![]() This redundancy problem, first proposed by Bernstein (1967), is still an important unresolved puzzle. ![]() Therefore, the central nervous system (CNS) must select a specific muscle activity pattern from infinite combinations to achieve the task. Since one joint is moved by multiple muscles, the same joint movements can be achieved by a variety of muscle activity pattern combinations. Humans must control multiple joints to perform a single motor task. This result suggests that the CNS can use extended muscle synergies to perform voluntary movements. Moreover, an average of 25.5% of the extended muscle synergy was significantly activatable. These results partly support the assumption of the muscle synergy hypothesis, i.e., that the conventional method can extract voluntarily and independently activatable muscle synergies by using the appropriate index of reconstruction. However, the proportion of activatable muscle synergies extracted by N CD and N AD was lower. As a result, an average of 90.8% of the muscle synergy extracted by the N VAF was independently activated. Subjects were visually feed-backed the index of independent activation, then instructed to generate muscle activity patterns similar to the conventional and extended muscle synergies. To examine whether the individual muscle synergy was voluntarily activatable or not, we calculated the index of independent activation, which reflects similarities between a selected single muscle synergy and the current muscle activation pattern of the subject. We also defined an extended muscle synergy as the muscle synergy extracted by the N EX> N AD. We defined a conventional muscle synergy as the muscle synergy extracted by the N VAF, N CD, and N AD. An additional muscle synergy model with N AD was also considered. The number ( N) of muscle synergies was determined by using the variability accounted for (VAF, N VAF) and the coefficient of determination (CD, N CD). We extracted muscle synergies during the task using electromyogram (EMG) data and the NMF method with varied numbers of muscle synergies. Subjects performed an isometric force production task with their right hand, and the 13 muscle activity patterns associated with their elbow and shoulder movements were measured. We defined the activation of a single muscle synergy as the generation of a muscle activity pattern vector parallel to the single muscle synergy vector. This study tested if human subjects can voluntarily activate individual muscle synergies extracted by non-negative matrix factorization (NMF), the standard mathematical method for synergy extraction. However, this assumption has not been empirically tested. The muscle synergy hypothesis assumes that individual muscle synergies are independent of each other and voluntarily controllable. 3Department of Psychology, The University of Tokyo, Tokyo, Japan.2Cognitive Mechanisms Laboratories, Advanced Telecommunications Research Institute International, Kyoto, Japan.1Graduate School of Informatics and Engineering, The University of Electro-Communications, Tokyo, Japan.
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