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Front. Comput. Neurosci. | doi: 10.3389/fncom.2018.00010

Muscle Synergies Control During Hand-Reaching Tasks in Multiple Directions Post-Stroke

  • 1University of Haifa, Israel
  • 2National Institute for Brain & Rehabilitation Sciences-Israel, Israel
  • 3Technion – Israel Institute of Technology, Israel

Purpose: A muscle synergies model was suggested to represent a simplifying motor control mechanism by the brainstem and spinal cord. The aim of the study was to investigate the feasibility of such control mechanisms in the rehabilitation of post-stroke individuals during the execution of hand-reaching movements in multiple directions, compared to non-stroke individuals.

Methods: Twelve non-stroke and 13 post-stroke individuals participated in the study. Muscle synergies were extracted from EMG data that was recorded during hand reaching tasks, using the NMF algorithm. The optimal number of synergies was evaluated in both groups using the Variance Accounted For (VAF) and the Mean Squared Error (MSE). A cross validation procedure was carried out to define a representative set of synergies. The similarity index and the K-means algorithm were applied to validate the existence of such a set of synergies, but also to compare the modulation properties of synergies for different movement directions between groups. The similarity index and hierarchical cluster analysis were also applied to compare between group synergies.

Results: Four synergies were chosen to optimally capture the variances in the EMG data, with mean VAF of 0.917±0.034 and 0.883±0.046 of the data variances, with respective MSE of 0.007 and 0.016, in the control and study groups respectively. The representative set of synergies was set to be extracted from movement to the center of the reaching space. Two synergies had different muscle activation balance between groups. Seven and 17 clusters partitioned the muscle synergies of the control and study groups. The control group exhibited a gradual change in the activation in the amplitude in the time domain (modulation) of synergies, as reflected by the similarity index, whereas the study group exhibited consistently significant differences between all movement directions and the representative set of synergies. The study findings support the existence of a representative set of synergies, which are modulated to execute movements in different directions.

Conclusions: Post-stroke individuals differently modulate the activation of synergies to different movement directions than do non-stroke individuals. The conclusion was supported by different muscle activation balances, similarity values and different classifications of synergies among groups.

Keywords: Muscle-synergies, Stroke, Motor-control, non-negative matrix factorization (NMF), electromyography (EMG), modulation, Variance accounted for, Hand reaching

Received: 20 Oct 2017; Accepted: 07 Feb 2018.

Edited by:

Ahmed A. Moustafa, Western Sydney University, Australia

Reviewed by:

NING JIANG, University of Waterloo, Canada
Shinya Aoi, Kyoto University, Japan  

Copyright: © 2018 Israely, Leisman, Machluf and Carmeli. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Mr. Sharon Israely, University of Haifa, Haifa, Israel,