title: Synergies in the space of control variables within the equilibrium-point hypothesis.
abstract: We use an approach rooted in the recent theory of synergies to analyze possible co-variation between two hypothetical control variables involved in finger force production based on the equilibrium-point (EP) hypothesis. These control variables are the referent coordinate (R) and apparent stiffness (C) of the finger. We tested a hypothesis that inter-trial co-variation in the {R; C} space during repeated, accurate force production trials stabilizes the fingertip force. This was expected to correspond to a relatively low amount of inter-trial variability affecting force and a high amount of variability keeping the force unchanged. We used the "inverse piano" apparatus to apply small and smooth positional perturbations to fingers during force production tasks. Across trials, R and C showed strong co-variation with the data points lying close to a hyperbolic curve. Hyperbolic regressions accounted for over 99% of the variance in the {R; C} space. Another analysis was conducted by randomizing the original {R; C} data sets and creating surrogate data sets that were then used to compute predicted force values. The surrogate sets always showed much higher force variance compared to the actual data, thus reinforcing the conclusion that finger force control was organized in the {R; C} space, as predicted by the EP hypothesis, and involved co-variation in that space stabilizing total force.
authors.affiliation: Department of Health and Kinesiology, Purdue University, West Lafayette, IN 47907, USA. Electronic address: sambike@purdue.edu.
authors.forename: S
authors.initials: S
authors.lastname: Ambike
authors.affiliation: Program in Occupational Therapy, Washington University School of Medicine, Saint Louis, MO 63110, USA.
authors.forename: D
authors.initials: D
authors.lastname: Mattos
authors.affiliation: Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA.
authors.forename: V M
authors.initials: VM
authors.lastname: Zatsiorsky
authors.affiliation: Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA.
authors.forename: M L
authors.initials: ML
authors.lastname: Latash
mesh_terms: D000328:Adult; D005260:Female; D005385:Fingers / Q000502:physiology*; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D008297:Male; D008959:Models, Neurological; D009043:Motor Activity / Q000502:physiology*; D012044:Regression Analysis
keywords: apparent stiffness; equilibrium-point hypothesis; finger force; isometric; synergy; uncontrolled manifold hypothesis