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Exp Brain Res. 2018 Jun;236(6):1545-1562. doi: 10.1007/s00221-018-5238-y. Epub 2018 Mar 22.

Stability of steady hand force production explored across spaces and methods of analysis.

Author information

1
Interdisciplinary Graduate Program in Healthy Sciences, Cruzeiro do Sul University, São Paulo, SP, Brazil.
2
Department of Kinesiology, Rec.Hall-267, The Pennsylvania State University, University Park, 16802, PA, USA.
3
Department of Neurology, Milton S. Hershey Medical Center, The Pennsylvania State University, Hershey, PA, USA.
4
Graduate Program in Physical Therapy, City University of São Paulo, São Paulo, SP, Brazil.
5
Department of Pharmacology, Milton S. Hershey Medical Center, The Pennsylvania State University, Hershey, PA, USA.
6
Department of Radiology, Milton S. Hershey Medical Center, The Pennsylvania State University, Hershey, PA, USA.
7
Department of Neurosurgery, Milton S. Hershey Medical Center, The Pennsylvania State University, Hershey, PA, USA.
8
Department of Kinesiology, Rec.Hall-267, The Pennsylvania State University, University Park, 16802, PA, USA. mll11@psu.edu.

Abstract

We used the framework of the uncontrolled manifold (UCM) hypothesis and explored the reliability of several outcome variables across different spaces of analysis during a very simple four-finger accurate force production task. Fourteen healthy, young adults performed the accurate force production task with each hand on 3 days. Small spatial finger perturbations were generated by the "inverse piano" device three times per trial (lifting the fingers 1 cm/0.5 s and lowering them). The data were analyzed using the following main methods: (1) computation of indices of the structure of inter-trial variance and motor equivalence in the space of finger forces and finger modes, and (2) analysis of referent coordinates and apparent stiffness values for the hand. Maximal voluntary force and the index of enslaving (unintentional finger force production) showed good to excellent reliability. Strong synergies stabilizing total force were reflected in both structure of variance and motor equivalence indices. Variance within the UCM and the index of motor equivalent motion dropped over the trial duration and showed good to excellent reliability. Variance orthogonal to the UCM and the index of non-motor equivalent motion dropped over the 3 days and showed poor to moderate reliability. Referent coordinate and apparent stiffness indices co-varied strongly and both showed good reliability. In contrast, the computed index of force stabilization showed poor reliability. The findings are interpreted within the scheme of neural control with referent coordinates involving the hierarchy of two basic commands, the r-command and c-command. The data suggest natural drifts in the finger force space, particularly within the UCM. We interpret these drifts as reflections of a trade-off between stability and optimization of action. The implications of these findings for the UCM framework and future clinical applications are explored in the discussion. Indices of the structure of variance and motor equivalence show good reliability and can be recommended for applied studies.

KEYWORDS:

Hand; Motor equivalence; Referent coordinate; Reliability; Synergy; Variance

PMID:
29564506
PMCID:
PMC5984153
[Available on 2019-06-01]
DOI:
10.1007/s00221-018-5238-y
[Indexed for MEDLINE]

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