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

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 change 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 versus recruitment threshold, motor unit action potential amplitude versus recruitment threshold, and motor unit firing rate versus action potential amplitude relationships of the biceps brachii (agonist) and triceps brachii (antagonist) muscles were analyzed. Moderate- to-strong relationships (vertical bar r vertical bar > 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 versus 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. NEW & NOTEWORTHY This is the first study to explore a potential motor unit control scheme and quantify changes in firing rates with increasing intensity of an antagonist muscle during isometric contractions. We demonstrate that the antagonist muscle possesses an organized motor unit firing rate and recruitment scheme similar to the agonist muscle during isometric forearm flexion, but unlike the agonist muscle, there was no significant increase in firing rates from a moderate- to higher-intensity isometric contraction.