title: On the Origin of Muscle Synergies: Invariant Balance in the Co-activation of Agonist and Antagonist Muscle Pairs.
abstract: Investigation of neural representation of movement planning has attracted the attention of neuroscientists, as it may reveal the sensorimotor transformation essential to motor control. The analysis of muscle synergies based on the activity of agonist-antagonist (AA) muscle pairs may provide insight into such transformations, especially for a reference frame in the muscle space. In this study, we examined the AA concept using the following explanatory variables: the AA ratio, which is related to the equilibrium-joint angle, and the AA sum, which is associated with joint stiffness. We formulated muscle synergies as a function of AA sums, positing that muscle synergies are composite units of mechanical impedance. The AA concept can be regarded as another form of the equilibrium-point (EP) hypothesis, and it can be extended to the concept of EP-based synergies. We introduce, here, a novel tool for analyzing the neurological and motor functions underlying human movements and review some initial insights from our results about the relationships between muscle synergies, endpoint stiffness, and virtual trajectories (time series of EP). Our results suggest that (1) muscle synergies reflect an invariant balance in the co-activation of AA muscle pairs; (2) each synergy represents the basis for the radial, tangential, and null movements of the virtual trajectory in the polar coordinates centered on the specific joint at the base of the body; and (3) the alteration of muscle synergies (for example, due to spasticity or rigidity following neurological injury) results in significant distortion of endpoint stiffness and concomitant virtual trajectories. These results indicate that muscle synergies (i.e., the balance of muscle mechanical impedance) are essential for motor control.
authors.affiliation: Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University , Toyonaka , Japan.
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Hirai
authors.affiliation: Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University , Toyonaka , Japan.
authors.forename: Fumio
authors.initials: F
authors.lastname: Miyazaki
authors.affiliation: Department of Neurology, Senri Chuo Hospital , Toyonaka , Japan.
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Naritomi
authors.affiliation: Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University , Toyonaka , Japan.
authors.forename: Keitaro
authors.initials: K
authors.lastname: Koba
authors.affiliation: Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University , Toyonaka , Japan.
authors.forename: Takanori
authors.initials: T
authors.lastname: Oku
authors.affiliation: Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University , Toyonaka , Japan.
authors.forename: Kanna
authors.initials: K
authors.lastname: Uno
authors.affiliation: Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University , Toyonaka , Japan.
authors.forename: Mitsunori
authors.initials: M
authors.lastname: Uemura
authors.affiliation: Department of Rehabilitation, Senri Chuo Hospital , Toyonaka , Japan.
authors.forename: Tomoki
authors.initials: T
authors.lastname: Nishi
authors.affiliation: Department of Rehabilitation, Senri Chuo Hospital , Toyonaka , Japan.
authors.forename: Masayuki
authors.initials: M
authors.lastname: Kageyama
authors.affiliation: Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University , Toyonaka , Japan ; Department of Mechanical Engineering, Massachusetts Institute of Technology , Cambridge, MA , USA ; Department of Neurology, University of Maryland School of Medicine , Baltimore, MD , USA ; Department of Rehabilitation Medicine I, School of Medicine, Fujita Health University , Toyoake , Japan ; Institute of Neuroscience, University of Newcastle , Newcastle upon Tyne , UK.
authors.forename: Hermano Igo
authors.initials: HI
authors.lastname: Krebs
keywords: electromyography; endpoint stiffness; mechanical impedance; motor primitives; muscle synergy; reference frame; virtual trajectory