Open-loop and closed-loop control of posture: Stabilogram-diffusion analysis of center-of-pressure trajectories among people with stroke

Sensor-Based Assessment of Mental Fatigue Effects on Postural Stability and Multi-Sensory Integration

OBJECTIVE

Mental fatigue (MF) induced by prolonged cognitive tasks poses significant risks to postural stability, yet its effects on multi-sensory integration remain poorly understood.

METHOD

This study investigated how MF alters sensory reweighting and postural control in 27 healthy young males. A 45 min incongruent Stroop task was employed to induce MF, validated via subjective Visual Analog Scale (VAS) scores and psychomotor vigilance tests. Postural stability was assessed under four sensory perturbation conditions (O-H: no interference; C-H: visual occlusion; O-S: proprioceptive perturbation; C-S: combined perturbations) using a Kistler force platform. Center of pressure (COP) signals were analyzed through time-domain metrics, sample entropy (SampEn), and Discrete Wavelet Transform (DWT) to quantify energy distributions across sensory-related frequency bands (visual: 0-0.1 Hz; vestibular: 0.1-0.39 Hz; cerebellar: 0.39-1.56 Hz; proprioceptive: 1.56-6.25 Hz).

RESULTS

MF significantly reduced proprioceptive energy contributions (p < 0.05) while increasing vestibular reliance under O-S conditions (p < 0.05). Time-domain metrics showed no significant changes in COP velocity or displacement, but SampEn decreased under closed-eye conditions (p < 0.001), indicating reduced postural adaptability. DWT analysis highlighted MF's interaction with visual occlusion, altering cerebellar and proprioceptive energy dynamics (p < 0.01).

CONCLUSION

These findings demonstrate that MF disrupts proprioceptive integration, prompting compensatory shifts toward vestibular and cerebellar inputs. The integration of nonlinear entropy and frequency-domain analyses advances methodological frameworks for fatigue research, offering insights into real-time sensor-based fatigue monitoring and balance rehabilitation strategies. This study underscores the hierarchical interplay of sensory systems under cognitive load and provides empirical evidence for optimizing interventions in high-risk occupational and clinical settings.

Balance Control Impairments in Usher Syndrome

OBJECTIVES

To explore postural disability in Usher Syndrome (USH) patients using temporal posturographic analysis to better elucidate sensory compensation strategies of deafblind patients for posture control and correlate the Activities-specific Balance Confidence (ABC) scale with posturographic variables.

DESIGN

Thirty-four genetically confirmed USH patients (11 USH1, 21 USH2, 2 USH 4) from the Otolaryngology Outpatient Clinic and 35 controls were prospectively studied using both classical and wavelet temporal analysis of center of pressure (CoP) under different visual conditions on static and dynamic platforms. The functional impact of balance was assessed with the ABC scale. Classical data in the spatial domain, Sensorial Organization Test, and frequency analysis of the CoP were analyzed.

RESULTS

On unstable surfaces, USH1 had greater CoP surface area with eyes open (38.51 ± 68.67) and closed (28.14 ± 31.64) versus controls (3.31 ± 4.60), p < 0.001 and (7.37 ± 7.91), p < 0.001, respectively. On an unstable platform, USH consistently showed increased postural sway, with elevated angular velocity versus controls with eyes open (USH1 [44.94 ± 62.54]; USH2 [55.64 ± 38.61]; controls [13.4 ± 8.57]) ( p = 0.003; p < 0.001) and closed (USH1 [60.36 ± 49.85], USH2 [57.62 ± 42.36]; controls [27.31 ± 19.79]) ( p = 0.002; p = 0.042). USH visual impairment appears to be the primary factor influencing postural deficits, with a statistically significant difference observed in the visual Sensorial Organization Test ratio for USH1 (80.73 ± 40.07, p = 0.04) and a highly significant difference for USH2 (75.48 ± 31.67, p < 0.001) versus controls (100). In contrast, vestibular ( p = 0.08) and somatosensory ( p = 0.537) factors did not reach statistical significance. USH exhibited lower visual dependence than controls (30.31 ± 30.08) (USH1 [6 ± 11.46], p = 0.004; USH2 [8 ± 14.15], p = 0.005). The postural instability index, that corresponds to the ratio of spectral power index and canceling time, differentiated USH from controls on unstable surface with eyes open USH1 (3.33 ± 1.85) p < 0.001; USH2 (3.87 ± 1.05) p < 0.002; controls (1.91 ± 0.85) and closed USH1 (3.91 ± 1.65) p = 0.005; USH2 (3.92 ± 1.05) p = 0.045; controls (2.74 ± 1.27), but not USH1 from USH2. The canceling time in the anteroposterior direction in lower zone distinguished USH subtypes on stable surface with optokinetic USH1 (0.88 ± 1.03), USH2 (0.29 ± 0.23), p = 0.026 and on unstable surface with eyes open USH1 (0.56 ± 1.26), USH2 (0.072 ± 0.09), p = 0.036. ABC scale could distinguish between USH patients and controls, but not between USH subtypes and it correlated with CoP surface area on unstable surface with eyes open only in USH1( ρ = 0.714, p = 0.047).

CONCLUSIONS

USH patients, particularly USH1, exhibited poorer balance control than controls on unstable platform with eyes open and appeared to rely more on proprioceptive information while suppressing visual input. USH2 seems to use different multisensory balance strategies that do not align as well with the ABC scale. The advanced analysis provided insights into sensory compensation strategies in USH subtypes.

Characteristics of unsuccessful reactive responses to lateral loss of balance in people with stroke

PURPOSE

The effectiveness of reactive responses to a sudden loss of balance is a critical factor that determines whether a fall will occur. We examined the strategies and kinematics associated with successful and unsuccessful balance recovery following lateral loss of balance in people with stroke (PwS).

METHODS

Eleven PwS were included in the analysis. They were exposed to unannounced right and left horizontal surface translations and demonstrated both successful and unsuccessful balance responses at the same perturbation magnitude. Reactive step strategies and kinematics were investigated comparatively in successful and unsuccessful recovery tests.

RESULTS

The crossover strategy was used in most of the unsuccessful tests (7/11) while the unloaded-leg side-step in the successful tests (6/11). There were no significant differences in the reactive step initiation time in unsuccessful vs. successful tests. However, the step execution time, step length and center of mass displacement were significantly higher during the first recovery step in unsuccessful tests.

CONCLUSIONS

PwS have difficulties in controlling and decelerating the moving center of mass following a lateral loss of balance. The increased step time and step length of the first reactive step in unsuccessful vs. successful tests suggest the crossover step strategy may be ineffective for PwS.

Imbalanced optimal feedback motor control system in spinocerebellar ataxia type 3

BACKGROUND AND PURPOSE

Human motor planning and control depend highly on optimal feedback control systems, such as the neocortex-cerebellum circuit. Here, diffusion tensor imaging was used to verify the disruption of the neocortex-cerebellum circuit in spinocerebellar ataxia type 3 (SCA3), and the circuit's disruption correlation with SCA3 motor dysfunction was investigated.

METHODS

This study included 45 patients with familial SCA3, aged 17-67 years, and 49 age- and sex-matched healthy controls, aged 21-64 years. Tract-based spatial statistics and probabilistic tractography was conducted using magnetic resonance images of the patients and controls. The correlation between the local probability of probabilistic tractography traced from the cerebellum and clinical symptoms measured using specified symptom scales was also calculated.

RESULTS

The cerebellum-originated probabilistic tractography analysis showed that structural connectivity, mainly in the subcortical cerebellar-thalamo-cortical tract, was significantly reduced and the cortico-ponto-cerebellar tract was significantly stronger in the SCA3 group than in the control group. The enhanced tract was extended to the right lateral parietal region and the right primary motor cortex. The enhanced neocortex-cerebellum connections were highly associated with disease progression, including duration and symptomatic deterioration. Tractography probabilities from the cerebellar to parietal and sensorimotor areas were significantly negatively correlated with motor abilities in patients with SCA3.

CONCLUSION

To our knowledge, this study is the first to reveal that disrupting the neocortex-cerebellum loop can cause SCA3-induced motor dysfunctions. The specific interaction between the cerebellar-thalamo-cortical and cortico-ponto-cerebellar pathways in patients with SCA3 and its relationship with ataxia symptoms provides a new direction for future research.

Overground Robotic Gait Trainer mTPAD Improves Gait Symmetry and Weight Bearing in Stroke Survivors

Stroke is a leading cause of disability, impairing the ability to generate propulsive forces and causing significant lateral gait asymmetry. We aim to improve stroke survivors' gaits by promoting weight-bearing during affected limb stance. External forces can encourage this; e.g., vertical forces can augment the gravitational force requiring higher ground reaction forces, or lateral forces can shift the center of mass over the stance foot, altering the lateral placement of the center of pressure. With our novel design of a mobile Tethered Pelvic Assist Device (mTPAD) paired with the DeepSole system to predict the user's gait cycle percentage, we demonstrate how to apply three-dimensional forces on the pelvis without lower limb constraints. This work is the first result in the literature that shows that with an applied lateral force during affected limb stance, the center of pressure trajectory's lateral symmetry is significantly closer to a 0% symmetry (5.5%) than without external force applied (-9.8%,p<0.05). Furthermore, the affected limb's maximum relative pressure (p) significantly increases from 233.7p to 234.1p (p<0.05) with an applied downward force, increasing affected limb loading. This work highlights how the mTPAD increases weight-bearing and propulsive forces during gait, which is a crucial goal for stroke survivors.

Integration of reginal shear measurements at the foot-ground interface during routine balance assessment of the elderly population

BACKGROUND

Force-plate posturography offers a convenient way for quantitative assessment of postural stability in the elderly. However, studies focusing on routine balance assessment have usually not taken reginal shear distributions (i.e., arising from horizontal forces) into consideration.

RESEARCH QUESTION

(1) Does plantar shear distribution differ between young and elderly subjects during upright standing? (2) How do the maximum plantar shear forces vary at different regions of the foot?

METHODS

The new reginal shear measurement (RSM) method can simultaneously capture the three-dimensional force distributions at regional plantar sites while subjects maintaining standing balance. The feasibility of the proposed method in characterizing the magnitude and distribution of plantar shear forces was tested in thirty-two normal young and nineteen elderly subjects. Statistical analysis was performed using the independent samples t-test for both the continuous and ordinal variables.

RESULTS

For regional AP shear forces, statistically significant differences were found between the two groups for the toe region of the right foot and the midfoot of both feet. For ML shear distributions, statistically significant differences were found at nearly all plantar sites expect for the hallux and lateral metatarsal. The maximum increase in ML shear forces occurred in the toe region of the right foot, where the peak shear values were 113.16% higher than those of the young subjects. The peak ML shear occurred in the midfoot were averagely 83.19% and 70.57% higher in the elderly's left and right feet, respectively.

SIGNIFICANCE

The RSM method may offer unique solutions to identify functional decline in postural control of the elderly. The plantar shear pattern has potential to become an important parameter in evaluating one's balance performance during upright standing.

Visual Effect on Brain Connectome That Scales Feedforward and Feedback Processes of Aged Postural System During Unstable Stance

Older adults with degenerative declines in sensory systems depend strongly on visual input for postural control. By connecting advanced neural imaging and a postural control model, this study investigated the visual effect on the brain functional network that regulates feedback and feedforward processes of the postural system in older adults under somatosensory perturbations. Thirty-six older adults conducted bilateral stance on a foam surface in the eyes-open (EO) and eyes-closed (EC) conditions while their center of pressure (COP) and scalp EEG were recorded. The stochastic COP trajectory was modeled with non-linear stabilogram diffusion analysis (SDA) to characterize shifts in postural control in a continuum of feedback and feedforward processes. The EEG network was analyzed with the phase-lag index (PLI) and minimum spanning tree (MST). The results indicated that visual input rebalanced feedforward and feedback processes for postural sway, resulting in a greater critical point of displacement (CD), short-term effective diffusion coefficients (D_s) and short-term exponent (H_s), but the smaller critical point of time (CT) and long-term exponent (H_l) for the EC state. The EC network demonstrated stronger frontoparietal-occipital connectivity but weaker fronto-tempo-motor connectivity of the theta (4–7 Hz), alpha (8–12 Hz), and beta (13–35 Hz) bands than did the EO network. MST analysis revealed generally greater leaf fraction and maximal betweenness centrality (BC_max) and kappa of the EC network, as compared with those of the EO network. In contrast, the EC network exhibited a smaller diameter and average eccentricity than those of the EO network. The modulation of long-term negative feedback gain of the aged postural system with visual occlusion was positively correlated with leaf fraction, BC_max, and kappa, but negatively correlated with the diameter and average eccentricity for all EEG sub-bands. In conclusion, the aged brain functional network in older adults is tuned to visual information for modulating long-term negative feedback of the postural system under somatosensory perturbations.