title: Task-specific stability of abundant systems: Structure of variance and motor equivalence.
abstract: Our main goal was to test a hypothesis that transient changes in performance of a steady-state task would result in motor equivalence. We also estimated effects of visual feedback on the amount of reorganization of motor elements. Healthy subjects performed two variations of a four-finger pressing task requiring accurate production of total pressing force (F TOT) and total moment of force (M TOT). In the Jumping-Target task, a sequence of target jumps required transient changes in either F TOT or M TOT. In the Step-Perturbation task, the index finger was lifted by 1cm for 0.5s leading to a change in both F TOT and M TOT. Visual feedback could have been frozen for one of these two variables in both tasks. Deviations in the space of finger modes (hypothetical commands to individual fingers) were quantified in directions of unchanged F TOT and M TOT (motor equivalent - ME) and in directions that changed F TOT and M TOT (non-motor equivalence - nME). Both the ME and nME components increased when the performance changed. After transient target jumps leading to the same combination of F TOT and M TOT, the changes in finger modes had a large residual ME component with only a very small nME component. Without visual feedback, an increase in the nME component was observed without consistent changes in the ME component. Results from the Step-Perturbation task were qualitatively similar. These findings suggest that both external perturbations and purposeful changes in performance trigger a reorganization of elements of an abundant system, leading to large ME change. These results are consistent with the principle of motor abundance corroborating the idea that a family of solutions is facilitated to stabilize values of important performance variables.
authors.affiliation: Biomechanics and Movement Science Program, University of Delaware, Newark, DE, United States; Program in Occupational Therapy, Washington University, Saint Louis, MO, United States; Department of Kinesiology, The Pennsylvania State University, University Park, PA, United States. Electronic address: mattosd@wusm.wustl.edu.
authors.forename: D
authors.initials: D
authors.lastname: Mattos
authors.affiliation: Institut für Neuroinformatik, Rühr University Bochum, Bochum, Germany.
authors.forename: G
authors.initials: G
authors.lastname: Schöner
authors.affiliation: Department of Kinesiology, The Pennsylvania State University, University Park, PA, United States.
authors.forename: V M
authors.initials: VM
authors.lastname: Zatsiorsky
authors.affiliation: Department of Kinesiology, The Pennsylvania State University, University Park, PA, United States.
authors.forename: M L
authors.initials: ML
authors.lastname: Latash
mesh_terms: D000328:Adult; D003627:Data Interpretation, Statistical; D056228:Feedback, Sensory / Q000502:physiology*; D005260:Female; D005385:Fingers; D006801:Humans; D008297:Male; D009043:Motor Activity; D011597:Psychomotor Performance
keywords: finger force; motor equivalence; perturbation; synergy; uncontrolled manifold hypothesis