Motor Unit Discharge Behavior Within Phasic Bursts From Higher Threshold Motor Units

Abstract

Our recent experiments on motor unit rate coding during sinusoidal force modulation tasks has focused on motor units demonstrating continued discharge without periods of de-recruitment. As these experiments continue, we are accumulating recordings of higher threshold motor units (∼20 %MVC) that contribute brief bursts of action potentials during peaks in these isometric force-matching conditions. The PURPOSE is to describe the discharge behavior of these motor units to better our understanding of how the brief activity of higher threshold motor units contributes to the gradation of muscular force. METHODS: Using indwelling electrodes, motor unit action potentials were recorded from the first dorsal interosseous muscle of 9 subjects as they performed 6 sinusoidal isometric force matching tasks .3, .6 and .9-Hz force oscillations were centered at 20% of maximum voluntary contraction (MVC) force and had amplitude of+/−3 and 6% MVC. The number of spikes, recruitment threshold force, and minimum and peak instantaneous firing rates were computed for each burst of action potentials within a recording. RESULTS: As task frequency increased, the relationship between recruitment threshold force and the number of spikes per burst strengthened (Pearson r = −.45, -.60, -.67 at .3, .6, .9-Hz, respectively). Within bursts, there was notable presence of single occurrences of high instantaneous discharge rates (>50 pps, inter-spike interval <20 ms) resulting in a skewed distribution of peak firing rates. The median peak firing rate of motor units demonstrating phasic discharge was 19.51 pps. This is less than the firing rate observed in lower threshold motor units exhibiting sustained discharge (24.2 pps). There were trends towards increases in firing rates with increases in task amplitude and frequency (p∼.1). The minimum discharge rate within a burst increased significantly with increases in task frequency (p<05: 5.9, 7.7, 9.7 pps at .3, .6 and .9-Hz, respectively). CONCLUSION: Consecutive motor unit action potentials with atypically brief inter-pulse intervals are known to enhance muscular force output. Along with the greater force output from higher threshold motor units, we need to consider the relative contributions of these brief bursts of motor unit discharge in the overall gradation of muscular force.