Abstract
Muscle synergy theory is a new appealing approach for different research fields. This study is aimed at evaluating the robustness of EMG reconstruction via muscle synergies and the repeatability of muscle synergy parameters as potential neurophysiological indices. Eight healthy subjects performed walking, stepping, running, and ascending and descending stairs' trials for five repetitions in three sessions. Twelve muscles of the dominant leg were analyzed. The “nonnegative matrix factorization” and “variability account for” were used to extract muscle synergies and to assess EMG goodness reconstruction, respectively. Intraclass correlation was used to quantify methodology reliability. Cosine similarity and coefficient of determination assessed the repeatability of the muscle synergy vectors and the temporal activity patterns, respectively. A 4-synergy model was selected for EMG signal factorization. Intraclass correlation was excellent for the overall reconstruction, while it ranged from fair to excellent for single muscles. The EMG reconstruction was found repeatable across sessions and subjects. Considering the selection of neurophysiological indices, the number of synergies was not repeatable neither within nor between subjects. Conversely, the cosine similarity and coefficient of determination values allow considering the muscle synergy vectors and the temporal activity patterns as potential neurophysiological indices due to their similarity both within and between subjects. More specifically, some synergies in the 4-synergy model reveal themselves as more repeatable than others, suggesting focusing on them when seeking at the neurophysiological index identification.
Highlights
Several locomotive activities, such as walking, running, and ascending and descending stairs, are commonly and continuously performed in daily life [1]
Taking into account the three sessions, no subject recruited the same muscle synergy model during all examined repetitions related to each task
To compare the results across the daily life tasks, we focused only on the 4-synergy model, as it resulted the most selected model considering all the subjects and all the tasks together
Summary
Several locomotive activities, such as walking, running, and ascending and descending stairs, are commonly and continuously performed in daily life [1]. These activities involve the coordination of a high number of muscles of lower limbs [2]. It is generally understood that the central nervous system (CNS) can reduce the dimensionality of neural activation outputs to control muscles and achieve a predefined movement [3, 4]. The reduction is obtained through the simultaneous coactivation of muscle groups, addressed as muscle synergies [5, 6]. Parameter estimation of muscle synergy models is usually obtained from the factorization of the electromyographic (EMG) signals by using decomposition algorithms [7]. The nonnegative matrix factorization [8]
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