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The relationship of agonist muscle single motor unit firing rates and elbow extension limb movement kinematics

Abstract

This study explored the relationship between single motor unit (MU) firing rates (FRs) and limb movement velocity during voluntary shortening contractions when accounting for the effects of time course variability between different kinematic comparisons. Single MU trains recorded by intramuscular electromyography in agonist muscles of the anconeus (n = 15 participants) and lateral head of the triceps brachii (n = 6) were measured during each voluntary shortening contraction. Elbow extension movements consisted of a targeted velocity occurring along the sagittal plane at 25, 50, 75 and 100% of maximum velocity. To account for the effect of differences in contraction time course between parameters, each MU potential was time locked throughout the shortening muscle contraction and linked with separated kinematic parameters of the elbow joint. Across targeted movement velocities, instantaneous FRs were significantly correlated with elbow extension rate of torque development (r = 0.45) and torque (r = 0.40), but FRs were not correlated with velocity (r = 0.03, p = n.s.). Instead, FRs had a weak indirect relationship with limb movement velocity and position assessed through multiple correlation of the stepwise kinematic progression. Results show that voluntary descending synaptic inputs correspond to a more direct relationship between agonist muscle FRs and torque during shortening contractions, but not velocity. Instead, FRs were indirectly correlated to preparing the magnitude of imminent movement velocity of the lagging limb through torque.

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Availability of data and material

Datasets described in this paper can be accessed by correspondence with the authors on reasonable request.

Code availability

All analyses were supported from open source software packages in R, specific code described in the current study can be accessed by correspondence with the authors.

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Acknowledgements

We thank all participants for their volunteered contribution.

Funding

This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) to E. A. Kirk (CGS-D) and C. L. Rice (No. 180970).

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Correspondence to Charles L. Rice.

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The authors have no competing interests.

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In agreement with the latest revision of the Declaration of Helsinki, the local University’s full research ethics board for human experimentation approved all procedures (no. 107505).

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Communicated by Winston D Byblow.

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Kirk, E.A., Rice, C.L. The relationship of agonist muscle single motor unit firing rates and elbow extension limb movement kinematics. Exp Brain Res 239, 2755–2766 (2021). https://doi.org/10.1007/s00221-021-06168-8

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Keywords

  • Concentric
  • Discharge rate
  • Human
  • Rate coding
  • Skeletal muscle
  • Voluntary