Multi-muscle control during bipedal stance: an EMG–EMG analysis approach

Adaptive changes in balance control strategies under continuous exposure to visual-somatosensory conflicts

Human postural control system has the capacity to adapt to balance-challenging perturbations. However, the characteristics and mechanisms of postural adaptation to continuous perturbation under the sensory conflicting environments remain unclear. We aimed to investigate the functional role of oscillatory coupling drive to lower-limb muscles with changes in balance control during postural adaptation under multisensory congruent and incongruent environments. We combined a platform moving sinusoidally (0.24 Hz) along the anterior-posterior (AP) axis and a virtual scene moving sinusoidally (0.24 Hz) either along the AP or the medio-lateral (ML) axis to present a 3-min visual-somatosensory congruent condition (n = 10) or incongruent condition (n = 12), respectively. We analyzed the kinematic data and performed intermuscular coherence analysis of surface EMG data from bilateral lower limbs. We found that the inter-limb coherence was larger under the congruent condition and decreased over the 3-min perturbation, while inter-limb coherence remained low and showed no changes under the incongruent condition over the 3-min perturbation. These results suggest that exposure to the incongruent condition disrupted inter-limb intermuscular coupling. Besides, we found the bilateral intra-limb coherence decreased over 3-min congruent and incongruent perturbation, with the bilateral ankle joint angular velocity decreased and the coupling strength (0.2-0.3 Hz) between whole body sway and sinusoidal stimuli in AP decreased. These findings suggest that continuous exposure to sinusoidal perturbation in AP under congruent and incongruent conditions decreased bilateral intermuscular coupling, contributing to flexibility in the sagittal plane. Overall, we suggested the postural control system adapts context specifically to different sensory environments, with distinct characteristics of neuromuscular control strategies.NEW & NOTEWORTHY Lower limb muscle coordination plays a vital role when facing continuous perturbation by updating sensorimotor mappings. However, it is unclear how muscle coordination adapts to visual-somatosensory congruent and incongruent perturbations. Here, we found that muscle coordination showed context-specific adaptive changes to visual-somatosensory congruent or incongruent environment.

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.

Differences in Ankle Neuromuscular Control Between the Preferred Speed and Fixed Speeds During Walking

Walking at the preferred speed, considered as a self-optimized gait pattern, is associated with improved energy conservation and cognitive abilities. However, the neuromuscular mechanisms underlying the benefits of the preferred walking speed remain unclear. Therefore, this study aimed to determine the differences in ankle neuromuscular control between the preferred and fixed speeds during walking. Eighteen healthy young adults were recruited to perform overground barefoot walking at the preferred speed, the prefer-matched control speed (PMCS), slower fixed speeds (1, 2, 3 and 4 km/h) and faster fixed speeds (5 and 6 km/h). Muscle synergies and intermuscular coherence were calculated using surface electromyography (EMG) signals of ankle muscles. Results showed that the preferred walking speed exhibited one less muscle synergy and higher intermuscular coherence in 8-42 Hz than the PMCS. Additionally, slow walking speeds performed more muscle synergies and weaker couplings between plantar flexors in 26-60 Hz than the preferred speed and faster fixed speeds. Our results demonstrate an impact of the preferred walking speed on ankle neuromuscular control during walking, which might influence energy consumption and brain resource occupation. Besides, the preferred walking speed and faster fixed speeds showed comparable modular control characteristics of ankle muscles, which might provide suggestions for experimental settings when examining individuals' natural neuromuscular control features.

Dynamical network-based evaluation for neuromuscular dysfunction in stroke-induced hemiplegia during standing

BACKGROUND

A given movement requires precise coordination of multiple muscles under the control of center nervous system. However, detailed knowledge about the changing characteristics of neuromuscular control for multi-muscle coordination in post-stroke hemiplegic patients during standing is still lacking. This study aimed to investigate the hemiplegia-linked neuromuscular dysfunction during standing from the perspective of multi-muscle dynamical coordination by utilizing a novel network approach - weighted recurrence network (WRN).

METHODS

Ten male hemiplegic patients with first-ever stroke and 10 age-matched healthy male adults were instructed to stand on a platform quietly for 30 s with eyes opened and eyes closed, respectively. The WRN was constructed based on the surface electromyography signals of 16 muscles from trunk, hips, thighs and calves. Relevant topological parameters, including clustering coefficient (C) and average shortest path length (L), were extracted to evaluate the dynamical coordination of multiple muscles. A measure of node centrality in network theory, degree of centrality (DC), was innovatively introduced to assess the contribution of single muscle in the multi-muscle dynamical coordination. The standing-related assessment metric, center of pressure (COP), was provided by the platform directly.

RESULTS

Results showed that the post-stroke hemiplegic patients stood with remarkably higher similarity of muscle activation and more coupled intermuscular dynamics, characterized by higher C and lower L than the healthy subjects (p < 0.05). The DC values and rankings of back, hip and calf muscles on the affected side were significantly decreased, whereas those on the unaffected side were significantly increased in hemiplegia group compared with the healthy group (p < 0.05). Without visual feedback, subjects exhibited enhanced muscle coordination and increased muscle involvement (p < 0.05). A decrease in C and an increase in L of WRN were observed with decreased COP areas (p < 0.05).

CONCLUSIONS

These findings revealed that stroke-induced hemiplegia could significantly influence the neuromuscular control, which was manifested as more coupled intermuscular dynamics, abnormal deactivation of muscles on affected side and compensation of muscles on unaffected side from the perspective of multi-muscle coordination. Enhanced multi-muscle dynamical coordination was strongly associated with impaired postural control. This study provides a novel analytical tool for evaluation of neuromuscular dysfunction and specification of responsible muscles for impaired postural control in stroke-induced hemiplegic patients, and could be potentially applied in clinical practice.

Acute responses of postural alignment and intermuscular coherence to anti-gravitational muscle engagement-A randomized crossover trial

INTRODUCTION

Posture is a facet of clinical assessment in several rehabilitative disciplines. Despite extensive clinical focus, the precision with which posture can be evaluated and intervened upon is limited by the very general terms used to describe it. The purpose of this crossover trial was to quantify the effects of targeted postural intervention motivated by theoretical sagittal gravitational collapsing (SGC) tendencies on: 1) distance from SGC, 2) intermuscular coherence (iCOH), and 3) kinematic chain connectivity.

METHODS

Ten healthy adults (24.50 ± 1.18 years, 172.72 ± 10.19 cm, 76.47 ± 14.60 kg) completed pre- and post-intervention testing on two occasions involving contrasting interventions: promote postural muscle (PPM) vs. reduce compensatory muscle (RCM) engagement. Distance from SGC, iCOH, and kinematic chain connectivity were quantified from electromyography and/or kinematic data acquired during tests administered before and after interventions. Effects of Treatment [PPM, RCM] and Time [Pre, Post] were tested with linear mixed models.

RESULTS

A Treatment*Time interaction was observed for distance from SGC. Post-intervention distance from SGC was greater following PPM only (p < 0.01). A Treatment*Time interaction was observed for hi-frequency trunk muscle iCOH, with a post-intervention increase corresponding to the RCM intervention (p < 0.007). Additional iCOH effects did not differ by intervention.

CONCLUSION

Distance from SGC is acutely modifiable and increases following exercises to facilitate anti-SGC muscles. Convergent findings related to kinematic chain connectivity and prescriptive neural binding were not observed. These observations suggest that it may be possible to describe, evaluate, and intervene upon posture in reference to a specific, mechanistic theory regarding the function of postural alignment.

Muscle synergies are associated with intermuscular coherence and cortico-synergy coherence in an isometric upper limb task

To elucidate the underlying physiological mechanisms of muscle synergies, we investigated long-range functional connectivity by cortico-muscular (CMC), intermuscular (IMC) and cortico-synergy (CSC) coherence. Fourteen healthy participants executed an isometric upper limb task in synergy-tuned directions. Cortical activity was recorded using 32-channel electroencephalography (EEG) and muscle activity using 16-channel electromyography (EMG). Using non-negative matrix factorisation (NMF), we calculated muscle synergies from two different tasks. A preliminary multidirectional task was used to identify synergy-preferred directions (PDs). A subsequent coherence task, consisting of generating forces isometrically in the synergy PDs, was used to assess the functional connectivity properties of synergies. Overall, we were able to identify four different synergies from the multidirectional task. A significant alpha band IMC was consistently present in all extracted synergies. Moreover, IMC alpha band was higher between muscles with higher weights within a synergy. Interestingly, CSC alpha band was also significantly higher across muscles with higher weights within a synergy. In contrast, no significant CMC was found between the motor cortex area and synergy muscles. The presence of a shared input onto synergistic muscles within a synergy supports the idea of neurally derived muscle synergies that build human movement. Our findings suggest cortical modulation of some of the synergies and the consequential existence of shared input between muscles within cortically modulated synergies.

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.

Coherence between electromyographic signals of anterior tibialis, soleus, and gastrocnemius during standing balance tasks

Introduction

Knowledge about the mechanics and physiological features of balance for healthy individuals enhances understanding of impairments of balance related to neuropathology secondary to aging, diseases of the central nervous system (CNS), and traumatic brain injury, such as concussion.

Methods

We examined the neural correlations during muscle activation related to quiet standing from the intermuscular coherence in different neural frequency bands. Electromyography (EMG) signals were recorded from six healthy participants (fs = 1,200 Hz for 30 s) from three different muscles bilaterally: anterior tibialis, medial gastrocnemius, and soleus. Data were collected for four different postural stability conditions. In decreasing order of stability these were feet together eyes open, feet together eyes closed, tandem eyes open, and tandem eyes closed. Wavelet decomposition was used to extract the neural frequency bands: gamma, beta, alpha, theta, and delta. Magnitude-squared-coherence (MSC) was computed between different muscle pairs for each of the stability conditions.

Results and discussion

There was greater coherence between muscle pairs in the same leg. Coherence was greater in lower frequency bands. For all frequency bands, the standard deviation of coherence between different muscle pairs was always higher in the less stable positions. Time-frequency coherence spectrograms also showed higher intermuscular coherence for muscle pairs in the same leg and in less stable positions. Our data suggest that coherence between EMG signals may be used as an independent indicator of the neural correlates for stability.

Long-term effects of mTBIs includes a higher dependency on visual inputs to control vertical posture

This study investigated the hypothesis that individuals living with long-term effects of mild traumatic brain injury (mTBI) develop an increased dependency on visual inputs to control upright posture. To test this hypothesis, we quantified visuo-postural dependency indices (VPDIs) calculated for multiple postural behavioral markers extracted from the body's center of pressure coordinates signals. These signals were recorded during the execution of a quiet bipedal stance under Vision and No-Vision experimental conditions. VPDIs were calculated as the normalized pair-wise subtraction of recordings obtained under Vision and No-Vision. A total of one hundred and twenty-nine volunteers were organized into two groups: mTBI group (n = 50) and neurotypical control group (n = 79). Consistent with our hypothesis, the results reveal that balance behavior of participants with mTBI deteriorate more abruptly in the absence of visual inputs when compared to neurotypical controls. These impairments might increase the likelihood of recurrent injuries and falls when time-constrained reactions are needed in daily activities, sports practice, or military operations. Additionally, the methodology used in this study shows to be potentially useful to aid future investigations of neural circuitry impaired by mTBI. It also provides indices of recovery for future clinical trials testing mTBI-related clinical interventions.

Modulation of Muscle Synergies in Lower-Limb Muscles Associated With Split-Belt Locomotor Adaptation

Humans have great locomotor adaptability to environmental demands, which has been investigated using a split-belt treadmill with belts on both the left and right sides. Thus far, neuromuscular control in split-belt locomotor adaptation has been evaluated by analyzing muscle activities at the individual muscle level. Meanwhile, in the motor control field, the muscle synergy concept has been proposed. Muscle synergies are considered the fundamental building blocks of movement and are groups of coactive muscles and time-varying activation patterns, thereby, reflecting the neurophysiological characteristics of movement. To date, it remains unclear how such muscle synergies change during the adaptation and de-adaptation processes on the split-belt treadmill. Hence, we chronologically extracted muscle synergies while walking on the split-belt treadmill and examined changes in the number, muscle weightings, and temporal activation patterns of muscle synergies. Twelve healthy young males participated, and surface electromyography (EMG) signals were recorded bilaterally from 13 lower-limb muscles. Muscle synergies were extracted by applying non-negative matrix factorization to the EMG data of each leg. We found that during split-belt walking, the number of synergies in the slow leg increased while an extra synergy appeared and disappeared in the fast leg. Additionally, the areas under the temporal activation patterns in several synergies in both legs decreased. When both belts returned to the same speed, a decrease in the number of synergies and an increase in the areas under the temporal activation patterns of several synergies were temporally shown in each leg. Subsequently, the number of synergies and the areas under the temporal activation patterns returned to those of normal walking before split-belt walking. Thus, changes in the number, muscle weightings, and temporal activation patterns of synergies were noted in the split-belt locomotor adaptation, suggesting that the adaptation and de-adaptation occurred at the muscle synergy level.

Inter-muscular coherence and functional coordination in the human upper extremity after stroke

Muscle coordination and motor function of stroke patients are weakened by stroke-related motor impairments. Our earlier studies have determined alterations in inter-muscular coordination patterns (muscle synergies). However, the functional connectivity of these synergistically paired or unpaired muscles is still unclear in stroke patients. The goal of this study is to quantify the alterations of inter-muscular coherence (IMC) among upper extremity muscles that have been shown to be synergistically or non-synergistically activated in stroke survivors. In a three-dimensional isometric force matching task, surface EMG signals are collected from 6 age-matched, neurologically intact healthy subjects and 10 stroke patients, while the target force space is divided into 8 subspaces. According to the results of muscle synergy identification with non-negative matrix factorization algorithm, muscle pairs are classified as synergistic and non-synergistic. In both control and stroke groups, IMC is then calculated for all available muscle pairs. The results show that synergistic muscle pairs have higher coherence in both groups. Furthermore, anterior and middle deltoids, identified as synergistic muscles in both groups, exhibited significantly weaker IMC at alpha band in stroke patients. The anterior and posterior deltoids, identified as synergistic muscles only in stroke patients, revealed significantly higher IMC in stroke group at low gamma band. On the contrary, anterior deltoid and pectoralis major, identified as synergistic muscles in control group only, revealed significantly higher IMC in control group in alpha band. The results of muscle synergy and IMC analyses provide congruent and complementary information for investigating the mechanism that underlies post-stroke motor recovery.

Unilateral transfemoral amputees exhibit altered strength and dynamics of muscular co-activation modulated by visual feedback

OBJECTIVE

Somatosensory perception is disrupted in patients with a lower limb amputation. This increases the difficulty to maintain balance and leads to the development of neuromuscular adjustments. We investigated how these adjustments are reflected in the co-activation of lower body muscles and are modulated by visual feedback.

APPROACH

We measured electromyography (EMG) signals of muscles from the trunk (erector spinae and obliquus external), and the lower intact/dominant leg (tibialis anterior and medial gastrocnemius) in eleven unilateral transfemoral amputees and eleven age-matched able-bodied controls during 30 seconds of upright standing with and without visual feedback. Muscle synergies involved in balance control were investigated using wavelet coherence analysis. We focused on 7 frequencies grouped in three frequency bands, a low-frequency band (7.56 and 19.86 Hz) representing more sub-cortical and spinal inputs to the muscles, a mid-frequency band (38.26 and 62.63 Hz) representing more cortical inputs, and a high-frequency band (92.90, 129 and 170.90 Hz) associated with synchronizing motor unit action potentials. Further, the dynamics of changes in intermuscular coupling over time were quantified using the Entropic Half-Life.

MAIN RESULTS

Amputees exhibited lower coherency values when vision was removed at 7.56 Hz for the muscle pair of the lower leg. At this frequency, the coherency values of the amputee group also differed from controls for the eyes closed condition. Controls and amputees exhibited opposite coherent behaviors with visual feedback at 7.56 Hz. For the eyes open condition at 129 Hz, the coherency values of amputees and controls differed for the muscle pair of the trunk, and at 170.90 Hz for the muscle pair of the lower leg. Amputees exhibited different dynamics of muscle co-activation at the low frequency band when vision was available.

SIGNIFICANCE

Altogether, these findings point to the development of neuromuscular adaptations reflected in the strength and dynamics of muscular co-activation.

Intra- and Intermuscular Coherence and Body Acceleration Control in Older Adults during Bipedal Stance

Our aim was to clarify the effect of aging on the coherence of electromyograms of plantar flexor pairs during bipedal stance and to clarify the relationship between coherence and center-of-mass acceleration (COMacc). The subjects were 16 adults and 18 older adults. Intra- and intermuscular coherence and phase analyses were used to analyze the muscle pairs of bilateral and unilateral plantar flexor muscle groups. The relationship between coherence value and anterior-posterior COMacc of the plantar flexor muscle pairs was also examined to determine whether the connectivity of the lower limb muscle pairs is functionally important. The older adults showed higher coherence in the frequency range of 0-4 Hz for muscle pairs than the younger adults. In phase analysis, the older adults showed a phase difference between bilateral heteronymous muscle pairs in the frequency range of 0-6 Hz, which was one of the characteristics not seen in the younger adults. Correlation analysis showed that all the muscle pairs were moderately correlated with COMacc in the older adults. Not only does aging affects the organization of the bilateral and unilateral postural muscle activity of the plantar flexors during bipedal stance, but such organization may also be related to the increased COMacc characteristics of older adults.

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.

The Changes of Motor Control Strategies in Non-specific Chronic Low Back Pain During Spinal Manipulation and Muscle Energy Techniques: A Beta-band Intermuscular Pair-Wise Coherence Analysis

Background: Until now, a variety of techniques have been introduced to address the adverse effects of NS-CLBP, including spinal manipulation technique (SMT) and muscle energy technique (MET). However, most of these techniques have focused on pain assessment and disability. In other words, the intermuscular synchronization between the co-contracting muscles was not considered, and hence, the effectiveness of these techniques on the corticospinal tract function was not studied. Objectives: This study aimed to compare the effects of SMT and MET on corticospinal tract function during four phases of standing, flexion, relaxation, and extension in flexion-extension task (F-ET) in NS-CLBP using pair-wise coherence of Beta-band intermuscular coherence (Bb-IMC). Methods: Twenty volunteer healthy male subjects and twenty-four male subjects with NS-CLBP (20 - 45 years of age) participated in this work. The patients had continuous or recurrent symptoms for three months or more without any referral pain to the lower extremities. The patients were randomly assigned to two equal intervention groups (SMT and MET), and the techniques were applied as described by Greenman. Surface electromyography (sEMGs) from lumbopelvic muscles was recorded for all participants (i.e., healthy group and the patient groups), while they performed three trials of F-ET, and the pair-wise coherence for all muscles was calculated using Bb-IMC analysis. Besides, in the patient's groups, sEMGs from the muscles were recorded before and after the interventional techniques (i.e., SMT and MET), while they performed three trials of F-ET, and the pair-wise coherence was calculated. Multivariate analysis of variance test was used to compare the healthy subjects and patient groups before the interventions in A and B muscle cross at different phases of F-ET task. Furthermore, in the NS-CLBP patients, comparisons were made before and after the interventions in each group (i.e., SMT and MET groups) as well as between the two groups in A and B muscle cross at different phases of the F-ET task. Results: In the standing phase of F-ET, there were no significant differences in the SMT and MET group before and after the intervention in PWC of A muscle cross and B muscle cross (P < 0.05). Considering the flexion phase, there were significant differences in the SMT group in all pair muscles as PWC (M1-M4), PWE (M1-M6), PWC (M4-M6) (P < 0.05), whereas there was one significantly in PWC (M4-M6) in the MET group (P < 0.05). In the relaxation phase, the SMT had significantly in PWC (M2-M5), whereas there was one significantly in PWC (M4-M6) in MET group (P < 0.05). In the extension phase, although the SMT was not significant (P < 0.05) in the MET intervention group, there were significant differences in the PWC (M2-M3) and PWC muscles (M2-M5). Conclusions: This study provided some pieces of evidence about the effects of one of the common manual therapy techniques on the primary motor cortex and corticospinal drive in the NS-CLBP patients. The results showed that, by increasing the pair-wise coherence in all phases of FE-T, SMT intervention was more effective than MET intervention. Therefore, the pair-wise coherence of Bb-IMC can be considered an approach for clinicians when designing the rehabilitation protocol to ensure optimal treatment.

Intermuscular coherence analysis in older adults reveals that gait-related arm swing drives lower limb muscles via subcortical and cortical pathways

KEY POINTS

Gait-related arm swing in humans supports efficient lower limb muscle activation, indicating a neural coupling between the upper and lower limbs during gait. Intermuscular coherence analyses of gait-related electromyography from upper and lower limbs in 20 healthy participants identified significant coherence in alpha and beta/gamma bands indicating that upper and lower limbs share common subcortical and cortical drivers that coordinate the rhythmic four-limb gait pattern. Additional directed connectivity analyses revealed that upper limb muscles drive and shape lower limb muscle activity during gait via subcortical and cortical pathways and to a lesser extent vice versa. The results provide a neural underpinning that arm swing may serve as an effective rehabilitation therapy concerning impaired gait in neurological diseases.

ABSTRACT

Human gait benefits from arm swing, as it enhances efficient lower limb muscle activation in healthy participants as well as patients suffering from neurological impairment. The underlying neuronal mechanisms of such coupling between upper and lower limbs remain poorly understood. The aim of the present study was to examine this coupling by intermuscular coherence analysis during gait. Additionally, directed connectivity analysis of this coupling enabled assessment of whether gait-related arm swing indeed drives lower limb muscles. To that end, electromyography recordings were obtained from four lower limb muscles and two upper limb muscles bilaterally, during gait, of 20 healthy participants (mean (SD) age 67 (6.8) years). Intermuscular coherence analysis revealed functional coupling between upper and lower limb muscles in the alpha and beta/gamma band during muscle specific periods of the gait cycle. These effects in the alpha and beta/gamma bands indicate involvement of subcortical and cortical sources, respectively, that commonly drive the rhythmic four-limb gait pattern in an efficiently coordinated fashion. Directed connectivity analysis revealed that upper limb muscles drive and shape lower limb muscle activity during gait via subcortical and cortical pathways and to a lesser extent vice versa. This indicates that gait-related arm swing reflects the recruitment of neuronal support for optimizing the cyclic movement pattern of the lower limbs. These findings thus provide a neural underpinning for arm swing to potentially serve as an effective rehabilitation therapy concerning impaired gait in neurological diseases.

Intermuscular coherence between homologous muscles during dynamic and static movement periods of bipedal squatting

Coordination of functionally coupled muscles is a key aspect of movement execution. Demands on coordinative control increase with the number of involved muscles and joints, as well as with differing movement periods within a given motor sequence. While previous research has provided evidence concerning inter- and intramuscular synchrony in isolated movements, compound movements remain largely unexplored. With this study, we aimed to uncover neural mechanisms of bilateral coordination through intermuscular coherence (IMC) analyses between principal homologous muscles during bipedal squatting (BpS) at multiple frequency bands (alpha, beta, and gamma). For this purpose, participants performed bipedal squats without additional load, which were divided into three distinct movement periods (eccentric, isometric, and concentric). Surface electromyography (EMG) was recorded from four homologous muscle pairs representing prime movers during bipedal squatting. We provide novel evidence that IMC magnitudes differ between movement periods in beta and gamma bands, as well as between homologous muscle pairs across all frequency bands. IMC was greater in the muscle pairs involved in postural and bipedal stability compared with those involved in muscular force during BpS. Furthermore, beta and gamma IMC magnitudes were highest during eccentric movement periods, whereas we did not find movement-related modulations for alpha IMC magnitudes. This finding thus indicates increased integration of afferent information during eccentric movement periods. Collectively, our results shed light on intermuscular synchronization during bipedal squatting, as we provide evidence that central nervous processing of bilateral intermuscular functioning is achieved through task-dependent modulations of common neural input to homologous muscles.

NEW & NOTEWORTHY It is largely unexplored how the central nervous system achieves coordination of homologous muscles of the upper and lower body within a compound whole body movement, and to what extent this neural drive is modulated between different movement periods and muscles. Using intermuscular coherence analysis, we show that homologous muscle functions are mediated through common oscillatory input that extends over alpha, beta, and gamma frequencies with different synchronization patterns at different movement periods.

Muscle Synergies and Coherence Networks Reflect Different Modes of Coordination During Walking

When walking speed is increased, the frequency ratio between the arm and leg swing switches spontaneously from 2:1 to 1:1. We examined whether these switches are accompanied by changes in functional connectivity between multiple muscles. Subjects walked on a treadmill with their arms swinging along their body while kinematics and surface electromyography (EMG) of 26 bilateral muscles across the body were recorded. Walking speed was varied from very slow to normal. We decomposed EMG envelopes and intermuscular coherence spectra using non-negative matrix factorization (NMF), and the resulting modes were combined into multiplex networks and analyzed for their community structure. We found five relevant muscle synergies that significantly differed in activation patterns between 1:1 and 2:1 arm-leg coordination and the transition period between them. The corresponding multiplex network contained a single module indicating pronounced muscle co-activation patterns across the whole body during a gait cycle. NMF of the coherence spectra distinguished three EMG frequency bands: 4-8, 8-22, and 22-60 Hz. The community structure of the multiplex network revealed four modules, which clustered functional and anatomical linked muscles across modes of coordination. Intermuscular coherence at 4-22 Hz between upper and lower body and within the legs was particularly pronounced for 1:1 arm-leg coordination and was diminished when switching between modes of coordination. These findings suggest that the stability of arm-leg coordination is associated with modulations in long-distant neuromuscular connectivity.

Intermuscular coupling and postural control in unilateral transfemoral amputees - a pilot study. ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. ANNUAL INTERNATIONAL CONFERENCE 2020;2020:3815-3818. [PMID: 33018832 DOI: 10.1109/embc44109.2020.9176850] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The dynamics of the adjustment of center of pressure (CoP) has been utilized to understand motor control in human pathologies characterized by impairments in postural balance. The control mechanisms that maintain balance can be investigated via the analysis of muscle recruitment using electromyography (EMG) signals. In this work, we combined these two techniques to investigate balance control during upright standing in transfemoral unilateral amputees wearing a prosthesis. The dynamics of the CoP adjustments and EMG-EMG coherence between four muscles of the trunk and lower limb of 5 unilateral transfemoral amputees and 5 age-matched able-bodied participants were quantified during 30 s of quiet standing using the entropic half-life (EnHL) method. Two visual conditions, eyes open and eyes closed, were tested. Overall, the group of amputees presented lower EnHL values (higher dynamics) in their CoP adjustments than controls, especially in their intact limb. The EnHL values of the EMG-EMG coherence time series in the amputee group were lower than the control group for almost all muscle pairs under both visual conditions. Different correlations between the EnHL values of the CoP data and the EMG-EMG coherence data were observed in the amputee and control groups. These preliminary results suggest the onset of distinct neuromuscular adaptations following a unilateral amputation.Clinical Relevance - Understanding neuromuscular adaptation mechanisms after an amputation may serve to design better rehabilitation treatments and novel prosthetic devices with sensory feedback.

The effects of aging on the distribution and strength of correlated neural inputs to postural muscles during unperturbed bipedal stance

The present study investigated the effects of aging on the distribution of common descending neural drives to main postural muscles acting on the ankle, knee, hip, and lower trunk. The presence, distribution, and strength of these drives were assessed using intermuscular coherence estimations at a low-frequency band (0-55 Hz). Ten healthy older adults (68.7 ± 3.5 years) with no recent history of falls and ten healthy younger adults (26.8 ± 2.7 years) performed bipedal stances with eyes either opened or closed. Electromyographic (EMG) signals of six postural muscles were recorded. Estimations of intermuscular coherence were obtained from fifteen muscle pairs and four muscle groups. In general, single-pair and pooled coherence analyzes revealed significant levels of signal synchronization within 1-10 Hz. Significant common drives to anterior, posterior, and antagonist muscle groups were observed for both cohorts of participants. However, older participants showed significantly stronger EMG-EMG synchronization in the frequency domain compared to younger participants. It seems that age-related sarcopenia, visual-vestibular-proprioceptive decline, cortical activation increase, presynaptic inhibition modulation decrease, and co-contraction increase had a major impact on strengthening the common drives to the aforementioned muscle groups. Differently from young adults, the absence of visual inputs did not reduce the magnitude of signal synchronization in older adults. These results suggest that the aging central nervous system seems to organize similar arrangements of common drives to postural antagonist muscles at different joints, and to postural muscles pushing the body either forward or backward when visual information is not available.

Smaller muscle mass is associated with increase in EMG-EMG coherence of the leg muscle during unipedal stance in elderly adults

Age-induced decline in the ability to perform daily activities is associated with a deterioration of physical parameters. Changes occur in neuromuscular system with age; however, the relationship between these changes and physical parameters has not been fully elucidated. Therefore, in this study, we aimed to determine the relationship between neuromuscular system evaluated using a coherence analysis of the leg muscles and physical parameters in community-dwelling healthy elderly adults. The participants were required to stand still in bipedal and unipedal stances on a force plate. Then, electromyography (EMG) was recorded from the tibialis anterior (TA) and medial and lateral gastrocnemius (MG/LG) muscles, and intermuscular coherence was calculated between the following pairs: TA and MG (TA-MG), TA and LG (TA-LG), and MG and LG (MG-LG). Furthermore, gait speed, unipedal stance time, and muscle mass were measured. EMG-EMG coherence for the MG-LG pair was significantly greater in the unipedal stance task than in the bipedal one (p = .001). Multiple linear regression analysis revealed that the muscle mass of the leg was negatively correlated with the change in the β-band coherence for the MG-LG pair from bipedal to unipedal stance (R² = 0.067, standard β = -0.345, p = .044). As the β-band coherence could reflect the corticospinal activity, the increased β-band coherence may be a compensation for the smaller muscle mass, or alternatively may be a sign of changes in the nervous system resulting in the loss of muscle mass.

Standing task difficulty related increase in agonist-agonist and agonist-antagonist common inputs are driven by corticospinal and subcortical inputs respectively

In standing, coordinated activation of lower extremity muscles can be simplified by common neural inputs to muscles comprising a functional synergy. We examined the effect of task difficulty on common inputs to agonist-agonist (AG-AG) pairs supporting direction specific reciprocal muscle control and agonist-antagonist (AG-ANT) pairs supporting stiffness control. Since excessive stiffness is energetically costly and limits the flexibility of responses to perturbations, compared to AG-ANT, we expected greater AG-AG common inputs and a larger increase with increasing task difficulty. We used coherence analysis to examine common inputs in three frequency ranges which reflect subcortical/spinal (0–5 and 6–15 Hz) and corticospinal inputs (6–15 and 16–40 Hz). Coherence was indeed higher in AG-AG compared to AG-ANT muscles in all three frequency bands, indicating a predilection for functional synergies supporting reciprocal rather than stiffness control. Coherence increased with increasing task difficulty, only in AG-ANT muscles in the low frequency band (0–5 Hz), reflecting subcortical inputs and only in AG-AG group in the high frequency band (16–40 Hz), reflecting corticospinal inputs. Therefore, common neural inputs to both AG-AG and AG-ANT muscles increase with difficulty but are likely driven by different sources of input to spinal alpha motor neurons.

Upright balance control strategies during pregnancy

BACKGROUND

Morphological and physiological changes during pregnancy are considered to interfere with the mechanisms of postural control and potentially increase the risk of falling. A clear understanding of these mechanisms is important to improve pre-natal care and reduce the fall risk in this population.

OBJECTIVES

This study focused on investigating how pregnancy affects postural control in each trimester of pregnancy by analyzing pelvic inclination and body sway behavior. Our main hypothesis was that balance control and posture would change during pregnancy. More specifically, pregnancy would increase sway amplitude, anterior pelvic tilt, and body sway regularity in time.

STUDY DESIGN

Forty women formed four groups: non-pregnant women (NP) and women at their first, second, and third trimester of pregnancy (P1, P2, and P3, respectively). All participants performed (1) postural evaluation of the pelvic inclination using a digital system of postural analysis and (2) instrumented posturography using a force platform to collect the coordinates of the body's center of pressure (COP) during quiet bipedal stance for 120 s. Kruskal-Wallis H test and post-hoc Mann-Whitney U tests were used to investigate the effects of pregnancy (NP, P1, P2, and P3) on pelvic inclination angle and postural indices computed from the COP signals.

RESULTS

Results revealed significant larger body sway accompanied by a more regular medial-lateral pattern of oscillation and a more synchronized anterior-posterior and medial-lateral sway already at the first trimester of pregnancy. The averaged COP migrated posteriorly at third trimester of pregnancy and the anterior pelvic tilt increased at second and third trimesters.

CONCLUSIONS

Our results indicate the existence of changes in posture and balance metrics even at early stages of pregnancy. We suggest the use of posturography as one of screening tools for postural instability and fall risk during pregnancy.

Effect of Task Failure on Intermuscular Coherence Measures in Synergistic Muscles

The term “task failure” describes the point when a person is not able to maintain the level of force required by a task. As task failure approaches, the corticospinal command to the muscles increases to maintain the required level of force in the face of a decreased mechanical efficacy. Nevertheless, most motor tasks require the synergistic recruitment of several muscles. How this recruitment is affected by approaching task failure is still not clear. The increase in the corticospinal drive could be due to an increase in synergistic recruitment or to overlapping commands sent to the muscles individually. Herein, we investigated these possibilities by combining intermuscular coherence and synergy analysis on signals recorded from three muscles of the quadriceps during dynamic leg extension tasks. We employed muscle synergy analysis to investigate changes in the coactivation of the muscles. Three different measures of coherence were used. Pooled coherence was used to estimate the command synchronous to all three muscles, pairwise coherence the command shared across muscle pairs and residual coherence the command peculiar to each couple of muscles. Our analysis highlights an overall decrease in synergistic command at task failure and an intensification of the contribution of the nonsynergistic shared command.

Modulation of EMG-EMG Coherence in a Choice Stepping Task

The voluntary step execution task is a popular measure for identifying fall risks among elderly individuals in the community setting because most falls have been reported to occur during movement. However, the neurophysiological functions during this movement are not entirely understood. Here, we used electromyography (EMG) to explore the relationship between EMG-EMG coherence, which reflects common oscillatory drive to motoneurons, and motor performance associated with stepping tasks: simple reaction time (SRT) and choice reaction time (CRT) tasks. Ten healthy elderly adults participated in the study. Participants took a single step forward in response to a visual imperative stimulus. EMG-EMG coherence was analyzed for 1000 ms before the presentation of the stimulus (stationary standing position) from proximal and distal tibialis anterior (TA) and soleus (SOL) muscles. The main result showed that all paired EMG-EMG coherences in the alpha and beta frequency bands were greater in the SRT than the CRT task. This finding suggests that the common oscillatory drive to the motoneurons during the SRT task occurred prior to taking a step, whereas the lower value of corticospinal activity during the CRT task prior to taking a step may indicate an involvement of inhibitory activity, which is consistent with observations from our previous study (Watanabe et al., 2016). Furthermore, the beta band coherence in intramuscular TA tended to positively correlate with the number of performance errors that are associated with fall risks in the CRT task, suggesting that a reduction in the inhibitory activity may result in a decrease of stepping performance. These findings could advance the understanding of the neurophysiological features of postural adjustments in elderly individuals.

The use of intermuscular coherence analysis as a novel approach to detect age-related changes on postural muscle synergy

The overall goal of this study was to investigate potential adaptations brought about by the natural processes of aging on the coordination of postural muscles. Considering the progressive and non-homogeneous deterioration of sensorimotor and neuromuscular systems as the individual grows older, it was hypothesized that aging is associated with a reorganization of synergistic mechanisms controlling postural muscles. Therefore, the presence, distribution, and strength of correlated neural inputs to three posterior postural muscles were measured by intermuscular coherence estimations at a low frequency band (0-55Hz). Nine healthy young adults and thirteen healthy older adults performed ten trials of a perturbed task: bipedal stance while holding a five kg load for fifteen seconds. Estimates of intermuscular coherence for each pair of electromyographic signals (soleus and biceps femoris, soleus and erector spinae, and biceps femoris and erector spinae) were computed. Results revealed significantly stronger levels of synchronization of posterior muscles within 0-10Hz in seniors compared to young adults. In addition, seniors presented similar spectra of intermuscular coherence within 0-55Hz for all three muscle pairs analyzed. These findings provide valuable information regarding compensatory mechanisms adopted by older adults to control balance. The age-related reorganization of neural drive controlling posterior postural muscles revealing a stronger synchronization within 0-10Hz might be related to the faster body sway and muscle co-activation patterns usually observed in this population. Finally, this study supports the use of Intermuscular Coherence Analysis as a sensitive method to detect age-related changes in multi-muscle control.

Spectral properties of multiple myoelectric signals: New insights into the neural origin of muscle synergies

It is still unclear if muscle synergies reflect neural strategies or mirror the underlying mechanical constraints. Therefore, this study aimed to verify the consistency of muscle groupings between the synergies based on the linear envelope (LE) of muscle activities and those incorporating the time-frequency (TF) features of the electromyographic (EMG) signals. Twelve healthy participants performed six 20-m walking trials at a comfort and fast self-selected speed, while the activity of eleven lower limb muscles was recorded by means of surface EMG. Wavelet-transformed EMG was used to obtain the TF pattern and muscle synergies were extracted by non-negative matrix factorization. When five muscle synergies were extracted, both methods defined similar muscle groupings whatever the walking speed. When accounting the reconstruction level of the initial dataset, a new TF synergy emerged. This new synergy dissociated the activity of the rectus femoris from those of the vastii muscles (synergy #1) and from the one of the tensor fascia latae (synergy #5). Overall, extracting TF muscle synergies supports the neural origin of muscle synergies and provides an opportunity to distinguish between prescriptive and descriptive muscle synergies.

Functional connectivity in the neuromuscular system underlying bimanual coordination

Neural synchrony has been suggested as a mechanism for integrating distributed sensorimotor systems involved in coordinated movement. To test the role of corticomuscular and intermuscular coherence in bimanual coordination, we experimentally manipulated the degree of coordination between hand muscles by varying the sensitivity of the visual feedback to differences in bilateral force. In 16 healthy participants, cortical activity was measured using EEG and muscle activity of the flexor pollicis brevis of both hands using high-density electromyography (HDsEMG). Using the uncontrolled manifold framework, coordination between bilateral forces was quantified by the synergy index R_V in the time and frequency domain. Functional connectivity was assessed using corticomuscular coherence between muscle activity and cortical source activity and intermuscular coherence between bilateral EMG activity. The synergy index increased in the high coordination condition. R_V was higher in the high coordination condition in frequencies between 0 and 0.5 Hz; for the 0.5- to 2-Hz frequency band, this pattern was inverted. Corticomuscular coherence in the beta band (16-30 Hz) was maximal in the contralateral motor cortex and was reduced in the high coordination condition. In contrast, intermuscular coherence was observed at 5-12 Hz and increased with bimanual coordination. Within-subject comparisons revealed a negative correlation between R_V and corticomuscular coherence and a positive correlation between R_V and intermuscular coherence. Our findings suggest two distinct neural pathways: 1) corticomuscular coherence reflects direct corticospinal projections involved in controlling individual muscles; and 2) intermuscular coherence reflects diverging pathways involved in the coordination of multiple muscles.

The difficulty of the postural control task affects multi-muscle control during quiet standing

The aim of this study was to compare the electromyographic (EMG) coherence between the lower limb and the core muscles when carrying out two postural tasks at different difficulty levels. EMG was recorded in 20 healthy male subjects while performing two independent quiet standing tasks. The first one involved a bipedal stance with the eyes open, while the second consisted of a dominant unipedal stance also with the eyes open. The obtained EMG signals were analysed by computing estimations of EMG–EMG coherence between muscle pairs, both singly (single-pair estimations) and combined (pooled estimations). Pooled and single coherence of anterior, posterior, core, antagonist and mixed pairs of muscles were significant in the 0–5 Hz frequency band. The results indicate that core and antagonist muscle groups, such as the anterior and posterior muscles, share low-frequency neural inputs (0–5 Hz) which could be responsible of the M-modes assembly. The core muscles could therefore provide the necessary synergy to maintain spine stability during the balancing exercise. Finally, differences in EMG–EMG coherence suggest that the muscle synergies formed during unipedal stance tasks are different from those established during bipedal stance.

Muscle networks: Connectivity analysis of EMG activity during postural control

Understanding the mechanisms that reduce the many degrees of freedom in the musculoskeletal system remains an outstanding challenge. Muscle synergies reduce the dimensionality and hence simplify the control problem. How this is achieved is not yet known. Here we use network theory to assess the coordination between multiple muscles and to elucidate the neural implementation of muscle synergies. We performed connectivity analysis of surface EMG from ten leg muscles to extract the muscle networks while human participants were standing upright in four different conditions. We observed widespread connectivity between muscles at multiple distinct frequency bands. The network topology differed significantly between frequencies and between conditions. These findings demonstrate how muscle networks can be used to investigate the neural circuitry of motor coordination. The presence of disparate muscle networks across frequencies suggests that the neuromuscular system is organized into a multiplex network allowing for parallel and hierarchical control structures.

The effect of force-controlled biting on human posture control

Several studies have confirmed the neuromuscular effects of jaw motor activity on the postural stability of humans, but the mechanisms of functional coupling of the craniomandibular system (CMS) with human posture are not yet fully understood. The purpose of our study was, therefore, to investigate whether submaximum biting affects the kinematics of the ankle, knee, and hip joints and the electromyographic (EMG) activity of the leg muscles during bipedal narrow stance and single-leg stance. Twelve healthy young subjects performed force-controlled biting (FB) and non-biting (NB) during bipedal narrow stance and single-leg stance. To investigate the effects of FB on the angles of the hip, knee, and ankle joints, a 3D motion-capture system (Vicon MX) was used. EMG activity was recorded to enable analysis of the coefficient of variation of the muscle co-contraction ratios (CVR) of six pairs of postural muscles. Between FB and NB, no significant differences were found for the mean values of the angles of the ankle, knee, and hip joints, but the standard deviations were significantly reduced during FB. The values of the ranges of motion and the mean angular velocities for the three joints studied revealed significant reduction during FB also. CVR was also significantly reduced during FB for five of the six muscle pairs studied. Although submaximum biting does not change the basic strategy of posture control, it affects neuromuscular co-contraction patterns, resulting in increased kinematic precision.