Age-related changes in static balance in older women aged in their early sixties to their late eighties: different aging patterns in the anterior-posterior and mediolateral directions.

Objective: The aim of this study was to cross-sectionally investigate how static balance changes throughout the aging process in older women aged from their early sixties to their late eighties. Methods: Forty-six older women (aged 62-89 years) were requested to stand barefoot and quietly on a force platform for 30 s with their eyes either open or closed. During the trials, the position of the center of foot pressure (CoP) and the acceleration of the body's center of mass (ACC) were measured. The root mean square (RMS) of the CoP and ACC values was calculated to evaluate the amplitude of postural sway and the level of regulatory activity, respectively. The mean power frequency of the ACC was also calculated to represent the temporal characteristics of regulatory activity. Results: In the anterior-posterior direction, there was no significant relationship between the RMS of CoP and the participants' age, whereas the RMS of ACC significantly increased with increasing age. In the mediolateral direction, however, the RMS of CoP significantly increased with increasing age, whereas the RMS of ACC did not change with age. The mean power frequency of ACC did not exhibit any age-related change in either the anterior-posterior or the mediolateral direction. Conclusion: The results indicate that static balance in older women aged in their early sixties to their late eighties exhibits distinctly contrasting aging patterns between the anterior-posterior and mediolateral directions. To prevent falls in older women, it is necessary to elucidate the physiological mechanisms responsible for the increase in mediolateral sway that occurs throughout old age.

A comprehensive multivariate analysis of the center of pressure during quiet standing in patients with Parkinson's disease.

BACKGROUND: We hypothesized that postural instability observed in individuals with Parkinson's disease (PD) can be classified as distinct subtypes based on comprehensive analyses of various evaluated parameters obtained from time-series of center of pressure (CoP) data during quiet standing. The aim of this study was to characterize the postural control patterns in PD patients by performing an exploratory factor analysis and subsequent cluster analysis using CoP time-series data during quiet standing. METHODS: 127 PD patients, 47 aged 65 years or older healthy older adults, and 71 healthy young adults participated in this study. Subjects maintain quiet standing for 30 s on a force platform and 23 variables were calculated from the measured CoP time-series data. Exploratory factor analysis and cluster analysis with a Gaussian mixture model using factors were performed on each variable to classify subgroups based on differences in characteristics of postural instability in PD. RESULTS: The factor analysis identified five factors (magnitude of sway, medio-lateral frequency, anterio-posterior frequency, component of high frequency, and closed-loop control). Based on the five extracted factors, six distinct subtypes were identified, which can be considered as subtypes of distinct manifestations of postural disorders in PD patients. Factor loading scores for the clinical classifications (younger, older, and PD severity) overlapped, but the cluster classification scores were clearly separated. CONCLUSIONS: The cluster categorization clearly identifies symptom-dependent differences in the characteristics of the CoP, suggesting that the detected clusters can be regarded as subtypes of distinct manifestations of postural disorders in patients with PD.

How do features of dynamic postural stability change with age during quiet standing, gait, and obstacle crossing?

Previous research has reported mixed findings regarding age-related changes in dynamic postural stability, quantified by margin of stability (MOS), during gait. However, age-related changes in MOS may be better elicited by tasks imposing greater challenges to the postural control system. Older adults' MOS during obstacle crossing, a destabilizing task, has previously been characterized, although studies comparing MOS during this task between younger and older adults remain sparse. This study investigated age-related changes in dynamic postural stability during quiet standing, gait, and obstacle crossing. Participants aged 20-30 (n = 20), 60-69 (n = 18), 70-79 (n = 15), and 80+ (n = 7; not analyzed statistically) years old performed these tasks while whole-body motion was tracked using motion capture. MOS in each direction was estimated throughout each trial, and integrals, transient ranges, and trial minima were extracted (as applicable). MOS time series were also ensemble averaged across age groups. No age-related differences were identified for quiet standing or gait. However, obstacle crossing metrics revealed greater stability (i.e., more positive MOS) and less instability (i.e., less negative MOS) in older adults, and reduced ranges during transients. These findings potentially arise from shorter step lengths, which may be the result of age-related physical declines; or may reflect a cautious strategy in older adults, which maximizes postural stability in the direction with the greatest consequences for foot-obstacle contact, as it changes throughout the task. This study supports the use of tasks imposing physical challenges and/or voluntary perturbations to study age-related changes in dynamic postural stability. Findings also contribute to our theoretical understanding of the time course of dynamic postural stability during functional tasks in relation to periods of transition in the base of support, and task-specific strategies adopted for obstacle crossing by older adults to maintain dynamic postural stability and mitigate fall risk.

Center of pressure palindromes reveals a wobbling standing balance in scoliotic girls.

BACKGROUND: This study characterized the center of pressure planar displacement by palindromic strings. The objective is to test if the center of pressure pathway of able-bodied girls and those with a moderate and severe scoliosis displayed similar palindromic tendencies. METHODS: The center of pressure excursions of 21 able-bodied girls were compared to 14 girls with a moderate scoliosis and 14 girls with severe one. Each girl was asked to stand upright on a force platform for 64 s. A crisscross grid of nine areas was centered around the mean center of pressure position (G) to define three other zones to use the MATLAB built-in nucleotide sequence analysis function. These were the antero-posterior extremities A, the coronal extremities C and the tilted or the four corners of the crisscross grid, T. The center of pressure positions were associated to any of the 4 zones using the GATC acronym. FINDINGS: For all groups center of pressure pattern in decreasing order was A, G, T and C. Able-bodied girls favored the A zones. Girls with moderate scoliosis displaced their center of pressure mostly in the A zones with shifts in the T sections (P ≤ 0.001). Girls with severe scoliosis, additionally displaced their center of pressure in the C zones (P ≤ 0.001). INTERPRETATION: An ankle modality characterized able-bodied girl's standing balance. Girls with a moderate scoliosis privilege the palindromic zones in the antero-posterior extremities with excursions in the corners of the base of support, girls with severe scoliosis further relied on the medio-lateral zones, suggesting a wobbling standing balance.

Quiet standing and anteroposterior limits of stability in adolescents and young adults with bilateral spastic cerebral palsy.

Stance stability in individuals with bilateral spastic cerebral palsy (BSCP) in various standing postures including the quiet standing (QS) and limits of stability (LoS) has been widely studied. However, the relationships between the QS and LoS remain unclear. This study aimed to determine the relationships between the positions and postural sway in the QS and anteroposterior LoS in individuals with BSCP. It included 27 adolescents and young adults with BSCP (BSCP group) and 27 adolescents and young adults without disability (control group). The position of center of pressure in the anteroposterior direction (CoPy position) and the path length of center of pressure (CoP path length) during the QS and the anterior and posterior LoS (A-LoS and P-LoS, respectively) were measured using a force platform. The CoPy positions in the A-LoS and P-LoS in the BSCP group were limited compared with those in the control group. In the BSCP group, the more anterior the CoPy position in the QS, the more anterior (i.e., limited) it was in the P-LoS. Although the CoP path length in the QS was larger in the BSCP group, those in the A-LoS and P-LoS were larger in the control group. The BSCP group also showed that the more anterior the CoPy position or the longer the CoP path length in the QS, the more decreased the anteroposterior LoS range was. Therefore, assessing various standing postures, including QS and anteroposterior LoS, is important to manage balance impairments in individuals with BSCP.

An improved intermittent control model of postural sway during quiet standing implemented by a data driven approach.

This paper proposes a new intermittent control model during human quiet standing, which is consisted of postulated "regular intervention" and "imminent intervention". The regular intervention is within the main control loop, and its trigger condition is equivalent to the switching frequency of center of pressure (COP) data calculated by wavelet transform. The imminent intervention will only be triggered after the postural sway angle exceeds a certain threshold. In order to prove the effectiveness of the new model, the simulation results of the new model and the model proposed by Asai et al. (2009) are compared with the experimental data. The setting parameters of both models are retrieved by Bayesian regression from the experimental data. The results show that the new model not only could exhibit two power law scaling regimes of power spectral density (PSD) of COP, but also show that indices of the probability density function distance, root mean square (RMS), Total Sway Path, displacement Range, 50% power frequency of center of mass (COP) between the simulation results and the experimental data are closer compared to the existing model. Moreover, the limit cycle oscillations (LCOs) obtained from the simulation results of the new model have a higher degree of matching with those retrieved from the experimental data.

Center of Mass Estimation Using a Force Platform and Inertial Sensors for Balance Evaluation in Quiet Standing.

Accurate estimation of the center of mass is necessary for evaluating balance control during quiet standing. However, no practical center of mass estimation method exists because of problems with estimation accuracy and theoretical validity in previous studies that used force platforms or inertial sensors. This study aimed to develop a method for estimating the center of mass displacement and velocity based on equations of motion describing the standing human body. This method uses a force platform under the feet and an inertial sensor on the head and is applicable when the support surface moves horizontally. We compared the center of mass estimation accuracy of the proposed method with those of other methods in previous studies using estimates from the optical motion capture system as the true value. The results indicate that the present method has high accuracy in quiet standing, ankle motion, hip motion, and support surface swaying in anteroposterior and mediolateral directions. The present method could help researchers and clinicians to develop more accurate and effective balance evaluation methods.

Subthreshold electrical stimulation with pink noise enhances feedback control as evaluated by scaling exponent of postural sway.

Subthreshold somatosensory stimulation with pink noise has been shown to attenuate postural sway better than stimulation with white noise. This might be due to the different frequency structures of the noise signals. However, their effects on the underlying somatosensory feedback pathway are still unknown. Thus, we aimed to determine whether pink noise enhances the somatosensory feedback pathway more effectively than other noises with different frequency structures, such as white and red noises. Sixteen young adults stood quietly for 65 s under four stimulation conditions: no stimulation and stimulations with white-, pink-, and red-noise-like signals. Based on a stabilogram-diffusion analysis, we calculated the long-term diffusion coefficient and scaling exponent in the radial direction to evaluate the effects of these noise signals on their somatosensory feedback control. The root mean square (CoPRMS) and mean velocity of the foot center of pressure were also computed to assess the amount of postural sway. The results showed that the stimulation condition had a significant effect on the scaling exponent, with the value under the pink-noise-like signal significantly lower than that under the no-stimulation condition. We also found that among the participants, the percentage of reduction in CoPRMS by the pink-noise-like signal was positively correlated with the CoPRMS value under the no-stimulation condition. Altogether, the somatosensory feedback control for balancing for quiet standing posture was improved by pink noise, and its effect on the variability of postural sway correlated with inherent postural sway variability.

The effect of ageing on between-limb centre of pressure coordination in standing balance: Is there evidence for reactive control challenges among older adults?

Interlimb temporal synchrony and spatial symmetry of centre of pressure (COP) displacements may be vital contributors to standing balance control. In previous work among stroke survivors, low-frequency COP displacements (< 0.4 Hz) were proposed to arise from centre of mass (COM) dynamics, or from proactive exploratory processes. COP displacements among higher frequencies (>0.4 Hz), in contrast, have been attributed to corrective balance responses to internal perturbations. The present study extends this work to explore age-related alterations in such stability control processes during standing balance. The combined COP displacements from both limbs (COPnet) in addition to individual-limb COP timeseries were calculated from synchronous force platform data obtained from 19 younger adults and 19 older adults during a 60 s trial of quiet standing. The discrete wavelet transform was used to decompose the anteroposterior and mediolateral COPnet, in addition to the individual-limb timeseries, into low-frequency and high-frequency bandwidths. Root-mean-squared (RMS) amplitudes of high- and low-frequency COPnet displacements were calculated. The cross-correlation coefficient was used to assess the extent of between-limb temporal synchronization, while the ratio of individual-limb RMS amplitudes was used to assess between-limb spatial symmetry within each high- and low-frequency bandwidth. We observed greater high-frequency anteroposterior COPnet displacements among older adults, without age related differences in the lower frequency bandwidth or in the mediolateral direction. Further, older adults exhibited greater high-frequency anteroposterior between-limb synchronization, without age-related differences in the low frequency bandwidth, or among any of the spatial symmetry variables. The present age-related alterations in COPnet could represent a conservative strategy to ensure stability, whereby age-related challenges in stability maintenance during standing are offset by greater demands on stability control. Further, increased high frequency between-limb temporal synchronization among older adults may suggest a loss of adaptability in balance corrective responses during standing.

Effects of Occasional and Habitual Wearing of High-Heeled Shoes on Static Balance in Young Women.

The purpose of this study was to examine the effects of occasional and habitual wearing of high-heeled shoes on static balance in young women. Groups of habitual high-heel wearers and non-wearers (n = 7 in both groups) were asked to stand quietly on a force platform without shoes (WS condition) or with high heels (heel area 1 cm2, heel height 7 cm) (HH condition). During the trials, the center-of-pressure (CoP) position in the anterior-posterior direction was measured, and its root mean square (as a measure of postural sway magnitude, CoPRMS) and mean velocity (as a measure of regulatory activity, CoPMV) were calculated. To further examine the effect of high-heel wearing on the temporal aspects of slow and fast processes in static balance, the CoP sway was decomposed into low- (below 0.5 Hz) and high- (above 0.5 Hz) frequency components, and then spectral analysis was performed. Results showed that the CoPRMS was not significantly different between the groups or between the shoe conditions, indicating that wearing high heels with a heel height of 7 cm did not increase the magnitude of postural sway, irrespective of high-heel experience. The CoPMV was significantly larger in the HH condition than in the WS condition, whereas it was not significantly different between the groups. This result indicates that wearing high heels increased the amount of regulatory activity in both habitual wearers and non-wearers. The spectral analysis further showed that habitual high-heel wearers showed significantly decreased rate of regulatory activity than non-wearers, both while standing with and without high heels. These results suggest that use-dependent changes in static balance control are evident in both high-heeled and without shoes conditions.

Effects of weighing phase duration on vertical force-time analyses and repeatability.

Force plate analyses of various activities sometimes require the average (WPav) and standard deviation (WPsd) of force across the Weighing Phase (i.e., quiet period) to calculate kinetic, temporal and kinematic metrics. Yet, the influence of weighing phase duration on these analyses has been scarcely investigated. This study investigated the effects of weighing phase duration on the agreement between vertical force-time variables and the repeatability of WPav and WPsd. Durations of 0.5, 1.0 and 1.5 s were compared to 2.0 s. Limits of agreement (LOA) for system weight, onset threshold, onset time, net impulse, take-off velocity and take-off displacement were calculated for 137 counter-movement, squat and single leg jumps. Repeatability coefficients for WPav and WPsd estimated the consistency between repeated trials. Shorter weighing phase durations produced small differences in WPav (LOA < ±0.25%), which accumulated during integration, affecting net impulse, take-off velocity (LOA ±2%) and take-off displacement (LOA ±23%). Differences were substantial using 5xWPsd as the onset threshold (LOA approximately ±25% to ±72%), consequently influencing onset time (LOA approximately ±6% to ±18%). WPav repeatability was high but the within-trial differences could augment with integration, requiring weighing phases longer than 2 s. WPsd had poor repeatability and its use requires further investigation.

Effects on intermittent postural control in people with Parkinson's due to a dual task.

OBJECTIVES: The aim of the present study was to determine the effects of performing a dual task on the sway density plot parameters in Parkinson's disease and control subjects. METHODS: A cross-sectional design was used to establish differences in the mean peak, mean time, and mean distance between a group with Parkinson's disease and a control group without Parkinson's disease. The subjects performed, in a unique measurement session, two trials under three different randomized conditions: i. eyes open, ii. eyes closed, and iii. Eyes open with foam base. One trial was performed as a single task (i.e., the subjects completed one of the balance test), while the other trial was performed as a dual task (i.e., the subjects performed a cognitive task at the same time that they maintained the static balance). RESULTS: There was a group x dual task x condition effect in mean peak (F1.5, 51.1 = 5.21; p = 0.015; η2p = 0.13) and mean time (F1.4, 47.3 = 4.43; p = 0.03; η2p = 0.11) variables. According dual-task cost analysis, there was a main effect of the condition (F6,134 = 2.44; p = 0.05; η2p = 0.34) on MD (F2,68 = 6.90; p < 0.01; η2p = 0.17). CONCLUSIONS: This result indicates differences in the dual task interference in the postural control mechanisms between the Parkinson's disease population and healthy pairs. For easy dual tasks, the Parkinson subjects used anticipatory control responses for longer periods of time, and for more difficult tasks, their control strategy did not change regarding single balance task.

Effects of subthreshold electrical stimulation with white noise, pink noise, and chaotic signals on postural control during quiet standing.

BACKGROUND: The stochastic resonance (SR) phenomenon has been used to improve postural control through the application of imperceptible noise to the somatosensory system. White noise signals have been applied in numerous SR studies on postural control. However, because the SR effect depends on the noise structure, the stimulation effects of signals with different structures, such as pink noise and chaotic signals, on postural control, must be determined to achieve better clinical applications of SR technology. RESEARCH QUESTION: During quiet standing, how is postural control affected by subthreshold electrical stimulation to the knee joints when signals with different structures (white noise, pink noise, and chaotic signals) are used? METHODS: Sixteen healthy young adults stood quietly for 40 s with their eyes closed. To evaluate postural sway, we calculated the mean velocity, root mean square (CoPRMS), and range (CoPRange) values for the center of pressure (CoP) in the anteroposterior direction. The standing task was conducted under subthreshold electrical stimulation with white noise, pink noise, and chaotic signals based on the Lorenz system, in addition to the no-stimulation condition. The four stimulation conditions were randomized within each set and repeated seven times. RESULTS: Significant effects of stimulation were observed in the CoPRMS and CoPRange values. The CoPRMS value under the pink noise signal was significantly lower than that under the no-stimulation condition. The CoPRange value also tended to decrease under the pink noise signal compared with the no-stimulation condition; however, the differences were not statistically significant. No significant changes were found with the white noise and chaotic signals compared with the no-stimulation condition. SIGNIFICANCE: We demonstrated that the pink noise signal was more effective in reducing postural sway than the white noise and chaotic signals based on the Lorenz system during quiet standing.

Effects of global postural alignment on posture-stabilizing synergy and intermuscular coherence in bipedal standing.

Clinicians frequently assess and intervene on postural alignment; however, notions of what constitutes good postural alignment are variable. Furthermore, the majority of current evidence appeals either to population norms or defines good postural alignment as the negation of what has been observed to correlate with pathology. The purpose of this study was to identify affirmative indicators of good postural alignment in reference to motor control theory. Electromyography (anterior leg, posterior leg, and trunk muscles) and motion capture data were acquired from 13 participants during 4 min bipedal standing trials in 4 conditions: control, - 10%, + 30%, and + 60% of subject-specific anterior limits of stability. Synergistic kinematic coordination was quantified via the uncontrolled manifold framework, and correlated neural drive was quantified in posture-relevant muscle groups (anterior, posterior, and trunk) via intermuscular coherence. Multilevel models assessed the effects of sagittal plane alignment on both outcomes. We observed a within-subjects fixed effect in which kinematic synergistic coordination decreased as subjects became more misaligned. We also observed within-subjects fixed effects for middle- and high-frequency intermuscular coherence in the posterior group (increased coherence with increased misalignment) and for trunk intermuscular coherence across all frequency bands (decreased coherence with increased misalignment). Our findings indicate that it may be possible to describe healthy postural alignment in light of referent control theory. Greater misalignment with respect to vertical is associated with compromises in synergistic control of posture and increased corticospinal drive to specific muscle groups. These results suggest that postural alignment may not simply be an empirical phenomenon.

Changes in postural control strategy during quiet standing in individuals with knee osteoarthritis.

BACKGROUND: Knee osteoarthritis (OA) impairs postural control and may affect how the lower limb joints are used for postural control. OBJECTIVE: To investigate how individuals with knee OA use lower limb joints for static postural control. METHODS: Ten patients with knee OA and thirteen healthy controls performed quiet standing for 30 s. The standard deviation of the center of mass (COM) and lower limb joint motions in the anterior-posterior (AP) and medial-lateral (ML) planes were calculated from three-dimensional marker trajectories. Pearson's correlation analysis and independent t-tests were conducted to investigate the relationship between COM and lower limb joint motion and to compare group difference, respectively. RESULTS: The AP hip angular velocity alone in the knee OA group and the AP hip and knee angular velocity in the control group were significantly correlated with the AP COM velocity. The ML hip angular velocity was significantly correlated with the ML COM velocity in both groups. The knee OA group exhibited a significantly larger standard deviation of AP COM velocity than the control group. CONCLUSIONS: Individuals with knee OA depended solely on the contribution of the hip to the AP COM velocity, which could not be successfully controlled by the knee.

Postural control during quiet standing and voluntary stepping response tasks in individuals post-stroke: a case-control study.

BACKGROUND: Postural control impairments following a stroke have an impact on mobility, reduce independence, and increase the risk of falls. Assessing these impairments during tasks representative of real-life situations, such as quiet standing (QS) and voluntary stepping response (VSR), will enhance our understanding of how the postural control system is impaired in individuals post-stroke (IPS). It will also inform the development of a more targeted and effective rehabilitation to prevent falls in IPS. OBJECTIVES: Identify the postural control impairments encountered by IPS during QS and VSR. METHODS: Twenty IPS and 16 healthy controls were recruited to perform QS and VSR tasks, while ground reaction forces and whole-body motion were measured. Displacement and speed variation of the COM, center of pressure (COP) displacement and spatiotemporal data were calculated and compared between groups. RESULTS: During QS, IPS exhibited greater maximal COP displacement in mediolateral direction, COM displacement in vertical direction and COM speed excursions compared to controls. During VSR, IPS exhibited smaller step length, braking force, posterior foot placement in relation to the pelvis and COM anteroposterior excursion compared to controls. IPS presented less static and dynamic postural stability compared to controls. CONCLUSIONS: Greater postural sway during QS, smaller anteroposterior COM displacement before losing balance and altered voluntary recovering steps during VSR could place IPS at more risk of falling when they face a postural challenge in the community. These novel results will improve the current knowledge base and should be considered in IPS rehabilitation.

A review of center of pressure (COP) variables to quantify standing balance in elderly people: Algorithms and open-access code.

Postural control is often quantified by recording the trajectory of the center of pressure (COP)-also called stabilogram-during human quiet standing. This quantification has many important applications, such as the early detection of balance degradation to prevent falls, a crucial task whose relevance increases with the aging of the population. Due to the complexity of the quantification process, the analyses of sway patterns have been performed empirically using a number of variables, such as ellipse confidence area or mean velocity. This study reviews and compares a wide range of state-of-the-art variables that are used to assess the risk of fall in elderly from a stabilogram. When appropriate, we discuss the hypothesis and mathematical assumptions that underlie these variables, and we propose a reproducible method to compute each of them. Additionally, we provide a statistical description of their behavior on two datasets recorded in two elderly populations and with different protocols, to hint at typical values of these variables. First, the balance of 133 elderly individuals, including 32 fallers, was measured on a relatively inexpensive, portable force platform (Wii Balance Board, Nintendo) with a 25-s open-eyes protocol. Second, the recordings of 76 elderly individuals, from an open access database commonly used to test static balance analyses, were used to compute the values of the variables on 60-s eyes-open recordings with a research laboratory standard force platform.

Association of bilateral lower limb coordination while standing with body sway control and aging.

PURPOSE: Coordinated movements of both lower limbs may be a clinically important indicator of motor control during quiet standing. From a neurological point of view, it is known that extensive coupling of muscles must be coordinated an upright posture. However, movement coordination between the lower limbs is the final motor output, is unknown. In this study, we focussed on the ground reaction force (GRF) vector and clarified the time and frequency characteristics of the force vectors of both lower limbs. MATERIALS AND METHODS: A total of 16 healthy young adults and 18 healthy older adults participated and placed each bare foot on one of two force plates to measure the GRF vectors (i.e., anteroposterior, mediolateral, and vertical) of each lower limb and determine the centre of mass (COM) acceleration in the anteroposterior direction (COMacc). Characteristics of the coordination of both lower limbs during movements were analysed using coherence analysis and cross-correlation function analysis (CCF). RESULTS: The coherence levels of the force vectors of both lower limbs were higher in all three directions and significantly increased in the older adults. CCF analysis showed that the force vectors of both lower limbs were negatively correlated at the zero-time lag. Moreover, a weak correlation was observed between COMacc and coherence values. CONCLUSIONS: The assessment of bilateral lower limb connectivity using force vectors can be used as an evaluation method to reflect changes in the ability to control bipedal standing during ageing.

Reproducing Human Arm Strategy and Its Contribution to Balance Recovery Through Model Predictive Control.

The study of human balance recovery strategies is important for human balance rehabilitation and humanoid robot balance control. To date, many efforts have been made to improve balance during quiet standing and walking motions. Arm usage (arm strategy) has been proposed to control the balance during walking motion in the literature. However, limited research exists on the contributions of the arm strategy for balance recovery during quiet standing along with ankle and hip strategy. Therefore, in this study, we built a simplified model with arms and proposed a controller based on nonlinear model predictive control to achieve human-like balance control. Three arm states of the model, namely, active arms, passive arms, and fixed arms, were considered to discuss the contributions of arm usage to human balance recovery during quiet standing. Furthermore, various indexes such as root mean square deviation of joint angles and recovery energy consumption were verified to reveal the mechanism behind arm strategy employment. In this study, we demonstrate to computationally reproduce human-like balance recovery with and without arm rotation during quiet standing while applying different magnitudes of perturbing forces on the upper body. In addition, the conducted human balance experiments are presented as supplementary information in this paper to demonstrate the concept on a typical example of arm strategy.

Identifying human postural dynamics and control from unperturbed balance.

BACKGROUND: Upright standing requires control of an inherently unstable multi-joint human body within a small base of support, despite biological motor and / or sensory noise which challenge balance. Without applying perturbations, system identification methods have been regarded as inadequate, because the relevant internal biological noise processes are not accessible to direct measurement. As a result, unperturbed balance studies have been limited to investigation of behavioral patterns rather than possible underlying control strategies. METHODS: In this paper, we present a mathemathically rigorous system identification method that is applicable to study the dynamics and control of unperturbed balance. The method is derived from autocorrelation matrices with non-zero time lags and identifies the system matrix of a discrete-time dynamic system in the presence of unknown noise processes, without requiring any information about the strength of the noise. RESULTS: Unlike reasonable 'least-squares' approaches, the performance of the new method is consistent across a range of different combinations of internal and measurement noise strengths, even when measurement noise is substantial. We present a numerical example of a model that simulates human upright balancing and show that its dynamics can be identified accurately. With a biomechanically reasonable choice of state and input variables, a state feedback controller can also be identified. CONCLUSIONS: This study provides a new method to correctly identify the dynamics of human standing without the need for known external perturbations. The method was numerically validated using simulation that included realistic features of human balance. This method avoids potential issues of adaptation or possible reflex responses evoked by external perturbations, and does not require expensive in-lab, high-precision measurement equipment. It may eventually enable diagnosis and treatment of individuals with impaired balance, and the development of safe and effective assistive and / or rehabilitative technologies.