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.

Background/aim: Patients with stroke who are suffering from impaired reaching movement experience insufficient spatial and temporal coordination, affecting upper limb functions and everyday life tasks. This study examines a new robot-assisted rehabilitation method for ameliorating arm reaching movements through velocity error enhancement training. The authors hypothesised that this robot-assisted rehabilitation training may encourage restoration of arm reaching abilities among post-stroke hemiparesis patients. Methods: Several clinical and kinematic measures were used to evaluate outcomes. Subjects were assigned either to an experimental group that underwent 5-week treatments with error enhanced forces, or to a control group that received passive treatment. The control group undertook reaching tasks over the same period while they were connected to the robot but without it applying any error enhancement forces to their upper limb. The robotic system was programmed based on previous kinematic data from healthy subjects, so any deviation from the relatively smooth, calculated, optimal trajectory, and velocity profile mean encountered error enhancing external forces. Results: The results showed an appreciable effect on smoothness and regularity of movement. After 5 weeks of velocity error enhancement treatment, all subjects in the experimental group displayed movements converging towards their optimal profiles, together with decreased variability in path trajectory. In contrast to the control group, their mean deviation was also significantly reduced. These positive changes in motor control patterns were paralleled by gains in functional capacity, as reflected by the Motor Assessment Scale test results. However, those results should be carefully inspected in regard to small sample size and un-matching of motor performance at the beginning of the trial between groups. Conclusion: The study demonstrates the potential of robotic rehabilitation that combines error enhancement and velocity component training to help stroke patients.

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AbstractMuscle synergies are proposed to function as motor primitives that are modulated by frontal brain areas to construct a large repertoire of movement. This paper reviews the history of the development of our current theoretical understanding of nervous system-based motor control mechanisms and more specifically the concept of muscle synergies. Computational models of muscle synergies, especially the nonnegative matrix factorization algorithm, are discussed with specific reference to the changes in synergy control post-central nervous system (CNS) lesions. An alternative approach for motor control is suggested, exploiting a combination of synergies control or flexible muscle control used for gross motor skills and for individualized finger movements. Rehabilitation approaches, either supporting or inhibiting the use of basic movement patterns, are discussed in the context of muscle synergies. Applications are discussed for the use of advanced technologies that can promote the recovery and functioning of the human CNS after stroke.

Most of the studies reviewed have significant methodological drawbacks that resulted in equivocal results. Therefore, we recommend that additional randomized controlled trials, with larger sample sizes and acceptable protocols be conducted to determine the long-term efficacy of EA training.