The central nervous system (CNS) is believed to utilize specific predefined modules, called muscle synergies (MS), to accomplish a motor task. Yet questions persist about how the CNS combines these primitives in different ways to suit the task conditions. The MS hypothesis has been a subject of debate as to whether they originate from neural origins or nonneural constraints. In this review article, we present three aspects related to the MS hypothesis: (1) the experimental and computational evidence in support of the existence of MS, (2) algorithmic approaches for extracting them from surface electromyography (EMG) signals, and (3) the possible role of MS as a neurorehabilitation tool. We note that recent advances in computational neuroscience have utilized the MS hypothesis in motor control and learning. Prospective advances in clinical, medical, and engineering sciences and in fields such as robotics and rehabilitation stand to benefit from a more thorough understanding of MS.

It is hypothesised that specific groups of muscles aka muscle synergies (MSs) are combined by the central nervous system to control a wide repertoire of movements and also simplify motor control. Therefore, studying MSs during human locomotion is of significance, as it may reveal neuromuscular strategies for postural stability. In this study, the authors aimed to use the hypothesis of MSs to identify specific muscle co-activations during overground walking and slacklining where postural perturbations were generated by the participants rather than being externally controlled. Nine participants were asked to walk overground and on a slackline while they recorded myoelectric activity of their leg muscles. They extracted synergies from the electromyography signals in the two tasks using factor analysis. The results showed adaptation in the shared MSs structure during walking on the slackline and these shared MSs across participants were recruited flexibly to meet the demand for stability. The modulation of synergies suggests adaptive neuromuscular strategies for stability while walking on a slackline. Specifically, higher activation of quadriceps during slacklining suggests a crouched gait to facilitate balance. During overground walking, lower leg muscles revealed higher activation compared to slacklining to support a more consistent toe-off during the stance phase.

In neurophysiology, a hypothesis under investigation relates to how neural modularity helps in learning of skills. Accordingly, we studied differences in muscle synergy (MS) organization at three different proficiency levels on a task more challenging than walking. Our study included slackline walking whereby the perturbations to evoke postural responses are generated by the participants rather than externally controlled. Furthermore, studying MS of individuals with different proficiency levels under such constraints will provide us an understanding of different strategies for dynamic postural stability. Hence, the main aim of our study is to identify MS associated with proficiency during slacklining. Muscle Synergies and their activation coefficients were extracted using factor analysis on electromyography that was recorded from lower limb muscles. The spatial and temporal profiles were analyzed to examine muscle co-activation patterns for stability across three different groups of slackliners (high, moderate, and nonproficient). We found three robust MS structures across all skill levels associated with crouched gait while slacklining. Higher activation of quadriceps, gastrocnemius, and hamstrings with tibialis anterior was observed for synergy one, two, and three, respectively. An additional proficiency-based synergy was recruited for highly proficient slackliners, and similarly for nonproficient ones. For highly proficient slackliners, the additional synergy was in relation to lowering of the center of mass for consistent stabilization. For nonproficient slackliners (PS), the recruitment of additional synergy was related to consistent knee flexion with the higher range of motion. Overall, our work showed alteration in the modular organization of MS at different proficiency levels that could be associated with differences in knee kinematics during slacklining. We think that the outcomes of our study regarding differences in the MS organization based on proficiency levels, and the underlying neuro-physiological features, will facilitate rehabilitation of individuals with balance disorders.

DC microgrids (DCMGs) integrate and coordinate various DC distribution generation units including various renewable energy sources and battery storage systems, and have been used in satellites, the International Space Station, telecom power stations, computer power supplies, electric aircraft, and electric ships. However, the presence of constant power loads (CPLs) can cause instability in DCMGs. Thus, this paper reviews the stabilization techniques that can resolve instability caused by CPLs, as well as various parameters of CPLs, such as bandwidth, and the frequency of the CPLs that can stabilize the DCMGs. It also discusses recent trends and future work in finding stability limits using the parameters of CPLs. It should be useful for directing research towards appropriate mathematical and experimental approaches for the stability of DCMGs with CPLs.