Transitions in persistence of postural dynamics depend on the velocity and structure of postural perturbations

DFA as a window into postural dynamics supporting task performance: does choice of step size matter?

Introduction: Detrended Fluctuation Analysis (DFA) has been used to investigate self-similarity in center of pressure (CoP) time series. For fractional gaussian noise (fGn) signals, the analysis returns a scaling exponent, DFA-α, whose value characterizes the temporal correlations as persistent, random, or anti-persistent. In the study of postural control, DFA has revealed two time scaling regions, one at the short-term and one at the long-term scaling regions in the diffusion plots, suggesting different types of postural dynamics. Much attention has been given to the selection of minimum and maximum scales, but the choice of spacing (step size) between the window sizes at which the fluctuation function is evaluated may also affect the estimates of scaling exponents. The aim of this study is twofold. First, to determine whether DFA can reveal postural adjustments supporting performance of an upper limb task under variable demands. Second, to compare evenly-spaced DFA with two different step sizes, 0.5 and 1.0 in log2 units, applied to CoP time series. Methods: We analyzed time series of anterior-posterior (AP) and medial-lateral (ML) CoP displacement from healthy participants performing a sequential upper limb task under variable demand. Results: DFA diffusion plots revealed two scaling regions in the AP and ML CoP time series. The short-term scaling region generally showed hyper-diffusive dynamics and long-term scaling revealed mildly persistent dynamics in the ML direction and random-like dynamics in the AP direction. There was a systematic tendency for higher estimates of DFA-α and lower estimates for crossover points for the 0.5-unit step size vs. 1.0-unit size. Discussion: Results provide evidence that DFA-α captures task-related differences between postural adjustments in the AP and ML directions. Results also showed that DFA-α estimates and crossover points are sensitive to step size. A step size of 0.5 led to less variable DFA-α for the long-term scaling region, higher estimation for the short-term scaling region, lower estimate for crossover points, and revealed anomalous estimates at the very short range that had implications for choice of minimum window size. We, therefore, recommend the use of 0.5 step size in evenly spaced DFAs for CoP time series similar to ours.

Characterizing stroke-induced changes in the variability of lower limb kinematics using multifractal detrended fluctuation analysis

Movement variability reflects the adaptation of the neuromuscular control system to internal or external perturbations, but its relationship to stroke-induced injury is still unclear. In this study, the multifractal detrended fluctuation analysis was used to explore the stroke-induced changes in movement variability by analyzing the joint angles in a treadmill-walking task. Eight healthy subjects and ten patients after stroke participated in the experiment, performing a treadmill-walking task at a comfortable speed. The kinematics data of the lower limbs were collected by the motion-capture system, and two indicators, the degree of multifractality (α) and degree of correlation [h(2)], were used to investigate the mechanisms underlying neuromuscular control. The results showed that the knee and ankle joint angles were multifractal and persistent at various scales, and there was a significant difference in the degree of multifractality and the degree of correlation at the knee and ankle joint angles among the three groups, with the values being ranked in the following order: healthy subjects < non-paretic limb < paretic limb. These observations highlighted increased movement variability and multifractal strength in patients after stroke due to neuromotor defects. This study provided evidence that multifractal detrended analysis of the angles of the knee and ankle joints is useful to investigate the changes in movement variability and multifractal after stroke. Further research is needed to verify and promote the clinical applications.

Adaptive treadmill walking encourages persistent propulsion

BACKGROUND

Adaptive treadmills allow real-time changes in walking speed by responding to changes in step length, propulsion, or position on the treadmill. The stride-to-stride variability, or persistence, of stride time during overground, fixed-speed, and adaptive treadmill walking has been studied, but persistence of propulsion during adaptive treadmill walking remains unknown. Because increased propulsion is often a goal of post-stroke rehabilitation, knowledge of the stride-to-stride variability may aid rehabilitation protocol design.

RESEARCH QUESTION

How do spatiotemporal and propulsive gait variables vary from stride to stride during adaptive treadmill walking, and how do they compare to fixed-speed treadmill walking?

METHODS

Eighteen young healthy subjects walked on an instrumented split-belt treadmill in the adaptive and fixed-speed modes for 10 minutes at their comfortable speed. Kinetic data was collected from the treadmill. Detrended fluctuation analysis was applied to the time series data. Shapiro-Wilk tests assessed normality and one-way repeated measures ANOVAs compared between adaptive, fixed-speed, and randomly shuffled conditions at a Bonferroni-corrected significance level of 0.0055.

RESULTS

Stride time, stride length, step length, and braking impulse were persistent (α > 0.5) in the adaptive and fixed-speed conditions. Adaptive and fixed-speed were different from each other. Stride speed was persistent in the adaptive condition and anti-persistent (α < 0.5) in the fixed-speed condition. Peak propulsive force, peak braking force, and propulsive impulse were persistent in the adaptive condition but not the fixed-speed condition (α ≈ 0.5). Net impulse was non-persistent in the adaptive and fixed-speed conditions. All variables were non-persistent in the shuffled condition.

SIGNIFICANCE

During adaptive treadmill walking, increases in propulsive force and impulse persist for multiple strides. Persistence was stronger on the adaptive treadmill, where increased propulsion translates into increased walking speed. For post-stroke gait rehabilitation where increasing propulsion and speed are goals, the stronger persistence of adaptive treadmill walking may be beneficial.

Center-of-pressure dynamics of upright standing as a function of sloped surfaces and vision

Upright postural control system exhibits dynamic behavior to produce flexible adaptations to a variety of internal and external perturbations. Understanding the range of postural adaptability in healthy individuals can index the overall state of the system and needs to be defined over various environmental and task constraints. The purpose of the current investigation was to understand the role of vision and support surface angle on the multiple time scales of control that maintain upright posture. Thirteen young, healthy adults performed quiet standing tasks on flat, inclined and declined support surfaces with either eyes open or closed. The variability of the anterior-posterior center of pressure (COP) trajectory was analyzed using linear (COP_length) and non-linear (multiscale entropy - MSE) approaches to index postural dynamics. Sway magnitude - COP_length - was greater in both sloped conditions compared to the flat support surface standing and with the removal of vision. Increased irregularity was revealed during the sloped conditions compared to flat surface standing with additional increases of COP complexity when vision was removed. Overall, a similar range of postural adaptability was revealed for both the singular and combined sensory manipulations suggesting limits to the degree of change of COP dynamics.

Comparison of a portable balance board for measures of persistence in postural sway

Measuring postural sway is important for determining functional ability or risk of falling. Gathering postural sway measures outside of controlled environments is desirable for reaching populations with limited mobility. Previous studies have confirmed the accuracy of the magnitude of postural sway using the Nintendo Wii Balance Board (WBB). However, it is unclear if the WBB can accurately measure persistence of postural sway, i.e., the pattern of center-of-pressure fluctuations over time. The purpose of this study was to compare measures of persistence of postural sway (through detrended fluctuation analysis) using WBB and a force platform (FP). Seventeen healthy individuals performed three standing conditions: eyes open, eyes closed, and one-leg standing. The WBB (30 Hz) was placed on top on the FP (600 Hz) to collect data simultaneously, then the FP data were downsampled to 100 Hz and 30 Hz. The agreement between WBB and FP for measures of postural sway were influenced by the sampling rate and postural sway direction. Intraclass correlation coefficient was excellent (range: 0.953-0.998) for long-term scaling regions in the anterior-posterior direction, but lower (range: 0.352-0.877) and inconsistent for medial-lateral direction and short-term scaling regions. The three comparison groups (WBB at 30 Hz, FP at 30 Hz, and FP at 100 Hz) showed dissimilar abilities in detecting differences in persistence of postural sway. In summary, the WBB is accurate for quantifying persistence of postural sway measurements in long-term scaling regions in the AP direction, but has limitations for short-term scaling regions and the ML direction.

Somatosensory perception sensitivity in voluntary postural sway movements: Age, gender and sway effect magnitudes

OBJECTIVES

1) to develop a reliable device for assessing somatosensory perception sensitivity in voluntary postural sway movement, specifically a sway discrimination apparatus (SwayDA) for testing voluntary lateral sway discrimination sensitivity (VLSDS); 2) to explore the relationship between mobility performance and VLSDS in older adults, and 3) to determine the effects of age, gender and sway magnitude on VLSDS.

METHODS

First, eighteen healthy young adults (8 males, 10 females, age ranging from 22 to 70) were recruited for a test-retest reliability study. During the SwayDA test, the participants were asked to discriminate between four possible medial-lateral sway extents when moving away from neutral standing. For Objective 2, twenty-five older participants (9 males, 16 females, mean age 70.1) undertook both the SwayDA and the mobility tests. The mobility testing battery consisted of single task and cognitive dual task timed-up and-go tests, and the 5 times sit-to-stand test. Pearson's correlation was calculated between SwayDA scores and mobility performance. For Objective 3, 20 community-dwelling adults over 65 years old (10 males, 10 females, mean age 71.3) and 20 young volunteers (10 males, 10 females, mean age 23.6) were recruited to study the effects of age, gender and sway magnitude on VLSDS. To obtain a bias-free measure of VLSDS, the probability of correct response was considered as the true-positive judgment, while the probability of incorrect response was considered as false-positive judgment, and these were cumulated across the response values. A receiver operating characteristic (ROC) curve was then generated and the Area Under the ROC Curve (AUC) was used to measure VLSDS.

RESULTS

There was no significant difference in AUC scores between Day 1 and Day 8 (p > 0.05). ICC(3,1) reliability indices were 0.750 for sway to the left and 0.879 for sway to the right. Pearson's correlation revealed a significant correlation between the SwayDA sores and timed-up-and-go (TUG), cognitive dual task TUG, 5 times sit-to-stand test (r = -0.456, -0.522, and - 0.416 respectively, all p < 0.05). Factorial ANOVA showed age and gender main effects (F = 8.144, p < 0.01, and F = 8.806, p < 0.01, respectively), suggesting older adults and females had worse VLSDS. In addition, a significant difference was found between the young and older participants in the inner range of VLSDS (t = -2.875, p < 0.017), indicating that the decline of somatosensory perception of postural sway in older people may be magnitude-specific, and greatest for small deviations from upright stance.

CONCLUSIONS

The SwayDA has good to excellent test-retest reliability. The finding that VLSDS score was significantly correlated with mobility performance in older adults highlights the importance of somatosensory perception in postural control. More importantly, the significantly worse VLSDS in older people observed in the inner lateral movement range may represent a unique characteristic of neuromuscular degeneration associated with aging, which should be monitored and addressed in rehabilitation programs.

Persistence in postural dynamics is dependent on constraints of vision, postural orientation, and the temporal structure of support surface translations

Activities of daily living require maintaining upright posture within a variety of environmental constraints. A healthy postural control system can adapt to different environmental constraints. Afferent sensory information is used to determine where the body is in relation to the gravitational vertical and efferent motor commands make corrections with the goal of keeping the center of mass within the base of support. The purpose of this research was to understand how vision, direction of translation, and the temporal correlation of the support surface stimuli affected the persistence characteristics of postural dynamics on short and long time scales. Ten healthy young adults performed a standing task with either eyes open or closed, oriented anteriorly or mediolaterally while the support surface underwent structured translations based on different levels of temporal correlation-white noise (no correlation), pink noise (moderate correlation), and red noise and sinusoidal movements (strong correlations). Center of pressure velocity was analyzed using fractal analysis to determine the dynamics of postural control. On the short time scale, persistence was shown to be stronger with eyes closed, in the mediolateral direction, and when the structure of translation contained stronger temporal correlation. On the long time scale, anti-persistence was stronger with eyes closed, in the mediolateral direction, and for all structures of movement except red noise. This study provides deeper insight into the flexibility existing in human movement responses to structured environmental stimuli through the fractal analysis of movement variability.