Abstract
The movement in a joint is facilitated by a pair of muscles that pull in opposite directions. The difference in the pair’s muscle force or reciprocal activity results in joint torque, while the overlapping muscle force or the cocontraction is related to the joint’s stiffness. Cocontraction knowingly adapts implicitly over a number of movements, but it is unclear whether the central nervous system can actively regulate cocontraction in a goal-directed manner in a short span of time. We developed a muscle interface where a cursor’s horizontal position was determined by the reciprocal activity of the shoulder flexion–extension muscle pair, while the vertical position was controlled by its cocontraction. Participants made goal-directed movements to single and via-point targets in the two-dimensional muscle space, learning to move the cursor along the shortest path. Simulations using an optimal control framework suggest that the reciprocal activity and the cocontraction may be controlled independently by the CNS, albeit at a rate orders of magnitude slower than the muscle’s maximal activation speed.
Highlights
The movement in a joint is facilitated by a pair of muscles that pull in opposite directions
Post-hoc comparisons using Tukey’s HSD revealed that the trajectory length when reaching targets (ii) (p < 0.001) and (iii) (p < 0.001) shortened with the block number. This learning effect was only observed for targets that required cocontraction, and not those where pure flexion (p = 0.99) or pure extension (p = 0.99) were needed. These results suggest that regulating either flexion or extension alone required little to no learning, while the concurrent control of reciprocal activity and cocontraction benefited from practice
We developed a computational model of the reaching task based on an optimal control framework in the muscle space to determine whether the reciprocal activity (RA) and the cocontraction were independently controlled by the central nervous system (CNS), and to examine how fast the CNS was able to modulate the activity in the muscle space to reach the targets
Summary
The movement in a joint is facilitated by a pair of muscles that pull in opposite directions. Participants made goal-directed movements to single and via-point targets in the two-dimensional muscle space, learning to move the cursor along the shortest path. The central nervous system (CNS) knowingly increases the arm’s cocontraction to accurately control the hand’s position during motion. Another study found that the cocontraction in the wrist could be controlled to a desired level set by the e xperimenter[12] These studies only tested how the CNS could maintain the cocontraction at a designated constant value, and did not require the participants to modulate their cocontraction to different values in a short span of time. (c) The cursor’s trajectory from a representative participant from the initial and final phases of reaching the point-to-point targets. (e) The time-series of the reciprocal activity and the cocontraction from the final phase of the point-to-point reaching task. Participants could activate each specific muscle by the correct magnitude to reach the target
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