An examination of a potential organized motor unit firing rate and recruitment scheme of an antagonist muscle during isometric contractions

Abstract

The primary purpose of the present study is to determine if an organized control scheme exists for the antagonist muscle during steady isometric torque. A secondary focus is to better understand how firing rates of the antagonist muscle changes from a moderate- to higher-contraction intensity. Fourteen subjects performed two submaximal isometric trapezoid muscle actions of the forearm flexors that included a linearly increasing, steady force at both 40% and 70% maximum voluntary contraction, and linearly decreasing segments. Surface electromyographic signals of the biceps and triceps brachii were collected and decomposed into constituent motor unit action potential trains. Motor unit firing rate vs. recruitment threshold, motor unit action potential amplitude vs. recruitment threshold, and motor unit firing rate vs. action potential amplitude relationships of the biceps brachii (agonist) and triceps brachii (antagonist) muscles were analyzed. Moderate- to-strong relationships (|r| ³ 0.69) were present for the agonist and antagonist muscles for each relationship with no differences between muscles (p = 0.716, 0.428, 0.182). The y-intercepts of the motor unit firing rate vs. recruitment threshold relationship of the antagonist did not increase from 40% to 70% maximal voluntary contractions (p = 0.96), unlike for the agonist (p = 0.009). The antagonist muscle exhibits a similar motor unit control scheme to the agonist. Unlike the agonist, however, the firing rates of the antagonist did not increase with increasing intensity. Future research should investigate how antagonist firing rates adapt to resistance training and changes in antagonist firing rates in the absence of peripheral feedback.

Velocity-Specific Coactivation and Neuromuscular Responses to Fatiguing, Reciprocal, Isokinetic, Forearm Flexion, and Extension Muscle Actions

“…In addition, using MMG MPF (12), and MMG AMP and MPF (19) to quantify coactivation, it has been shown that there were similar responses between agonist and antagonist muscles during fatiguing isometric contractions of the flexors/extensors of thumb and forearm, respectively. The results of this study extended the findings of Lévénez et al (25), De Luca and Mambrito (12), and Reece and Herda (30) to maximal, reciprocal, fatiguing, isokinetic forearm flexion, and extension muscle actions. The parallel fatigue-induced responses observed within each muscle during both movements may be attributable to a descending "common drive" to the agonist and antagonist muscles (12,28,30) by the musculocutaneous and radial nerves, which both stem from the C6 spinal nerve (6).…”

“…Thus, in this study, it is possible that the parallel fatigue-induced responses in activation between the agonist and antagonist muscles were the result of a “common drive” to provide elbow joint stability (12,13,28). Furthermore, the current findings, in conjunction with those of previous studies (12,19,25,30), suggested that although coactivation can be examined using EMG, the simultaneous examination of MMG responses may be helpful in assessing coactivation.…”

“…The results of this study extended the findings of Lévénez et al (25), De Luca and Mambrito (12), and Reece and Herda (30) to maximal, reciprocal, fatiguing, isokinetic forearm flexion, and extension muscle actions. The parallel fatigue-induced responses observed within each muscle during both movements may be attributable to a descending “common drive” to the agonist and antagonist muscles (12,28,30) by the musculocutaneous and radial nerves, which both stem from the C6 spinal nerve (6). Therefore, when performing the same task, the motor neuron pools of the muscle pair are controlled as if they are one motor neuron pool (12).…”

An examination of motor unit firing rates during steady torque of maximal efforts with either an explosive or slower rate of torque development

“…The analysis of MU firing rates in earlier phases (0-200 ms) of torque development would add considerably to our understanding of differences in MU control schemes between the contractions performed in the present study (Maffiuletti et al, 2016) MUs within the observed action potential ranges (Contessa et al, 2018;Hu et al, 2013;Miller et al, 2018Miller et al, , 2019Miller et al, , 2020Reece & Herda, 2021;Sterczala et al, 2018b). Figures 9 and 10 illustrate the MFR versus MUAP AMP relationships along with 95% confidence intervals for each subject and muscle included in the firing rate ANOVA models.…”

“…This MU firing rate versus action potential amplitude relationship has been demonstrated in submaximal contractions in both agonist (Hu et al, 2013;Miller et al, 2019;Trevino et al, 2019) and antagonist muscles (Reece & Herda, 2021). In order to gain a better understanding of activation of the agonist (Miller et al, 2018(Miller et al, , 2019(Miller et al, , 2020Parra et al, 2020;Sterczala et al, 2020;Trevino et al, 2019) and antagonist muscles (Reece & Herda, 2021), previous studies have quantified MU firing rates as a function of action potential amplitude on a subject-by-subject and contraction-by-contraction basis. Therefore, we examined firing rates and action potential amplitudes of MUs recorded from the biceps brachii (BB) and triceps brachii (TB) during isometric explosive and ramp MVCs of the elbow flexors.…”