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

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.

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.

Age-Related Differences in Intermuscular Coherence EMG-EMG of Ankle Joint Antagonist Muscle Activity during Maximal Leaning

BACKGROUND

Intermuscular synchronization is one of the fundamental aspects of maintaining a stable posture and is of great importance in the aging process. This study aimed to assess muscle synchronization and postural stabilizer asymmetry during quiet standing and the limits of stability using wavelet analysis. Intermuscular synchrony and antagonistic sEMG-sEMG (surface electromyography) coherence asymmetry were evaluated in the tibialis anterior and soleus muscles.

METHODS

The study involved 20 elderly (aged 65 ± 3.6) and 20 young (aged 21 ± 1.3) subjects. The task was to perform a maximum forward bend in a standing position. The prone test was divided into three phases: quiet standing (10 s), dynamic learning, and maintenance of maximum leaning (20 s). Wavelet analysis of coherence was performed in the delta and beta bands.

RESULTS

Young subjects modulated interface coherences to a greater extent in the beta band. Analysis of postural stability during standing tasks showed that only the parameter R2b (the distance between the maximal and minimal position central of pressure), as an indicator for assessing the practical limits of stability, was found to be significantly associated with differences in aging.

CONCLUSION

The results showed differences in the beta and delta band oscillations between young and older subjects in a postural task involving standing quietly and leaning forward.

Age-related difference in muscle metabolism patterns during upper limb's encircling exercise: a near-infrared spectroscopy study

Aging is usually accompanied by decrease in limb motor function and change in muscle metabolism patterns. However, few studies have investigated the aging effect on muscle hemodynamics of the upper extremity. This study aims to explore the aging effect on muscle metabolism patterns during upper limb's exercise. Twelve middle-aged and elderly subjects and 12 young subjects were recruited, and muscle oxygenation signals from these subjects' biceps brachii muscles were collected during active and passive upper limb's encircling exercise with near-infrared spectroscopy (NIRS). The old group showed stronger muscle hemodynamic metabolism than the young group. The multiscale fuzzy approximate entropy and multiscale transfer entropy analyses indicated higher complexity and stronger interlimb coupling of the muscle oxygenation signals for the old group. Based on the selected muscle metabolism features, the constructed support vector machine model showed a high accuracy rate for classifying the two groups of subjects: 91.6% for the passive mode and 87.5% for the active mode. Our results proved the specific muscle metabolism patterns in the upper limb's exercise for old subjects, promoting the understanding of the aging effect on muscle hemodynamics.

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.

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.

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.