muscular fatigue have been suggested to be factors contributing to an increased injury risk. Although it has been widely shown that physical fatigue affects static postural sway, it is still questionable as to what extent these adaptations are relevant for dynamic, sports-related situations. The objective of this study was to determine the effects of whole-body and localized fatigue on postural control in stable and unstable conditions. Nineteen male team handball players were assessed in 2 sessions separated by 1 week. Treadmill running and single-leg step-up exercises were used to induce physical fatigue. The main outcome measures were center of pressure (COP) sway velocity during a single-leg stance on a force plate and maximum reach distances of the star excursion balance test (SEBT). The COP sway velocity increased significantly (p < 0.05) after general (+47%) and localized fatigue (+10%). No fatigue effects were found for the SEBT. There were no significant correlations between COP sway velocity and SEBT mean reach in any condition. The results showed that although fatigue affects static postural control, sensorimotor mechanisms responsible for regaining dynamic balance in healthy athletes seem to remain predominantly intact. Thus, our data indicate that the exclusive use of static postural sway measures might not be sufficient to allow conclusive statements regarding sensorimotor control in the noninjured athlete population.     

Effect of hip and ankle muscle fatigue on unipedal postural control

Fatigue and deficits in postural control may predispose musculoskeletal injury. The purpose of this study was to examine the effects of fatigue at the hip and ankle during frontal plane movements on postural control during single-leg stance. Thirteen healthy volunteers completed two testing sessions 1 week apart consisting of isokinetic fatigue of the frontal plane movers of either the ankle or hip with measures of static unipedal postural control taken before and after fatigue. Postural control was assessed during three 30-s trials of unilateral stance with eyes open before and after the fatigue protocol at each testing session. Mean center of pressure (COP) excursion velocity in the sagittal and frontal planes was compared between pre- and post-fatigue across the two joints. Fatigue of the hip musculature led to postural control impairments in the frontal and sagittal planes, while fatigue of the ankle musculature did not significantly impair postural control in either plane. Our results suggest that there is a greater effect of localized fatigue of the frontal plane movers of the hip compared to the ankle on maintenance of a postural control in single-leg stance.     

Postural stability during static strain before and after submaximal aerobic bycycle test in athletes

The aim of study was to investigate static balance control in wrestlers (n = 31) and effects of fatigue on postural regulation in two position: bipedal stance and squat position with open eyes prior to and 2 min after bicycle PWC(170) test. A force platform ("Ritm". Russia) was used to determine static balance control. We found a minimum difference in postural control between wrestlers and controls prior to exercise in bipedal stance. In squat position linear and angular (all p < 0.001) sway velocity of centre of pressure were lower in wrestlers and were negatively correlated to PWC(170) index (r = 0.454 and r = 455, p < 0.001 with linear and angular sway velocities respectively) indicating that 20% dispersion of sway velocities in static strain position may be determined to physical working capacity. After PWC(170) test all parameters of sway were increased in both groups both in bipedal stance (p < 0.01) and squat position (p < 0.001) indicating an important role of muscular fatigue in the decrease of postural stability in both groups. Linear velocity after PWC(170) in bipedal stance increased to an equal extent in both groups but the increases of velocities of sway of centre of pressure in squat position were lower in athletes (ANOVA, p = 0.037 for linear and p = 0.008 for angular sway velocities respectively) and were negatively correlated to an extent of recovery of heart rate after PWC(170) indicating some contribution (6.5-14.2% of dispersion of linear and angular sway velocities) of an recovery rate to maintenance of a high level of postural stability in an static strain position in wrestlers during physical fatigue.     

The effects of early stages of aging on postural sway: A multiple domain balance assessment using a force platform

Technical advancements in instrumentation and analytical methods have improved the ability of assessing balance control. This study investigated the effects of early stages of aging on postural sway using traditional and contemporary postural indices from different domains. Eleven healthy young adults and fourteen healthy non-faller older adults performed two postural tasks: (a) functional limits of stability and (b) unperturbed bipedal stance for 120 s. Postural indices from spatial, temporal, frequency, and structural domains were extracted from the body’s center of pressure (COP) signals and its Rambling and Trembling components. Results revealed a preservation of functional limits of upright stability in older adults accompanied by larger, faster, and shakier body sway in both anterior-posterior and medio-lateral directions; increased medio-lateral sway frequency; increased irregularity of body sway pattern in time in both directions; and increased area, variability, velocity, and jerkiness of both rambling and trembling components of the COP displacement in the anterior-posterior direction (p < 0.02). Such changes might be interpreted as compensatory adjustments to the age-related decline of sensory, neural, and motor functions. In conclusion, balance assessment using postural indices from different domains extracted from the COP displacement was able to capture subtle effects of the natural process of aging on the mechanisms of postural control. Our findings suggest the use of such indices as potential markers for postural instability and fall risk in older adults.     

Static Postural Stability in Women during and after Pregnancy: A Prospective Longitudinal Study

This longitudinal study aimed to compare static postural stability in women between early pregnancy, advanced pregnancy, and at 2 and 6 months postpartum. Forty-five pregnant women were enrolled and 31 completed the protocol. Data were collected at 7–16 and 34–39 weeks gestation, and at 6–10 and 26–30 weeks postpartum. For each subject, the center of foot pressure path length and mean velocity (with directional subcomponents) were computed from 30-s long quiet-standing trials on a stationary force plate with eyes open or closed. The body mass, stance width, and sleep duration within 24 h before testing were also recorded. Static postural stability was not different between pregnancy and postpartum, except for the anterior posterior sway tested in the eyes-closed condition, which was significantly increased in late pregnancy compared to that at 2 and 6 months postpartum. Pregnant/postpartum women’s body mass weakly positively correlated with anterior-posterior sway in the eyes-closed condition and their stance width weakly positively correlated with the anterior-posterior sway in the eyes-open condition. No effect of sleep duration on postural sway was found. Our findings indicate that under visual deprivation conditions women in advanced pregnancy may have decreased static stability compared to their non-pregnant state.     

Cross-correlations of center of mass and center of pressure displacements reveal multiple balance strategies in response to sinusoidal platform perturbations

Compared to static balance, dynamic balance requires a more complex strategy that goes beyond keeping the center of mass (COM) within the base of support, as established by the range of foot center of pressure (COP) displacement. Instead, neuromechanics must accommodate changing support conditions and inertial effects. Therefore, because they represent body's position and changes in applied moments, relative COM and COP displacements may also reveal dynamic postural strategies. To investigate this concept, kinetics and kinematics were recorded during three 12 cm, 1.25 Hz, sagittal perturbations. Forty-one individual trials were classified according to averaged cross-correlation lag between COM and COP displacement (lag(COM:COP)) and relative head-to-ankle displacement (Δ(head)/Δ(ankle)) using a k-means analysis. This process revealed two dominant patterns, one for which the lag(COM:COP) was positive (Group 1 (n=6)) and another for which it was negative (Group 2 (n=5)) . Group 1 (G1) absorbed power from the platform over most of the cycle, except during transitions in platform direction. Conversely, Group 2 (G2) participants applied power to the platform to maintain a larger margin between COM and COP position and also had larger knee flexion and ankle dorsiflexion, resulting in a lower stance. By the third repetition, the only kinematic differences were a slightly larger G2 linear knee displacement (p=0.008) and an antiphasic relationship of pelvis (linear) and trunk (angular) displacements. Therefore, it is likely that the strategy differences were detected by including COP in the initial screening method, because it reflects the pattern of force application that is not detectable by tracking body movements.     

Mechanism of reducing knee adduction moment by shortening of the knee lever arm via medio-lateral manipulation of foot center of pressure: A pilot study

Prominent conservative treatment options for medial-compartment knee osteoarthritis include footwear that reduces knee adduction moment (KAM) correlated with detrimental loads in the medial compartment of the knee, thus providing clinical benefit. The proposed mechanism by which they reduce KAM is a lateral shift in foot center of pressure (COP) and a consequent shortening of the knee lever arm (KLA), thereby reducing KAM, which can be simply calculated as KLA multiplied by the frontal plane ground reaction force (FP-GRF). The present study investigated this mechanism for a unique biomechanical device capable of shifting COP by means of moveable convex elements attached to the shoe. Fourteen healthy young male subjects underwent gait analysis in two COP configurations of the device for comparison: (1) laterally and (2) medially deviated. Average midstance KLA and KAM were decreased by 8.2% and 8.7%, respectively, in the lateral COP compared to medial. Ground reaction force parameters, frontal plane knee angle (FP-KA), and spine lateral flexion angle (SLF) did not differ between COP configurations. No study parameters differed for terminal stance. Linear mixed effects models showed that COP and FP-GRF components, but not FP-KA and SLF, were significant predictors of KLA. In addition, KLA and FP-GRF were significant predictors of KAM; although, FP-GRF did not change significantly with medio-lateral COP shift, while KLA did. This suggests that the mechanism by which the study device reduces KAM is primarily through shortening of KLA brought on by a lateral shift in COP.     

Effect of long-duration spaceflight on postural control during self-generated perturbations.

This report is the first systematic evaluation of the effects of prolonged weightlessness on the bipedal postural control processes during self-generated perturbations produced by voluntary upper limb movements. Spaceflight impacts humans in a variety of ways, one of which is compromised postflight postural control. We examined the neuromuscular activation characteristics and center of pressure (COP) motion associated with arm movement of eight subjects who experienced long-duration spaceflight (3--6 mo) aboard the Mir space station. Surface electromyography, arm acceleration, and COP motion were collected while astronauts performed rapid unilateral shoulder flexions before and after spaceflight. Subjects generally displayed compromised postural control after flight, as evidenced by modified COP peak-to-peak anterior-posterior and mediolateral excursion, and pathlength relative to preflight values. These changes were associated with disrupted neuromuscular activation characteristics, particularly after the completion of arm acceleration (i.e., when subjects were attempting to maintain upright posture in response to self-generated perturbations). These findings suggest that, although the subjects were able to assemble coordination modes that enabled them to generate rapid arm movements, the subtle control necessary to maintain bipedal equilibrium evident in their preflight performance is compromised after long-duration spaceflight.     

Specific and Nonspecific Visual Influences on the Stability of the Vertical Posture in Humans

We studied physiological mechanisms of vision-related stabilization of the vertical posture in humans using a stabilographic technique; spontaneous deviations of the projection of the center of gravity during quiet stance and magnitudes of the postural response to vibratory stimulation of proprioceptors of the lower leg muscles under varied conditions of visual control were measured. The stability of quiet stance, as estimated according to the root mean square value of the sagittal component of the stabilogram, was the best with eyes open. Vibration-induced postural responses were the smallest also under these conditions. Spontaneous postural sway and the amplitude of response to vibratory stimulation increased when only a central sector of visual field (20 ang. deg) was preserved and, especially, under conditions of closed eyes and horizontal inversion of visual perception using prismatic spectacles. Parallel changes in the quantitative stabilographic indices and amplitude of vibration-induced postural responses show that the intensity of the latter is probably determined by the background stiffness of the musculoskeletal system. We tried to estimate separately the contributions of the stiffness factor, on the one hand, and specific visual influences, on the other hand, by testing the parameters of quiet stance and postural responses under conditions of standing while lightly touching a support with the index finger. We found that the influence of the conditions of visual control on the stability of quiet stance while touching the support was eliminated. At the same time, the magnitude of postural responses to vibratory stimulation decreased but, nonetheless, changed with visual conditions in the same manner as when standing without additional support. We conclude that vision performs a dual function in the control of the vertical posture; it forms the basis for the spatial reference system and serves the source of information on the movements of one's body.