Proficiency-based recruitment of muscle synergies in a highly perturbed walking task (slackline)

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.