The effect of rate of torque development on motor unit recruitment and firing rates during isometric voluntary trapezoidal contractions

Abstract

It is common practice to examine motor unit (MU) activity according to mean firing rate (MFR) and action potential amplitude (MUAPAMP) vs. recruitment threshold (RT) relationships during isometric trapezoidal contractions. However, it is unknown whether the rate of torque development during the linearly increasing torque phase affects the activity of MUs during such contractions. Sixteen healthy males and females performed two isometric trapezoidal muscle actions at 40% of maximum voluntary contraction (MVC), one at a rate of torque development of 5% MVC/s (SLOW40) and one at 20% MVC/s (FAST40) during the linearly increasing torque phase. Surface electromyography (EMG) was recorded from the vastus lateralis (VL) via a 5-pin surface array sensor and decomposed into action potential trains of individual MUs, yielding MFRs and MUAPAMP which were regressed against RT separately for each contraction. Surface EMG amplitude recorded from leg extensors and flexors was used to quantify muscle activation and coactivation. MFR vs. RT relationship slopes were more negative (P = 0.003) for the SLOW40 (− 0.491 ± 0.101 pps/%MVC) than FAST40 (− 0.322 ± 0.109 pps/%MVC) and the slopes of the MUAPAMP vs. RT relationships (P = 0.022, SLOW40 = 0.0057 ± 0.0021 mV/%MVC, FAST40 = 0.0041 ± 0.0023 mV/%MVC) and muscle activation of the extensors (P < 0.001, SLOW40 = 36.3 ± 7.82%, FAST40 = 34.0 ± 6.26%) were greater for SLOW40 than FAST40. MU firing rates were lower and action potential amplitudes were larger in relation to recruitment thresholds for a contraction performed at a slower rate compared to a faster rate of isometric torque development. Differences in MU activity can exist as a function of rate of torque development during commonly performed isometric trapezoidal contractions.

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Acknowledgements

The authors would like to thank the undergraduate research assistants who aided in the collection and analysis of data. We would also like to thank the subjects for their selfless participation.

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JDM and TJH developed the research design. CJL, MDG, KLS, MEW collected and analyzed data. JDM analyzed data, performed statistical procedures, and prepared the figures and manuscript. TJH aided in manuscript preparation. All authors edited the manuscript and approved the final version.

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Correspondence to Trent J. Herda.

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New and Noteworthy: It is common practice to characterize motor unit activity during submaximal isometric trapezoidal contractions, however, it is unknown whether the rate of torque development during such contractions influences motor unit activity. The current study revealed larger action potential amplitudes and decreased firing rates in relation to recruitment thresholds for contractions performed with a slower compared to a faster rate of torque development, suggesting motor unit activity is altered by rate of torque development.

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Miller, J.D., Lund, C.J., Gingrich, M.D. et al. The effect of rate of torque development on motor unit recruitment and firing rates during isometric voluntary trapezoidal contractions. Exp Brain Res 237, 2653–2664 (2019). https://doi.org/10.1007/s00221-019-05612-0

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Keywords

  • Rate of torque development
  • Motor unit decomposition
  • Vastus lateralis
  • Recruitment
  • Firing rate