Abstract
A central question in Neuroscience is that of how the nervous system generates the spatiotemporal commands needed to realize complex gestures, such as handwriting. A key postulate is that the central nervous system (CNS) builds up complex movements from a set of simpler motor primitives or control modules. In this study we examined the control modules underlying the generation of muscle activations when performing different types of movement: discrete, point-to-point movements in eight different directions and continuous figure-eight movements in both the normal, upright orientation and rotated 90°. To test for the effects of biomechanical constraints, movements were performed in the frontal-parallel or sagittal planes, corresponding to two different nominal flexion/abduction postures of the shoulder. In all cases we measured limb kinematics and surface electromyographic activity (EMG) signals for seven different muscles acting around the shoulder. We first performed principal component analysis (PCA) of the EMG signals on a movement-by-movement basis. We found a surprisingly consistent pattern of muscle groupings across movement types and movement planes, although we could detect systematic differences between the PCs derived from movements performed in each shoulder posture and between the principal components associated with the different orientations of the figure. Unexpectedly we found no systematic differences between the figure eights and the point-to-point movements. The first three principal components could be associated with a general co-contraction of all seven muscles plus two patterns of reciprocal activation. From these results, we surmise that both “discrete-rhythmic movements” such as the figure eight, and discrete point-to-point movement may be constructed from three different fundamental modules, one regulating the impedance of the limb over the time span of the movement and two others operating to generate movement, one aligned with the vertical and the other aligned with the horizontal.
Highlights
Following the quantitative definitions for discrete and rhythmic gestures proposed by Hogan and Sternad (2007), handwriting movements, in terms of behavioral and observational features, are special cases of discrete movements because they have rhythmic phases but last a finite duration, with the hand starting and ending at zero velocity
We previously demonstrated that for rapid execution of a single figure-eight movement the isochrony principle and the 2/3 power law between angular velocity and curvature are respected, and that the tangential velocity profile is invariant relative to the initial direction of movement (Cheron et al, 1999), electromyographic activity (EMG) analyses have shown that muscular activations present temporal modulation related to the figure as a whole, in contrast to directional pattern of tuning that would point to a segmented control
We first looked to see if the principal component analysis (PCA), which we applied to each movement one-by-one, was able to identify regular patterns of muscle involvement across the different movement directions and movement planes
Summary
Following the quantitative definitions for discrete and rhythmic gestures proposed by Hogan and Sternad (2007), handwriting movements, in terms of behavioral and observational features, are special cases of discrete movements because they have rhythmic phases but last a finite duration, with the hand starting and ending at zero velocity. Making the distinction between discrete and rhythmic movements is central because their underlying neural control could be different (Hollerbach, 1981). An opposing view is that rhythmic movements represent the fundamental class and that discrete movements are abbreviated rhythmic movements (Sternad and Schaal, 1999; Sternad et al, 2000; Schaal and Sternad, 2001; Sternad and Dean, 2003). Both of these viewpoints would suggest that only a single, common control mechanism is used to achieve both types of movement. A third possibility is that rhythmic and discrete movements represent two distinct movement classes that are mediated by separate neural control circuitry
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
