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.

Hand synergies: Integration of robotics and neuroscience for understanding the control of biological and artificial hands

“…It should be noted that the analysis of hand synergies at the muscular or kinematic level are likely to reveal phenomena that lie on a functional continuum, i.e., dynamic and static hand postural synergies may be related to, or mediated by, hand muscle synergies. This view is supported by a recent study showing a close correspondence between muscle synergies and kinematic synergies in a reach-grasp-pull task [32]. …”

Analysis of Finger Motion Coordination during Packaging Interactions

“…Anatomical factors, such as interdigit webbings, connections between various tendons, insertions of extrinsic finger muscles and neuronal connections result in mechanical and neural couplings between various joints. The sum of mechanical and neural coupling generates coordinated movements between various joints . Thus, the proficient grasping of an object entails simultaneous motion at multiple joints, with correlated rotations .…”

Analyzing finger interdependencies during the Purdue Pegboard Test and comparative activities of daily living

“…However, there are just a few comprehensive quantitative descriptions of the movement patterns from a kinematic perspective under multi-finger, multi-joint tasks 20,47,15 . The current study was an attempt to contribute to these studies by utilising the latest measurement technology to obtain kinematic data and investigate the robustness of a traditional dexterity test, the Purdue Pegboard Test, by comparing finger movement correlation patterns during the performance of the test with those measured during tasks related to activities of daily living.…”

“…Anatomical factors, such as interdigit webbings, connections between various tendons, insertions of extrinsic finger muscles, and neuronal connections result in mechanical and neural couplings between various joints. The sum of mechanical and neural coupling generates coordinated movements between various joints [12][13][14][15] . Thus the proficient grasping of an object entails simultaneous motion at multiple joints, with correlated rotations 13 .…”