Abstract
BackgroundRecently, muscle synergy analysis has become a standard methodology for extracting coordination patterns from electromyographic (EMG) signals, and for the evaluation of motor control strategies in many contexts. Most previous studies have characterized upper-limb muscle synergies across a limited set of reaching movements. With the aim of future uses in motor control, rehabilitation and other fields, this study provides a comprehensive characterization of muscle synergies in a large set of upper-limb tasks and also considers inter-individual and environmental variability.MethodsSixteen healthy subjects performed upper-limb hand exploration movements for a comprehensive mapping of the upper-limb workspace, which was divided into several sectors (Frontal, Right, Left, Horizontal, and Up). EMGs from representative upper-limb muscles and kinematics were recorded to extract muscle synergies and explore the composition, repeatability and similarity of spatial synergies across subjects and movement directions, in a context of high variability of motion.ResultsEven in a context of high variability, a reduced set of muscle synergies may reconstruct the original EMG envelopes. Composition, repeatability and similarity of synergies were found to be shared across subjects and sectors, even if at a lower extent than previously reported.ConclusionExtending the results of previous studies, which were performed on a smaller set of conditions, a limited number of muscle synergies underlie the execution of a large variety of upper-limb tasks. However, the considered spatial domain and the variability seem to influence the number and composition of muscle synergies. Such detailed characterization of the modular organization of the muscle patterns for upper-limb control in a large variety of tasks may provide a useful reference for studies on motor control, rehabilitation, industrial applications, and sports.
Highlights
In recent years, the study of motor control has focused on a human-centered perspective by emphasizing the importance of evaluating muscle or kinematic coordination patterns
While many data processing factors may affect the extraction of muscle synergies, we focus a further discussion of the results on the impact of normalization, which we hypothesized might be significant in our study because of the high variability explored in our dataset
With the main distinctive feature of our study, which is to provide a mapping of muscle synergies in a wide portion of the workspace, we chose to normalize the EMG envelopes across all the maximum EMG values found in all the trials, because we wanted to refer our results to all the high variability of motions and EMG patterns that we explored throughout the workspace
Summary
The study of motor control has focused on a human-centered perspective by emphasizing the importance of evaluating muscle or kinematic coordination patterns Such view focuses on the premise that the Central Nervous System (CNS) relies on a limited number of modules (Bizzi et al, 2008), possibly implemented at neural level (Bizzi and Cheung, 2013), to simplify the production of movement. By properly recruiting spatial modules with temporal activation coefficients, the CNS exploits a reduced set of pre-shaped neural pathways, called synergies, to achieve a large variety of motor commands This view implicitly assumes that, if synergies are encoded at neural level, a unique set should be used across a variety of movements or, at least, task-specific sets should underlie movements requiring similar motor commands. With the aim of future uses in motor control, rehabilitation and other fields, this study provides a comprehensive characterization of muscle synergies in a large set of upperlimb tasks and considers inter-individual and environmental variability
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