Muscle Synergy Assessment During Single-Leg Stance

Characteristics of static balance performance in 4-Stage Balance test in the Healthy Older Adults

Background: The 4-Stage Balance test is one of the most commonly used tests to assess balance for older adults. Although it is generally accepted that the four positions (including side-by-side (SBSS), semi-tandem (STS), tandem (TS), and single-leg stance (SLS)) in this test are progressively more difficult, there are no studies comparing the balance parameters of the four positions in older adults to prove this result. The purpose of this study is to determine the difficulty of 4 positions in the 4-Stage Balance test and the effect of the dominant and non-dominant lower extremities on static balance among healthy older adults; Methods: A total of 115 community-dwelling healthy older adults were included. The postural parameters (including sway range standard deviation (SR), velocity of body sway (V), total sway area (TSA) and sway perimeter (TSP) of the center of pressure) were measured during 8 static postures (including SBSS, left STS, right STS, left TS, right TS, left SLS, right SLS and comfortable stance (CS)). Repeated measures ANOVA was used to analyze the postural parameters in 8 static postures; Results: The static balance stability of the five stances in older adults can be ranked in the following sequence: CS > SBSS/STS > TS > SLS. Moreover, changing foot placement in STS, TS and SLS tasks has no influence on stability. This study has been registered in China Clinical Trial Registry (ChiCTR2200065803). Conclusions: Our findings suggest that it is feasible to simplify the 4-Stage Balance test to a 3-Stage Balance test in the older adults.

Effects of dominance and vision on unipedal balance tests in futsal players using a triaxial accelerometer

Optimal postural control improves performance and reduces the risk of injury in futsal. In this context, wearable accelerometers may detect velocity changes of the centre of mass during a task, enabling the analysis of postural control in different environments. This work aimed to determine the influence of vision and dominance on unipodal static postural balance in non-professional athletes. Twenty-four university male futsal players performed a unipodal balance test to assess their body sway using a triaxial accelerometer. To assess dominance, the preferred limb for kicking the ball was considered, while vision was manipulated by asking participants to close their eyes during the test. Root mean square (RMS) and sample entropy (SaEn) of centre of mass variables were analysed. For statistical analysis, a multivariate analysis of variance model was used. Our results suggest an effect of vision, but not of dominance nor the interaction between vision and limb dominance. Specifically, a higher-acceleration RMS in the mediolateral axis was observed, as well as an increased SaEn in the three axes. To conclude, unipodal postural demand in futsal players under visual input suppression was not influenced by their limb dominancy.

Age-related differences in the capacity and neuromuscular control of the foot core system during quiet standing

The foot core system is essential for upright stability. However, aging-induced changes in the foot core function remain poorly understood. The present study aimed to examine age-related differences in postural stability from the perspective of foot core capacity and neuromuscular control during quiet standing. Thirty-six older and 25 young adults completed foot core capacity tests including toe flexion strength, muscle ultrasonography, and plantar cutaneous sensitivity. The center of pressure (COP) and electromyography (EMG) of abductor hallucis (ABH), peroneus longus (PL), tibialis anterior (TA) and medial gastrocnemius (GM) were simultaneously recorded during double-leg and single-leg standing (SLS). EMG data were used to calculate muscle synergy and intermuscular coherence across three frequency bands. Compared to young adults, older adults exhibited thinner hallucis flexors, weaker toe strength, and lower plantar cutaneous sensitivity. The ABH thickness and plantar cutaneous sensitivity were negatively associated with the COP mean peak velocity in older adults, but not in young adults. Besides, older adults had higher cocontraction of muscles spanning the arch (ABH-PL) and ankle (TA-GM), and had lower beta- and gamma-band coherence of the ABH-PL and TA-PL during SLS. Foot core capacities became compromised with advancing age, and the balance control of older adults was susceptible to foot core than young adults in balance tasks. To compensate for the weakened foot core, older adults may adopt arch and ankle stiffening strategies via increasing muscle cocontraction. Furthermore, coherence analysis indicated that aging may increase the demand for cortical brain resources during SLS.

Balance and Muscle Synergies During a Single-Limb Stance Task in Individuals With Chronic Ankle Instability

The aim of this study was to investigate balance performance and muscle synergies during a Single-Limb Stance (SLS) task in individuals with Chronic Ankle Instability (CAI) and a group of healthy controls. Twenty individuals with CAI and twenty healthy controls were asked to perform a 30-second SLS task in Open-Eyes (OE) and Closed-Eyes (CE) conditions while standing on a force platform with the injured or the dominant limb, respectively. The activation of 13 muscles of the lower limb, hip, and back was recorded by means of surface electromyography. Balance performance was assessed by identifying the number and the duration of SLS epochs, and the Root-Mean-Square (RMS) in Antero-Posterior (AP) and Medio-Lateral (ML) directions of the body-weight normalized ground reaction forces. The optimal number of synergies, weight vectors, and activation coefficients were also analyzed. CAI group showed a higher number and a shorter duration of SLS epochs and augmented ground reaction force RMS in both AP and ML directions compared to controls. Both groups showed an increase in the RMS in AP and ML forces in CE compared to OE. Both groups showed 4 optimal synergies in CE, while controls showed 5 synergies in OE. CAI showed a significantly higher weight of knee flexor muscles in both OE and CE. In conclusion, muscle synergies analysis provided an in-depth knowledge of motor control mechanisms in CAI individuals. They showed worse balance performance, a lower number of muscle synergies in a CE condition and abnormal knee flexor muscle activation compared to healthy controls.

Why do children with unilateral cerebral palsy struggle with the single leg stance test? A kinematic and centre of pressure analysis

BACKGROUND

Children with unilateral cerebral palsy often report difficulty with balance in everyday life. The single leg stance test is a challenging task, requiring rapid sensory input and precise motor adjustment. The purpose of this study was to examine how children with cerebral palsy perform this test, compared to typically developing children.

METHODS

Three-dimensional kinematics of the trunk and lower limbs of 10 children with cerebral palsy and 15 children with typical development were captured as they performed a single leg stance test on their non-dominant leg on a force platform. Stance time, joint kinematics and centre of pressure sway were measured and examined.

FINDINGS

There was evidence of shorter single leg stance performance and increased mediolateral centre of pressure sway in children with cerebral palsy. Coronal plane movement at the subtalar joint and foot was reduced (-6.0° (-10.9, -1.2°)), while proximally there was greater trunk movement in the coronal (13.5° (2.4°, 24.5°)) and transverse planes (9.9° (0.7, 19°)) and pelvis movement in the transverse plane (6.1° (1.7, 10.5°). An association existed between stance time and mediolateral centre of pressure sway (p < 0.01), with an average reduction in stance time of 0.15 s for every 1 mm/s increase in mediolateral sway.

INTERPRETATION

Children with cerebral palsy showed poor mediolateral control of centre of pressure sway, leading to shorter stance time. They have a less effective coronal foot-tilt strategy and excessive trunk and pelvis movement. Interventions aimed at improving single leg stance performance should consider addressing both ankle / foot and trunk motor control.

Using different matrix factorization approaches to identify muscle synergy in stroke survivors

Over the past several decades, many scholars have investigated muscle synergy as a promising tool for evaluating motor function. However, it is challenging to obtain favorable robustness using the general muscle synergy identification algorithms, namely non-negative matrix factorization (NMF), independent component analysis (ICA), and factor analysis (FA). Some scholars have proposed improved muscle synergy identification algorithms to overcome the shortcomings of these approaches, such as singular value decomposition NMF (SVD-NMF), sparse NMF (S-NMF), and multivariate curve resolution-alternating least squares (MCR-ALS). However, performance comparisons of these algorithms are seldom conducted. In this study, experimental electromyography (EMG) data collected from healthy individuals and stroke survivors were applied to assess the repeatability and intra-subject consistency of NMF, SVD-NMF, S-NMF, ICA, FA, and MCR-ALS. MCR-ALS presented higher repeatability and intra-subject consistencies than the other algorithms. More synergies and lower intra-subject consistencies were observed in stroke survivors than in healthy individuals. Thus, MCR-ALS is considered a favorable muscle synergy identification algorithm for patients with neural system disorders.

Muscle synergies in Parkinson's disease before and after the deep brain stimulation of the bilateral subthalamic nucleus

The aim of this study is to quantitatively assess motor control changes in Parkinson's disease (PD) patients after bilateral deep brain stimulation of the subthalamic nucleus (STN-DBS), based on a novel muscle synergy evaluation approach. A group of 20 PD patients evaluated at baseline (before surgery, T₀), at 3 months (T₁), and at 12 months (T₂) after STN-DBS surgery, as well as a group of 20 age-matched healthy control subjects, underwent an instrumented gait analysis, including surface electromyography recordings from 12 muscles. A smaller number of muscle synergies was found in PD patients (4 muscle synergies, at each time point) compared to control subjects (5 muscle synergies). The neuromuscular robustness of PD patients-that at T₀ was smaller with respect to controls (PD T₀: 69.3 ± 2.2% vs. Controls: 77.6 ± 1.8%, p = 0.004)-increased at T₁ (75.8 ± 1.8%), becoming not different from that of controls at T₂ (77.5 ± 1.9%). The muscle synergies analysis may offer clinicians new knowledge on the neuromuscular structure underlying PD motor types of behavior and how they can improve after electroceutical STN-DBS therapy.

The Pre-Exhaustion Method Does Not Increase Muscle Activity in Target Muscle During Strength Training in Untrained Individuals

This study investigated the effects of the pre-exhaustion method on electromyographic activity (root mean square and median frequency) at different interval sets. Twenty adults with little or no experience in strength training performed the seated row exercise until muscular failure, with or without completing a previous pre-exhaustion set in a pullover exercise. Surface electrodes were placed over the latissimus dorsi, teres major, biceps brachii, and posterior deltoid muscles of the dominant side. The results showed no effect of pre-exhaustion on EMG activity of latissimus dorsi target muscle for any interval sets. Regarding median frequency, our results showed a consistent decrease throughout the sets (p>0.05). Also, our results revealed a decrease in posterior deltoid median frequency with pre-exhaustion (p=0.001). We conclude that the pre-exhaustion method seems ineffective in increasing the EMG activity of the desired muscle throughout all repetitions of strength training. Furthermore, pre-exhaustion seems to be a good way to increase fatigue in the posterior deltoid muscle.

Muscle synergies for the control of single-limb stance with and without visual information in young individuals

Purpose

Single-limb stance is a demanding postural task featuring a high number of daily living and sporting activities. Thus, it is widely used for training and rehabilitation, as well as for balance assessment. Muscle activations around single joints have been previously described, however, it is not known which are the muscle synergies used to control posture and how they change between conditions of normal and lack of visual information.

Methods

Twenty-two healthy young participants were asked to perform a 30 s single-limb stance task in open-eyes and closed-eyes condition while standing on a force platform with the dominant limb. Muscle synergies were extracted from the electromyographical recordings of 13 muscles of the lower limb, hip, and back. The optimal number of synergies, together with the average recruitment level and balance control strategies were analyzed and compared between the open- and the closed-eyes condition.

Results

Four major muscle synergies, two ankle-dominant synergies, one knee-dominant synergy, and one hip/back-dominant synergy were found. No differences between open- and closed-eyes conditions were found for the recruitment level, except for the hip/back synergy, which significantly decreased (p = 0.02) in the closed-eyes compared to the open-eyes condition. A significant increase (p = 0.03) of the ankle balance strategy was found in the closed-eyes compared to the open-eyes condition.

Conclusion

In healthy young individuals, single-limb stance is featured by four major synergies, both in open- and closed-eyes condition. Future studies should investigate muscle synergies in participants with other age groups, as well as pathological conditions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13102-021-00392-z.

An Algorithm for Choosing the Optimal Number of Muscle Synergies during Walking

In motor control studies, the 90% thresholding of variance accounted for (VAF) is the classical way of selecting the number of muscle synergies expressed during a motor task. However, the adoption of an arbitrary cut-off has evident drawbacks. The aim of this work is to describe and validate an algorithm for choosing the optimal number of muscle synergies (ChoOSyn), which can overcome the limitations of VAF-based methods. The proposed algorithm is built considering the following principles: (1) muscle synergies should be highly consistent during the various motor task epochs (i.e., remaining stable in time), (2) muscle synergies should constitute a base with low intra-level similarity (i.e., to obtain information-rich synergies, avoiding redundancy). The algorithm performances were evaluated against traditional approaches (threshold-VAF at 90% and 95%, elbow-VAF and plateau-VAF), using both a simulated dataset and a real dataset of 20 subjects. The performance evaluation was carried out by analyzing muscle synergies extracted from surface electromyographic (sEMG) signals collected during walking tasks lasting 5 min. On the simulated dataset, ChoOSyn showed comparable performances compared to VAF-based methods, while, in the real dataset, it clearly outperformed the other methods, in terms of the fraction of correct classifications, mean error (ME), and root mean square error (RMSE). The proposed approach may be beneficial to standardize the selection of the number of muscle synergies between different research laboratories, independent of arbitrary thresholds.