Title : The flexible recruitment of muscle synergies depends on the 1 required force generating capability

19 To simplify redundant motor control, the central nervous system (CNS) may 20 modularly organize and recruit groups of muscles as “muscle synergies”. However, 21 smooth and efficient movements are expected to require not only low-dimensional 22 organization but also flexibility in the recruitment or combination of synergies 23 depending on force-generating capability of individual muscles. In this study, we 24 examined how the CNS controls activations of muscle synergies as changing joint 25 angles. Subjects performed multi-directional isometric force generations around right 26 ankle and extracted the muscle synergies using non-negative matrix factorization 27 (NMF) across various knee and hip joint angles. As a result, muscle synergies were 28 selectively recruited with merging or decomposition as changing the joint angles. 29 Moreover, the activation profiles, including activation levels and the direction 30 indicating the peak, of muscle synergies across force directions depended on the joint 31 angles. Therefore, we suggested that the CNS selects appropriate muscle synergies 32 and controls their activation patterns based on the force-generating capability of 33 muscles with merging or decomposing descending neural inputs. 34