Inter-joint coordination of posture on a seesaw device

Analysis of Connectivity in Electromyography Signals to Examine Neural Correlations in the Activation of Lower Leg Muscles for Postural Stability: A Pilot Study

In quiet standing, the central nervous system implements a pre-programmed ankle strategy of postural control to maintain upright balance and stability. This strategy comprises a synchronized common neural drive delivered to synergistically grouped muscles. This study evaluated connectivity between EMG signals of the unilateral and bilateral homologous muscle pairs of the lower legs during various standing balance conditions using magnitude-squared coherence (MSC). The leg muscles examined included the right and left tibialis anterior (TA), medial gastrocnemius (MG), and soleus (S). MSC is a frequency domain measure that quantifies the linear phase relation between two signals and was analyzed in the alpha (8-13 Hz), beta (13-30 Hz), and gamma (30-100 Hz) neural frequency bands for feet together and feet tandem, with eyes open and eyes closed conditions. Results showed that connectivity in the beta and lower and upper gamma bands (30-100 Hz) was influenced by standing balance conditions and was indicative of a neural drive originating from the motor cortex. Instability was evaluated by comparing less stable standing conditions with a baseline-eyes open feet together stance. Changes in connectivity in the beta and gamma bands were found to be most significant in the muscle pairs of the back leg during a tandem stance regardless of dominant foot placement. MSC identified the MG:S muscle pair as significant for the right and left leg. The results of this study provided insight into the neural mechanism of postural control.

Comparison of inter-joint coordination strategies during activities of daily living with prosthetic and anatomical limbs

While healthy individuals have redundant degrees of freedom of the joints, they coordinate their multi-joint movements such that the redundancy is effectively reduced. Achieving high inter-joint coordination may be difficult for upper limb prosthesis users due to the lack of proprioceptive feedback and limited motion of the terminal device. This study compared inter-joint coordination between prosthesis users and individuals without limb loss during different upper limb activities of daily living (ADLs). Nine unilateral prosthesis users (five males) and nine age- and sex-matched controls without limb loss completed three unilateral and three bilateral ADLs. Principal component analysis was applied to the three-dimensional motion trajectories of the trunk and arms to identify coordinative patterns. For each ADL, we quantified the cumulative variance accounted for (VAF) of the first five principal components (pcs), which was the lowest number of pcs that could achieve 90% VAF in control limb movements across all ADLs (5 ≤ n ≤ 9). The VAF was lower for movements involving a prosthesis compared to those completed by controls across all ADLs (p < 0.001). The pc waveforms were similar between movements involving a prosthesis and movements completed by control participants for pc1 (r > 0.78, p < 0.001). The magnitude of the relationship for pc2 and pc3 differed between ADLs, with the strongest correlation for symmetric bilateral ADLs (0.67 ≤ r ≤ 0.97, p < 0.001). Collectively, this study demonstrates that activities of daily living were completed with distinct coordination strategies in prosthesis users compared to individuals without limb loss. Future work should explore how device features, such as the availability of sensory feedback or motorized wrist joints influence multi-joint coordination.

EMG-EMG coherence during voluntary control of human standing tasks: a systematic scoping review

Background

Intra- or inter-muscular (EMG-EMG) coherence is a simple and non-invasive method for estimating central nervous system control during human standing tasks. Although this research area has developed, no systematic literature review has been conducted.

Objectives

We aimed to map the current literature on EMG-EMG coherence during various standing tasks to identify the research gaps and summarize previous studies comparing EMG-EMG coherence between healthy young and elderly adults.

Methods

Electronic databases (PubMed, Cochrane Library, and CINAHL) were searched for articles published from inception to December 2021. We incorporated studies that analyzed EMG-EMG coherence of the postural muscles in various standing tasks.

Results

Finally, 25 articles fulfilled the inclusion criteria and involved 509 participants. Most participants were healthy young adults, while only one study included participants with medical conditions. There was some evidence that EMG-EMG coherence could identify differences in standing control between healthy young and elderly adults, although the methodology was highly heterogeneous.

Conclusion

The present review indicates that EMG-EMG coherence may help elucidate changes in standing control with age. In future studies, this method should be used in participants with central nervous system disorders to understand better the characteristics of standing balance disabilities.

Assessment of Dynamic Balancing Performance of Synchronized Ice Skaters With Sudden Provocation Test via Principal Component Analysis

Background: Balancing performance can be affected by regular and high-level athletic training, which has not been fully explored in synchronized ice skaters. This study aimed to analyze the dynamic balancing performance by assessing the principal and compensatory movements performed during the sudden provocation tests and evaluating the parameters that characterize the platform’s motion. Method: Twelve young female synchronized ice skaters and 12 female age-matched controls participated. Sudden provocation tests were completed three times in bipedal stance and in single-leg stances, and sport-specific fatigue session was inserted between the repetitions. Results: Significantly more time was necessary to recover balance for both groups after the fatiguing sessions (p < .05). Interestingly, skaters performed less effectively in the simplest condition (bipedal stance) than the control group (p < .05). The principal component analysis showed that the first principal movement was the same for both groups. The skater group used the upper body and arms more often to compensate, while the control group’s recovery strategy consisted mainly of abduction of the elevated leg. The damping ratio and the relative variance of the first principal movement showed a negative correlation (p < .05), suggesting that those with superior balancing effectiveness recruited more compensatory movements.

Multi-segmental postural control patterns in down syndrome

BACKGROUND

Patients with Down Syndrome (DS) exhibit less efficient and unstable standing postural control. The specificities of somatosensorial deficits might result in a different utilization of resources and in distinct whole-body kinematic patterns, to date still unexplored. In this paper we aim at addressing multi-segmental coordination patterns in people with DS while maintaining standing balance under different visual conditions (open and closed eyes).

METHODS

This cross-sectional observational cohort study involved two groups of 23 patients with DS and 12 healthy controls. A 30-s standing balance test allowed to extract (i) the length of the trajectory of the center-of-pressure sway and 95% confidence ellipse area from Ground Reaction forces, and (ii) Principal Movement (PM) components from full-body motion kinematics; the latter were obtained exploiting a Principal Component Analysis-based approach, also embracing a motor-control perspective through the evaluation of the number of modifications applied by the neuromuscular controller on segments' acceleration.

FINDINGS

Trajectory length was significantly higher in patients; 95% ellipse confidence area did not differ between groups/condition. Postural movement components differed in people with DS from healthy controls not only in the "observable", behavioural phenotype (PM3 and PM8), but also in the amount of activation of the associated control (PM1 to PM8, over-activated in DS) in all spatial directions.

INTERPRETATION

Results reinforced the prevalence of a medio-lateral hip strategy (instead of an ankle strategy) in maintaining postural stability. Most important, they revealed a less frequent activation of postural patterns in all spatial directions.

Changes in postural strategy of the lower limb under mechanical knee constraint on an unsteady stance surface

Joint constraint could limit the available degrees of freedom in a kinematic chain for maintaining postural stability. This study investigated adaptive changes in postural synergy due to bracing of bilateral knee joints, usually thought to have a trifling impact on upright stance. Twenty-four young adults were requested to maintain balance on a stabilometer plate as steadily as possible while wearing a pair of knee orthoses, either unlocked (the non-constraint (NC) condition) or locked to restrict knee motion (the knee constraint (KC) condition). Knee constraint led to a significant increase in the regularity of the stabilometer angular velocity. More than 95% of the variance properties of the joint angular velocities in the lower limb were explained by the first and second principal components (PC1 and PC2), which represented the ankle strategy and the combined knee and hip strategy, respectively. In addition to the increase trend in PC1 regularity, knee constraint enhanced the mutual information of the stabilometer angular velocity and PC1 (MI_STBV-PC1) but reduced the mutual information of the stabilometer angular velocity and PC2 (MI_STBV-PC2). The MI_STBV-PC1 was also positively correlated to stance steadiness on the stabilometer in the KC condition. In summary, in the knee constraint condition, postural synergy on the stabilometer was reorganized to increase reliance on ankle strategies to maintain equilibrium. In particular, a stable stabilometer stance under knee constraint is associated with a high level of coherent ankle–stabilometer interaction.

Standing on a Double-Seesaw Device is an Easy Way to Modify the Coordination Between the Two Feet for Controlling Upright Stance: Assessment Through Weight-Bearing Asymmetry

Healthy young subjects were instructed to modify their weight-bearing asymmetry when standing on a double-seesaw device. The results indicated decreased and unchanged amplitudes in the center-of-pressure movements under the unloaded and loaded legs, respectively. In addition, a concomitant increased contribution of the more loaded leg and a decreased contribution of the pressure distribution mechanism along the mediolateral axis were observed in the production of the resultant center of pressure, its amplitude remaining constant. Thus, contrary to what was previously reported for stance control on solid ground, one of the main characteristics of a double-seesaw device, by preventing increased amplitudes on the loaded side during weight-bearing asymmetry, would be to facilitate a greater independency of the feet in the stance control process.

Impact of Visual Biofeedback of Trunk Sway Smoothness on Motor Learning during Unipedal Stance

The assessment of trunk sway smoothness using an accelerometer sensor embedded in a smartphone could be a biomarker for tracking motor learning. This study aimed to determine the reliability of trunk sway smoothness and the effect of visual biofeedback of sway smoothness on motor learning in healthy people during unipedal stance training using an iPhone 5 measurement system. In the first experiment, trunk sway smoothness in the reliability group (n = 11) was assessed on two days, separated by one week. In the second, the biofeedback group (n = 12) and no-biofeedback group (n = 12) were compared during 7 days of unipedal stance test training and one more day of retention (without biofeedback). The intraclass correlation coefficient score 0.98 (0.93–0.99) showed that this method has excellent test–retest reliability. Based on the power law of practice, the biofeedback group showed greater improvement during training days (p = 0.003). Two-way mixed analysis of variance indicates a significant difference between groups (p < 0.001) and between days (p < 0.001), as well as significant interaction (p < 0.001). Post hoc analysis shows better performance in the biofeedback group from training days 2 and 7, as well as on the retention day (p < 0.001). Motor learning objectification through visual biofeedback of trunk sway smoothness enhances postural control learning and is useful and reliable for assessing motor learning.

Effects of performing dual tasks on postural sway and postural control complexity in people with haemophilic arthropathy

INTRODUCTION

People with haemophilic arthropathy (PWHA) have impairments in postural control. However, little is known about the effects of demanding conditions, including the unipedal stance and dual tasks, on postural control in PWHA.

AIM

Determine the effects of performing dual tasks while in the one-leg stance on postural sway and postural control complexity in PWHA vs. healthy active (HAG) and non-active (HNAG) groups of individuals.

METHODS

Fifteen PWHA and 34 healthy subjects (18 active and 16 non-active) were recruited. Vertical (V), mediolateral (ML) and anteroposterior (AP) centre of mass signals were acquired using a 3-axis accelerometer placed at the L3/L4 vertebrae of subjects as they performed the one-leg stance under single and dual-task conditions. Sway balance and the complexity of postural control were studied via root mean square (RMS) acceleration and sample entropy, respectively. Increased complexity of postural sway was attributed to increased automatism of postural control.

RESULTS

RMS values for PWHA were higher than HAG under both conditions for the V and ML axes, and higher than HNAG under the dual-task condition for the ML axis. Sample entropy was lower in PWHA than healthy individuals under the dual-task condition for V and ML axes, and the single-task condition for the ML axis (P < .05).

CONCLUSION

PWHA had poorer postural sway and decreased postural control complexity when performing a one-leg stance than healthy people, especially when the dual-task condition was applied. These results may help to design new approaches to assess and improve postural control in PWHA.

Adaptation of kinematic synergy and postural control to mechanical ankle constraint on an unsteady stance surface

Joint constraint interferes with the coordinative structure in joint movements used to optimize postural stability. This study aimed to investigate changes in postural synergy when the ankle joints were bilaterally braced during a stabilometer stance. Twenty-four young adults stood on a stabilometer plate while wearing a pair of ankle-foot orthoses, which were either unlocked or locked to restrict ankle motion (the ankle constraint (AC) and non-constraint (NC) conditions). Although ankle constraint did not significantly affect the dynamics of the stabilometer movements, the size and regularity of the first principal component (PC1), which explained more than 80% of the variance of joint movements in the lower limb, were increased. In addition, PC1 exhibited higher communalities with angular movements of the knee and hip joints in the AC condition than in the NC condition. Those subjects who exhibited a constraint-induced increase in postural sway (the I group) showed greater increases in the size and regularity of PC1 than did those who exhibited reduced postural sway during ankle constraint (the D group). Constraint-induced changes in postural synergy were group-dependent. Only the I group exhibited an increase of communality of PC1 with the hip angular movement following bilateral ankle constraint. In summary, bilateral ankle constraint altered the coordination solution, with increasing reliance on compensatory knee movement to maintain a balanced posture on the stabilometer. However, accessory hip movement due to ankle constraint was not economical and was disadvantageous to stance stability.