Muscle synergy is the activation of a group of muscles that contribute to a particular movement. Several studies have shown that electromyography (EMG) from a variety of tasks can be described by data that may reflect synergies. These studies use algorithms, such as nonnegative matrix factorization (NNMF), to identify synergies from EMG. Experimental constraints hinder researchers ability to take EMG from all muscles involved in a task and make it unclear as to whether the number and type of muscles included in the analysis impacts estimated synergies.
The purpose of Tresch et al’s study was to evaluate the impact of the number and choice of muscles on synergy analysis. This study sought to determine if the number and type of muscles selected with EMG in an experimental protocol had any impact on the synergies identified using matrix factorization algorithms, such as NNMF. The study found that the number and choice of muscles does impact the structure of synergies and the amount of variance in muscle activity accounted for by a given set of synergies. For this study, Steele used a master set of synergies calculated using all muscles. Steele then randomly selected subsets of between 5 and 29 muscles from the master subset and compared the similarities calculated from each subset to the master set of synergies. This comparison showed that the structure of synergies is dependent upon the number and choice of muscles included in the analysis. The analysis showed that when five muscles were included in the analysis, the similarity of the synergies to the master set was only 0.57 ± 0.54. This similarity improved to over 0.8 when more than ten muscles were included in the analysis.
The results of the study showed that analysis that included fewer muscles could overestimate the total variance accounted for compared to an analysis that included all muscles involved in a task. Synergies identified from a subset of muscles could potentially describe the variance in EMG activity but may not accurately reflect how muscles are recruited or activated together which can impact the functional interpretations of synergies.
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