title: Unsteady steady-states: central causes of unintentional force drift.
abstract: We applied the theory of synergies to analyze the processes that lead to unintentional decline in isometric fingertip force when visual feedback of the produced force is removed. We tracked the changes in hypothetical control variables involved in single fingertip force production based on the equilibrium-point hypothesis, namely the fingertip referent coordinate (RFT) and its apparent stiffness (CFT). The system's state is defined by a point in the {RFT; CFT} space. We tested the hypothesis that, after visual feedback removal, this point (1) moves along directions leading to drop in the output fingertip force, and (2) has even greater motion along directions that leaves the force unchanged. Subjects produced a prescribed fingertip force using visual feedback and attempted to maintain this force for 15 s after the feedback was removed. We used the "inverse piano" apparatus to apply small and smooth positional perturbations to fingers at various times after visual feedback removal. The time courses of RFTand CFTshowed that force drop was mostly due to a drift in RFTtoward the actual fingertip position. Three analysis techniques, namely hyperbolic regression, surrogate data analysis, and computation of motor-equivalent and non-motor-equivalent motions, suggested strong covariation in RFTand CFTstabilizing the force magnitude. Finally, the changes in the two hypothetical control variables {RFT; CFT} relative to their average trends also displayed covariation. On the whole, the findings suggest that unintentional force drop is associated with (a) a slow drift of the referent coordinate that pulls the system toward a low-energy state and (b) a faster synergic motion of RFTand CFTthat tends to stabilize the output fingertip force about the slowly drifting equilibrium point.
authors.affiliation: Department of Health and Kinesiology, Purdue University, 800 West Stadium Ave, West Lafayette, IN, 47907, USA. sambike@purdue.edu.
authors.forename: Satyajit
authors.initials: S
authors.lastname: Ambike
authors.affiliation: Program in Occupational Therapy, Washington University School of Medicine, Saint Louis, MO, USA.
authors.forename: Daniela
authors.initials: D
authors.lastname: Mattos
authors.affiliation: Department of Kinesiology, The Pennsylvania State University, University Park, PA, USA.
authors.forename: Vladimir M
authors.initials: VM
authors.lastname: Zatsiorsky
authors.affiliation: Department of Kinesiology, The Pennsylvania State University, University Park, PA, USA.
authors.forename: Mark L
authors.initials: ML
authors.lastname: Latash
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D001696:Biomechanical Phenomena; D056228:Feedback, Sensory; D005260:Female; D005385:Fingers / Q000502:physiology*; D006801:Humans; D033182:Intention; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D009068:Movement / Q000502:physiology*; D011597:Psychomotor Performance / Q000502:physiology*; D055815:Young Adult
keywords: Apparent stiffness; Finger force; Isometric; Uncontrolled manifold