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Synergies at the level of motor units in single-finger and multi-finger tasks

Abstract

We explored the organization of motor units recorded in the flexor digitorum superficialis into stable groups (MU-modes) and force-stabilizing synergies in spaces of MU-modes. Young, healthy participants performed one-finger and three-finger accurate cyclical force production tasks. Two wireless sensor arrays (Trigno Galileo, Delsys, Inc.) were placed over the proximal and distal portions of the muscle for surface recording and identification of motor unit action potentials. Principal component analysis with Varimax rotation and factor extraction was used to identify MU-modes. The framework of the uncontrolled manifold hypothesis was used to analyze inter-cycle variance in the space of MU-modes and compute the index of force-stabilizing synergy. Multiple linear regression between the first MU-mode in the three-finger task and the first MU-modes in the three single-finger tasks showed no differences between the data recorded by the two electrodes suggesting that MU-modes were unlikely to be synonymous with muscle compartments. Multi-MU-mode synergies stabilizing task force were documented across all tasks. In contrast, there were no force-stabilizing synergies in the three-finger task analyzed in the space of individual finger forces. Our results confirm the synergic organization of motor units in single-finger tasks and, for the first time, expand this result to multi-finger tasks. We offer an interpretation of the findings within the theoretical scheme of control with spatial referent coordinates expanded to the analysis of individual motor units. The results confirm trade-offs between synergies at different hierarchical levels and expand this notion to intra-muscle synergies.

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Availability of data and materials

The original data are available from the corresponding author at a reasonable request.

Code availability

The original codes in Matlab are available from the corresponding author at a reasonable request.

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Acknowledgements

We are very much grateful to Paola Contessa and Nicholas Ducey for the important discussions and insights.

Funding

The Galileo system used in the study was on loan from Delsys, Inc. Shirin Madarshahian was supported for two months in 2019 by a fellowship from Delsys, Inc.

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Authors

Contributions

SM: design of the study, acquisition, analysis, and interpretation of data, writing the draft. MLL: conception and design of the study, analysis and interpretation of data, writing the draft. Both authors approved the final version of the manuscript, agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved; both persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.

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Correspondence to Mark L. Latash.

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All the procedures were approved by the Office for Research Protection of the Pennsylvania State University (protocol #33393) in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki.

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All of the subjects provided their informed consent based on the procedures approved by the Office for Research Protection of Pennsylvania State University.

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Communicated by Francesco Lacquaniti.

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Madarshahian, S., Latash, M.L. Synergies at the level of motor units in single-finger and multi-finger tasks. Exp Brain Res 239, 2905–2923 (2021). https://doi.org/10.1007/s00221-021-06180-y

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Keywords

  • Finger
  • Hand
  • Uncontrolled manifold
  • Referent coordinate
  • Hierarchy