Abstract
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.
Highlights
In the upper limb, numerous muscles with thousands of motor units require the control of the motor system even when executing simple daily tasks, such as hand reaching
In the first stage of analysis we aimed to compare the number of synergies which are required to reconstruct the EMG data in both groups, before determining the optimal number of synergies, and further analyzing the properties of these synergies
The quality of the negative Matrix Factorization (NMF) to reconstruct the EMG data was evaluated by two measures: mean squared errors (MSE) and VAF
Summary
Numerous muscles with thousands of motor units require the control of the motor system even when executing simple daily tasks, such as hand reaching. Direct control on these structures requires simultaneous control over large degrees of freedom, which in turn, imposes a significant computational burden on the Central Nervous System (CNS). In the context of this paper, modulation of synergies will be referred to changes in the amplitude of activations of synergies in the time-domain This combinatorial system is a composite of discrete numbers of building blocks, i.e., muscle synergies within the brainstem and spinal cord. During execution of compound movement the CNS flexibly combines these synergies, and modulates their amplitude and timing of activation (Tresch and Jarc, 2009)
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