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

Abstract

The extent to which muscle synergies represent the neural control of human behavior remains unknown. Here, we tested whether certain sets of muscle synergies that are fundamentally necessary across behaviors exist. We measured the electromyographic activities of 26 muscles, including bilateral trunk and lower limb muscles, during 24 locomotion, dynamic and static stability tasks, and we extracted the muscle synergies using non-negative matrix factorization. Our results show that 13 muscle synergies that may have unique functional roles accounted for almost all 24 tasks by combinations of single and/or merging of synergies. Therefore, our results may support the notion of the low dimensionality in motor outputs, in which the central nervous system flexibly recruits fundamental muscle synergies to execute diverse human behaviors. Further studies are required to validate the neural representation of the fundamental components of muscle synergies.

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

“…The EMG signals were bandpass ltered (20-450 Hz), ampli ed (with a 300-gain preampli er), and sampled at 1000 Hz. The starting point was initiated with the verbal cue "go" when the examiner manually pressed the electrical trigger once 9,33 . After participants completed the tasks and returned to a resting posture for approximately 1 s, the examiner manually pressed the electrical trigger twice with the verbal cue "end" to de ne the end of the movement 9,33 .…”

“…Because NMF utilizes the random initialization of W and C, the algorithm may have been stuck in a suboptimal local minimum. Thus, each synergy extraction was repeated 50 times and the iteration with the highest VAF was maintained 9,33,39,40 . In this study, we extracted trunk muscle synergies from all-task EMG matrices.…”

“…First, we sorted the extracted muscle synergies into ve groups using a hierarchical cluster analysis in MATLAB ("linkage" function, MathWorks, Inc., Natick, MA, USA) 20 . To determine if the ve extracted muscle synergies during the 11 selected trunk motor tasks were similar to those during the 24 locomotion and stability tasks in the trunk and lower limbs in our previous study 9 , we calculated the similarity using the scalar product of the cluster centroids of muscle synergies in the current study and those extracted in our previous studies, excluding muscle weighting components in the lower limbs. Regarding the variability of trunk muscle synergies and their temporal patterns between participants, we computed the similarity by averaging the values of all pairwise dot products between muscle synergies and the r max coe cient using the cross-correlation function between temporal patterns in each cluster as the variability between participants (the within-cluster similarity) 20,43 .…”

“…We have previously identi ed ve basic units of trunk muscle synergies from 12 trunk muscles (i.e., six muscles bilaterally) during 24 locomotion and stability tasks in the trunk and lower limbs 9 . In this study, it was hypothesized that the CNS exibly selects and modi es the degree of contribution to a few sets of basic trunk muscle synergies rather than framing a new pattern to overcome several human locomotion and stability motor behaviors 9 .…”

“…Furthermore, our previous study included 11 functional tasks of multidirectional trunk movements and several of the four positional tasks that require trunk stability 9 . Each motor task has largely been utilized in research and clinical elds to evaluate and improve motor control [12][13][14][15][16] .…”