title: Differences between kinematic synergies and muscle synergies during two-digit grasping.
abstract: The large number of mechanical degrees of freedom of the hand is not fully exploited during actual movements such as grasping. Usually, angular movements in various joints tend to be coupled, and EMG activities in different hand muscles tend to be correlated. The occurrence of covariation in the former was termed kinematic synergies, in the latter muscle synergies. This study addresses two questions: (i) Whether kinematic and muscle synergies can simultaneously accommodate for kinematic and kinetic constraints. (ii) If so, whether there is an interrelation between kinematic and muscle synergies. We used a reach-grasp-and-pull paradigm and recorded the hand kinematics as well as eight surface EMGs. Subjects had to either perform a precision grip or side grip and had to modify their grip force in order to displace an object against a low or high load. The analysis was subdivided into three epochs: reach, grasp-and-pull, and static hold. Principal component analysis (PCA, temporal or static) was performed separately for all three epochs, in the kinematic and in the EMG domain. PCA revealed that (i) Kinematic- and muscle-synergies can simultaneously accommodate kinematic (grip type) and kinetic task constraints (load condition). (ii) Upcoming grip and load conditions of the grasp are represented in kinematic- and muscle-synergies already during reach. Phase plane plots of the principal muscle-synergy against the principal kinematic synergy revealed (iii) that the muscle-synergy is linked (correlated, and in phase advance) to the kinematic synergy during reach and during grasp-and-pull. Furthermore (iv), pair-wise correlations of EMGs during hold suggest that muscle-synergies are (in part) implemented by coactivation of muscles through common input. Together, these results suggest that kinematic synergies have (at least in part) their origin not just in muscular activation, but in synergistic muscle activation. In short: kinematic synergies may result from muscle synergies.
authors: Neuroscience Research Federation FR3636, CNRS, Université Paris Descartes Paris, France ; Centre de Neurophysique, Physiologie et Pathologie, UMR 8119, CNRS, Université Paris Descartes Sorbonne Paris Cité, Paris, France.
authors: Michele
authors: M
authors: Tagliabue
authors: Laboratory of Biomedical Robotic and Biomicrosystem, Università Campus Bio-Medico di Roma Roma, Italy.
authors: Anna Lisa
authors: AL
authors: Ciancio
authors: Institut de Neurosciences de la Timone, UMR 7289, CNRS, Aix-Marseille Université Marseille, France.
authors: Thomas
authors: T
authors: Brochier
authors: Neuroscience Research Federation FR3636, CNRS, Université Paris Descartes Paris, France ; Life Sciences Department, Université Paris Diderot Sorbonne Paris Cité, Paris, France.
authors: Selim
authors: S
authors: Eskiizmirliler
authors: Neuroscience Research Federation FR3636, CNRS, Université Paris Descartes Paris, France ; Life Sciences Department, Université Paris Diderot Sorbonne Paris Cité, Paris, France.
authors: Marc A
authors: MA
authors: Maier
keywords: EMG; kinematic synergy; movement kinematics; muscle synergy; precision grip