Individual preferences in motor coordination seen across the two hands: relations to movement stability and optimality

Abstract

The framework of the uncontrolled manifold (UCM) hypothesis was used to explore variables related to stability of task performance in the two hands of young healthy individuals. Fourteen young adults performed four-finger accurate constant force production tasks interrupted by a voluntary quick force pulse production and by an externally imposed displacement of all fingers. Three groups of variables were used to quantify stability of steady force production: (1) indices of the inter-trial variance were computed within the UCM and orthogonal to the UCM; (2) indices of motor equivalence were computed between steady-state intervals separated by the force pulse and by the inverse piano action; and (3) referent coordinate and apparent stiffness were computed using the data during the ascending phase of the finger-lifting episode. In another task, the subjects performed accurate constant force production with visual feedback removal after the 8th s and the drop in the total force after the removal was computed. There were differences between the right and left hand in some outcome variables such as variance within the UCM, and the timing of anticipatory synergy adjustments prior to the force pulse, consistent with the dynamic dominance hypothesis. There were significant correlations between the two hands for indices that were unrelated to accuracy of performance: variance within the UCM, index of motor equivalence, referent coordinate, apparent stiffness, and the drop of total force after visual feedback removal. We interpret these findings within the concept of stability-optimality trade-off. In particular, we conclude that individual subjects select particular, person-specific solutions within the spectrum allowed by the explicit task constraints, and this choice is consistent between the two hands. We conclude with a hypothesis that selecting specific solutions within the stability-optimality trade-off may represent an individual’s personal preference consistent between the two hands.

Visual Feedback Improves Bimanual Force Control Performances at Planning and Execution Levels

“…Accordingly, the current findings confirmed the potential relation between bilateral synergetic coordination strategies at the planning level (i.e., between multiple trials) and traditional and temporal structural of motor variability of DOFs produced by two hands at the execution level (i.e., within a single trial). Moreover, our correlation findings supported a proposition that the motor system may prefer the stability strategy from no-vision to vision conditions because we observed greater bilateral motor synergies with reduced bad variability related to advanced task stabilization patterns such as less variable and regular bimanual force outputs 37 . As revealed in a neuroimaging study, force control improvements in the vision condition were significantly related to visuomotor processing with greater parietal cortical activations 44 .…”

“…The presence of online visual feedback increased bilateral motor synergies implicating more cooperative motor actions between two hands with the reduction of bad variability components across multiple trials. Freitas and colleagues proposed the stabilityoptimality trade-off phenomenon in human behaviors 37 . Specifically, an optimality strategy represents that the central nervous system selects an optimal motor solution from numerous motor elements (i.e., motor redundancy) by minimizing the variance of force elements produced by two limbs along the UCM line.…”

“…On the other hand, a stability strategy indicates the organization of numerous motor elements (i.e., motor abundance) for stabilizing task performance by decreasing the variance of force elements along the ORT line resulting in greater index of bilateral motor synergies (i.e., higher good variability relative to bad variability). These two motor control strategies may independently influence actions generating synergetic interlimb movements 28,[37][38][39] . Taken together, our findings suggest that when the simultaneous task-related visual feedback is available, the performer may prefer the stability strategy for successful task performances across multiple trials via increasing bilateral motor synergies while minimizing bad variability components.…”

Visual feedback improves bimanual force control performances at planning and execution levels

“…The presence of online visual feedback increased bilateral motor synergies implicating more cooperative motor actions between two hands with the reduction of bad variability components across multiple trials. Freitas et al proposed the stability-optimality trade-off phenomenon in human behaviors 27 . Specifically, an optimality strategy postulates that the central nervous system selects an optimal motor solution from numerous motor elements by minimizing the variance of force elements produced by two limbs along the UCM line.…”

Motor Control: Creating a Natural Science of Biological Movement

“…Hence, there is an inherent trade-off between stability and optimality (Park et al, 2010). A recent study has shown that healthy individuals show person-specific signatures when dealing with this trade-off (de Freitas et al, 2019), which may be viewed as personal traits, so far barely explored.…”