However, recent studies have revealed that torque generated at joints other than the ankle (e.g., hip or knee) is negligible in explaining the mechanism for maintaining balance in reaction against perturbation ( Kuo, 1995 Bloem et al., 2000 Alexandrov et al., 2005) and in a quiet stance ( Aramaki et al., 2001 Creath et al., 2005 Hsu et al., 2007 Pinter et al., 2008). In postural control during quiet standing, some studies have described human standing as a single inverted pendulum that pivots at the ankle joint ( Winter et al., 1998 Peterka, 2002). Humans maintain their bipedal standing posture by controlling their center of foot pressure (COP) to keep the projection of their center of mass on the ground within the base of support. In terms of the composition of muscle synergies and the ADs of individual muscles, we confirmed that muscle synergies in multi-directional postural control comprised a combination of several muscles, including various ADs, that generate torque at different joints. The results revealed that the ADs of muscle synergies were almost uniformly distributed in the task space in most of the subjects, which indicates that mechanical characteristics reduce the redundancy in postural control. Then the directional properties, such as the mechanical role (i.e., action directions, we use ADs as abbreviation below), of muscle synergies and muscles were estimated using an electromyogram-weighted averaging (EWA) method, which is based on a cross-correlation between the fluctuations in the activation of muscle synergies and the COP. ![]() Muscle synergies were extracted using non-negative matrix factorization (NMF), and the structure of coordinative modules to keep the posture leaning toward various directions was disclosed. Electromyograms (EMGs) of 18 muscles and COPs were recorded in the experiment. The task for healthy, young subjects was to shift and align their center of pressure (COP) to targets dispersed in 12 different directions in the horizontal plane by leaning their bodies for 10 s. In this article, the mechanical contribution of muscle synergies and coordinative structures of muscles in voluntary multi-directional postural control were investigated. 4School of Health and Sport Sciences, Chukyo University, Nagoya, JapanĪ muscle synergy is a coordinative structure of muscles that has been proposed as a strategy to reduce the number of variables that the central nervous system (CNS) has to address in motor tasks. ![]() 3Graduate School of Education, The University of Tokyo, Tokyo, Japan.2Japan Society for the Promotion of Science, Tokyo, Japan.1Laboratory of Neurophysiology, Graduate School of Human and Environmental Studies, Kyoto University, Kyoto, Japan. ![]() Akari Kubo 1 Shota Hagio 2,3 Benio Kibushi 1,2 Toshio Moritani 4 Motoki Kouzaki 1*
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