Flexible Recruitments of Fundamental Muscle Synergies in the Trunk and Lower Limbs for Highly Variable Movements and Postures

Shoulder kinematics and muscle synergy during multi-plane humeral elevation and lowering

Shoulder kinematics and muscle activity vary depending on the elevation plane of the upper limb. However, how muscle coordination, which plays a crucial role in controlling shoulder kinematics, differs among elevation planes remains unclear. This study compared shoulder kinematics, muscle synergies, and muscle activation levels across different elevation planes to better understand the neuromuscular mechanisms underlying shoulder kinematics. Shoulder kinematics and muscle activity were recorded during three upper limb elevation tasks (sagittal, scapular, and frontal plane elevation) in 12 subjects (7 males and 5 females). Muscle synergies were extracted using nonnegative matrix factorization, and individual muscle activity levels were calculated as a percentage of maximum voluntary contraction. Glenohumeral elevation was greatest during the sagittal plane elevation task and smallest during the frontal plane elevation task (maximum difference of 14.1°). The differences in kinematics among these elevation planes were suggested to be attributable to the early-stage activity level during elevation of one of the two extracted muscle synergies-specifically, the synergy believed to contribute to humeral head stabilization-and the activation amplitude of the anterior deltoid. Differences in scapular kinematics among three elevation plane tasks could not be explained by variations in muscle synergies but were instead suggested to result primarily from differences in the activation amplitudes of the three parts of the trapezius. To results suggest that shoulder kinematics are controlled by subtle changes in muscle synergy activation patterns and individual muscle activation levels.

Generalizability of motor modules across walking-based and in-place tasks - a distribution-based analysis on total knee replacement patients

Introduction: There are evidences that the nervous system produces motor tasks using a low-dimensional modular organization of muscle activations, known as motor modules. Previous studies have identified characteristic motor modules across similar tasks in healthy population. This study explored the generalizability of motor modules across two families of walking-based (level-walking, downhillwalking and stair-decent), in-place ascending (sit-to-stand, squat-to-stand), and in-place descending (stand-to-sit and stand-to-squat) motor tasks in a group of six individuals undergone total knee replacement (TKR) surgery. Methods: Motor modules were extracted from the EMG data of CAMS-Knee dataset using non-negative matrix factorization technique. A distribution-based approach, employing three levels of k-means clustering, was then applied to find the shared and task-specific modules, and assess their representability among the whole task-trial data. Results and Discussion: Results indicated a four- and a seven-subcluster arrangement of the shared and task-specific motor modules, depending upon the membership criteria. The first arrangement revealed motor modules which were shared across all tasks (min coverage index: 76%; modules' distinctness range: 7.08-8.91) and the latter among tasks of the same family mainly, although there remained some interfamily shared modules (min coverage index: 81%; modules' distinctness range: 7.17-9.89). It was concluded that there are shared motor modules across walking-based and in-place tasks in TKR individuals, with their generalizability and representability depending upon the analysis method. This finding highlights the importance of the analysis method in identifying the shared motor modules, as the main building blocks of motor control.

Effects of initial foot position on neuromuscular and biomechanical control during the stand-to-sit movement: Implications for rehabilitation strategies

BACKGROUND

Initial foot position (IFP) is one of the important movement strategies that influence neuromuscular and biomechanical control during sit-to-stand (STS) movements. Similarly, stand-to-sit (StandTS) is vital in rehabilitation settings for evaluating strength and balance control during descending movements. Understanding how IFP impacts changes in biomechanical and neuromuscular movement control factors during StandTS can provide valuable insights for designing effective rehabilitation programs.

METHODS

Twelve healthy young adults participated in this study, examining three symmetric IFPs: (1) REF (reference); (2) wide: each foot was shifted outwards by 30% from REF; (3) TO (toes-out): symmetric toes-out angle of 30° from REF. Kinematic and kinetic differences among the three IFPs during StandTS were analyzed, along with the characteristics of muscle activation patterns using muscle synergy analysis.

RESULTS

In the wide IFP, trunk flexion angle was reduced, and valgus angle was greater than in the other IFPs. The TO IFP resulted in greater dorsiflexion and knee flexion angles compared to the REF and wide IFPs. Compared to the REF IFP, both wide and TO IFPs showed greater eccentric work at the hip joint in the anterior-posterior (power absorption) and the vertical direction (gravitational force control) and demonstrated reduced postural sway in medio-lateral and vertical directions. Muscle synergy analysis of EMG activity revealed increased activation of back and plantar flexor muscle in the wide IFP, and increased contribution of hip joint muscles in the TO IFP.

CONCLUSION

The wide IFP increased the valgus angle, leading to reduced trunk flexion with increased back muscle activation. The TO IFP enabled greater angular displacement at the ankle and knee joints, enhancing hip joint muscle involvement in StandTS movement control. Both the wide and TO IFPs facilitated hip joint work, improving postural sway control during the descent phase of StandTS. These findings provide valuable insights for designing rehabilitation strategies tailored to specific patient needs.

Muscle synergy in several locomotor modes in chimpanzees and Japanese macaques, and its implications for the evolutionary origin of bipedalism through shared muscle synergies

Recent evidence indicates that human ancestors utilized a combination of quadrupedal walking, climbing, and bipedal walking. Therefore, the origin of bipedalism may be linked to underlying mechanisms supporting diverse locomotor modes. This study aimed to elucidate foundations of varied locomotor modes from the perspective of motor control by identifying muscle synergies and demonstrating similarities in synergy compositions across different locomotor modes in chimpanzees and Japanese macaques. Four muscle synergies were extracted for bipedal and quadrupedal walking in both the chimpanzees and macaques, as well as for vertical climbing in the chimpanzees. Bipedal walking synergies were generally analogous to those observed in quadrupedal walking and vertical climbing. Specifically, the bipedal walking synergies during the stance and swing phase in the chimpanzees were substitutable with those of vertical climbing and quadrupedal walking, respectively. For the macaque, not all bipedal walking synergies exhibited similarities to quadrupedal walking synergies, likely due to instability during the single support phase of bipedalism. These findings suggest that synergies from vertical climbing and quadrupedal walking might be transferred to bipedal walking, as seen in the chimpanzees, and that this sharing of synergies might form a foundation for a diverse range of locomotor capacities including bipedal walking.

Comparison of muscle synergies in walking and pedaling: the influence of rotation direction and speed

Background

Understanding the muscle synergies shared between pedaling and walking is crucial for elucidating the mechanisms of human motor control and establishing highly individualized rehabilitation strategies. This study investigated how pedaling direction and speed influence the recruitment of walking-like muscle synergies.

Methods

Twelve healthy male participants pedaled at three speeds (60 RPM, 30 RPM, and 80 RPM) in two rotational directions (forward and backward). Additionally, they completed walking tasks at three different speeds (slow, comfortable, and fast). Surface electromyography (EMG) was recorded on 10 lower limb muscles during movement, and muscle synergies were extracted from each condition using non-negative matrix factorization. The similarities between the muscle synergies during walking and each pedaling condition were examined using cosine similarity.

Results

The results confirmed that the composition of muscle synergies during pedaling varied depending on the rotational direction and speed. Furthermore, one to three muscle synergies, similar to those observed during walking, were recruited in each pedaling condition, with specific synergies dependent on direction and speed. For instance, synergy involving the quadriceps and hip extensors was predominantly observed during pedaling at 30 RPM, regardless of the direction of rotation. Meanwhile, synergy involving the hamstrings was more pronounced during forward pedaling at 60 RPM and backward pedaling at 80 RPM.

Conclusion

These findings suggest that walking-like muscle synergies can be selectively recruited during pedaling, depending on the rotational direction and speed.

The influence of smoothness and speed of stand-to-sit movement on joint kinematics, kinetics, and muscle activation patterns

Background

Stand-to-sit (StandTS) is an important daily activity widely used in rehabilitation settings to improve strength, postural stability, and mobility. Modifications in movement smoothness and speed significantly influence the kinematics, kinetics, and muscle activation patterns of the movement. Understanding the impact of StandTS speed and smoothness on movement control can provide valuable insights for designing effective and personalized rehabilitation training programs.

Research question

How do the smoothness and speed of StandTS movement affect joint kinematics, kinetics, muscle activation patterns, and postural stability during StandTS?

Methods

Twelve healthy younger adults participated in this study. There were two StandTS conditions. In the reference condition, participants stood in an upright position with their feet positioned shoulder-width apart on the force plate. Upon receiving a visual cue, participants performed StandTS at their preferred speed. In the smooth condition, participants were instructed to perform StandTS as smoothly as possible, aiming to minimize contact pressure on the seat. Lower leg kinetics, kinematics, and coordination patterns of muscle activation during StandTS were measured: (1) angular displacement of the trunk, knee, and hip flexion; (2) knee and hip extensor eccentric work; (3) muscle synergy pattern derived from electromyography (EMG) activity of the leg muscles; and (4) postural sway in the anterior-posterior (A-P), medio-lateral (M-L), and vertical directions.

Results

Compared to the reference condition, the smooth condition demonstrated greater eccentric knee extensor flexion and increased joint work in both the knee and hip joints. Analysis of specific muscle synergy from EMG activity revealed a significant increase in the relative contribution of hip joint muscles during the smooth condition. Additionally, a negative correlation was observed between knee extensor and vertical postural sway, as well as hip extensor work and M-L postural sway.

Conclusion

Smooth StandTS facilitates enhanced knee eccentric control and increased joint work at both the hip and knee joints, along with increased involvement of hip joint muscles to effectively manage falling momentum during StandTS. Furthermore, the increased contributions of knee and hip joint work reduced postural sway in the vertical and M-L directions, respectively. These findings provide valuable insights for the development of targeted StandTS rehabilitation training.

Direction-Specific Changes in Trunk Muscle Synergies in Individuals With Extension-Related Low Back Pain

Background Identifying altered trunk control is critical for treating extension-related low back pain (ERLBP), a common subgroup classified by clinical manifestations. The changed coordination of trunk muscles within this group during particular trunk tasks is still not clearly understood. Objectives The objective of this study is to investigate trunk muscle coordination during 11 trunk movement and stability tasks in individuals with ERLBP compared to non-low back pain (LBP) participants. Methods Thirteen individuals with ERLBP and non-LBP performed 11 trunk movement and stability tasks. We recorded the electromyographic activities of six back and abdominal muscles bilaterally. Trunk muscle coordination was assessed using the non-negative matrix factorization (NMF) method to identify trunk muscle synergies. Results The number of synergies in the ERLBP group during the cross-extension and backward bend tasks was significantly higher than in the non-LBP group (p<0.05). The cluster analysis identified the two trunk synergies for each task with strikingly similar muscle activation patterns between groups. In contrast, the ERLBP group exhibited additional trunk muscle synergies that were not identified in the non-LBP group. The number of synergies in the other tasks did not differ between groups (p>0.05). Conclusion Individuals with ERLBP presented directionally specific alterations in trunk muscle synergies that were considered as increased coactivations of multiple trunk muscles. These altered patterns may contribute to the excessive stabilization of and the high frequency of hyperextension in the spine associated with the development and persistence of ERLBP.

Age-related differences in lower limb muscle activation patterns and balance control strategies while walking over a compliant surface

Substantial evidence demonstrates that falls in older adults are leading causes of fatal and non-fatal injuries and lead to negative impacts on the quality of life in the aging population. Most falls in older fallers result from unrecoverable limb collapse during falling momentum control in the single limb support (SLS) phase. To understand why older adults are more likely to fall than younger adults, we investigated age-related differences in knee extensor eccentric control, lower limb muscle activation patterns, and their relation to balance control. Ten older and ten younger healthy adults were compared during balance control while walking on a compliant surface. There was a positive correlation between knee extensor eccentric work in the perturbed leg and the swinging leg's speed and margin of stability. In comparison to younger adults, older adults demonstrated (1) less eccentric work, reduced eccentric electromyography burst duration in the perturbed leg, (2) higher postural sway during SLS, and (3) impaired swinging leg balance control. The group-specific muscle synergy showed that older adults had a prominent ankle muscle activation, while younger adults exhibited a more prominent hip muscle activation. These findings provide insight into targeted balance rehabilitation directions to improve postural stability and reduce falls in older adults.

Muscle synergy patterns as altered coordination strategies in individuals with chronic low back pain: a cross-sectional study

BACKGROUND

Chronic low back pain (CLBP) is a highly prevalent disease with poorly understood underlying mechanisms. In particular, altered trunk muscle coordination in response to specific trunk tasks remains largely unknown.

METHODS

We investigated the muscle synergies during 11 trunk movement and stability tasks in 15 healthy individuals (8 females and 7 males, aged 21. 3 (20.1-22.8) ± 0.6 years) and in 15 CLBP participants (8 females and 7 males, aged 20. 9 (20.2-22.6) ± 0.7 years) by recording the surface electromyographic activities of 12 back and abdominal muscles (six muscles unilaterally). Non-negative matrix factorization was performed to extract the muscle synergies.

RESULTS

We found six trunk muscle synergies and temporal patterns in both groups. The high similarity of the trunk synergies and temporal patterns in the groups suggests that both groups share the common feature of the trunk coordination strategy. We also found that trunk synergies related to the lumbar erector spinae showed lower variability in the CLBP group. This may reflect the impaired back muscles that reshape the trunk synergies in the fixed structure of CLBP. Furthermore, the higher variability of trunk synergies in the other muscle regions such as in the latissimus dorsi and oblique externus, which were activated in trunk stability tasks in the CLBP group, represented more individual motor strategies when the trunk tasks were highly demanding.

CONCLUSION

Our work provides the first demonstration that individual modular organization is fine-tuned while preserving the overall structures of trunk synergies and temporal patterns in the presence of persistent CLBP.

Variability of trunk muscle synergies underlying the multidirectional movements and stability trunk motor tasks in healthy individuals

Muscle synergy analysis is useful for investigating trunk coordination patterns based on the assumption that the central nervous system reduces the dimensionality of muscle activation to simplify movement. This study aimed to quantify the variability in trunk muscle synergy during various trunk motor tasks in healthy participants to provide reference data for evaluating trunk control strategies in patients and athletes. Sixteen healthy individuals performed 11 trunk movement and stability tasks with electromyography (EMG) recording of their spinal and abdominal muscles (6 bilaterally). Non-negative matrix factorization applied to the concatenated EMG of all tasks identified the five trunk muscle synergies (W) with their corresponding temporal patterns (C). The medians of within-cluster similarity defined by scalar products in W and rmax coefficient using the cross-correlation function in C were 0.73-0.86 and 0.64-0.75, respectively, while the inter-session similarities were 0.81-0.96 and 0.74-0.84, respectively. However, the lowest and highest values of both similarity indices were broad, reflecting the musculoskeletal system's redundancy within and between participants. Furthermore, the significant differences in the degree of variability between the trunk synergies may represent the different neural features of synergy organization and strategies to overcome the various mechanical demands of a motor task.

A computational method for estimating trunk muscle activations during gait using lower extremity muscle synergies

One of the surgical treatments for pelvic sarcoma is the restoration of hip function with a custom pelvic prosthesis after cancerous tumor removal. The orthopedic oncologist and orthopedic implant company must make numerous often subjective decisions regarding the design of the pelvic surgery and custom pelvic prosthesis. Using personalized musculoskeletal computer models to predict post-surgery walking function and custom pelvic prosthesis loading is an emerging method for making surgical and custom prosthesis design decisions in a more objective manner. Such predictions would necessitate the estimation of forces generated by muscles spanning the lower trunk and all joints of the lower extremities. However, estimating trunk and leg muscle forces simultaneously during walking based on electromyography (EMG) data remains challenging due to the limited number of EMG channels typically used for measurement of leg muscle activity. This study developed a computational method for estimating unmeasured trunk muscle activations during walking using lower extremity muscle synergies. To facilitate the calibration of an EMG-driven model and the estimation of leg muscle activations, EMG data were collected from each leg. Using non-negative matrix factorization, muscle synergies were extracted from activations of leg muscles. On the basis of previous studies, it was hypothesized that the time-varying synergy activations were shared between the trunk and leg muscles. The synergy weights required to reconstruct the trunk muscle activations were determined through optimization. The accuracy of the synergy-based method was dependent on the number of synergies and optimization formulation. With seven synergies and an increased level of activation minimization, the estimated activations of the erector spinae were strongly correlated with their measured activity. This study created a custom full-body model by combining two existing musculoskeletal models. The model was further modified and heavily personalized to represent various aspects of the pelvic sarcoma patient, all of which contributed to the estimation of trunk muscle activations. This proposed method can facilitate the prediction of post-surgery walking function and pelvic prosthesis loading, as well as provide objective evaluations for surgical and prosthesis design decisions.

On the Applications of EMG Sensors and Signals

The ability to execute limb motions derives from composite command signals (or efferent signals) that stem from the central nervous system through the highway of the spinal cord and peripheral nerves to the muscles that drive the joints [...].

Measurement, Evaluation, and Control of Active Intelligent Gait Training Systems—Analysis of the Current State of the Art

Gait recognition and rehabilitation has been a research hotspot in recent years due to its importance to medical care and elderly care. Active intelligent rehabilitation and assistance systems for lower limbs integrates mechanical design, sensing technology, intelligent control, and robotics technology, and is one of the effective ways to resolve the above problems. In this review, crucial technologies and typical prototypes of active intelligent rehabilitation and assistance systems for gait training are introduced. The limitations, challenges, and future directions in terms of gait measurement and intention recognition, gait rehabilitation evaluation, and gait training control strategies are discussed. To address the core problems of the sensing, evaluation and control technology of the active intelligent gait training systems, the possible future research directions are proposed. Firstly, different sensing methods need to be proposed for the decoding of human movement intention. Secondly, the human walking ability evaluation models will be developed by integrating the clinical knowledge and lower limb movement data. Lastly, the personalized gait training strategy for collaborative control of human–machine systems needs to be implemented in the clinical applications.

Evidence for basic units of upper limb muscle synergies underlying a variety of complex human manipulations

Manipulations require complex upper-limb movements in which the central nervous system (CNS) must deal with many degrees of freedom. Evidence suggests that the CNS utilizes motor primitives called muscle synergies to simplify the production of movements. However, the exact neural mechanism underlying muscle synergies to control a wide array of manipulations is not fully understood. Here, we tested whether there are basic units of muscle synergies that can explain a diverse range of manipulations. We measured the electromyographic activities of 20 muscles across the shoulder, elbow, and wrist and fingers during 24 manipulation tasks. As a result, non-negative matrix factorization identified nine basic units of muscle synergies derived from the upper limb muscles that are shared across all tasks. The high similarity between muscle synergies of each of the 24 tasks and various combinations of nine basic unit muscle synergies in a single and/or merging state provides evidence that the CNS flexibly selects and modifies the degree of contribution of the nine basic units of muscle synergies to overcome different mechanical demands of tasks.