title: Dimensionality of joint torques and muscle patterns for reaching.
abstract: Muscle activities underlying many motor behaviors can be generated by a small number of basic activation patterns with specific features shared across movement conditions. Such low-dimensionality suggests that the central nervous system (CNS) relies on a modular organization to simplify control. However, the relationship between the dimensionality of muscle patterns and that of joint torques is not fixed, because of redundancy and non-linearity in mapping the former into the latter, and needs to be investigated. We compared the torques acting at four arm joints during fast reaching movements in different directions in the frontal and sagittal planes and the underlying muscle patterns. The dimensionality of the non-gravitational components of torques and muscle patterns in the spatial, temporal, and spatiotemporal domains was estimated by multidimensional decomposition techniques. The spatial organization of torques was captured by two or three generators, indicating that not all the available coordination patterns are employed by the CNS. A single temporal generator with a biphasic profile was identified, generalizing previous observations on a single plane. The number of spatiotemporal generators was equal to the product of the spatial and temporal dimensionalities and their organization was essentially synchronous. Muscle pattern dimensionalities were higher than torques dimensionalities but also higher than the minimum imposed by the inherent non-negativity of muscle activations. The spatiotemporal dimensionality of the muscle patterns was lower than the product of their spatial and temporal dimensionality, indicating the existence of specific asynchronous coordination patterns. Thus, the larger dimensionalities of the muscle patterns may be required for CNS to overcome the non-linearities of the musculoskeletal system and to flexibly generate endpoint trajectories with simple kinematic features using a limited number of building blocks.
authors.affiliation: Laboratory of Neuromotor Physiology, Santa Lucia Foundation Rome, Italy.
authors.forename: Marta
authors.initials: M
authors.lastname: Russo
authors.affiliation: Laboratory of Neuromotor Physiology, Santa Lucia Foundation Rome, Italy.
authors.forename: Mattia
authors.initials: M
authors.lastname: D'Andola
authors.affiliation: Laboratory of Neuromotor Physiology, Santa Lucia Foundation Rome, Italy ; Center of Space Biomedicine, University of Rome "Tor Vergata," Rome, Italy.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Portone
authors.affiliation: Laboratory of Neuromotor Physiology, Santa Lucia Foundation Rome, Italy ; Center of Space Biomedicine, University of Rome "Tor Vergata," Rome, Italy ; Department of Systems Medicine, University of Rome "Tor Vergata," Rome, Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Lacquaniti
authors.affiliation: Laboratory of Neuromotor Physiology, Santa Lucia Foundation Rome, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: d'Avella
keywords: EMGs; human subjects; inverse dynamics; joint torques; modularity; muscle synergies; reaching movements; temporal components