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Motor unit-based synergies in a non-compartmentalized muscle

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Abstract

The concept of synergies has been used to address the grouping of motor elements contributing to a task with the covariation of these elements reflecting task stability. This concept has recently been extended to groups of motor units with parallel scaling of the firing frequencies with possible contributions of intermittent recruitment (MU-modes) in compartmentalized flexor and extensor muscles of the forearm stabilizing force magnitude in finger pressing tasks. Here, we directly test for the presence and behavior of MU-modes in the tibialis anterior, a non-compartmentalized muscle. Ten participants performed an isometric cyclical dorsiflexion force production task at 1 Hz between 20 and 40% of maximal voluntary contraction and electromyographic (EMG) data were collected from two high-density wireless sensors placed on the skin over the right tibialis anterior. EMG data were decomposed into individual motor unit frequencies and resolved into sets of MU-modes. Inter-cycle analysis of MU-mode magnitudes within the framework of the uncontrolled manifold (UCM) hypothesis was used to quantify force-stabilizing synergies. Two or three MU-modes were identified in all participants and trials accounting, on average, for 69% of variance and were robust to cross-validation measurements. Strong dorsiflexion force-stabilizing synergies in the space of MU-modes were present in all participants and for both electrode locations as reflected in variance within the UCM (median 954, IQR 511–1924) exceeding variance orthogonal to the UCM (median 5.82, IQR 2.9–17.4) by two orders of magnitude. In contrast, MU-mode-stabilizing synergies in the space of motor unit frequencies were not present. This study offers strong evidence for the existence of synergic control mechanisms at the level of motor units independent of muscle compartmentalization, likely organized within spinal cord circuitry.

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Data are available by reasonable request to the corresponding author.

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Acknowledgements

We are grateful to Dr. Matteo Bertucco for contributing necessary equipment and expertise for data collection. We are also very much grateful to Delsys, Inc. for providing the Trigno system and associated software on free loan.

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Authors and Affiliations

  1. Department of Kinesiology, Rec.Hall-20, The Pennsylvania State University, University Park, PA, 16802, USA

    Joseph M. Ricotta, Sayan D. De, Jinrui Jiang, William Graziani & Mark L. Latash

  2. Clinical and Translational Science Institute, Penn State College of Medicine, Hershey, PA, 17033, USA

    Joseph M. Ricotta

  3. Department of Neurosciences, Biomedicine and Movement Sciences, University of Verona, Verona, Italy

    Mauro Nardon

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  1. Joseph M. Ricotta
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Correspondence to Joseph M. Ricotta.

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Ricotta, J.M., Nardon, M., De, S.D. et al. Motor unit-based synergies in a non-compartmentalized muscle. Exp Brain Res 241, 1367–1379 (2023). https://doi.org/10.1007/s00221-023-06606-9

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