Anticipatory modulation of motor unit discharge rate before rapid isometric elbow flexion force production

Surface electromyography (EMG) studies have demonstrated anticipatory muscle activation prior to predictable voluntary actions. However, the mechanism through which this preparation is expressed, whether through motor-unit recruitment or discharge-rate modulation, remains to be elucidated. We employed surface decomposition EMG to quantify motor-unit behavior prior to self-paced rapid isometric elbow-flexion force pulses. Twelve healthy young men were instructed to generate elbow-flexion pulse force at 30%, 40%, and 50% of the maximal voluntary contraction (MVC) at a self-selected time. Motor-unit activity was decomposed from the biceps brachii and triceps brachii muscles, and the normalized active motor-unit number and mean discharge rate were analyzed prior to pulse onset. In the agonist, the pre-pulse increase mean discharge rate exhibited a higher value than the change in detected motor-unit count, particularly at the 40% and 50% MVC targets. The discharge-rate increase scaled with the target force, with a heightened response observed in high-threshold as compared to low-threshold motor units. Antagonist recordings with sufficient decomposition yield exhibited a similar discharge-rate-dominant pattern; however, these data were available from a smaller sample size. Present findings suggest that discharge-rate modulation is a primary motor-unit-level feature of anticipatory preparation for rapid isometric force production.