title: Action Direction of Muscle Synergies in Three-Dimensional Force Space.
abstract: Redundancy in the musculoskeletal system was supposed to be simplified by muscle synergies, which modularly organize muscles. To clarify the underlying mechanisms of motor control using muscle synergies, it is important to examine the spatiotemporal contribution of muscle synergies in the task space. In this study, we quantified the mechanical contribution of muscle synergies as considering spatiotemporal correlation between the activation of muscle synergies and endpoint force fluctuations. Subjects performed isometric force generation in the three-dimensional force space. The muscle-weighting vectors of muscle synergies and their activation traces across different trials were extracted from electromyogram data using decomposing technique. We then estimated mechanical contribution of muscle synergies across each trial based on cross-correlation analysis. The contributing vectors were averaged for all trials, and the averaging was defined as action direction (AD) of muscle synergies. As a result, we extracted approximately five muscle synergies. The ADs of muscle synergies mainly depended on the anatomical functions of their weighting muscles. Furthermore, the AD of each muscle indicated the synchronous activation of muscles, which composed of the same muscle synergy. These results provide the spatiotemporal characteristics of muscle synergies as neural basis.
authors.affiliation: Japan Society for the Promotion of Science , Tokyo , Japan ; Laboratory of Neurophysiology, Graduate School of Human and Environmental Studies, Kyoto University , Kyoto , Japan.
authors.forename: Shota
authors.initials: S
authors.lastname: Hagio
authors.affiliation: Laboratory of Neurophysiology, Graduate School of Human and Environmental Studies, Kyoto University , Kyoto , Japan.
authors.forename: Motoki
authors.initials: M
authors.lastname: Kouzaki
keywords: cross-correlation analysis; electromyogram; force fluctuations; mechanical pulling direction; muscle activity; non-negative matrix factorization