Stiffness and Damping in Postural Control Increase With Age

Upright balance is believed to be maintained through active and passive mechanisms, both of which have been shown to be impacted by aging. A compensatory balance response often observed in older adults is increased co-contraction, which is generally assumed to enhance stability by increasing joint stiffness. We investigated the effect of aging on standing balance by fitting body sway data to a previously developed postural control model that includes active and passive stiffness and damping parameters. Ten young (24 $pm$ 3 years) and seven older (75 $pm$ 5 years) adults were exposed during eyes-closed stance to perturbations consisting of lateral pseudorandom floor tilts. A least-square fit of the measured body sway data to the postural control model found significantly larger active stiffness and damping model parameters in the older adults. These differences remained significant even after normalizing to account for different body sizes between the young and older adult groups. An age effect was also found for the normalized passive stiffness, but not for the normalized passive damping parameter. This concurrent increase in active stiffness and damping was shown to be more stabilizing than an increase in stiffness alone, as assessed by oscillations in the postural control model impulse response.      

A Quiet Standing Index for Testing the Postural Sway of Healthy and Diabetic Adults Across a Range of Ages

A quiet standing index is developed for tracking the postural sway of healthy and diabetic adults over a range of ages. Several postural sway features are combined into a single composite feature C that increases with age a. Sway features are ranked based on the r ²-values of their linear regression models, and the composite feature is a weighted sum of selected sway features with optimal weighting coefficients determined using principal component analysis. A performance index based on both reliability and sensitivity is used to determine the optimal number of features. The features used to form C include power and distance metrics. The quiet standing index is a scalar that compares the composite feature C to a linear regression model f(a) using C ^'(a) = C/f(a). For a motionless subject, C ^' = 0, and when the composite feature exactly matches the healthy control (HC) model, C ^' = 1. Values of C ^' >> 1 represent excessive postural sway and may indicate impaired postural control. Diabetic neurologically intact subjects, nondiabetic peripheral neuropathy subjects (PN), and diabetic PN subjects (DPN) were evaluated. The quiet standing indexes of the PN and DPN groups showed statistically significant increases over the HC group. Changes in the quiet standing index over time may be useful in identifying people with impaired balance who may be at an increased risk of falling.     

Galvanic vestibular stimulation elicits consistent head-neck motion in seated subjects

Humans actively stabilize the head-neck system based on vestibular, proprioceptive and visual information. Galvanic vestibular stimulation (GVS) has been used previously to demonstrate the role of vestibular feedback in standing balance. This study explores the effect of GVS on head-neck kinematics and evaluates the approach to investigate the vestibular contribution to head-neck stabilization. GVS was applied to 11 seated subjects using seven different stimuli (single sinusoids and multisines) at amplitudes of 0.5-2 mA and frequencies of 0.4-5.2 Hz using a bilateral bipolar configuration while 3-D head and torso kinematics were recorded using motion capture. System identification techniques were used evaluating coherence and frequency response functions (FRFs). GVS resulted in significant coherence in roll, yaw and lateral translation, consistent with effects of GVS while standing as reported in the literature. The gain of the FRFs varied with frequency and no modulation was observed across the stimulus amplitudes, indicating a linear system response for the stimulations considered. Compared to single sine stimulation, equivalent FRFs were observed during unpredictable multisine stimulation, suggesting the responses during both stimuli to be of a reflexive nature. These results demonstrate the potential of GVS to investigate the vestibular contribution to head-neck stabilization.     

New Foot Pressure Activated Sensory Compensation System for Posture-Control Enhancement in Amputees

A computerized foot pressure activated sensory compensation system using subsensory electrical stimulation combined with visual-auditory biofeedback was developed. The proposed system was used for enhancing standing balance and gait performance for amputees. In this pilot study, we hypothesized that the static balance with single limb support and gait performance during treadmill walking could be improved by providing proprioceptive neuromuscular facilitation using subsensory stimulation and visual-auditory biofeedback in amputee, respectively. To test this hypothesis, five unilateral transtibial amputees who consecutively wore prosthetics over two years were recruited. Experimental results show a reduction in all of the postural sway indexes and increase in single-leg support time index during single-leg quiet standing by applying subsensory stimulation. With visual-auditory biofeedback for providing clue for heel contact and toe push-off condition during treadmill ambulation, an improvement in all four dynamic gait performance indexes in amputees was verified. These findings suggest that the proposed system with subsensory electrical stimulation and visual-auditory biofeedback mechanisms may be effective in compensating sensory loss and improving posture control for amputees.     

Galvanic vestibular stimulation for analysis of postural adaptationand stability

Human postural dynamics was investigated in 12 normal subjects by means of a force platform recording body sway, induced by bipolar transmastoid galvanic stimulation of the vestibular nerve and labyrinth. The model adopted was that of an inverted segmented pendulum, the dynamics of postural control being assumed to be reflected in the stabilizing forces actuated by the feet as a result of complex muscular activity subject to state feedback of body sway and position. Time-series analysis demonstrates that a transfer function from stimulus to sway-force response with specific parameters can be identified. In addition, adaptation to the vestibular stimulus is demonstrated to exist, and the authors describe this phenomenon using quantification in terms of a postural adaptation time constant in the range of 40-50 s. The results suggest means to evaluate adaptive behavior and postural control in the erect human being which may be useful in the rehabilitation of individuals striving to regain upright stance     

Postural stability and stereo-ambiguity in man-designed visualenvironments

Our modern rectilinear visual environment contains visual stimuli for which evolution has not had time to optimally shape visual processing. One such stimulus, periodic stripes, is known to lead to visual depth ambiguity. In this paper we show that postural instability, as measured by the variance of fore and aft sway, is increased by viewing such stimuli. This instability may be the precursor of falls. Designers must evaluate the visual impressions conveyed by their systems in order to avoid postural instability due to visual ambiguity.     

Postural control adaptation during galvanic vestibular and vibratory proprioceptive stimulation

The objective for this study was to investigate whether the adaptation of postural control was similar during galvanic vestibular stimulation and during vibratory proprioceptive stimulation of the calf muscles. Healthy subjects were tested during erect stance with eyes open or closed. An analysis method designed to consider the adaptive adjustments was used to evaluate the motion dynamics and the evoked changes of posture and stimulation response. Galvanic vestibular stimulation induced primarily lateral body movements and vibratory proprioceptive stimulation induced anteroposterior movements. The lateral body sway generated by the galvanic stimulation was proportionally smaller and contained more high-frequency movements (> 0.1 Hz) than the anteroposterior body sway induced by the vibratory stimulation. The adaptive adjustments of the body sway to the stimulation had similar time course and magnitude during galvanic and vibratory stimulation. The perturbations induced by stimulation were gradually reduced within the same time range (15-20 s) and both kinds of stimulation induced a body leaning whose direction was dependent on stimulus. The similarities in the adjustment patterns suggest that postural control operates in the same way independent of the receptor systems affected by the disturbance and irrespective of whether the motion responses were induced in a lateral or anteroposterior direction.     

Analysis of the posture control system under fixed andsway-referenced support conditions

To delineate the relative roles of each of the feedback sensors in the posture control system such as the visual, vestibular, and proprioceptive sensors, an identification technique was applied to measurements of antero-posterior sway angles of the body and ankle moments under the following conditions: standing on a fixed support with eyes open (ox), standing on a fixed support with eyes closed (cx), standing on a sway-referenced support with eyes open (os), and standing on a sway-referenced support with eyes closed (cs). Frequency response functions from the sway angle to the ankle moment were calculated. Gain and phase characteristics for conditions (os) and (cs) were similar to those of Nashner's (1972) vestibular model in the high-frequency range, which shows that the vestibular system may be dominant. The gain was higher under condition (cx) than under (ox). Judging from the phase characteristics, this was probably due to increased weighting of the proprioceptive sensor over the vestibular sensor. There was a tendency for gain to increase as balance tasks became more demanding     

Invariant density analysis: modeling and analysis of the postural control system using Markov chains

In this paper, a novel analysis technique, invariant density analysis (IDA), is introduced. IDA quantifies steady-state behavior of the postural control system using center of pressure (COP) data collected during quiet standing. IDA relies on the analysis of a reduced-order finite Markov model to characterize stochastic behavior observed during postural sway. Five IDA parameters characterize the model and offer physiological insight into the long-term dynamical behavior of the postural control system. Two studies were performed to demonstrate the efficacy of IDA. Study 1 showed that multiple short trials can be concatenated to create a dataset suitable for IDA. Study 2 demonstrated that IDA was effective at distinguishing age-related differences in postural control behavior between young, middle-aged, and older adults. These results suggest that the postural control system of young adults converges more quickly to their steady-state behavior while maintaining COP nearer an overall centroid than either the middle-aged or older adults. Additionally, larger entropy values for older adults indicate that their COP follows a more stochastic path, while smaller entropy values for young adults indicate a more deterministic path. These results illustrate the potential of IDA as a quantitative tool for the assessment of the quiet-standing postural control system.     

Balance prosthesis based on micromechanical sensors using vibrotactile feedback of tilt

A prototype balance prosthesis has been made using miniature, high-performance inertial sensors to measure lateral head tilt and vibrotactile elements mounted on the body to display head tilt to the user. The device has been used to study the feasibility of providing artificial feedback of head tilt to reduce postural sway during quiet standing using six healthy subjects. Two vibrotactile display schemes were used: one in which the individual vibrating elements, called tactors, were placed on the shoulders (shoulder tactors); another in which columns of tactors were placed on the right and left sides of the trunk (side tactors). Root-mean-square head-tilt angle (Tilt) and center of pressure displacement (Sway) were measured for normal subjects standing in a semi-tandem Romberg position with eyes closed, under four conditions: no balance aids; shoulder tactors; side tactors; and light touch. Compared with no balance aids, the side tactors significantly reduced Tilt (35%) and Sway (33%). Shoulder tactors also significantly reduced Tilt (44%) and Sway (17%). Compared with tactors, light touch resulted in less Sway, but more Tilt. The results suggest that healthy normal subjects can reduce their lateral postural sway using head tilt information as provided by a vibrotactile display. Thus, further testing with balance-impaired subjects is now warranted.     

Measures of postural steadiness: differences between healthy youngand elderly adults

Measures of postural steadiness are used to characterize the dynamics of the postural control system associated with maintaining balance during quiet standing. The objective of this study was to evaluate the relative sensitivity of center-of-pressure (COP)-based measures to changes in postural steadiness related to age. A variety of time and frequency domain measures of postural steadiness were compared between a group of twenty healthy young adults (21-35 years) and a group of twenty healthy elderly adults (66-70 years) under both eyes-open and eyes-closed conditions. The measures that identified differences between the eyes-open and eyes-closed conditions in the young adult group were different than those that identified differences between the eye conditions in the elderly adult group. Mean velocity of the COP was the only measure that identified age-related changes in both eye conditions, and differences between eye conditions in both age groups. The results of this study will be useful to researchers and clinicians using COP-based measures to evaluate postural steadiness     

Application of cross time-frequency analysis to postural sway behavior: the effects of aging and visual systems

In this paper, the effects of visual feedback and aging on postural sway systems and signals are investigated by analyzing the transient phase difference between "input" and "output" which correspond to center of pressure (COP) and center of mass (COM), respectively. In order to analyze the transient phase difference characteristics of COP and COM, a relatively new cross time-frequency analysis technique that provides time- and frequency-localized phase difference information is utilized. The feedback control process in the postural sway is interpreted in terms of a feedback compensator which is characterized in terms of a phase difference. Using the experimental results of the transient phase difference obtained from the cross time-frequency distribution, it is demonstrated that the postural control of young persons are more stable and rely more on visual sensory feedback to stabilize postural control compared to that of the elderly persons.     

Early recognition of postural disorders in multiple sclerosisthrough movement analysis: a modeling study

In the present study, spontaneous postural behavior has been analyzed in freely standing multiple sclerosis (MS) patients, exhibiting no clinically assessable abnormalities of postural control. This population has been compared with two other groups, healthy people and hemiparetic patients. This latter group represents a situation where the central nervous system (CNS) lesion is precisely localized in one anatomical site and no signal-conduction disorders are present; i.e., it has an opposite anatomical character with respect to the MS at a preclinical stage. The hypothesis underlying the modeling study is the presence of a controller block working in a feedback posture control system. This controller block receives the body sway as input, and produces the corresponding ankle torque stabilizing the body, the latter being modeled as an inverted pendulum. The CNS damage, caused by MS, is supposed to be reflected in some detectable change in the structure of the controller of the posture control system. The identification of the controller has been performed by means of a parametric estimation procedure which employed as input sequences, data recorded by means of a movement-analysis (MA) system. Reported findings show a structural change of the model of the controller block in the posture control system. This result may suggest the presence of an MS-specific reorganization of the posture control system. Some speculation is finally made on the black-box approach in comparison with traditional posturography, to arrive at hypothesizing a progression path for postural disorders     

Nonstationary spectral analysis based on time-frequency operator symbols and underspread approximations

We present a unified framework for time-varying or time-frequency (TF) spectra of nonstationary random processes in terms of TF operator symbols. We provide axiomatic definitions and TF operator symbol formulations for two broad classes of TF spectra, one of which is new. These classes contain all major existing TF spectra such as the Wigner-Ville, evolutionary, instantaneous power, and physical spectrum. Our subsequent analysis focuses on the practically important case of nonstationary processes with negligible high-lag TF correlations (so-called underspread processes). We demonstrate that for underspread processes all TF spectra yield effectively identical results and satisfy several desirable properties at least approximately. We also show that Gabor frames provide approximate Karhunen-Loeve (KL) functions of underspread processes and TF spectra provide a corresponding approximate KL spectrum. Finally, we formulate simple approximate input-output relations for the TF spectra of underspread processes that are passed through underspread linear time-varying systems. All approximations are substantiated mathematically by upper bounds on the associated approximation errors. Our results establish a TF calculus for the second-order analysis and time-varying filtering of underspread processes that is as simple as the conventional spectral calculus for stationary processes.     

Computational methods for real-time Fourier synthesis

In Fourier synthesis, natural musical sounds are produced by summing time-varying sinusoids. Sounds from individual instruments are analyzed to find the amplitude and frequency characteristics for their sinusoids; interpolation between the characteristics of several sounds is used to produce intermediate timbres. An ensemble can be synthesized by summing all the sinusoids for several sounds, but in practice it is difficult to perform such computations in real time. To solve this problem on inexpensive hardware, the author takes advantage of the masking effects of the auditory system. By avoiding the computations for perceptually unimportant sinusoids, and by using other computation reduction techniques to achieve interpolation, a large ensemble can be synthesized in real time. Unlike existing computation reduction techniques, the techniques described in this correspondence do not sacrifice independent fine control over the amplitude and frequency characteristics of each sinusoid     

Interference Analysis of Filtered Multitone Modulation Over Time-Varying Frequency- Selective Fading Channels

We consider in this paper filtered multitone (FMT) modulation over frequency-selective time-varying fading channels. Due to the phase and amplitude distortion introduced by the fading channel, not only is the orthogonality among different subcarriers destroyed, but also the perfect Nyquist sampling condition of the baseband matched filters is no longer valid. Consequently, interchannel, as well as intersymbol, interference will cause distortions to the transmitted signals. In this paper, the interference caused by the channel frequency selectivity and time variance is quantified by analyzing the demodulated signals at the receiver under several different fading-channel conditions. An analysis of the average carrier-to-interference (C/I) ratio of the FMT system is provided in order to demonstrate the underlying tradeoff between spectral efficiency and system performance. For comparison purposes with other multicarrier communication systems (or modulation techniques), the C/I ratio of the conventional orthogonal frequency-division multiplexing system is also provided and compared with that of the FMT system under the same channel conditions and spectral efficiency. Finally, numerical and simulation results are given that confirm the C/I ratio results obtained     

Data-adaptive evolutionary spectral estimation

We present a novel data-adaptive estimator for the evolutionary spectrum of nonstationary signals. We model the signal at a frequency of interest as a sinusoid with a time-varying amplitude, which is accurately represented by an orthonormal basis expansion. We then compute a minimum mean-squared error estimate of this amplitude and use it to estimate the time-varying spectrum at that frequency, all while minimizing the interference from the signal components at other frequencies. Repeating the process over all frequencies, we obtain a power distribution that is consistent with the Wold-Cramer evolutionary spectrum and reduces to Capon's (1969) method for the stationary case. Our estimator possesses desirable properties in terms of time-frequency resolution and positivity and is robust in the spectral estimation of noisy nonstationary data. We also propose a new estimator for the autocorrelation of nonstationary signals. This autocorrelation estimate is needed in the data-adaptive spectral estimation. We illustrate the performance of our estimator using simulation examples and compare it with the recently presented evolutionary periodogram and the bilinear time-frequency distribution with exponential kernels     

Signal Adaptive Pipelined Hardware Design of Time-Varying Optimal Filter for Highly Nonstationary FM Signal Estimation

In this paper we develop a multiple-clock-cycle signal adaptive hardware design of an optimal nonstationary (time-varying) filtering system. The proposed design is based on the real-time results of time-frequency (TF) analysis and the estimation of instantaneous frequency (IF). It permits multiple detection of the local filter’s region of support (FRS) in the observed increment of time, resulting in the efficient filtering of multicomponent frequency modulated (FM) signals. The proposed design takes a variable number of clock (CLK) cycles–the only necessary ones regarding the highest quality of IF estimation–in different TF points within the execution. In this way it allows the implemented system to optimize the computational cost, as well as the time required for execution. Further, the proposed serial design optimizes critical design performances, related to the hardware complexity, making it a suitable system for real-time implementation on an integrated chip. Also, by applying the pipelining technique, it allows overlapping between different TF points within the execution, additionally improving the time required for time-varying filtering. The design has been verified by a field-programmable gate array (FPGA) circuit design, capable of performing filtering of nonstationary FM signals in real-time.     

Stabilization and modulation of laser oscillators by internal time-varying perturbation

The paper reviews the application of internal time-varying perturbation to the problem of laser mode control and stabilization. The spectral characteristics and time domain behavior obtained by means of phase type and loss type perturbations are considered. Two techniques which allow the attainment of high single frequency output powers from normally multimode lasers are described. A method for the absolute frequency stabilization of an FM laser is considered, and a brief discussion of an efficient method of internal modulation, termed coupling modulation, is given.